copyright by xiangyu liu 2017
TRANSCRIPT
Copyright
by
Xiangyu Liu
2017
The Dissertation Committee for Xiangyu Liu Certifies that this is the approved
version of the following dissertation:
Mud-to-Cement Conversion of Synthetic-Based Drilling Muds using
Geopolymers
Committee:
Eric van Oort, Supervisor
Paul M. Bommer
Hugh C. Daigle
David N. Espinoza
Maria G. Juenger
Sriramya D. Nair
Mud-to-Cement Conversion of Synthetic-Based Drilling Muds using
Geopolymers
by
Xiangyu Liu
Dissertation
Presented to the Faculty of the Graduate School of
The University of Texas at Austin
in Partial Fulfillment
of the Requirements
for the Degree of
Doctor of Philosophy
The University of Texas at Austin
August 2017
Dedication
Dedicated to my wonderful parents, Haimi Liu and Min Wang,
and my dear husband, Jun Lu,
for their endless love, support and encouragement.
v
Acknowledgements
First of all I would like to thank my supervisor, Prof. Eric van Oort for the
opportunity to work on this project and for introducing me to the world of well
cementing. Thank you for your invaluable guidance, support and encouragement as I
pursue my degree. Your invaluable suggestions and guidance has inspired and motivated
me to keep exploring research ideas with both fundamental and practical values.
I would like to extend my gratitude to my committee members: Dr. Paul Bommer,
Dr. Hugh Daigle, Dr. Maria Juenger, Dr. Nicolas Espinoza and Dr. Sriramya Nair. Thank
you for your friendly interactions, and for the insightful suggestions and comments on my
work.
To the lab ladies, Sriramya Nair, Qian Wu, Katherine Aughenbaugh, Michelle
Shuck and Hanna Lee - I would not have finished without your support. You all made the
lab full of fun and encouragement. I truly enjoyed the memorable and productive time we
have spent together. It would be a difficult journey without all your support.
I thank Matthew Ramos for helping me run the triaxial compressive strength test
and Besmir Bez Buranaj and Bence Tóth for running the pressure transmission test. The
results were very important for characterizing the materials. I would also like to thank Dr.
Raissa Ferron from the Civil, Architechtural, and Environmental Engineering Department
for allowing me the use of the particle size analyzer in her laboratory.
Special thanks go to Tesse Smitherman, Frankie Hart, Amy Stewart, Glen Baum,
Gary Miscoe and Daryl Nygaard for their technical and administrative support.
vi
I would like to acknowledge the contribution of undergraduate assistants who
have aided in the experimental work and data analysis. A special recognition for Hanna
Lee, Marjorie Dininger, and Mateo Valencia.
I would like to thank ConocoPhillips for providing research funding towards this
project. Thank you to PQ Corporation and SEFA Group for providing materials and
technical support.
Finally, my heartfelt gratitude goes to my parents, Haimi Liu and Min Wang, and
my brother, Xiangyi Liu, for their unconditional love and support. To my beloved
husband, Jun Lu, thank you for being so patient, caring and supportive throughout this
process, and for making my graduate life so memorable. You have been - and will always
be - a source of strength and inspiration for me.
vii
Mud-to-Cement Conversion of Synthetic-Based Drilling Muds using
Geopolymers
Xiangyu Liu, Ph.D.
The University of Texas at Austin, 2017
Supervisor: Eric van Oort
When constructing wells ranging from simple land wells to complex deepwater
wells, incompatibility between oil-based and synthetic-based muds (OBM / SBM) and
Portland cements can lead to poor cementation and loss of cement integrity, which in turn
may compromise zonal isolation. An alternative cementitious material based on
geopolymers has been developed with improved OBM / SBM compatibility for primary
cementing and lost circulation control as well as well abandonment. Benefits of using
geopolymers go beyond mere OBM / SBM compatibility: it is in fact possible to solidify
non-aqueous drilling fluids (NAF) such as SBM and OBM using geopolymer
formulations. This also means that such NAFs can be disposed of in a more cost-effective
way, which presents a viable option for environmentally acceptable on-site or off-site
disposal of drilling muds and cuttings. In the following, focus will be primarily on the
compatibility between SBM and geopolymers, with the understanding that the results
obtained for SBM can generally be extrapolated to OBM as well.
Geopolymer is a type of alkali-activated material that forms when an
aluminosilicate precursor powder (such as fly ash) is mixed with an alkaline-activating
solution (such as sodium hydroxide). A novel SBM solidification method was developed
viii
by blending varied amounts of geopolymer and SBM. The consolidated mud was named
a “geopolymer hybrid cement”.
In an effort to develop the geopolymer hybrid system as a novel well cementing
material, the solidification method was comprehensively studied with various sources of
precursor powders, activators, as well as SBM and OBM formulations. Fresh state
properties, such as slurry rheology and thickening time, and hardened state mechanical
properties, such as compressive strength (under both uniaxial and triaxial confinement
conditions), as well as the self-healing capabilities of the geopolymer hybrid cement were
evaluated.
Strength testing results showed that geopolymer cement can solidify up to a 60/40
geopolymer/SBM ratio by volume. The incorporation of SBM greatly improved the
rheological properties of the geopolymer hybrid, allowing for the otherwise non-
pumpable slurry to become pumpable for well cementation and lost circulation control
purposes. The laboratory evaluations showed that the geopolymer hybrid cement could
meet typical requirements as a well cementing slurry. By changing the amount of
geopolymer and SBM in the slurry, the geopolymer hybrid can be deliberately designed
with high compressive strength for primary cementation, or with lower compressive
strength for lost circulation control. Moreover, geopolymer and geopolymer hybrid
cements reveal true self-healing capability, which means that they can recover and even
increase their strength after prior yielding. This ability would possibly allow such
cements to better adapt to subsurface stress changes acting on abandoned wells, making
them better suited for use in permanent barriers in plug and abandonment operations.
ix
Table of Contents
List of Figures ...................................................................................................... xiii
List of Tables .........................................................................................................xx
Chapter 1: Introduction ...................................................................................1
Motivation .................................................................................................1 1.1
Objectives .................................................................................................3 1.2
Dissertation Organization .........................................................................4 1.3
Chapter 2: Background ...................................................................................6
Geopolymer: Definition and Terminology ...............................................6 2.1
Synthesis of Alkali-Activated Fly Ash-based Geopolymer ......................7 2.2
Aluminosilicate source..................................................................7 2.2.1
Activating solution ......................................................................11 2.2.2
Hydroxide Activation..................................................................11
Silicate Activation .......................................................................12
Proportioning ..............................................................................12 2.2.3
Curing regime .............................................................................13 2.2.4
Polymerization Mechanism and Microstructure .....................................14 2.3
Properties and Admixture .......................................................................16 2.4
Compressive strength ..................................................................17 2.4.1
Rheological properties ................................................................17 2.4.2
Setting time control .....................................................................18 2.4.3
Self-healing capability ................................................................20 2.4.4
Other properties ..........................................................................21 2.4.5
Geopolymer in Oilwell Cementing .........................................................22 2.5
Primary cementation ...................................................................22 2.5.1
Well abandonment ......................................................................26 2.5.2
Lost circulation control ...............................................................29 2.5.3
Mud Solidification Technique ................................................................31 2.6
x
BFS-based drilling mud solidification ........................................31 2.6.1
Geopolymer-based solidification for organic waste disposal .....33 2.6.2
Summary .................................................................................................34 2.7
Chapter 3: Materials and Methods ................................................................35
Raw Materials .........................................................................................35 3.1
Fly ash composition ....................................................................35 3.1.1
Particle size distribution (PSD) of fly ashes ...............................36 3.1.2
Activator .....................................................................................37 3.1.3
Admixtures ..................................................................................39 3.1.4
Portland cement ..........................................................................40 3.1.5
Alkali-activated slag ...................................................................40 3.1.6
SBM / OBM ................................................................................41 3.1.7
Methods...................................................................................................43 3.2
Compressive strength ..................................................................43 3.2.1
Ultrasonic compressive strength .................................................44 3.2.2
Rheology .....................................................................................44 3.2.3
Thickening time ..........................................................................45 3.2.4
Confined compressive strength ...................................................45 3.2.5
Unconfined self-healing test .......................................................47 3.2.6
Confined self-healing test ...........................................................47 3.2.7
Pipe-in-pipe shear bond strength test ..........................................48 3.2.8
Pressure transmission test ...........................................................49 3.2.9
Porosity and pore size distribution ............................................53 3.2.10
Chapter 4: Hydroxide Activation..................................................................55
Contamination Resistance .......................................................................55 4.1
Properties of Geopolymer Hybrid Cement .............................................61 4.2
Compressive Strength .................................................................61 4.2.1
Downhole and Surface Rheology ...............................................62 4.2.2
Thickening Time .........................................................................64 4.2.3
Validation ................................................................................................65 4.3
xi
Effect of Changing Activator Molarity .......................................65 4.3.1
Effect of Changing Aluminosilicate Source ...............................67 4.3.2
Effect of Seawater .......................................................................70 4.3.3
Effect of SBM Composition .......................................................72 4.3.4
Effect of Pressure ........................................................................76 4.3.5
Stability Control ......................................................................................78 4.4
Solidification of Non-Aqueous Drilling Muds .......................................82 4.5
Summary .................................................................................................84 4.6
Chapter 5: Silicate Activation .......................................................................86
Rheological Properties ............................................................................86 5.1
Compressive Strength .............................................................................93 5.2
Thickening Time .....................................................................................95 5.3
Summary .................................................................................................99 5.4
Chapter 6: Mechanical Properties and Self-Healing Capability .................100
Confined Compressive Strength ...........................................................100 6.1
Mechanical Properties ...........................................................................103 6.2
Self-Healing Properties .........................................................................105 6.3
Cement-to-Pipe Bond Strength .............................................................113 6.4
Hydraulic Conductivity .........................................................................116 6.5
Porosity .................................................................................................119 6.6
Ultrasonic Cement Strength ..................................................................122 6.7
Summary ...............................................................................................125 6.8
Chapter 7: Conclusions and future work ....................................................128
Conclusions ...........................................................................................129 7.1
Future Work ..........................................................................................134 7.2
xii
List of Abbreviations ...........................................................................................137
List of Key Symbols ............................................................................................139
List of Publications ..............................................................................................140
Bibliography ........................................................................................................141
xiii
List of Figures
Figure 2.1 - Ternary phase diagram showing the composition of OPC, blast furnace
slag (BFS), fly ash (FA), silica fume and metakaolin.........................9
Figure 2.2 - Schematic representation of the alkali activation reaction process
(Juenger et al., 2011) .........................................................................15
Figure 2.3 - Effects of calcium and magnesium compounds (at the molar dosage of
0.09 mol) on the setting time of three different alkali-activated fly
ash/kaolinite blends ((Lee and van Deventer, 2002a) .......................20
Figure 2.4 - Primary cementation illustration ........................................................23
Figure 2.5 - Cement plugs in an abandonment well (image adopted from Global CCS
Institute) ............................................................................................27
Figure 2.6 - Lost circulation scenario with (a) partial loss, and (b) total loss (figure
adopted from petrowiki.org) .............................................................30
Figure 3.1 - Volume weighted particle size distribution of all fly ash particles. For the
x-axis, 40 size intervals were generated logarithmically between 0.1 and
1000. Y-axis shows the volume fraction of particles between those sizes.
...........................................................................................................37
Figure 3.2 - Volume weighted particle size distribution of limestone dust (admixture
A) ......................................................................................................40
Figure 3.3 - Schematic diagram of the pipe-in-pipe shear bond strength set-up ...48
Figure 3.4 - Schematic of the pressure transmission test set-up ............................50
Figure 3.5 - Picture of sample assembly in pressure transmission test ..................51
xiv
Figure 4.1 - Compressive strength values of hardened Portland cement slurry (P1) and
geopolymer slurry (G1) with SBM (S1) contamination (replacement by
volume) at 24 hours, 170 °F and 3,000 psi. ......................................56
Figure 4.2 - Normalized compressive strength of hardened Portland cement slurry
(P1) and geopolymer slurry (G1) with SBM (S1) contamination
(replacement by volume) at 24 hours, 170 °F and 3,000 psi. ...........57
Figure 4.3 - Rheological properties of (a) Portland cement (P1) and (b) geopolymer
(G1) slurries replaced with various dosages of SBM (S1) by volume at
70 °F ..................................................................................................59
Figure 4.4 – Numerical simulation of drilling mud displacement with cement slurry.
The color bar shows the volume fraction of the cement slurry. The color
gradient at the interface indicates mixing of the two fluids (Enayatpour
and van Oort, 2017) ..........................................................................60
Figure 4.5 - Compressive strength of neat geopolymer (G1) and geopolymer hybrids
(G1S1) at 170 °F and 3,000 psi .........................................................62
Figure 4.6 - Rheological properties of neat geopolymer (G1), geopolymer hybrid
(G1S1-20) and Portland cement (P1-R) at 70 °F and 125 °F ...........63
Figure 4.7 - Thickening time of geopolymer hybrids at 125 °F ............................64
Figure 4.8 - 1-day compressive strength of geopolymer hybrids activated by 6M or
8M NaOH activator...........................................................................65
Figure 4.9 - Rheological properties of geopolymer hybrids activated by 6M or 8M
NaOH activator at 70 °F ...................................................................66
Figure 4.10 - (a) 1-day and (b) 3-day compressive strength of geopolymer hybrids
with three different types of fly ashes ...............................................68
Figure 4.11 - Rheological properties of G2S1 hybrids at 70 °F ............................69
xv
Figure 4.12 - Effect of changing aluminosilicate source on the thickening time of
G1S1-20 and G2S1-20 slurries at 125 °F and 3,000 psi ...................70
Figure 4.13 - Effect of using seawater vs. DI water on the compressive strength of G1
and G1S1 hybrids at 70 °F and 3,000 psi .........................................71
Figure 4.14 - Effect of using seawater vs. Di water on the rheological properties of
G1 and G1S1 hybrids at 70 °F ..........................................................71
Figure 4.15 - Rheology of the original SBM (75/25 SWR, 23% CaCl2) as well as
modified SBMs at (a) 80° F, (b) 120° F, and (c) 150° F ..................73
Figure 4.16 - Effect of changing SWR and internal brine CaCl2 concentration of SBM
on (a) rheology and (b) gel strength of G2S1-30-2A at 125 °F and 3,000
psi ......................................................................................................74
Figure 4.17 - Effect of changing SWR and internal brine CaCl2 concentration of SBM
on thickening time of G2S1-30-2A. (BHCT of 125 °F and BHP of 3,000
psi).....................................................................................................75
Figure 4.18 - Effect of changing SWR and internal brine CaCl2 concentration of SBM
on 1-day compressive strength of G2S1-30-2A at 170 °F and 3,000 psi
...........................................................................................................76
Figure 4.19 - Effect of pressure on thickening time of G2S1-30-2A at BHCT of 125
°F .......................................................................................................77
Figure 4.20 - (a) 1-day and (b) 3-day compressive strength of G2S1 hybrids with
varying dosages of stability modifier (A) .........................................79
Figure 4.21 - Effect of adding 1.5% stability modifier (A) on the rheological
properties of (a) G2 and G2S1-20 hybrid, (b) G2S1-30 and G2S1-40
hybrids at 70 °F .................................................................................80
xvi
Figure 4.22 - Effect of different dosages of stability modifier (A) on the rheological
properties of G2S1-40 hybrids at 125 °F ..........................................81
Figure 4.23 - Effect of adding 0.75% of stability modifier (A) on thickening time for
G1S1-30 hybrid .................................................................................82
Figure 4.24 - Normalized compressive strength of hardened Portland cement slurries
(P1) and geopolymer slurries (G1) with 20% mud replacement (by
volume) .............................................................................................83
Figure 4.25 - Picture of a geopolymer/mud sample containing 20% (a) S1 and (b) O2
...........................................................................................................84
Figure 5.1 - (a) rheology and (b) gel strength of liquid-form sodium silicate (LSS)
activated geopolymer (0% SBM) at room temperature ....................88
Figure 5.2 - (a) rheology and (b) gel strength of liquid-form sodium silicate (LSS)
activated geopolymer blended with 20% SBM at room temperature90
Figure 5.3 - (a) rheology and (b) gel strength of solid form sodium silicate (SSS)
activated geopolymer (0% SBM) at room temperature ....................91
Figure 5.4 - (a) rheology and (b) gel strength of solid form sodium silicate (SSS)
activated geopolymer blended with 20% SBM at room temperature92
Figure 5.5 - Compressive strength of liquid sodium silicate (LSS) activated
geopolymer for (a) fly ash 1 and (b) fly ash 2 in comparison to liquid
sodium hydroxide (LSH) activated geopolymer without any SBM at 170
°F and 3,000 psi ................................................................................94
Figure 5.6 - Compressive strength of solid sodium silicate (SSS) activated
geopolymer for FA 2 in comparison to liquid sodium hydroxide (LSH)
activated geopolymer without any SBM at 170 °F and 3,000 psi ....95
xvii
Figure 5.7 - Effect of various activating solutions on the thickening time of neat
geopolymer slurry (G2) at 125 °F and 3,000 psi ..............................96
Figure 5.8 - Effect of adding 20% original SBM on thickening time of (a) LSH-8M
and (b) SSS-0.24 geopolymer slurries (G2) at 125 °F and 3,000 psi98
Figure 6.1 - Differential stress vs. axial strain for two samples each for geopolymer
subjected to a confining pressure of 100 psi and 500 psi at 7 days 102
Figure 6.2 - Differential stress vs. axial strain for two samples each for geopolymer
hybrid subjected to a confining pressure of 100 psi and 500 psi at 7 days
.........................................................................................................102
Figure 6.3 - Differential stress vs. axial strain for two samples each for Portland
cement subjected to a confining pressure of 100 psi and 500 psi at 7 days
.........................................................................................................103
Figure 6.4 - Self-healing properties of geopolymer and Portland cement pre-loaded at
(a) 7 days and (b) 28 days under atmospheric conditions. Cylindrical
samples were prepared at 170 °F ....................................................107
Figure 6.5 - Self-healing capability of geopolymer (G) under 500 psi confining stress,
for two samples (a) and (b). The G-7 sample was loaded beyond its yield
point at 7 days, and the same sample was re-tested to failure at 28 days
(G-7-28). Note the evident increase in peak stress observed for the
samples at 28 days ..........................................................................109
Figure 6.6 - Self-healing capability of geopolymer hybrid (GH) under 500 psi
confining stress, for two samples (a) and (b). The GH-7 sample was
loaded beyond its yield point at 7 days and the same sample was re-
tested to failure at 28 days (GH-7-28). Note the evident increase in peak
stress observed for the samples at 28 days ......................................110
xviii
Figure 6.7 - Self-healing capability of Portland cement (P) under 500 psi confining
stress, for two samples (a) and (b). The P-7 sample was loaded beyond
its yield point at 7 days and the same sample was re-tested to failure at
28 days (P-7-28). Note the evident reduction in peak stress observed for
the samples at 28 days.....................................................................111
Figure 6.8 - Cement-to-pipe shear bond strength for two samples of Portland cement
and geopolymer with clean steel pipe at 170 °F on day-7 ..............114
Figure 6.9 - Cement-to-pipe shear bond strength for two samples of Portland cement
and geopolymer with SBM coated steel pipe at 170 °F on day-7 ...115
Figure 6.10 - Cross-section view from bottom of the cement-to-pipe shear bond test
samples for (a) Portland cement and (b) geopolymer .....................115
Figure 6.11 - Pressure transmission curves, linear fit and the delay factor for (a)
Portland cement, (b) geopolymer, and (c) slag at 28 days ..............117
Figure 6.12 - Hydraulic conductivity (HC) and linear fit trend lines of Portland
cement, geopolymer, and slag at 28 days .......................................118
Figure 6.13 - Pore size distribution of Portland cement, geopolymer and slag obtained
from MIP test, samples were cured at 170 °F for 28 days ..............121
Figure 6.14 - Geopolymer sonic compressive strength and transit time obtained from
UCA test with built-in empirical correlations developed for Portland
cement, test was conducted at 189 °F and 5,000 psi .......................123
Figure 6.15 - Portland cement sonic compressive strength and transit time obtained
from UCA test with built-in empirical correlations developed for
Portland cement, test was conducted at 170 °F and 3,000 psi ........124
xix
Figure 6.16 - Geopolymer sonic compressive strength and transit time obtained from
UCA test with empirical correlations developed for geopolymer, test
was conducted at 189 °F and 5,000 psi ...........................................125
Figure 7.1 - Circular flow diagram showing the design philosophy of
geopolymer/mud hybrid cement .....................................................133
xx
List of Tables
Table 2.1 - Empirical correlations for geopolymer specimens prepared with various
NaOH concentrations at two different temperatures. Specimens were
cured under a pressure of 5,000 psi (Khalifeh et al. 2014) ...............29
Table 3.1 - Composition of fly ashes (weight %) ..................................................36
Table 3.2 - Geopolymer mix design ......................................................................39
Table 3.3 - Properties of the SBMs (S1 & S2) ......................................................42
Table 3.4 - Properties of the OBMs (O1 & O2) ....................................................43
Table 3.5 - Parameters used in the MIP measurement ...........................................54
Table 6.1 - Confined compressive strength at 7 days ..........................................103
Table 6.2 - Mechanical properties of hardened slurries at 500 psi confining pressure
.........................................................................................................105
Table 6.3 - Peak stress values at 7-days and after 21-day waiting period (at 500 psi
confining stress) ..............................................................................112
Table 6.4 - Porosity and pore size evaluation of Portland, geopolymer and slag from
MIP measurement at 7 days ............................................................120
1
Chapter 1: Introduction
MOTIVATION 1.1
Field experience and laboratory investigations have established that a major cause
of oilwell cementing failure is contamination of the cement slurry by drilling mud
(Aughenbaugh et al., 2014; Beach and Goins, 1957; Miranda et al., 2007; Morgan and
Dumbauld, 1952). Intermixing of cement slurry and drilling mud will almost inevitably
occur even under nearly ideal displacement conditions. Numerical simulation has shown
that high levels of cement contamination occur when interface instability occurs during
mud displacement (Enayatpour and van Oort, 2017). With advancements in drilling in
more challenging environments, including deepwater, high-pressure / high temperature
(HPHT), unconventional plays accessed by directional / horizontal drilling etc.,
progressively more oil / gas reservoirs are becoming accessible to exploration and
production. This in turn leads to the more difficult cementing operations, and it is a fair
assessment to state that cementing technology has not kept pace with the advancement of
drilling technology. Highly deviated wellbores, wellbore enlargement stemming from
hole instability, poor casing centralization, improper displacement, etc. all increase the
potential of cement contamination (Nair et al., 2015; Nelson and Guillot, 2006), in turn
resulting in poor / insufficient hydraulic zonal isolation by cement. This can lead to a
variety of “knock-on” problems, including sustained annular casing pressure, intermixing
of reservoir fluids, potential contamination of shallow aquifers, increased well control
incident risks, etc. Cement contamination may also play a prominent role in well
abandonment operations, when leftover drilling mud in the well can mix with - and
contaminate - the cement slurry used for abandonment plugs.
2
Targeting the contamination issue, a system named “universal fluid (UF)” was
developed over 25 years ago. This system was based on a water-based mud (WBM)
which could be converted into a well cement (Hale and Cowan, 1991; Nahm et al., 1994).
A UF, when used as a drilling mud, delivers all of the necessary characteristics of a
drilling mud. As an added dimension, however, it contains a hydraulic material (blast
furnace slag), which can be activated to set up like a cementitious material when an
alkaline activator is added. The main advantage of a UF in comparison to an ordinary
Portland cement (OPC) system is its improved inherent compatibility with water-based
drilling muds. With the use of a UF, any undisplaced drilling mud or mud filter cake may
be incorporated into the cement, thereby largely eliminating the effect of contamination.
The UF system, however, never found widespread industry acceptance after some
promising early trials and field applications.
To date, a similar mud-to-cement conversion technique has not been successfully
developed and applied for NAFs. Contamination of cement slurry by NAFs is well-
known to be detrimental to important cement slurry properties such as pumping time and
compressive strength. Studies have shown that Portland cement slurries are particularly
sensitive to NAF contamination. With even a relatively small quantity of SBM
contamination, Portland cement slurries can completely lose their compressive strength
(Aughenbaugh et al., 2014; Miranda et al., 2007; Patel et al., 1999). Changing the
industry’s dependence on Portland cement, however, will require a paradigm shift that is
supported by the development of suitable and competent non-Portland based alternatives
that address the shortcomings of Portland cement.
This study will focus on the development of high performance geopolymers for
mud-to-cement conversion of SBMs. Development of geopolymers to be the next
generation of oilwell cements is a promising approach to meet current well cementing
3
challenges. Geopolymer cement is a type of alkali-activated material (AAM) that can be
formed by blending an alkaline solution with an aluminosilicate powder such as fly ash
(Duxson et al., 2006). Geopolymers are receiving increasing attention as a replacement
for Portland cement in various industries (e.g. building and road construction) due to their
versatile chemistry, low environmental impact (e.g. the manufacturing of AAMs usually
leads to no or low CO2 emissions, see Provis and van Deventer, 2014), and the fact that
AAMs such as fly-ash are typically waste streams and side-products of other industries
(e.g. energy production from coal burning), allowing relatively inexpensive waste to be
(re-)employed for a useful purpose. The different chemical makeup makes geopolymers
appropriate for use in cases where OPC is less suitable. Geopolymers have largely been
focused on construction applications over the last few decades. A point of special interest
is that researchers have, until now, not been successful in identifying a geopolymer slurry
formulation that is readily pumpable (note that the high viscosity of most geopolymer
slurries allows them to be poured, e.g. for road bases and construction templates, but not
pumped without excessive pumping pressures), which is essential for oilwell cementing
(Khalifeh et al., 2014). Moreover, open literature regarding formulation of these slurries
for oil / gas well applications remains very limited.
OBJECTIVES 1.2
The main objective of this research is to explore the feasibility of using alkali-
activated fly ash (also known as geopolymer) to convert SBM into cementitious
materials. The specific R&D objectives and questions can be broken down as follows:
1. Can geopolymer solidify SBM at various slurry-to-mud mixing ratios?
4
2. Can the compressive strength of the geopolymer/mud hybrid system be tailored to
meet various applications such as primary cementing, remedial cementing, or lost
circulation control?
3. Will geopolymer/mud hybrid systems have acceptable pumpability and
rheological properties at both surface and downhole conditions for efficient mud
displacement and cement placement?
4. Will the geopolymer/mud hybrid system be stable at various pressure and
temperature conditions?
5. What are critical hardened state properties of the geopolymer/mud hybrid system
including mechanical properties, porosity and hydraulic conductivity?
6. Can geopolymer and geopolymer/mud hybrid systems be developed for
application in well abandonment and decommissioning? In particular, do these
systems exhibits “self-healing” characteristics, i.e. strength recovery after
yielding, which will contribute to long-term integrity of well abandonment plugs?
DISSERTATION ORGANIZATION 1.3
This dissertation is organized into several sections. Chapter 2 provides a succinct
literature review with introduction to the definition, composition and synthesis of
geopolymers. The geopolymer characteristics that are of importance to oilwell cementing
applications are also reviewed in detail. Chapter 3 gives detailed description of the
materials used in this study and the experimental methods used to characterize the new
cement/mud hybrid system. Chapter 4 presents the results and discussion on hydroxide-
activated geopolymers and their effectiveness in solidifying SBM. This chapter also gives
details about the influence of activator dosage, aluminosilicate source, different types and
composition of mud, and effect of downhole pressure. Chapter 5 evaluates the
5
effectiveness of using silicate activation in comparison with hydroxide activation in the
mud solidification method. Chapter 6 focuses on the hardened state properties of the
geopolymer/mud hybrid cement with investigation of the mechanical properties, self-
healing capability, porosity and hydraulic conductivity of the geopolymer system.
Finally, chapter 7 gives an overall summary of the novel mud solidification method
presented in this dissertation, and suggests ideas for future work.
6
Chapter 2: Background
The first part of this chapter clarifies geopolymer definition and terminology. The
general rules of geopolymer synthesis are introduced. The reaction process and
microstructure of geopolymers are also discussed, as they influence their behavior and
properties. Next, critical properties of geopolymer including compressive strength,
rheological properties, setting time control and self-healing behavior, which are all of
great importance to oilwell cementing, are reviewed in detail. Subsequently, the current
state of geopolymer research aimed at oilwell cementing applications is presented, with
introduction of basic and essential concepts regarding primary cementing, well
abandonment plug cementation and remedial cementing. Finally, a brief discussion of the
mud solidification technique is provided, showing the potential of using geopolymers to
solidify non-aqueous drilling muds.
GEOPOLYMER: DEFINITION AND TERMINOLOGY 2.1
The blending of an alkali solution with an alumina- and silica-containing powder
leads to a binder phase that can harden and develop compressive strength like an OPC.
This reaction was first patented by Kühl in 1908, and was later on studied in more detail
by Purdon during the development of an alkali-activated blast furnace slag (Kühl, 1908;
Purdon, 1940). Since the 1990s, research into binders with alkali activation has expanded
rapidly all over the world, primarily driven by the potential for substantial reduction in
greenhouse gas emissions that appears to be possible by moving away from using OPC,
the manufacturing of which globally produces a large amount of CO2 (Duxson et al.,
2007b; Juenger et al., 2011).
7
A broad classification of “alkali-activated materials” (AAM) is applied to any
binder system formed from an alkali activator and a silicate precursor powder. By this
definition, an AAM can be an alkali-activated calcium silicate such as alkali-activated
slag (AAS) or a more aluminosilicate-rich material such as alkali-activated fly ash. The
name “geopolymer” was first applied by Davidovits in 1978 to a group of mineral
polymer resins derived from the reaction of metakaolin with soluble silicate (Davidovits,
2015). In most recent publications, “geopolymer” is considered to refer to a more specific
type of AAM; although it is still used to describe a wide range of alkali-activated binders,
it mostly refers to alkali-activated low-calcium or calcium-free aluminosilicates (Juenger
et al., 2011; Provis and van Deventer, 2014). Note that the use of the term “geopolymer”
to characterize those alkali-activated materials is still a topic of considerable controversy.
Further discussion on the definition of geopolymer is, however, beyond the scope of this
work, which will use the generally accepted definition as outlined in the 2014 RILEM
report (Provis and van Deventer, 2014). The current study will focus on developing low-
calcium fly ash-based geopolymer cements for oil / gas applications. Background details
on structure, properties, synthesis and applications of fly-ash based geopolymer will be
reviewed in detail in the following sections.
SYNTHESIS OF ALKALI-ACTIVATED FLY ASH-BASED GEOPOLYMER 2.2
Aluminosilicate source 2.2.1
To create a geopolymer slurry, an aluminosilicate precursor powder is mixed with
an alkaline activating solution. Common sources of aluminosilicate include metakaolin
and fly ash. Metakaolin is a dehydroxylated form of kaolinite clay. Alkaline-activated
metakaolin forms a strong and durable binder material (Davidovits, 2015). Fly ash,
8
although having more variability in its composition, delivers a more favorable rheology
than metakaolin-based binders at a much lower cost (Kong et al., 2007). It was therefore
selected as the preferred aluminosilicate source for this study.
Fly ash is a by-product generated from coal burning power plants. The
composition of any fly ash greatly depends on the source of coal and the operating
conditions of the boiler. ASTM Standard C618-18 (2015) defines two main classes of fly
ash. The sum of SiO2, Al2O3, and FeO must be above 50% for a class C fly ash and must
be above 70% for a class F fly ash. Class C fly ashes typically have higher total CaO
content than class F fly ashes (Figure 2.1). Due to the rapid cooling in the flue, fly ash
particles typically contain 50-90% of amorphous phases (Ward and French, 2006). The
amorphous phases in the fly ashes are mainly aluminosilicate glasses with a tetrahedral
structure (Hemmings and Berry, 1987; Williams et al., 2005; Williams and van Riessen,
2010). The remaining parts of the fly ash are crystalline phases including quartz,
hematite, mullite, magnetite, etc. (Hemmings and Berry, 1987).
Unlike Portland cement, which is manufactured specifically as a construction
material according to industrial standards, fly ash is a by-product with more variability in
the raw materials, therefore can vary appreciably in reactivity. It has been commonly
accepted that the main characteristics of a fly ash for optimal geopolymer formulation
include a high vitreous content, more specifically high reactive aluminate and high
reactive silicate, since these are the main building blocks of the geopolymer reaction
product (Duxson et al., 2006; Provis and van Deventer, 2014). The quantification of the
crystalline phases in fly ash material can be obtained from X-ray diffraction (XRD). The
determination of glassy phases, however, is difficult. Different elements can occur in
different ways within the ash. For example, silicon, aluminum and iron elements may
exist as crystalline phases such as quartz or magnetite, as aluminosilicate minerals such
9
as mullite, or as amorphous aluminosilicate glasses. The amorphous glassy phases do not
have regular arrays of atoms that can be characterized by the XRD measurements;
instead, they show as a “hump” in the X-ray diffraction patterns.
Figure 2.1 - Ternary phase diagram showing the composition of OPC, blast furnace slag
(BFS), fly ash (FA), silica fume and metakaolin
A considerable amount of prior research has been focused on the quantitative
analysis of the glassy constituents and the chemical reactivity of fly ash particles. One
way to quantify the total fraction of amorphous phase is by subtracting the total
crystalline fraction obtained from XRD from the bulk oxide content of the fly ash
(Aughenbaugh, 2013; Ward and French, 2006). This method, however, usually fails to
distinguish between different amorphous phases. Selective dissolution is a chemical
Al2O
3
SiO2
CaO
Silica Fume
OPC
Metakaolin
10
method that can also provide information on the glassy content. When fly ash particles
are dissolved in acids (such as hydrofluoric acid, acetic acid or oxalic acid) or caustic
solutions (such as sodium hydroxide), the glassy phases are dissolved and the unreactive
crystalline phases are assumed to be unaffected (Aughenbaugh, 2013; Hemmings and
Berry, 1987; Williams and van Riessen, 2010). The bulk glassy composition can be
estimated by measuring the change in weight during dissolution, i.e., by subtracting the
unreactive crystalline content from the total mass prior to dissolution, the amount of
glassy phases can be determined.
One way to identify the chemical composition of glassy phases in fly ash particles
is to use Energy Dispersive Spectroscopy (EDS) along with Scanning Electron
Microscope (SEM) (Aughenbaugh, 2013; Durdziński et al., 2015). Using EDS, a
spectrum for every pixel can be plotted into a set of element intensity maps. Multi-
spectral image analysis software can be used to assist the analysis and quantification of
groups that has similar chemical compositions. Insight into the microstructure and
location of phases can thus be obtained.
The characterization of fly ash particles can provide very useful information in
geopolymer design. More research is ongoing to analyze the individual glassy phases in
fly ash. Based on literature review, it can be summarized that a suitable fly ash precursor
has to contain sufficient reactive aluminum, as it is the main component in the
aluminosilicate gel (Fernández-Jiménez et al., 2006b; Fernández-Jiménez and Palomo,
2005a). The total reactive silica in the fly ash precursor should be between 40-50% in the
fly ash and the reactive Si/Al ratio should be below 2 (Fernández-Jiménez and Palomo,
2003, Fernández-Jiménez et al., 2006). Other properties that may influence the reactivity
of the fly ash include (1) the morphology and size of the fly ash particles; (2) the calcium
content; (3) the concentration of unburned coal (which preferably should be lower than
11
5%); (4) the iron content (which preferably should be no higher than 10%) (Fernández-
Jiménez and Palomo, 2003).
Activating solution 2.2.2
Typical alkaline activators include alkali metal hydroxides, silicates, or a blend of
the two. Carbonate and sulfate activators have also been tested by researchers but are less
effective in low-calcium AAMs (Fernández-Jiménez and Palomo, 2005b; Shi and Day,
1995). The alkali cations in the activator can be sodium, potassium and lithium, with
sodium being the most studied. The choice of cation and the concentration of the alkaline
solution will result in varying properties in the hardened geopolymer (Lizcano et al.,
2012).
Hydroxide Activation
Alkali hydroxides are usually prepared in the form of an aqueous solution.
Typical hydroxide solutions for geopolymer activation range from 8 g/mol to 15 g/mol
(Provis and van Deventer, 2014). Upon mixing with the precursor, the hydroxide ion
(OH-) works as a catalyst in dissolving the solids. The alkali will be incorporated into the
geopolymer structure and acts to balance the negative charges. The structure of the
geopolymer gel will be discussed in more detail later in this review. Hydroxide solutions
have a very high pH, thus they require extreme caution during handling. Once the
geopolymer gels start to form and the hydroxides become chemically bound in the
structure, the resulting hardened material does not pose a hazard to living beings or the
environment.
12
Silicate Activation
Alkali silicate solution, or “waterglass”, is also commonly used for geopolymer
activation. Commercially available sodium silicate solutions can be used, or alternatively
they can be prepared in custom form by dissolving amorphous silica (silica fume) in
appropriate alkali hydroxide solutions. The alkali silicate solution provides additional
silicon that is immediately available to interact with other dissolved ions. Studies showed
that the use of sodium silicate leads to a denser geopolymer structure compared to a
geopolymer formed using sodium hydroxide solution (Ma et al., 2012).
Proportioning 2.2.3
Geopolymer can be proportioned based on the water-to-solid ratio or solution-to-
powder ratio in the same way as Portland cement concretes are proportioned (Al Bakri et
al., 2011; Bakharev, 2005; Fernández-Jiménez et al., 2008, 2006b; Oh et al., 2010; Ruiz-
Santaquiteria et al., 2012). Another way to proportion geopolymer is based on molar ratio
of the aluminum and silicon constituents. Khale and Chaudhary (Khale and Chaudhary,
2007) recommend the following oxide ratio for proper polymerization and strength
development for a metakaolin-based geopolymer: M2O/SiO2 = 0.2-0.48, SiO2/Al2O3 =
3.3-4.5, H2O/M2O = 10-25, and M2O/Al2O3 = 0.8-1.6, where M is the alkali metal (such
as sodium or potassium). Formulations with compositional ranges of SiO2/Al2O3 < 1 or
SiO2/Al2O3 > 5 may result in interesting properties for certain specific applications, but
will not be discussed in this literature review. In addition, the alkali composition also
affects the development of geopolymer structure (Duxson et al., 2007a, 2005a).
Davidovits (Davidovits, 1982) recommends (Na2O, K2O)/SiO2 ratio in the range of 0.2-
0.28, and (Na2O, K2O)/Al2O3 ratio in the range of 0.8-1.2. A wide range of recommended
molar ratios can be found in literature (Aughenbaugh et al., 2014; Barbosa et al., 2000;
13
Chindaprasirt et al., 2012; Duxson et al., 2007a; Fletcher et al., 2005; Rowles and
O’Connor, 2003). The proportioning of geopolymer solution and precursor must be based
on the composition of the specific aluminosilicate, augmented by trial and error testing.
Compared to metakaolin, proportioning for fly ash-based geopolymer is more
complicated due to greater compositional variability in the latter. Only the reactive
portion of the fly ash should be used in deciding the proportions. As mentioned in the
previous section, finding the amount of reactive phases in a fly ash is very challenging.
Considerable effort has been made on this topic, which still remains an active area of
research (Aughenbaugh, 2013; Chancey et al., 2010; Durdziński et al., 2015). For
simplicity, the non-crystalline part of the fly ash can be considered as the reactive portion
regardless of the varying level of reactivity of different glassy phases.
Curing regime 2.2.4
Curing conditions have a large effect on hardened geopolymer properties. Most
sources report that the synthesis of low-calcium alkali-activated binders requires elevated
temperature. The curing temperature regimes are typically between 40 °C and 100 °C
(Davidovits, 1989; Duxson et al., 2007a; Fernández-Jiménez et al., 2008; Rowles and
O’Connor, 2003; Singh et al., 2015). Room temperature curing is possible, but may
require longer than desirable time to develop measureable strength (Somna et al., 2011).
In literature, both short-term and long-term heat curing conditions have been
studied. Short-term heat curing means that the samples were cured at elevated
temperature for up to 24 hours, followed by room temperature curing until the samples
were tested. Long-term heat curing requires the samples to be cured at elevated
temperature until tested. Palomo et al. (Palomo et al., 1999) studied the effect of short-
14
term heat curing at 65 °C and 85 °C using a fly ash based geopolymer. They found that
the higher temperature resulted in higher compressive strength when the samples were
heat cured for 2 or 5 hours. Longer curing time did not result in greater strength. Najafi
Kani and Allahverdi (2009) found that longer pre-curing at room temperature before the
application of heat is beneficial for strength development of the geopolymer.
POLYMERIZATION MECHANISM AND MICROSTRUCTURE 2.3
The chemistry and reaction mechanism of alkali activation is currently a hot topic
in non-Portland cement research. It is widely agreed that the geopolymer reaction is a
dissolution-precipitation process. Upon mixing, both silicate and aluminate species from
the aluminosilicate powder dissolve into the basic solution. Silicon exists in the solution
in the form of HSiO43-
until a critical concentration is reached. Precipitation happens
concurrently forming an aluminum-rich gel in the first stage, and a more silicon-rich gel
in the second stage (Duxson et al., 2006; Duxson and Provis, 2008; Fernández-Jiménez
and Palomo, 2005b).
The micro-structure of the hardened geopolymer binder is a highly cross-linked
aluminosilicate network structure that is very similar to zeolite frameworks (Bell et al.,
2008a, 2008b; Fernández-Jiménez et al., 2008; Juenger et al., 2011; Provis and van
Deventer, 2014). This structure is X-ray amorphous and is oftentimes described as N-A-
S-H gel, where N is Na2O, A is Al2O3, S is SiO2 and H represents water (García-Lodeiro
et al., 2007). The tetrahedral framework of silicon and aluminum is linked by oxygen and
charge-balanced by sodium or other cations (Davidovits, 1989). This aluminosilicate gel
contains a very low level of chemically bound water. It therefore is often characterized as
N-A-S-(H) (Duxson et al., 2005a; Rahier et al., 1996).
15
When the precursor solid contains higher amounts of calcium, for instance when
Class C fly ash or blast furnace slag is used in combination with a low-calcium
geopolymer system, a binder phase of C-(A)-S-H gel that is partially crystalline and
partially amorphous will coexist with the N-A-S-(H) gel (Richardson et al., 1994; Wang
and Scrivener, 1995). Figure 2.2 shows a conceptual geopolymer reaction process for
both high-calcium and low-calcium fly ashes in a most general sense. The two stages of
gel evolution are represented by “solidification and hardening” and “ongoing gel
evolution” respectively.
Figure 2.2 - Schematic representation of the alkali activation reaction process (Juenger et
al., 2011)
Dissolution of solid
aluminosilicate source
Silicate species in
activating solution
Rearrangement and exchange
among dissolved species
Gel nucleation
C-(A)-S-H gel N-A-S-(H) gel
Solidification and hardening
Ongoing gel evolution with progression
towards crystallization
16
PROPERTIES AND ADMIXTURE 2.4
Here, the compressive strength, rheological properties, setting time control and
self-healing capability of geopolymers are reviewed in more detail, since these are of
great importance to the development of slurries suitable for oilwell cementing. The
admixtures that modify each of the relevant geopolymer properties are also discussed.
In Portland cement systems, “admixture” refers to the additional components that
are added to give special properties to the fresh or hardened cement slurry. Admixtures
may enhance the workability, strength or durability of a given cement mixture. In the
context of an alkali-activated system, the alkaline activators are not considered to be an
admixture since they are a part of the binder chemistry. An admixture will be defined as
the additives that are purposely added to alter the properties of the AAM.
Unlike the Portland cement admixture chemistries, which are well-researched and
documented, admixtures to AAM or geopolymer have not been widely addressed in the
literature. Moreover, the laboratory results can be largely different or even contradictory
among different researchers. This can be attributed to the complexity and variation in the
precursor, the activator, and the type and dosage of the admixture. It is also clear that the
admixtures that are developed for Portland cement chemistries behave very differently in
an AAM, due to the distinct hardening mechanism. For low-calcium alkaline activated
material especially, the majority of the well-known OPC admixtures have been found
ineffective for property modification. In the next section, the admixture chemistries and
their effectiveness on altering geopolymer performance will be discussed in detail for
properties that are critical to oilwell cementing applications.
17
Compressive strength 2.4.1
To successfully utilize geopolymers on a significant scale in the construction of
buildings and other infrastructure, their compressive strength was developed to be close
to that of a hardened Portland cement. In literature, studies have reported compressive
strength values higher than 70 MPa (more than 10,000 psi) for geopolymer concrete
cured at temperatures in the range of 50-80 °C (van Deventer et al., 2012). Note that the
compressive strength is expected to increase as the curing temperature increases. Room
temperature curing leads to lower strength in the range of 20-23 MPa (around 3,000 psi)
(Somna et al., 2011). All of these reported values meet the minimum required
compressive strength for oil/gas wells. With the confining pressure present in a downhole
condition, the failure strength of geopolymer will be even higher than the unconfined
compressive strength values, providing sufficient strength for supporting casing and
isolating different zones.
Nasvi et al. (2012a) observed some level of strength reduction at curing
temperatures that are higher than 100 °C (212 °F). They believed that is due to the
breaking up of the intergranular structure of geopolymer at those high temperatures.
Portland cement also suffered from strength retrogression when temperature exceeds 60
°C based on their laboratory evaluations. Nevertheless, the strength reduction rate of
geopolymer is less than that of a Portland cement. The issue of strength retrogression
needs to be specifically addressed when applications for high temperature / high pressure
(HPHT) oil / gas wells are considered.
Rheological properties 2.4.2
Flowability is another important characteristic for oil / gas applications.
Previously reported geopolymer slurries are in general too viscous to be pumped in a
18
typical oil / gas well. Adding additional water or a superplasticising admixture are the
two main ways to improve the pumpability of a cement slurry. Similar to Portland cement
slurries, adding more water in geopolymer mixture will improve the workability.
However, excess water will negatively influence the strength and porosity of the
hardened material. Therefore the amount of water requires regulation and optimization
(Barbosa et al., 2000). Effective admixtures for reducing viscosity in Portland cement
slurries, including lignosulfonate-, polynaphthalene- and polycarboxylate-based
superplasticizers, have been tested in AAM slurries. Very distinct, if not contradictory,
effects were observed when using superplasticizers in alkali-activated slags (Criado et al.,
2009; Palacios et al., 2009; Palacios and Puertas, 2005; Puertas et al., 2003). The
contrasting behavior of geopolymers with superplasticizers can be explained by the
chemical instability of superplasticizers at pH greater than 13. Some metakaolin or
bottom ash based slurries showed improvement in workability or slump with
superplastisizer. However, none of those slurries achieved a thin enough viscosity for
pumping the geopolymer slurry down oil / gas wells (Hardjito and Rangan, 2005; Kong
and Sanjayan, 2010; Nematollahi and Sanjayan, 2014). Therefore, designing a
geopolymer slurry with appropriate rheological properties and finding effective
admixtures for the high alkaline environment has remained a big challenge. The topic is
discussed in more detail in the remainder of this dissertation.
Setting time control 2.4.3
Setting time control of geopolymers has been studied by various researchers. It
has been well-documented that borate species act as effective retarders for Portland
cement. Nicholson et al. (2005) studied the effect of using borate in silicate-activated
19
class C fly ash and discovered that borates can significantly delay the onset of setting at
7% by weight of fly ash or higher. At these high dosages, however, the binder suffered a
significant drop in compressive strength. The borate is not expected to work as
effectively in lower calcium AAMs, as the borate only resides in the tetrahedral BO4
sides of the geopolymer structure when there is limited amount of calcium in the system.
Other admixtures including potassium salt, phosphoric acid and K2HPO4 have been
identified as effective retarders for alkali-activated high calcium AAMs (Chang, 2003;
Lee and van Deventer, 2002a).
Salt is another class of admixtures that alters the setting time of Portland cement
slurries. For silicate-activated fly ash mixtures, Lee and van Deventer (Lee and van
Deventer, 2002a, 2002b) tested a range of salts and summarized the results in a chart
(Figure 2.3). As shown in the figure, calcium salts generally showed an accelerating
effect, whereas magnesium compounds did not significantly affect the setting time for all
three systems tested.
Other salts have been evaluated for high-Ca alkali-activated fly ash and alkali-
activated blast furnace slag (BFS) cements. Brough et al. (2000) observed that NaCl at
levels up to 4% has an accelerating effect on silicate-activated BFS cements, whereas
dosages higher than 4% of NaCl retards the reaction. In another study, little effect of
NaCl on the setting behavior was observed on a BFS system up to 20% addition
(Sakulich et al., 2009). In contrast, in OPC slurries, NaCl acts as an accelerator at
concentrations up to 15% and as a retarder at concentrations above 20%. Between
dosages of 15% - 20%, NaCl is essentially neutral (Nelson and Guillot, 2006).
20
Figure 2.3 - Effects of calcium and magnesium compounds (at the molar dosage of 0.09
mol) on the setting time of three different alkali-activated fly ash/kaolinite blends ((Lee
and van Deventer, 2002a)
Self-healing capability 2.4.4
Ahn and Kishi (2010) reported that geopolymer has a self-healing capability,
where the geopolymer matrix can heal when a micro-crack is present. If a geopolymer
indeed self-heals, it will be a great candidate for well abandonment plugs where
maintaining long-term cement sheath integrity is critical. However, literature about
geopolymer re-healing capability is quite sparse and warrants the need for a thorough
21
study. Since remediation of cement in an oil / gas well is costly, difficult, and rarely
successful, it would be beneficial to use a cementitious material that can re-heal in-situ. A
good understanding of the re-healing process and mechanism is imperative for providing
adequate zonal isolation throughout the life of the well.
Other properties 2.4.5
To evaluate the potential of using geopolymer in carbon sequestration and in
offshore operations, Nasvi et al. (2012b) Giasuddin et al. (2013) and Duran (Duran,
2015) studied the effects of saline water curing on geopolymer cement strength and
durability. The results showed that the strength of geopolymer increases with increasing
salinity, possibly because the NaCl content provided more resistance against alkali
leaching from the geopolymer matrix. By comparison, hardened Portland cement slurries
can lose 50% strength when cured in saline water (Nasvi et al., 2012b).
Nasvi et al. (2014a, 2014b, 2013) studied the effect of temperature on the
permeability of class F fly ash-based geopolymers for carbon capture and storage. The
permeability increases with increasing curing temperature during setting and hardening.
The maximum apparent CO2 permeability at any temperature was 0.04 µD, lower than
the CO2 permeability of a typical OPC used in oil / gas wells (0.1-200 µD, see Bachu and
Bennion, 2009; Laudet et al., 2011; Le-Minous et al., 2017), which is well below the
permeability limit (200 µD) recommended by API (Kutchko et al., 2009). These findings
indicate that geopolymer is a viable alternate to ordinary Portland cement for long-term
well integrity as it delivers good durability and low permeability.
Overall, geopolymer has significant potential to be used as an alternative to
Portland cement for oil well cementing. Other than the properties and qualities that have
22
been discussed above, geopolymer has also been reported to provide additional benefits:
(1) good durability under harsh conditions (Bakharev, 2005; Lloyd, 2008); (2) resistance
to acid and chemical attack (Bakharev, 2005; Uehara, 2010); (3) low permeability
(Chung et al. 2010); (4) resistance to high temperature (Duxson et al., 2007b; Nasvi et al.,
2012a); (5) good volumetric stability after hardening (Papakonstantinou et al., 2001); (6)
adhesion and binding to multiple surfaces including metallic substrates (Bell et al., 2005);
(7) low cost; (8) low environmental impact. Of course, all of these great properties were
obtained for different mixes as well as a variety of curing conditions. Application-
specific mix design and associated optimization is required to attain the desired
properties.
GEOPOLYMER IN OILWELL CEMENTING 2.5
Geopolymers have been researched during the past few decades for potential as a
replacement for OPC in a wide range of applications. Geopolymer, for instance, can be
designed for building and repairing infrastructure, for making fire resistant concrete, for
hazardous and radioactive waste encapsulation, etc. The application in oilwell cementing,
however, is much less studied. In the following, the basic concepts of oilwell cementing
operations are introduced, along with a critical review of the current state of geopolymer
research in well cementing applications.
Primary cementation 2.5.1
Primary cementation is a technique for placing cement slurries in the annulus
between casing and formation rock or between casing strings (Figure 2.4). In principle,
the primary cementing techniques are similar regardless of the type and size of casing
23
string. Cement slurry is pumped down through the string to be cemented, displaces
drilling mud as it moves back up the annulus, and is left to set in the annulus. The
foremost goal of the hardened cement sheath is to provide complete and durable zonal
isolation in the wellbore for the lifetime of the well, meaning that the hydraulic bond
formed by the cement will prevent the migration of reservoir fluids (brine, oil, gas)
between strata and up to the surface.
Figure 2.4 - Primary cementation illustration
Successful primary cementing requires accurate knowledge and preparation of the
well (creating a “cementable” borehole), meticulous planning and testing of the cement
slurry, and proper job execution according to plan. There are many facets to the design of
Casing
Production Liner
Drilling Liner
Tie-back Liner
Cement Sheath
24
a well-specific cement slurry. Some of the critical characteristics of a cement slurry
include:
Proper rheological characteristics for mixability and pumpability.
Optimized mud removal and displacement by the cement slurry.
Sufficient pumping time for circulation and placement.
Right-angle setting to prevent formation fluid/gas invasion into the cement.
Sufficient compressive strength and flexural strength.
Good hydraulic bonding capability to seal the casing and formation interfaces.
Appropriate mechanical properties for long-term durability.
To achieve these desired properties, Portland cement systems have been
rigorously studied and additive chemistry has been well developed to make it possible to
satisfy such a wide range of requirements. Geopolymer, on the other hand, is much less
studied, especially under downhole well conditions. Research regarding geopolymer-
based well cementing has been mostly in-house within service companies, and has just
started to receive more attention in academia in recent years.
Salehi et al. (2016a, 2016b) conducted a series of laboratory tests dedicated to the
application of geopolymer for primary cementing. They evaluated the pumping time of a
class F fly ash-based geopolymer at various temperatures and saw strong accelerating
effect when cured at an elevated temperature. Enhancement of thickening time of the
geopolymer slurries were achieved which allowed good pumpability for temperatures up
to 250 °F. A polycarboxymethyl superplasticizer and retarder was also tested in the same
paper and showed a significant retardation effect on thickening time. In the same study,
compressive strength, shear bond strength and durability of hardened geopolymer were
compared with Class H cement mixtures. Laboratory investigation showed that the
geopolymer developed more than 6,000 psi compressive strength by two weeks. The
25
compressive strength of geopolymer mixtures increased with temperature, whereas
Portland cement slurries showed strength retrogression. Additionally, geopolymer
mixtures had comparable shear bond strength and better durability compared to Portland
cement at elevated temperatures.
Sugumaran (2015) also conducted a study that looked into the properties of class
F fly ash based geopolymer with different compositions and their acid resistance at
different acid concentrations for oilwell cementing. According to their results, at 60 °C
curing temperature, the optimum compressive strength was achieved when the water-to-
fly ash ratio was kept at 0.3 and when 12 M sodium hydroxide solution was used at an
alkaline-to-fly ash ratio of 0.4. The fly ash geopolymer cement was found to be more
resistant to acid attack when exposed to up to 10% sulfuric acid solution at room
temperature.
Pershikova et al. (Pershikova et al., 2012, 2011; Porcherie et al., 2011; Porcherie
and Pershikova, 2010) patented a pumpable geopolymer formulation for oilfield
application. The composition of the geopolymer was comprised of an aluminosilicate
source, an alkali activator, a rheology modifier containing aluminum compounds (such as
bauxite, aluminum salts aluminum oxide), a strength reinforcing agent (such as fiber or
magnesium silicate), and a filler material for density control. The example slurries
disclosed in the patents showed good pumpability, good mechanical strength, and low
water permeability (< 80 μD) at temperatures up to 90 °C and at a pressure of 3,000 psi.
The thickening time was manipulated by altering the silicon versus aluminum ratio or by
adding lithium compounds.
Overall, these preliminary studies evaluated the strength, rheological properties,
thickening time and acid resistance of geopolymer at downhole conditions. The
performance of geopolymer indicated by these preliminary studies certainly demonstrates
26
great potential for using geopolymer as an alternative to current oilwell cement, as it
addresses many of the requirements for cementing a typical oil / gas well. Improved
performance is to be expected once the formulation of geopolymer is optimized and when
the admixture chemistry is more developed for geopolymer mixtures.
Well abandonment 2.5.2
When a well is decommissioned at the end of its life-cycle, usually a set of
cement plugs are placed in the well to maintain and guarantee isolation between
geological layers (Figure 2.5). The primary importance of cement plugs in an abandoned
well is to prevent contamination of groundwater aquifers by hydrocarbon seepage or
formation brine coming from below the aquifer. Contamination during plug cementing
has long been recognized as a serious problem for well abandonment (Nelson and
Guillot, 2006). In abandonment plug operations, a considerable amount of drilling mud
can exist and mix with cement slurry. Consequently, the setting time and mechanical
properties of the cement plug could vary significantly from the designed values, leading
to a highly compromised plug cementation.
Seeking a better well abandonment strategy, previous work has evaluated the use
of Class C fly ash slurry for plugging abandoned wells using coiled tubing (Shah and
Cho, 2001; Shah and Jeong, 2003). Since Class C fly ash has sufficient calcium and
aluminosilicate content, the slurry was activated with pure water instead of an alkaline
solution. Five fly ashes were tested for their thickening time, rheology, shear bond
strength, hydraulic bond strength and gas permeability. The experimental results revealed
that Class C fly ash slurries had sufficient thickening time (longer than 2 hours) and
could be pumped through coiled tubing for well abandonment. The viscosity of the
27
slurries decreases as the temperature increases. Shear bond strength values ranged from
100 psi to 1,130 psi among different fly ash sources. The magnesium oxide content in the
fly ash provided good swelling characteristics and increased the shear bond strength. The
hydraulic bond strength values ranged from 1,500 psi to 2,100 psi, comparable to a
typical Class H cement sample under similar curing conditions. The gas permeability of
the geopolymer sample were of the same order of magnitude as Class H cement, ranging
from 0.062 md to 0.197 md, indicating minimal gas migration through the hardened plug.
Figure 2.5 - Cement plugs in an abandonment well (image adopted from Global CCS
Institute)
In another study, Khalifeh et al. (2016, 2015, 2014) investigated Class C fly ash-
based geopolymer slurries that were activated by silicate/hydroxide mixed solutions for
28
plug and abandonment (P&A) operation. Thickening time test, rheological properties,
bulk shrinkage, uniaxial compressive strength and sonic compressive strength buildup
were evaluated. In the thickening time test, the geopolymer slurry showed high plateau
consistency around 50 Bc before the consistency finally reached 100 Bc in 40 minutes.
The slurries exhibited a viscous nature with Newtonian-like viscosity and fast gelation. A
maximum bulk shrinkage of 2% was observed for the samples that were activated by
10M NaOH and cured at 125 °C and 5000 psi.
Most importantly, empirical correlations were developed for the first time for
testing geopolymer slurries in an Ultrasonic Cement Analyzer (UCA). An UCA measures
the travel time of ultrasonic waves through a cement sample while it cures under
temperature and pressure. The transit time is correlated to compressive strength using
empirical relationships that were originally established for OPC. An UCA test protocol
has been included in the API Recommended Practice (API RP 10B-2, 2010) as
nondestructive testing method to evaluate cement strength. Such empirical correlations,
however, cannot be applied indiscriminately to other cementitious materials, such as
geopolymer, due to distinct structural differences that affect the ultrasonic travel times.
Khalifeh et al. (2014, 2015) developed a set of equations for geopolymers in an UCA test
by correlating data from unconfined compressive strength (UCS) for specimens that were
prepared with different concentrations of NaOH activator at 87 °C and 125 °C. Table 2.1
shows the acquired empirical correlations.
29
Table 2.1 - Empirical correlations for geopolymer specimens prepared with various
NaOH concentrations at two different temperatures. Specimens were cured under a
pressure of 5,000 psi (Khalifeh et al. 2014)
Concentration
of NaOH
(M)
Temperature
(°C)
Empirical Correlations
x: transit time (μsec/in); y: Compressive strength (psi)
6 125 y = -769.81x2 + 13,151x - 48,085
8 125 y = -2,654.8x2 + 50,357x - 229,847
10 125 y = -10,062x2
+ 191,272x – 893,329
6 87 y = -2,154.8x2 + 43,332x - 211,942
8 87 y = -1,144.8x2 + 18,924x - 68,972
10 87 y = 490.19x2 - 13,529x + 88,881
Lost circulation control 2.5.3
Lost circulation, in particular, is a serious well construction problem, and
geopolymer offers great potential to treat such scenarios. If a highly permeable formation,
cavernous formation or fractures are encountered while drilling or cementing, drilling
fluids or cement slurries may totally or partially be lost into these zones instead of
returning back to the surface (Figure 2.6), i.e. circulation is “lost”. Such scenarios will
first be treated with lost circulation materials (LCM) such as particulates and fibers to
temporarily plug the lost zones. If unsuccessful, cement plugs can be placed and slightly
squeezed to seal and consolidate the formation. Note that the probability of a successful
30
cement squeeze job for lost circulation control is usually low, often because of
incompatibility between OPC and OBM/SBM used for drilling.
(a) Partial loss (b) Total loss
Figure 2.6 - Lost circulation scenario with (a) partial loss, and (b) total loss (figure
adopted from petrowiki.org)
Miller et al. (2013) developed a geopolymer-based pill as an engineered solution
to treat lost circulation problems. In this approach, an aqueous alkali alumino silicate
(AAAS) was formulated to serve as a chemical sealant. AAAS is a pre-primed silicate
that is in liquid form. When AAAS is subjected to high shear or a reduced pH
environment, it can be triggered to polymerize and form a solid crystalline phase in a
controllable manner. In the study, AAAS was mixed with OBM to form a highly viscous
Flow Flow
31
fluid prior to solidification for controlling circulation losses. Setting time was controlled
by adding propylene carbonate into the OBM. When higher compressive strength is
desirable to seal and block a fracture, the AAAS can be formulated into a cementitious
material by adding slag or fly ash. Experimental results showed that a cement plug
containing AAAS and class F fly ash developed good early compressive strength in 3
hours at temperatures higher than 25 °C.
MUD SOLIDIFICATION TECHNIQUE 2.6
Techniques for mud solidification were first introduced to the industry in the late
1980s / early 1990s. As mentioned in the introduction chapter, the conversion of drilling
mud into a cement offers a lot of advantages compared to a conventional cementing
process (Hale and Cowan, 1991):
Better drilling mud removal.
Improved compatibility between drilling mud and cement slurries, with less
negative effect from any mud contamination.
Ability to convert any undisplaced drilling mud and mud filtercake into cement.
Re-cycle drilling mud in-situ.
BFS-based drilling mud solidification 2.6.1
There are multiple approaches to converting a drilling mud into a cementitious
slurry. For instance, Wilson et al. (1989) proposed to add special copolymer dispersants
and accelerators into drilling mud and then add cement to achieve a settable
characteristic. A more developed method that was extensively studied and even field
tested was the slag-based mud solidification method, also called as the “universal fluid”
32
(UF) system, in which a WBM was diluted and treated with a chemical activator. BFS
was mixed into the pretreated drilling mud as the hydraulic material to consolidate the
mud (Cowan et al., 1992; Hale et al., 1995; Hale and Cowan, 1994, 1991, Nahm et al.,
1995, 1994, 1993). This method has been deployed for use in several cementing
operations including primary cementing, temporary abandonment plugs and sidetracking
plugs in the last two decades. Laboratory evaluations and field tests showed that the BFS-
based WBM-to-cement conversion technique not only provided good-quality cement, but
also effectively solved gas-migration and lost-circulation problems (Leimkuhler et al.,
1994; Pessier et al., 1994; Song et al., 2000; Wu et al., 1998).
There are several limitations associated with the BFS-based mud solidification
method. In laboratory testing, micro-cracking has been observed in the set cement and the
cause of such cracking remains unclear. Benge and Webster (1994) believed that the set
cement containing BFS and drilling mud have no fibers or crystals connecting the grains,
making the set material less ductile and giving it a tendency to crack. Mueller and
Dickerson (1994) investigated the physical and mechanical properties of the set drilling
mud and observed that the brittleness and stress-cracking of the slag-converted drilling
muds are heavily influenced by factors such as the type and concentration of the
activator. Over-activation of the system could result in detrimental effects such as
abnormal thickening time, excessive heat buildup, high viscosities and diminished
compressive strength.
Another complex factor in the design of a BFS-based mud solidification system is
the varied composition of the drilling mud. The cuttings and chemicals that are added to
the drilling mud continually changes when the bit penetrates different formations to
maintain desired mud properties. Such compositional changes may strongly affect the
cement properties once the mud is converted to a cement slurry. To properly design a
33
specific job, a portion of the mud samples has to be isolated and shipped to a laboratory
long before the cementing. This is a limitation that has to be overcome with any mud-
solidification technique.
Despite the progress made on utilizing BFS-based material for WBM
solidification, the same technique has not yet been developed and applied on SBMs or
OBMs. The industry’s dependence on SBM has gone up significantly in the past two
decades. In deepwater drilling especially, SBMs are preferred over WBM due to the
superior performance properties in borehole stabilization, lubrication and improved rate
of penetration. SBMs and OBMs are expensive and are typically recycled whenever
possible to the point where the mud can no longer be reused. The disposal of these NAFs
is governed by strict environmental regulations. Offshore operations, in particular, have
to follow regulations including the OSPAR Commission in the northeastern Atlantic and
the Environmental Protection Agency (EPA) in the Gulf of Mexico. According to EPA,
NAFs as whole mud cannot be discharged in the ocean and NAF (SBM only) left on
cuttings has to be reduced to specific levels before cuttings can be discharged. If a mud
solidification technique becomes feasible, then NAFs that can no longer be recycled may
be processed and consolidated on-site with no need to transport it back to land.
Geopolymer-based solidification for organic waste disposal 2.6.2
The solidification / stabilization (S / S) of industrial wastes using cement-based
materials is a widely applied technique (Wiles, 1987). Traditional cements, however,
have proven to be ineffective for immobilizing organic wastes, as they may inhibit
cement hydration and in general do not chemically bond with the binder (Pollard et al.,
1991; Trussell and Spence, 1994). In a study that investigated the disposal of radioactive
34
waste in the nuclear industry, Cantarel et al. (2015) discovered that geopolymer acts as a
strong candidate to encapsulate liquid oil waste effectively. This technique involves the
mixing of oil in an alkali silicate solution to form an emulsion, followed by mixing with
an alumino-silicate source (e.g. metakaolin) to allow for the setting of a geopolymer. The
experimental results showed that metakaolin-based geopolymers can successfully
encapsulate oil waste, with oil droplets uniformly distributed throughout the matrix of the
hardened binder. Good rigidity and mechanical strength were observed. Leaching tests
showed very limited release of oil from the composite material. The geopolymer/oil
composites have been successfully tested at up to 20% by volume. Based on these results,
geopolymer should also have potential to effectively solidify NAFs in a similar manner.
SUMMARY 2.7
Overall, geopolymer cements have significant potential for improved long-term
zonal isolation and well integrity as indicated by previous research studies. Many
geopolymer systems that have been developed require high temperature curing, which
naturally occurs in oil / gas wells. Very limited progress has been achieved in the
following areas limiting its application in oil / gas industry: (1) verifying the performance
of geopolymer paste at various pressure and temperature conditions; (2) study the
compatibility between geopolymer and drilling mud for displacement efficiency; (3)
develop effective methods or identify admixtures to control rheology and workability of
the slurry paste. The current work will focus on addressing these limitations.
35
Chapter 3: Materials and Methods
This chapter provides a brief overview about the materials and experimental
procedures. Section 3.1 provides general information about all the materials that were
used in this study. Section 3.2 presents all the experimental methods that were used to
evaluate the geopolymer slurries under various temperature and pressure conditions.
RAW MATERIALS 3.1
Fly ash composition 3.1.1
To create a geopolymer slurry, an aluminosilicate precursor powder is mixed with
an alkaline activating solution. As mentioned in the literature review section, fly ash-
based AAM delivers a more favorable rheology than metakaolin-based binders at a much
lower cost. Thus, in this study, three different class F fly ashes were selected as the
aluminosilicate precursor (ASTM C618-18, 2015). The main composition of each fly ash
(FA) is shown in Table 3.1. Fly ashes FA1 and FA3 were heat treated through a thermal
beneficiation process to increase the reactive component of the ashes. FA3 was passed
through a number 100 sieve to remove large particles. Geopolymer slurries that were
formed from fly ashes FA1, FA2 and FA3 will be referred to in the following as G1, G2
and G3 respectively.
36
Table 3.1 - Composition of fly ashes (weight %)
Beneficiated SiO2 Al
2O
3 Fe
2O
3 (SiO
2+Al
2O
3+Fe
2O
3) CaO MgO
Alkalis
(Na2O+0.658K
2O)
SO3
FA1 Yes 49.9 25.3 15.1 90.3 3.0 0.91 0.73 0.44
FA2 No 48.8 20.3 16.33 85.48 6.1 1.1 0.44 2.07
FA3 Yes 53.5 27.0 10.4 90.86 2.7 1.0 0.5 0.24
Particle size distribution (PSD) of fly ashes 3.1.2
The particle size distribution of each fly ash was measured using a Mastersizer
2000 particle size analyzer with a Hydro MU 2000 (Malvern, Worcestershire, United
Kingdom) wet dispersion unit. The specific gravity of fly ash was 2.56 and was obtained
from the manufacturer. Refractive index was picked to be 1.56 based on literature data
(Jewell and Rathbone, 2009). The absorption value was chosen such that the residual
weighted PSD calculated by the Mastersizer software was less than 1. Fly ash particles
were dispersed in isopropyl alcohol (IPA) to prevent hydration throughout the
measurement. The refractive index of IPA was assumed to be 1.39. During testing, the
particles were circulated at a pump speed of 2000 rpm and were sonicated with an
ultrasonic probe for 30 seconds. The particles were then circulated in the particle size
analyzer at a pump speed of 2000 rpm without sonication. Figure 3.1 shows the average
PSD based on five measurements for all the three fly ashes. As shown in the figure, all
the fly ashes showed a d50 between 10-20 µm (meaning 50% of the particles are smaller
than 10-20 µm). For PSD of FA3, a bimodal distribution was observed with peaks at 11.5
µm and 65.0 µm, whereas the data for FA1 and FA2 exhibited a unimodal distribution
with peaks at 14.5 µm and 16.0 µm respectively.
37
Figure 3.1 - Volume weighted particle size distribution of all fly ash particles. For the x-
axis, 40 size intervals were generated logarithmically between 0.1 and 1000. Y-axis
shows the volume fraction of particles between those sizes.
Activator 3.1.3
The alkaline activator solution was either hydroxide (sodium hydroxide) or
silicate, where both solid-form (SSS) and liquid-form sodium silicate (LSS) were used.
For hydroxide activation, 8 M NaOH solution was selected as the primary activator. It
has been recurrently reported in literature that 8 M NaOH can successfully activate class
F fly ash-based geopolymer (Aughenbaugh, 2013; Duxson et al., 2006; Fernández-
Jiménez et al., 2006a; Fernández-Jiménez and Palomo, 2005b; Provis and Deventer,
2009). For comparison purposes, 4 M and 6 M NaOH were also tested. NaOH solution
was prepared by weighing reagent grade NaOH pellets and dissolving them in ultrapure
0
1
2
3
4
5
0.1 1 10 100 1000
Volu
me
(%)
Particle Size (μm)
FA1
FA2
FA3
38
water (resistivity of 18 MΩ-cm) to the desired concentration. The solution was cooled
down to room temperature prior to use. The activator solution to fly ash ratio was
proportioned to be 0.485 based on preliminary tests for optimum strength and
workability.
For LSS activation, sodium hydroxide pellets and deionized water were added to
a commercially available LSS solution to achieve SiO2/Na2O weight ratios of 0.12, 0.24
or 0.48 (Table 3.2). For SSS activation, the SSS powder was blended with fly ash, and
was activated with a pre-mixed sodium hydroxide solution containing an appropriate
amount of sodium ions to achieve the same SiO2/Na2O ratios of 0.12, 0.24 or 0.48. These
three ratios, although are lower than the values recommended in literature, were selected
because the slurries showed the greatest potential for pumpability based on viscometer
and consistometer measurements. The Na2O/FA ratio was fixed at 0.1 for all the mixes
shown in Table 3.2, leading to a constant Na2O/Al2O3 ratio for any given fly ash. The
geopolymer slurry was hand-mixed with a spatula to incorporate all wet and dry
components, and was then mixed with a paddle stirrer at 480 rpm for 30 seconds.
39
Table 3.2 - Geopolymer mix design
Notation Activator SiO2/Na2O water/solid Na2O/fly ash
LSH-8M Liquid Sodium Hydroxide (8 M) - 0.33 0.1
LSS-0.12 Liquid Sodium Silicate 0.12 0.36 0.1
LSS-0.24 Liquid Sodium Silicate 0.24 0.36 0.1
LSS-0.48 Liquid Sodium Silicate 0.48 0.36 0.1
SSS-0.12 Solid Sodium Silicate 0.12 0.36 0.1
SSS-0.24 Solid Sodium Silicate 0.24 0.36 0.1
SSS-0.48 Solid Sodium Silicate 0.48 0.36 0.1
Admixtures 3.1.4
Limestone dust was used as a stability-enhancing additive (A) up to 2% by weight
of fly ash when instability was observed in the slurry. The particle size distribution of
limestone dust was also measured with the Mastersizer 2000 particle size analyzer and
the PSD data is shown in Figure 3.2. The d50 of limestone dust is approximately 3 µm and
d90 is 12 µm, indicating the limestone particles are smaller in comparison with the fly ash
particles (d50 between 10-20 µm).
40
Figure 3.2 - Volume weighted particle size distribution of limestone dust (admixture A)
Portland cement 3.1.5
For comparison, a Portland slurry (P1) consisting of Class H Portland cement and
38.5% by weight of cement (bwoc) water was prepared in accordance with API standard
RP 10B-2 (2010). For Portland slurries, sodium glucoheptonate retarder (R) was added at
a dosage of 0.025% bwoc for downhole rheology and thickening time measurements.
Alkali-activated slag 3.1.6
For the sake of comparison, properties of an alkali-activated slag cement were
measured using pressure transmission tester and mercury intrusion porosimetry. The slag
slurry was formed with a grade 120 blast furnace slag and 50% mix water by weight of
slag (bwos). The alkali activator contains 1.5% (bwos) NaOH and 4.5% (bwos) dense
soda ash. When preparing the slurry, NaOH pallets were first dissolved in water in a high
0
1
2
3
4
5
0.1 1 10 100 1000
Vo
lum
e (%
)
Particle Size (µm)
Admixture A
41
shear blender. The slag powder and soda ash were premixed and added to the blender.
Once all dry ingredients and liquids were combined together, the slurry was mixed at
12,000 rpm for 35 seconds.
SBM / OBM 3.1.7
The solidification of drilling mud was achieved by directly blending drilling mud
and geopolymer slurry with a paddle stirrer at 480 rpm for 30 seconds and allowing it to
set for various periods of time. In this study, 5-20% of the Portland slurry and 10-40% of
the geopolymer slurry was replaced with SBM / OBM by volume. A SBM (S1) that is
commonly used in the Gulf of Mexico was selected to conduct an in-depth study to
understand various properties of the geopolymer / mud hybrid. Another SBM (S2) and
two spent mineral oil-based muds (O1 and O2) were also solidified using 8 M NaOH
activated fly ash-based geopolymers and their compressive strength was measured. For
the sake of brevity, when 10% of the G1 slurry is replaced with S1, the slurry will be
referred to as G1S1-10. The properties of the four drilling muds used in this study are
shown in Table 3.3 and Table 3.4.
42
Table 3.3 - Properties of the SBMs (S1 & S2)
S1 S2
Density (ppg) 9.7 9.7
Water (%) 27% 25%
Oil (%) 63% 59%
Solid (%) 10% 16%
70 °F 120 °F 150 °F 70 °F 120 °F 150 °F
Viscometer Dial
Reading
600 rpm 94 65 48 70 50 36
300 rpm 58 40 28 44 29 21
200 rpm 43 28 20 32 21 15
100 rpm 25 18 15 21 14 10
6 rpm 11 10 9 9 6 4
3 rpm 10 9 8 9 6 4
Plastic Viscosity (cp) 36 25 20 26 21 15
Yield Point (lbf/100sqft) 22 15 8 18 7 6
S: synthetic-based mud
43
Table 3.4 - Properties of the OBMs (O1 & O2)
O1 O2
Density (ppg) 12 11.54
Water (%) 24% 18%
Oil (%) 56% 58%
Solid (%) 20% 24%
70 °F 120 °F 150 °F 70 °F 120 °F 150 °F
Viscometer Dial
Reading
600 rpm 349 239 148 349 213 133
300 rpm 317 151 90 246 122 78
200 rpm 241 117 68 178 90 58
100 rpm 156 79 42 104 56 36
6 rpm 43 20 7 23 14 10
3 rpm 34 16 6 18 12 9
Plastic Viscosity (cp) 32 88 58 103 91 55
Yield Point (lbf/100sqft) 285 63 32 143 31 23
O: mineral oil-based mud
METHODS 3.2
Compressive strength 3.2.1
The unconfined compressive strength (UCS) of hardened OPC slurries and
geopolymer slurries was obtained by crushing 2” × 2” cubes that were cured in a
pressurized curing chamber (API RP 10B-2, 2010). The curing temperature was
increased from room temperature to bottom hole circulating temperature (BHCT) of 125
°F in 2 hours and to bottom hole state temperature (BHST) of 170 °F in 10 hours. The
44
curing pressure was maintained at bottom hole pressure (BHP) of 3,000 psi. All
compressive strength values reported here are based on the average of 8 cubes. The error
bars in the plots represent one standard deviation. Please note that in order to apply a
confining pressure, a different sample size and sample shape was used for the self-healing
tests as mentioned in a later section.
Ultrasonic compressive strength 3.2.2
The ultrasonic compressive strength of geopolymers was obtained with an UCA
that is commercially manufactured for testing Portland cement. As mentioned in the
background chapter, strength of cement can be estimated ultrasonically with UCA. With
this method, the travel time of ultrasonic energy through a cement sample was measured
and empirical correlations were used to estimate the strength values. In the present study,
a geopolymer sample was tested in UCA with both built-in correlations that were
developed for Portland cement, and with correlations that were developed by Khalifeh et
al. (2014). To test a geopolymer sample in UCA, the geopolymer slurry was formed with
FA1 and 8 M NaOH solution. The slurry was placed in UCA upon mixing. Temperature
was ramped from room temperature to 189 °F (87 °C) in two hours and pressure was
maintained at 5,000 psi (34.5 MPa).
Rheology 3.2.3
Rheological properties of slurries were measured at room temperature with a
typical viscometer with F1 spring and R1B1 rotor configuration following the API
standard API RP 10B-2 (2010). When measuring the downhole rheological properties,
the slurry was pre-conditioned in a HPHT consistometer where the temperature and
45
pressure values were ramped to 125 °F and 3,000 psi, respectively, in 80 minutes and
stirred for an additional 30 minutes. At the end of the pre-conditioning period, the slurry
was taken out of the consistometer and poured into a pre-heated viscometer cup to
measure the rheological properties.
Thickening time 3.2.4
Thickening time, or pumping time, of the slurry was measured with a HPHT
consistometer following the API standard RP 10B-2 (2010). The ramping schedule was
80 minutes to BHCT of 125 °F and BHP of 3,000 psi.
Confined compressive strength 3.2.5
The confined compressive strength of hardened slurries was measured using a
triaxial load frame and a confinement vessel. Cylindrical samples(2 in. length × 1 in.
diameter) were prepared and cured in a water bath at 170 °F for 7 days. Prior to testing,
all samples were cut and a surface grinder was used to ensure the top and bottom surfaces
of the samples were smooth and parallel to each other as well as perpendicular to the
circumference of the cylinder. This was done to ensure the load was being applied along
the axis of the cylinder. Each sample was placed between two stainless steel endcaps and
wrapped with heat shrink tubing, which served as an impermeable barrier to the confining
fluid. Axial and radial strain gauges were placed to measure the triaxial displacement.
The confining stress (σ3), or confining pressure (Pc), acts isotropically on the sample.
Hence, the total axial stress (σ1) is a summation of the axial stress applied by the piston
and the confining stress. The difference between the axial stress and the confining stress
is defined as the differential stress (σ1- σ3).
46
At the start of each triaxial test, the sample was first loaded with a differential
stress of 30 psi. The sample was then loaded hydrostatically to the target confining
pressure of either 100 psi or 500 psi. The confining pressure was held constant for 10
minutes to ensure the change in axial strain due to creep was negligible compared to the
strain rate during measurement. After the system reached equilibrium, the differential
stress was increased at a constant axial strain rate of 10-5
per second. Loading was
continued until the post-peak regime was adequately defined. Two samples were
measured for each slurry type at both confining pressures. The stress-strain curves were
plotted showing axial and radial strains as a function of differential stress. Elastic
constants were determined over the linear sections of the stress-strain curves using the
following equations:
E =∆σa
∆εa (3.1)
ν =∆εr
∆εa (3.2)
where,
E = Young’s Modulus (psi),
ν = Poisson’s ratio (dimensionless),
σa = Axial stress (psi),
εa = Axial strain (inch/inch), and,
εr = Radial strain (inch/inch).
47
Unconfined self-healing test 3.2.6
Self-healing tests were conducted to study the re-healing capability of geopolymer
samples that were subjected to pre-loading. Cylindrical samples (4 in. length × 2 in
diameter) were cured in a water bath at 170 °F. On day 7, four samples were loaded to
failure to measure the UCS. Four samples each were loaded to either 30%, 50% or 70%
of the 7-day UCS, which likely causes varying levels of damage to the internal
microstructure. These partially damaged samples were placed in the water bath at 170 °F
for an additional 21 days. At the end of the re-healing period, the samples were loaded to
failure. By comparing the ultimate strength of the pre-damaged samples to the 28-day
UCS of an undamaged sample, the self-healing capability could be evaluated. Similar
measurements were conducted on samples that were pre-damaged to either 30%, 50% or
70% of the 28-day UCS and re-healed for an additional 28 days. The error bars in the
plots represent one standard deviation. The self-healing capability of the geopolymer
slurry was compared with that of Portland slurry.
Confined self-healing test 3.2.7
The confined self-healing test was performed with the triaxial load frame and a
confinement vessel. Cylindrical samples (4 in. length × 2 in. diameter) were cured for 7
days in a water bath at 170 °F. The samples were cooled down and loaded under 500 psi
confining pressure at a constant axial strain rate of 10-5
per second. The samples were
unloaded after 2% axial strain was reached, and were allowed to cure at 170 °F in a water
bath for an additional 21 days. After this re-healing period, the samples were loaded at
the same axial strain rate until failure. For each sample, the peak stress at 28 days was
compared to the peak stress at 7 days to evaluate the self-healing capability of Portland as
well as geopolymer slurries.
48
Pipe-in-pipe shear bond strength test 3.2.8
The cement-to-pipe shear bond strength was measured with a customized pipe-in-
pipe experimental set-up, as shown in Figure 3.3. A steel bar (8 in. length × 1 in.
diameter) was first polished with a medium grade emery cloth and then placed in a 3 in.
inner diameter plastic pipe. Cement slurry was poured into the plastic pipe to a depth of 6
inches around the steel bar. The sample was placed in a humidity controlled environment
at 170 °F for 7 days. After curing, the bottom plate was removed. The sample was placed
on a hollow base and the steel bar was pushed out of the cement sheath. The peak force
divided by the contact surface area of the steel bar yields the shear bond strength of the
cement-to-pipe interface.
Figure 3.3 - Schematic diagram of the pipe-in-pipe shear bond strength set-up
8” 7” 6”
1”
3”
Steel Bar
Cement
Steel Base
Plate
Apply vertical load at
constant rate
49
Pressure transmission test 3.2.9
The hydraulic conductivity (HC) of hardened geopolymer was indirectly
measured via pressure transmission testing. For the purpose of comparison, a Portland
cement slurry and an alkali-activated slag cement were also tested.
Pressure transmission test (PTT) was introduced to the oil / gas industry for
testing the tendency of a fluid filtrate, applied at overbalance (differential) pressure, to
invade the sample matrix and elevate the near wellbore pore pressure (Bol et al., 1994;
Darley, 1969; Stowe et al., 2001; Tare and Mody, 2000; van Oort, 1994). The schematic
of the pressure transmission test set-up is shown in Figure 3.4. A detailed test protocol
has been described in literature (van Oort et al., 1996, 2016). Simply explained, in a
pressure transmission test, a testing fluid is injected at a differential pressure across the
sample, allowing the fluid to transmit and penetrate through the sample. The downstream
pressure buildup due to pressure transmission is recorded and the data is processed to
obtain the hydraulic conductivity.
In this study, one sample each was tested for geopolymer, Portland cement, and
slag. The samples (2 in. length × 1 in. diameter) were cured at 170 °F for 28 days. All the
samples were submerged in water throughout curing and sample preparation period until
testing. Figure 3.5 shows a picture of the sample assembly. Porous aluminium frits were
placed on each end of the hardened cement sample to ensure uniform fluid distribution.
The sample stack was then placed between two flow heads, and was wrapped with heat
shrink tube and a viton sleeve. The sample was placed in an iso-static coreholder and
subjected to confining pressure of 1000 psi in a temperature controlled oven operated at
100 °F (35 °C). Tests were performed with two distinct cycles: in the first cycle 3%
artificial seawater (ASW) was used to characterize the conductivity of the artificial pore
fluid, and in a second cycle (after re-equilibrating the sample to initial conditions) light
50
mineral oil (LMO) was used. In both cycles, the upstream pressure was maintained at 300
psi and the downstream pressure was set to 50 psi to create a differential pressure of 250
psi.
Figure 3.4 - Schematic of the pressure transmission test set-up
Hardened cement
Confining fluid
Downstream (50 spi)
Viton Sleeve
Upstream (300 psi)
Porous top frit
Porous bottom frit
Top flow head Iso-static coreholder
Bottom flow head
51
Figure 3.5 - Picture of sample assembly in pressure transmission test
Simply explained, the downstream pressure build-up behavior due to pressure
transmission through the sample is similar to the charging of a capacitor in a resistor-
capacitor (RC) circuit. The pressure transmission is essentially described as follows (van
Oort et al., 2016):
P(l, t) − Po
Pm − Po= 1 − exp [−
Akt
μβVl] (3.3)
Where
Po = initial pore pressure (Pa),
Pm = upstream fluid pressure (Pa),
P(l, t) = downstream pressure transmission at sample end as a function of time
(Pa),
l = sample length (m),
A = sample cross-sectional area (m2),
V = volume of downstream reservoir (m3),
β = fluid compressibility (Pa-1
),
μ = fluid viscosity (Pa·s),
52
k = relative permeability of sample (m2).
The viscosity µ and compressibility β of the test fluid are generally unknown (or
provided to be indeterminate via calculations). The hydraulic conductivity (HC = 𝑘/𝜇𝛽
(m2/s)) is calculated for each of the cycles with ASW and LMO. Rearranging equation
(3.3):
𝐻𝐶 =k
μβ=
Vl
Atln
Pm − Po
Pm − P(l, t) (3.4)
The left side of this equation is hydraulic conductivity, which measures the
diffusivity of the fluids through the samples. Both the hydraulic conductivity
measurements are then compared to yield a “delay factor”:
Delay Factor =Hydraulic Conductivity (LMO)
Hydraulic Conductivity (ASW) (3.5)
The data obtained from PTT is processed in accordance with equation (3.4) and
fitted with a least-squares linear fit. From the slope of the fitted lines obtained for the
pore fluid (ASW) cycle and the test fluid (LMO) cycle, a delay factor can be calculated
using equation (3.5). The PTT data can be used to compare pressure transmission delays
(pore fluid vs. test fluid) and thus characterize the ability of fluid pressure to transmit
through a sample. The delay factor value indirectly reflects by how much the dynamic
pressure transmission can be slowed down. Overall, the pressure transmission test result
demonstrates the sample’s diffusive capability and thus exhibits its permeability
characteristics.
53
Porosity and pore size distribution 3.2.10
The porosity and pore size distribution of neat hardened geopolymer, Portland
cement, and alkali-activated slag were measured with mercury intrusion porosimetry
(MIP) test using AutoPore IV Automated Mercury Porosimeter (from Micromeritics
Instrument Corporation). A mercury porosimeter characterizes a material’s porosity (ϕ)
by applying various levels of pressure to a sample immersed in mercury. The pressure
required to intrude mercury into the pores of a sample is inversely proportional to the size
of the pores, and thus the pore size distribution of the porous material can be
characterized.
For each slurry formulation, a cylindrical sample (0.5 in. length × 1 in. diameter)
was cured at 170 °F for 28 days, and was then dried in an oven at 200 °F for at least 24
hours to ensure the weight reaches an equilibrium. The sample was then immersed in
mercury in a pressure-sealed chamber that was connected to a capillary stem with a
capacitor. The pressure in the chamber was increased incrementally from 0 to 60,000 psi.
This pressure range corresponds to pore sizes from a few nanometers (high pressure) to
hundreds of micrometers (low pressure). Each pressure step was maintained until the
volume equilibrium was reached. The bulk sample volume can be determined by the
capacitance when immersed in mercury. The pore volume and porosity can be computed
based on the capacitance measurements. The volume of mercury injected and the pore
volume at each incremental pressure can be used to construct pore size distribution.
Detailed MIP experimental protocols and data interpretation methods can be found in
Bear (1972), Peters (2012), and Purcell (1949). The pore throat radius (rp) can be
obtained from mercury injection pressure (PHg) using the following equation:
rp =2σHg cos θHg
PHg
(3.6)
54
where σHg is the air-mercury interfacial tension, θHg is the mercury contact angle.
The values for each parameter used in the pore size distribution and capillary pressure
calculation were summarized in Table 3.5.
In this study, the estimated porosity, pore area and average pore diameter of the
three samples were reported. The pore size distribution curves for each slurry were
plotted and compared.
Table 3.5 - Parameters used in the MIP measurement
Hg advancing angle 130 °
Hg receding angle 130 °
Air-Hg interfacial tension (N/cm) 0.485
Hg density (g/ml) 13.5335
55
Chapter 4: Hydroxide Activation
This chapter primarily focuses on evaluating the ability of geopolymer to convert
SBM into a cementitious material. In order to develop the mud solidification method, it is
imperative to first evaluate the compatibility between geopolymer and SBM. The
geopolymer slurry was formed by blending class F fly ash with 8 M sodium hydroxide
activator. Once the enhanced compatibility between geopolymer and SBM was
confirmed, a mud solidification method was achieved by simply incorporating various
quantities of SBM into the geopolymer slurry. The critical path for development of the
geopolymer hybrid cement included the evaluation of both the fresh state properties (such
as rheology and thickening time) and the hardened state properties (such as compressive
strength). To validate the applicability of the proposed solidification method, the effect of
changing the aluminosilicate powder (fly ash) source and the molarity of the hydroxide
activator was studied. The effect of varying the synthetic/water ratio (SWR) and internal
brine composition of the SBM was also analyzed. Seawater was tested as the mixing
water to assess the applicability of this method in offshore deepwater operations. The
thickening behavior of the geopolymer hybrid was evaluated under various BHPs.
Finally, the newly proposed solidification method was tested on various types of SBMs
and OBMs, verifying the compatibility between geopolymer and NAFs.
CONTAMINATION RESISTANCE 4.1
As described in the introduction chapter, OPC in general is incompatible with
NAFs. Contamination of Portland cement slurries with SBM leads to a significant
decrease in compressive strength. Figure 4.1 shows the substantial drop in compressive
strength values of Portland cement by almost 3500 psi with 10% SBM contamination,
56
whereas, geopolymer only had a strength reduction of 400 psi at the same contamination
level. Although neat Portland cement had much higher compressive strength than neat
geopolymer, at 20% SBM contamination the two curves intersected, showing equal
compressive strength. Further contamination displayed higher compressive strength for
geopolymer. In addition, the geopolymer can be designed with comparable compressive
strength values to OPC as mentioned in the background chapter. Therefore it is more
appropriate to compare the strength reduction in percentage rather than comparing the
actual strength value.
Figure 4.1 - Compressive strength values of hardened Portland cement slurry (P1) and
geopolymer slurry (G1) with SBM (S1) contamination (replacement by volume) at 24
hours, 170 °F and 3,000 psi.
0
1000
2000
3000
4000
5000
6000
7000
0% 10% 20% 30% 40% 50%
Com
pre
ssiv
e S
tren
gth
(psi
)
% of SBM
P1S1
G1S1
57
For both OPC and geopolymer, the change in compressive strength from their
neat state is displayed by plotting the percentage of strength reduction. When neat
Portland cement slurry was contaminated with 5% SBM (replacement by volume), the
compressive strength decreased by 65% and 10% contamination resulted in a 70%
strength reduction (Figure 4.2). Conversely, the geopolymer slurry only had a 30%
strength reduction with 10% SBM contamination. The geopolymer sample retained
measureable compressive strength up to the maximum contamination level tested of 40%
SBM while the Portland slurries had no measurable strength from contamination level of
30% SBM.
Figure 4.2 - Normalized compressive strength of hardened Portland cement slurry (P1)
and geopolymer slurry (G1) with SBM (S1) contamination (replacement by volume) at
24 hours, 170 °F and 3,000 psi.
0%
20%
40%
60%
80%
100%
0 10 20 30 40 50
% o
f U
nco
nta
min
ated
Sam
ple
Com
pre
ssiv
e
Str
ength
% of SBM
P1S1
G1S1
58
In addition to the issues with compressive strength, Portland cement slurries also
suffer from loss of pumpability (increase in viscosity) when contaminated with SBM. For
the sake of brevity, when 10% of the G1 slurry is replaced with S1, the slurry will be
referred to as G1S1-10. As shown in Figure 4.3 (a), Portland cement slurries became
progressively more viscous as the level of contamination increased. Above 20%
contamination with S1, Portland cement slurry became too viscous to be measured with a
typical viscometer using a F1 spring and a R1B1 rotor configuration. As shown in Figure
4.4, numerical simulation results have shown that high levels of cement contamination
occur at the leading edge of the cement slurry during displacement when interface
instability occurs (Enayatpour and van Oort, 2017). In a real life scenario, the increase in
viscosity caused by contamination could lead to serious problems including high
pumping pressure and unwanted fracturing of the well.
Geopolymer, on the contrary, showed a reverse rheological trend. As shown in
Figure 4.3 (b), neat geopolymer was too viscous and the upper limit of the viscometer
was reached above 100 s-1
shear rate. Typical neat geopolymer slurries can be placed into
formwork for civil engineering purposes, but cannot be pumped for petroleum
engineering purposes due to their viscous nature. The present work showed that by
adding S1 into geopolymer slurries, at room temperature the rheological properties were
greatly improved. At contamination levels of 30% and 40% the geopolymer slurries
showed rheological profiles that approached that of the neat Portland slurry, allowing a
pumpable geopolymer for well cementation. In addition, it was found that the rheological
properties of geopolymer could further be improved at higher temperatures. These
experimental results will be shown in the following section.
Both compressive strength and rheology measurements proved that geopolymer
slurries exhibit better contamination resistance compared to Portland cement slurries. In
59
fact, because a large volume of SBM can be incorporated into geopolymer to form a
pumpable slurry with the ability to harden, it demonstrates the possibility of a mud-to-
cement conversion technique using geopolymer.
Figure 4.3 - Rheological properties of (a) Portland cement (P1) and (b) geopolymer (G1)
slurries replaced with various dosages of SBM (S1) by volume at 70 °F
0
50
100
150
200
250
0 100 200 300 400 500 600
Shea
r S
tres
s (P
a)
Shear Rate (s-1)
P1S1-15
P1S1-10
P1S1-5
P1
(a)
0
50
100
150
200
250
0 100 200 300 400 500 600
Sh
ear
Str
ess
(Pa)
Shear Rate (s-1)
G1 G1S1-10G1S1-20 G1S1-30G1S1-40 P1
(b)
60
Figure 4.4 – Numerical simulation of drilling mud displacement with cement slurry. The
color bar shows the volume fraction of the cement slurry. The color gradient at the
interface indicates mixing of the two fluids (Enayatpour and van Oort, 2017)
In the remainder of this dissertation, SBM (or OBM) was purposely added to
geopolymer at various quantities. The resulting mixture will be referred to as geopolymer
hybrid. In a real life scenario, this geopolymer hybrid will be the cement slurry that will
be injected to a well. Since geopolymer demonstrates the ability to solidify drilling mud
at various levels of contamination, even if this geopolymer hybrid is further contaminated
at the leading edge, where non-uniform drilling mud contamination is most likely to
occur, the mixture should still have the ability to harden and develop compressive
strength.
In the following sections, properties including strength, rheology and thickening
time of the geopolymer hybrid will be discussed in detail, evaluating the capability of
geopolymer to solidify NAFs for both primary and remedial cementing applications.
61
PROPERTIES OF GEOPOLYMER HYBRID CEMENT 4.2
Compressive Strength 4.2.1
The compressive strength development from 16 hours to 14 days is presented in
Figure 4.5 for hardened geopolymer (G1) and geopolymer/SBM hybrids (G1S1) at up to
40% replacement by volume. For samples G1, G1S1-20, and G1S1-30, compressive
strength was measured as early as 16 hours after loading the cubes in the curing chamber.
At 16 hours, G1S1-40 sample was omitted from data collection due to the low strength
measured for the G1S1-30 sample. The strengths were also measured at 24 hours, 48
hours, 72 hours, 7 days, and 14 days. For G1 and G1S1-20 samples, the compressive
strength developed at a fairly rapid rate during the first three days. The G1S1-20 sample
had a 14-day compressive strength slightly above 2000 psi, which is close to half of that
of the neat G1 sample. These values indicated that the G1S1-20 slurry could provide
sufficient compressive strength for most primary cementing applications. In chapter 6, it
will be further proved that the confined compressive strength values are even higher than
the UCS values when the samples are subjected to confining stresses.
On the other hand, the G1S1-30 sample had rapid strength development over the
first 2 days, while the G1S1-40 sample increased in strength very gradually past day-1.
Both slurries reached a plateau after 2 days. The ultimate strength for G1S1-30 slurry was
close to 1000 psi, while for G1S1-40, this value was approximately 400 psi. The G1S1-
30 and G1S1-40 samples can, for instance, be developed into a lost circulation treatment
with a constantly low compressive strength, a desirable quality for such a treatment.
Based on the distinct strength development profiles, the blending of geopolymer slurry
with SBM presents a primary benefit: by deliberately changing the amount of SBM in the
slurry, the compressive strength of the final mix can be fine-tuned to meet the strength
requirement for a specific application.
62
Figure 4.5 - Compressive strength of neat geopolymer (G1) and geopolymer hybrids
(G1S1) at 170 °F and 3,000 psi
Downhole and Surface Rheology 4.2.2
The rheological properties of geopolymer hybrids at 70 °F were previously shown
in Figure 4.3 (b). The same measurements were also conducted at elevated temperature.
The neat geopolymer slurry (G1) was too viscous to be conditioned in a consistometer,
therefore was conditioned in a roller oven at 125 °F for 110 minutes. All geopolymer
hybrids were conditioned in a HPHT consistometer as described in experimental
methods. As shown in Figure 4.6, in comparison with data at room temperature (70 °F),
the geopolymer hybrid G1S1-20 had lower shear stresses at each shear rate after
conditioning at temperature and pressure. Neat geopolymer slurry also exhibited
0
1000
2000
3000
4000
5000
0 2 4 6 8 10 12 14
Co
mp
ress
ive
Str
ength
(p
si)
Time (days)
G1
G1S1-20
G1S1-30
G1S1-40
63
improved rheological properties after conditioning at 125 °F compared to the room
temperature sample, and although it reached the maximum reading for the F1 spring
(R1B1 configuration), it reached the maximum at a higher shear rate than it did at room
temperature. These results are in contrast with the results for the Portland slurry (see
Figure 4.6), which had increased shear stresses at each shear rate at elevated temperature.
The data showed that improved rheological properties of the geopolymer and geopolymer
hybrids can be expected at elevated temperatures.
Figure 4.6 - Rheological properties of neat geopolymer (G1), geopolymer hybrid (G1S1-
20) and Portland cement (P1-R) at 70 °F and 125 °F
0
50
100
150
200
250
0 100 200 300 400 500 600
Shea
r S
tres
s (P
a)
Shear Rate (s-1)
G1 (70 °F) G1S1-20 (70 °F) P1-R (70 °F)
G1 (125 °F) G1S1-20 (125 °F) P1-R (125 °F)
64
Thickening Time 4.2.3
The thickening time of neat geopolymer could not be tested due to its high
viscosity. However, two geopolymer hybrids, G1 with 20% S1 (G1S1-20) and 30% S1
(G1S1-30) slurries, were tested and the results are shown in Figure 4.7. For both slurries,
around 6-18 hours the liquid state consistency maintained at approximately 25 Bc. After
this time period the consistency increased at a steady rate. The results showed that the
thickening time for geopolymer hybrids was very long, with the G1S1-20 sample
reaching 70 Bc at 21 hours. The test for the G1S1-30 sample was terminated at 23 hours,
at which point the consistency was 60 Bc. These results showed that incorporation of
higher dosages of SBM into geopolymers prolongs the thickening time. This behavior is
expected since a higher volume of SBM was added to replace the reactive geopolymer
component. Note that geopolymer hybrids will require acceleration for actual field use,
given these long thickening time values.
Figure 4.7 - Thickening time of geopolymer hybrids at 125 °F
0
10
20
30
40
50
60
70
0:00 4:00 8:00 12:00 16:00 20:00 24:00
Consi
sten
cy (
Bc)
Time (hh:mm)
G1S1-20
G1S1-30
65
VALIDATION 4.3
Effect of Changing Activator Molarity 4.3.1
The concentration of alkali activator is a factor that influences the reaction of
geopolymer, and consequently the microstructure of hardened geopolymer (Salehi et al.,
2016a; Somna et al., 2011). In general for NaOH activation, the optimum molarity is
between 6 M and 10 M, while higher molarity results in slightly higher compressive
strength but less favorable rheology. In the present study, 4 M, 6 M and 8 M NaOH
solutions were used to compare their effectiveness in activating fly ash, and in creating
practical geopolymer hybrid cements. As shown in Figure 4.8, geopolymer activated by 6
M or 8 M developed similar 1-day compressive strength values in presence or absence of
SBM. 4 M NaOH activation was also attempted, however, the specimen showed
instability with a large amount of free water separating from the mixture right after
mixing.
Figure 4.8 - 1-day compressive strength of geopolymer hybrids activated by 6M or 8M
NaOH activator
0
500
1000
1500
2000
0% 20%
1-d
ay C
om
pre
ssiv
e S
tren
gth
(psi
)
% of SBM
8M NaOH
6M NaOH
66
The effect of NaOH molarity on rheological properties of geopolymer hybrid was
evaluated, and the results are shown in Figure 4.9. As shown, the neat G1 slurry activated
by 6 M NaOH solution, G1(6 M), showed slightly lower rheological readings at lower
shear rates in comparison to the G1(8 M) slurry. When 20% SBM was added, the G1S1-
20(6 M) slurry showed much higher rheological readings when compared with 8 M
NaOH activation. The 4 M NaOH activated slurries were tested and showed significant
separation, thus are not shown on the plot. Based on the rheological measurements it can
be concluded that the 8 M NaOH activation provided the best strength and pumpability.
Therefore, the remaining work described in this dissertation was all conducted with 8 M
NaOH solution.
Figure 4.9 - Rheological properties of geopolymer hybrids activated by 6M or 8M NaOH
activator at 70 °F
0
50
100
150
200
250
0 100 200 300 400 500 600
Shea
r S
tres
s (P
a)
Shear Rate (s-1)
G1(8M) G1S1-20(8M)
G1(6M) G1S1-20(6M)
67
Effect of Changing Aluminosilicate Source 4.3.2
In order to show that there are many aluminosilicate sources that are appropriate
raw materials for the proposed mud-to-cement conversion technique, three different
sources of class F fly ash were tested. As described in the materials section, two fly ashes
(FA1 and FA3) were heat treated by the manufacturer to improve their reactivity, while
the other (FA2) was used as produced from the power plant. The results in Figure 4.10
showed that the slurries made from the three different sources did not significantly vary
in strength when cured for 1-day for any amount of SBM. By day-3, G2 was significantly
stronger by itself, but it had similar strengths to the other geopolymers when blended
with SBM. These results lead to the conclusion that even if the composition of the fly ash
varies between sources, the compressive strength of the geopolymer hybrid would not
change significantly. This however has to be verified for more sources of fly ash.
The rheological properties of the G1S1 hybrid (from FA1) were tested at 70 °F
and presented in Figure 4.3. Geopolymer slurry G3 showed comparable rheological
profiles to slurry G1 when no SBM was added. However, G3S1 hybrids (from FA3) had
an instability issue when more than 30% S1 was added, therefore the data are not shown
here. Results for G2S1 hybrids are shown in Figure 4.11. Compared with G1 slurries
shown in Figure 4.3 (b), neat G2 slurry had a significantly lower rheological profile.
Furthermore, the rheological properties of G2S1-30 and G1S1-40 slurries were nearly
identical to those of the neat Portland slurry, P1. These results show that the source
material can have a big impact on the viscosity as well as stability of the hybrid slurry.
Critical evaluation of the precursor is essential during the development and routine
testing (e.g. for field applications) of a geopolymer hybrid.
68
Figure 4.10 - (a) 1-day and (b) 3-day compressive strength of geopolymer hybrids with
three different types of fly ashes
0
1000
2000
3000
4000
5000
0% 20% 30% 40%
1-d
ay C
om
pre
ssiv
e S
tren
gth
(p
si)
% of SBM
G1S1
G2S1
G3S1
(a)
0
1000
2000
3000
4000
5000
0% 20% 30% 40%
3-d
ay C
om
pre
ssiv
e S
tren
gth
(psi
)
% of SBM
G1S1
G2S1
G3S1
(b)
69
Figure 4.11 - Rheological properties of G2S1 hybrids at 70 °F
The thickening time of G1S1-20 and G2S1-20 slurries are shown in Figure 4.12.
The liquid state consistencies of the two slurries were similar and were both around 20
Bc. After 17 hours, the G1S1-20 slurry started to thicken and reached 100 Bc by 23
hours. The G2S1-20 slurry exhibited a much longer thickening time. As shown in the
materials and methods chapter, the PSD of FA1 and FA2 are very similar to each other.
The differences in the rheological properties and thickening time behavior between G1
and G2 slurries could be attributed to the compositional differences between the fly
ashes. Notice that FA2 has a lower aluminum content (20.3%) while the other two fly
ashes have aluminum content values exceeding 25%. The thinner rheological behavior
and longer thickening time behavior of G2S1-20 in comparison to G1S1-20 could be a
result of fewer aluminum species present in the liquid state.
0
50
100
150
200
250
0 100 200 300 400 500 600
Sh
ear
Str
ess
(Pa)
Shear Rate (s-1)
G2
G2S1-20
G2S1-30
G2S1-40
P1
70
Figure 4.12 - Effect of changing aluminosilicate source on the thickening time of G1S1-
20 and G2S1-20 slurries at 125 °F and 3,000 psi
Effect of Seawater 4.3.3
For offshore deepwater cementing operations, it is not uncommon to use seawater
instead of fresh water to mix cement slurries since seawater is more readily available in
such situation than fresh water. For such applications, it is important to study the effect of
using seawater as mixing fluid on the compressive strength and rheological properties of
geopolymer slurries. As shown in Figure 4.13 and Figure 4.14, even when the mix water
was changed from deionized (DI) water to seawater, both compressive strength and
rheological properties remained unchanged for neat geopolymer and geopolymer hybrids.
Therefore, it appears to be feasible to use seawater as the mixing water for geopolymer
and geopolymer hybrids while still achieving reliable slurry properties. This helps reduce
cost of the expensive deepwater operation.
0
20
40
60
80
100
120
140
0:00 12:00 24:00 36:00 48:00
Co
nsi
sten
cy (
Bc)
Time (hh:mm)
G2S1-20
G1S1-20
71
Figure 4.13 - Effect of using seawater vs. DI water on the compressive strength of G1 and
G1S1 hybrids at 70 °F and 3,000 psi
Figure 4.14 - Effect of using seawater vs. Di water on the rheological properties of G1
and G1S1 hybrids at 70 °F
0
500
1000
1500
2000
0% 20%
Co
mp
ress
ive
Str
ength
(p
si)
% of SBM
G1 (DI)
G1 (seawater)
0
50
100
150
200
250
0 100 200 300 400 500 600
Sh
ear
Str
ess
(Pa)
Shear Rate (s-1)
G1 (DI) G1S1-20 (DI) G1S1-30 (DI)
G1 (seawater) G1S1-20 (seawater) G1S1-30 (seawater)
72
Effect of SBM Composition 4.3.4
Previous results were all conducted with a field SBM that had a mud weight of
9.7 ppg, 23% CaCl2 and 75/25 SWR. To study the effect of SWR on the properties of the
geopolymer hybrid, additional brine was added to the field SBM to achieve a 60/40 SWR
while maintaining constant CaCl2 concentration in the internal phase. To study the effect
of internal brine phase composition on the properties of the geopolymer hybrid, the field
mud was diluted with water such that the CaCl2 content in the SBM was lowered from
23% to 13%.
Figure 4.15 reports the rheology readings for the three variations of the SBM,
which showed that SBM with higher SWR was less viscous at all shear rates at 80 °F,
120 °F and 150 °F. Changing the brine concentration in the internal phase had no impact
on the rheological behavior.
Figure 4.16 shows the rheological responses of geopolymer hybrids at 125 °F for
the three variations of SBM. As shown in the figure, the average shear stress of
geopolymer hybrid containing 30% original field SBM (75/25 SWR, 23% CaCl2) varied
from 12 Pa (at 5.1 s-1
) to 70 Pa (at 511 s-1
) with 10 sec gel strength of 15 Pa and 10 min
gel strength of 27 Pa. Altering the SWR ratio of the SBM from 75/25 to 60/40 while
maintaining the internal brine composition had no significant change in the rheological
profiles nor the gel strengths within the experimental error. Similar behavior was
observed by altering the internal brine composition, i.e., changing the CaCl2 content from
23% to 13%. In general, drilling muds with lower SWR usually exhibited higher
rheological readings (Figure 4.15). The geopolymer hybrid, on the other hand, was not
affected by the compositional changes in the drilling mud. Thus, the mud solidification
method is expected to be applicable to drilling muds with various SWR’s without
jeopardizing pumpability.
73
Figure 4.15 - Rheology of the original SBM (75/25 SWR, 23% CaCl2) as well as
modified SBMs at (a) 80° F, (b) 120° F, and (c) 150° F
0
50
100
150
200
250
0 300 600 900 1200
Sh
ear
Str
ess
(Pa)
Shear Rate (s-1)
(a) S1 at 80 °F 75/25, CaCl2=23%
60/40, CaCl2=23%
60/40, CaCl2=13%
0
10
20
30
40
50
60
0 300 600 900 1200
Shea
r S
tres
s (P
a)
Shear Rate (s-1)
(b) S1 at 120 °F 75/25, CaCl2=23%
60/40, CaCl2=23%
60/40, CaCl2=13%
0
10
20
30
40
50
60
0 300 600 900 1200
Sh
ear
Str
ess
(Pa)
Shear Rate (s-1)
(c) S1 at 150 °F 75/25, CaCl2=23%
60/40, CaCl2=23%
60/40, CaCl2=13%
74
Figure 4.16 - Effect of changing SWR and internal brine CaCl2 concentration of SBM on
(a) rheology and (b) gel strength of G2S1-30-2A at 125 °F and 3,000 psi
0
50
100
150
200
250
0 100 200 300 400 500 600
Sh
ear
Str
ess
(Pa)
Shear Rate (s-1)
75/25 CaCl2=23%
60/40 CaCl2=23%
60/40 CaCl2=13%
(a) G2S1-30-2A
Rheology
0
50
100
150
200
250
75/25 CaCl2=23% 60/40 CaCl2=23% 60/40 CaCl2=13%
Shea
r S
tres
s (P
a)
10 sec 10 min(b) G2S1-30-2A
Gel Strength
75
Thickening time of geopolymer hybrids containing 30% field SBM (original as
well as modified) was measured at BHCT of 125 °F and BHP of 3,000 psi. It can be seen
that at the beginning, the liquid state consistencies of all three slurries were
approximately 20-30 Bc (Figure 4.17) and during the first part of the test the samples
maintained low consistency values. Beyond 1 hour 30 minutes, the consistency of the
sample with the original field SBM (75/25 SWR with 23% CaCl2) increased slowly and
reached 70 Bc at approximately 4 hour 50 minutes. When the SWR was changed to
60/40, the time to reach 70 Bc shortened by only 20 minutes, which is within the
experimental error. However, when the internal brine composition was reduced to 13%,
the pumping time further decreased by an additional 50 minutes. Thus, it can be
concluded that changing the SWR of the SBM does not impact pumping time
significantly, but lowering the internal brine content lowers the pumping time.
Figure 4.17 - Effect of changing SWR and internal brine CaCl2 concentration of SBM on
thickening time of G2S1-30-2A. (BHCT of 125 °F and BHP of 3,000 psi)
0
10
20
30
40
50
60
70
0:00 1:00 2:00 3:00 4:00 5:00
Consi
sten
cy (
Bc)
Time (h:min)
75/25 CaCl2=23%
60/40 CaCl2=23%
60/40 CaCl2=13%
76
The 1-day compressive strength of geopolymers blended with 20% and 30% of
SBM with varying compositions was measured (Figure 4.18). When blended with 20%
original SBM, the compressive strength decreased from 2240 psi to 1210 psi and with
30% replacement, further decreased to 630 psi. For the other two variations of SBM, a
similar reduction in strength was observed.
Figure 4.18 - Effect of changing SWR and internal brine CaCl2 concentration of SBM on
1-day compressive strength of G2S1-30-2A at 170 °F and 3,000 psi
Effect of Pressure 4.3.5
Having a good understanding of the thickening time behavior of geopolymer
slurries at elevated temperature and pressure is of great importance in the development of
geopolymers for oilwell cementing applications. Salehi et al. (2016) reported that
0
500
1000
1500
2000
2500
3000
75/25 CaCl2=23% 60/40 CaCl2=23% 60/40 CaCl2=13%
Co
mp
ress
ive
Str
ength
(p
si)
0% SBM 20% SBM 30% SBM
77
temperature has a big impact on the pumpability of geopolymer slurries, where the
consistency plateaued at higher values at higher temperature within a given time period.
In the present work, pumpability of geopolymer slurries at various pressures was
evaluated. Thickening time of G2S1-30 was measured at different BHPs. Figure 4.19
shows the trend for one representative slurry of three replicates at each pressure. It can be
seen that at the beginning of the test, the liquid state consistency at all three BHPs was
approximately 25-30 Bc. All the samples exhibited similar profiles until reaching a point
of departure around 1 hour 30 minutes. Beyond this point the consistency of the sample
with BHP of 3,000 psi increased slowly and reached 70 Bc at approximately 4 hour 40
minutes. In contrast, the sample subjected to 12,000 psi reached 70 Bc in 3 hours. Thus, it
can be concluded that the pumping time of geopolymer hybrid decreases with increasing
pressure. This acceleration in thickening time with pressure is also observed in Portland
cement slurries, but the degree to which it is affected is based on the specific ramp rate to
reach BHP (Nelson and Guillot, 2006).
Figure 4.19 - Effect of pressure on thickening time of G2S1-30-2A at BHCT of 125 °F
0
10
20
30
40
50
60
70
0:00 1:00 2:00 3:00 4:00 5:00
Co
nsi
sten
cy (
Bc)
Time (h:min)
3000 psi
6000 psi
12000 psi
78
STABILITY CONTROL 4.4
A powder form stability modifier (A) was mixed into the slurries to aid with
stability of the blended slurry, which tended to separate at the highest SBM amounts,
particularly in stirred tests such as the thickening time test. The results presented in
Figure 4.20 showed that for G2 and G2S1-20 hybrid, the strengths with addition of 0%,
1.5% or 5% of the stability modifier (A) were nearly the same when considering the error
in the test. For slurries containing G2S1-30 and G2S1-40 samples, with addition of 1.5%
or 5% of the modifier the compressive strength increased in value at both day-1 and day-
3. These results show that not only does the powder work as a stability enhancer, but it
also has the added benefit of increased strength for the slurries with higher levels of
SBM.
The room temperature rheologies of G2 slurries mixed with 1.5% of the stability
modifier were measured, and the results are shown in Figure 4.21. The data for the
geopolymer hybrids with and without the additive are shown for ease of comparison. G2
was selected for this purpose, since it was thinner than G1 and its properties could be
measured without reaching the maximum for the viscometer with a F1 spring. In general,
the shear stresses at each shear rate increased with inclusion of the modifier, which was
expected due to the fineness of the powder additive (refer to PSD shown in Figure 3.2).
The downhole rheological profiles were also measured for the G2S1-40 hybrids with 1%,
1.5%, or 3% of the stability modifier and are presented in Figure 4.22. It can be seen that
at elevated temperatures, there was a significant improvement in the rheologies, and the
profiles were nearly similar to that of the Portland slurry with retarder (P1-R). These
results showed that the stability-enhancing modifier did affect the surface rheology when
added to slurries with high amounts of S1. However, the influence of the modifier on the
slurries was minimum with excellent rheological properties at elevated temperatures.
79
Figure 4.20 - (a) 1-day and (b) 3-day compressive strength of G2S1 hybrids with varying
dosages of stability modifier (A)
0
1000
2000
3000
4000
5000
6000
0% 20% 30% 40%
1-d
ay C
om
pre
ssiv
e S
tren
gth
(p
si)
% of SBM
(a) G2S1-0A
G2S1-1.5A
G2S1-5A
0
1000
2000
3000
4000
5000
6000
0% 20% 30% 40%
3-d
ay C
om
pre
ssiv
e S
tren
gth
(psi
)
% of SBM
(b) G2S1-0A
G2S1-1.5A
G2S1-5A
80
Figure 4.21 - Effect of adding 1.5% stability modifier (A) on the rheological properties of
(a) G2 and G2S1-20 hybrid, (b) G2S1-30 and G2S1-40 hybrids at 70 °F
0
50
100
150
200
250
0 100 200 300 400 500 600
Shea
r S
tres
s (P
a)
Shear Rate (s-1)
(a) G2-1.5A G2
G2S1-20-1.5A G2S1-20
P1-R
0
50
100
150
200
250
0 100 200 300 400 500 600
Shea
r S
tres
s (P
a)
Shear Rate (s-1)
(b) G2S1-30-1.5A G2S1-30
G2S1-40-1.5A G2S1-40
P1-R
81
Figure 4.22 - Effect of different dosages of stability modifier (A) on the rheological
properties of G2S1-40 hybrids at 125 °F
The stability modifier was used in G1 slurries, as well, and thickening time curves
with and without the stability modifier of the G1S1-30 hybrid are shown in Figure 4.23.
The consistency of the slurry containing the modifier was slightly higher at all ages. The
thickening time for the slurry was shortened with addition of 0.75% of the modifier, with
the slurry reaching 70 Bc at 19 hours. The shorter thickening time of the slurry measured
with the stability modifier is a beneficial result from the use of the modifier, however, the
higher consistency values are not.
0
50
100
150
200
250
0 100 200 300 400 500 600
Sh
ear
Str
ess
(Pa)
Shear Rate (s-1)
G2S1-40-3A
G2S1-40-1.5A
G2S1-40-1A
P1-R
82
Figure 4.23 - Effect of adding 0.75% of stability modifier (A) on thickening time for
G1S1-30 hybrid
SOLIDIFICATION OF NON-AQUEOUS DRILLING MUDS 4.5
The previous sections were all focused on the solidification of one type of SBM
(S1). In this section, compressive strength of geopolymer hybrid was also measured for
another SBM (S2) and two OBMs (O1, O2) and the values were compared with that of
Portland cement (P1). From Figure 4.24 it can be seen geopolymer performed better than
Portland cement with all four muds. At 20% replacement volume, Portland cement
retained less than 30% compressive strength, while geopolymer retained more than 50%
of its compressive strength with all four muds. The S2 and O2 muds required addition of
3% stability modifier. Based on these data, geopolymer demonstrates the possibility of
converting different types of NAFs into cementitious material that may be used for a
0
20
40
60
80
100
0:00 4:00 8:00 12:00 16:00 20:00 24:00
Co
nsi
sten
cy (
Bc)
Time (hh:mm)
G1S1-30
G1S1-30-0.75A
83
variety of applications. If desired, the geopolymer formulation can be further optimized
for improved compressive strength. Figure 4.25 shows pictures of two hardened
geopolymer hybrid samples that contained 20% of S1 and O2 muds. No layering or
separation was observed, demonstrating a good sample integrity and uniformity.
Figure 4.24 - Normalized compressive strength of hardened Portland cement slurries (P1)
and geopolymer slurries (G1) with 20% mud replacement (by volume)
0%
20%
40%
60%
80%
100%
S1 S2 O1 O2
% o
f U
nco
nta
min
ated
Com
pre
ssiv
e S
tren
gth
G1-20
P1-200% Mud
84
(a) (b)
Figure 4.25 - Picture of a geopolymer/mud sample containing 20% (a) S1 and (b) O2
SUMMARY 4.6
This work presents a new versatile technique to solidify NAFs into cementitious
materials using sodium hydroxide-activated geopolymer. Although the concept of drilling
mud solidification is not new, this is the first time this technology has become applicable
to SBM or OBM systems. The key findings include:
1. Geopolymers showed much better compatibility with NAFs compared to Portland
cement slurries in terms of both UCS and rheological properties.
2. The geopolymer hybrid cement provided sufficient compressive strength and
acceptable rheological properties for well cementing applications.
3. The short term and long term compressive strength of geopolymer can be tailored
to the application with the use of different dosages of SBM. More specifically,
lower dose hybrids may be suitable for primary cementing applications and higher
dose hybrids may find application in lost circulation treatments.
85
4. This mud-to-cement conversion technique is not limited to a single source
material. All three types of fly ashes tested were found to be viable candidates for
the solidification method.
5. The three fly ashes with varying compositions did not affect compressive strength
but showed a big influence on the viscosity as well as stability of the hybrid
slurry. Therefore, critical evaluation of the precursor is essential during the
development of a geopolymer hybrid.
6. For offshore deepwater cementing operations, seawater can be used instead of
fresh water without any negative effect on strength or rheology.
7. Changing SBM SWR or calcium chloride concentration showed no significant
effect on the compressive strength or rheology of geopolymer hybrid.
8. Changing the SBM SWR from 70/30 to 60/40 did not alter the thickening time,
but lowering calcium concentration slightly accelerated the thickening behavior.
9. Similar to Portland cement slurries, increased pressure shortened the thickening
time of geopolymer hybrids.
The primary benefits of this solidification method include but are not limited to:
1. Improved compatibility of the slurry with NAFs, and thus enhanced zonal
isolation in wells that have been drilled or abandoned with NAFs.
2. Reduced risk of poor cementation when NAF-based drilling mud displacement is
incomplete and/or when filter cake is present.
3. Inexpensive source material.
4. Environmentally friendly on-site/in-situ or off-site disposal of non-recyclable
NAFs.
86
Chapter 5: Silicate Activation
In this chapter, geopolymers activated with silicate alkaline solutions were
evaluated for their applicability in the mud solidification process. To achieve favorable
strength and mechanical properties, sodium silicate was combined with sodium
hydroxide solution in geopolymer synthesis as described in chapter 3. Studies have
shown that silicate-activated geopolymers could lead to a higher-strength and lower-
porosity binder, as the alkali silicate solution provides additional silicate species that
results in the formation of a larger volume of aluminosilicate gel (Duxson et al., 2005;
Criado et al., 2007). For silicate-activated geopolymers, the optimum Na2O/Al2O3 ratio is
reported to be around 1, and SiO2/Na2O ratio is between 0.1 and 2 (Chindaprasirt et al.,
2012; Criado et al., 2008, 2007). The silicate species in the solution control the rate of
structural reorganization and densification during polymerization as well as the degree of
reaction, especially at early age. Thus, engineering the right amount of silicate in the
alkaline solution is a critical task to optimize the slurry design for cementing purposes.
Geopolymer slurries activated by sodium silicate in both liquid form and solid
form were tested for their applicability in the mud solidification method. The rheological
properties, thickening time, and compressive strength of silicate-activated slurries were
compared with hydroxide-activated slurries. Results for neat silicate-activated
geopolymers are first presented, followed by results for geopolymer hybrids formulated
with the original field SBM.
RHEOLOGICAL PROPERTIES 5.1
Figure 5.1 shows the rheological readings of a neat geopolymer slurry activated
with liquid-form sodium silicate (LSS). As can be seen in Figure 5.1 (a), the silicate mix
87
with 0.12 SiO2/Na2O ratio (LSS-0.12) had very similar shear stress readings at low shear
rates and gel strength values compared with the sodium hydroxide mix (LSH). The LSS-
0.12 slurry became more viscous beyond 100 s-1
shear rate. The silicate mix with 0.24
SiO2/Na2O ratio (LSS-0.24) had similar rheological readings at higher shear rates
compared to the mix with 0.12 SiO2/Na2O ratio (LSS-0.12). However, the LSS-0.24 mix
was more viscous at lower shear rates and had much higher 10 sec and 10 min gel
strength values (Figure 5.1 b), which raises concerns about pumpability. As the
SiO2/Na2O ratio was increased to 0.48, the slurry became unpumpable and the gel
strength was significantly higher at 10 minutes. These results concur with findings
previously reported in the literature, which indicate that liquid-form sodium silicate tends
to generate more viscous geopolymer mixtures with a tendency to stick to mixing
equipment rather than flowing easily (Lloyd, 2009).
88
Figure 5.1 - (a) rheology and (b) gel strength of liquid-form sodium silicate (LSS)
activated geopolymer (0% SBM) at room temperature
0
50
100
150
200
0 100 200 300 400 500 600
Sh
ear
Str
ess
(Pa)
Shear Rate (s-1)
LSS-0.12
LSS-0.24
LSS-0.48
LSH-8M
(a)
0
25
50
75
100
125
150
LSS-0.12 LSS-0.24 LSS-0.48 LSH-8M
Shea
r S
tres
s (P
a)
10 sec
10 min
(b)
89
Figure 5.2 shows the effect of SBM on liquid silicate-activated geopolymer.
Unlike sodium hydroxide-activated geopolymer, which showed improvement in rheology
when blended with SBM, all three LSS-activated geopolymer slurries showed higher
rheological readings. The calcium ions from the internal phase of SBM likely interacted
with the silicate and formed a gel leading to the higher readings. For LSS-0.24 mix, the
addition of SBM reduced the “stickiness” of the mixture and showed slippage against the
viscometer rotor; however, this mixture was somewhat unstable.
Since the geopolymer activated by LSS was too viscous to be pumped, solid
sodium silicate (SSS) was tested. It exhibited significantly improved rheological
properties. As shown in Figure 5.3, all three SSS-activated geopolymer slurries showed
rheological readings comparable to LSH. The high gelation tendency was also inhibited,
allowing for a pumpable slurry. When SBM was added to SSS-activated geopolymer,
both rheological readings and gel strength values remained unaffected, regardless of the
SiO2/Na2O ratio in the activator (Figure 5.4). Based on these observations, it can be
concluded that LSS-activated geopolymer in general showed poor pumpability. However,
when solid-form silicate was used, the dissolution of silicate species from the solid
silicate activator was delayed, resulting in good pumpability and delayed gelation.
90
Figure 5.2 - (a) rheology and (b) gel strength of liquid-form sodium silicate (LSS)
activated geopolymer blended with 20% SBM at room temperature
0
50
100
150
200
0 100 200 300 400 500 600
Shea
r S
tres
s (P
a)
Shear Rate (s-1)
LSS-0.12
LSS-0.24
LSS-0.48
LSH-8M
(a)
0
25
50
75
100
125
150
LSS-0.12 LSS-0.24 LSS-0.48 LSH-8M
Shea
r S
tres
s (P
a)
10 sec
10 min
(b)
91
Figure 5.3 - (a) rheology and (b) gel strength of solid form sodium silicate (SSS)
activated geopolymer (0% SBM) at room temperature
0
50
100
150
200
0 100 200 300 400 500 600
Sh
ear
Str
ess
(Pa)
Shear Rate (s-1)
SSS-0.12
SSS-0.24
SSS-0.48
LSH-8M
(a)
0
25
50
75
100
125
150
SSS-0.12 SSS-0.24 SSS-0.48 LSH-8M
Shea
r S
tres
s (P
a)
10 sec
10 min
(b)
92
Figure 5.4 - (a) rheology and (b) gel strength of solid form sodium silicate (SSS)
activated geopolymer blended with 20% SBM at room temperature
0
50
100
150
200
0 100 200 300 400 500 600
Sh
ear
Str
ess
(Pa)
Shear Rate (s-1)
SSS-0.12
SSS-0.24
SSS-0.48
LSH-8M
(a)
0
25
50
75
100
125
150
SSS-0.12 SSS-0.24 SSS-0.48 LSH-8M
Shea
r S
tres
s (P
a)
10 sec
10 min
(b)
93
COMPRESSIVE STRENGTH 5.2
The effect of silicate activation on compressive strength has been well
documented in the open literature (Duxson et al., 2005; Criado et al., 2007). In general,
moving from hydroxide activation to silicate activation tends to increase the compressive
strength. Here the same trend was observed with fly ash FA1 (Figure 5.5 a), with the
LSS-0.24 mix almost doubling in compressive strength compared to the LSH-activated
slurry. The fly ash FA 2, on the other hand, had lower strength value with either liquid-
form (Figure 5.5 b) or solid-form (Figure 5.6) silicate activation. Therefore, the effect of
activator on strength highly depends on the composition of the aluminosilicate precursor.
Factors including glassy content, structure, and particle size distribution could all play a
role and influence the reactivity of fly ash. In addition, for FA1, increasing the amount of
silicate had no further improvements in the compressive strength, which indicates that
there is an optimum SiO2/Na2O ratio in the activator for desired compressive strength.
These results show that analogous to Portland slurry design, a full suite of tests is
necessary using the specific aluminosilicate powder during the geopolymer slurry design
process.
94
Figure 5.5 - Compressive strength of liquid sodium silicate (LSS) activated geopolymer
for (a) fly ash 1 and (b) fly ash 2 in comparison to liquid sodium hydroxide (LSH)
activated geopolymer without any SBM at 170 °F and 3,000 psi
0
500
1000
1500
2000
2500
3000
LSH-8M LSS-0.24 LSS-0.72
1-d
ay C
om
pre
ssiv
e S
tren
gth
(p
si) (a) FA 1
0
500
1000
1500
2000
2500
3000
LSH-8M LSS-0.12 LSS-0.24
1-d
ay C
om
pre
ssiv
e S
tren
gth
(psi
) (b) FA 2
95
Figure 5.6 - Compressive strength of solid sodium silicate (SSS) activated geopolymer
for FA 2 in comparison to liquid sodium hydroxide (LSH) activated geopolymer without
any SBM at 170 °F and 3,000 psi
THICKENING TIME 5.3
Another important parameter in the development of any cement slurry is
thickening time. For this, silicate activation can be successfully utilized as a means to
accelerate / retard the thickening time. As shown in Figure 5.7, in the absence of SBM,
neat LSH-activated geopolymer reached 70 Bc in 53 hours. For sample LSS-0.12 with
SiO2/Na2O ratio of 0.12, the thickening time was shortened to 22 hours. The thickening
time test for LSS-0.24 was attempted, but the slurry was too viscous to be tested in a
consistometer. Clearly, the presence of dissolved silicate in the solution greatly
accelerated the reaction rate. The rapid gelation and setting behavior of LSS-activated
geopolymer has also been documented by other researchers (Antoni et al., 2016). The
0
500
1000
1500
2000
2500
3000
LSH-8M SSS-0.12 SSS-0.24 SSS-0.48
1-d
ay C
om
pre
ssiv
e S
tren
gth
(p
si)
FA2
96
SSS-0.24 sample, on the other hand, had a considerably longer thickening time of 69
hours, indicating the delayed gelation of the aluminate and silicate species. Solely based
on the thickening time data it might appear that the SSS-0.24 sample does not set for a
long period of time. Note that under static conditions all the slurries developed more than
1,000 psi compressive strength by 24 hours (Figure 5.5, Figure 5.6). The dynamic
conditioning process in a consistometer breaks down the gel, thus significantly delaying
the restructuring of reaction products. Once pumping operations have concluded and the
slurry is in place, the setting time will become significantly shortened.
Figure 5.7 - Effect of various activating solutions on the thickening time of neat
geopolymer slurry (G2) at 125 °F and 3,000 psi
Figure 5.8 shows the effect of adding SBM on thickening time of both LSH- and
SSS-activated geopolymer. A very pronounced accelerating effect was observed for both
0
10
20
30
40
50
60
70
0:00 12:00 24:00 36:00 48:00 60:00 72:00
Co
nsi
sten
cy (
Bc)
Time (h:min)
SSS-0.24 LSS-0.12 LSH-8M
97
activation chemistries. The thickening time of LSH-activated G2 slurry with 20% SBM
was 3.5 hours, which is 50 hours shorter than for the slurry without SBM (Figure 5.8 a).
The accelerating effect can be attributed to the interaction between calcium ions in the
internal phase of SBM and silicate ions from the activating solution. The SSS-0.24 slurry
showed an initial peak of 65 Bc at 3.5 hours, but continued stirring broke down the initial
gel (Figure 5.8 b). The slurry plateaued after about 14 hours, and the liquid state
consistency lasted for about 5 hours before the slurry began to thicken. The final
thickening time to 70 Bc was 39 hours. As shown in Figure 5.6, the SSS-0.24 slurry
developed a compressive strength of 980 psi by 24 hours, well before the thickening time
reached 70 Bc. This unique behavior of SSS-activated geopolymer is promising for
treating lost circulation in particular. By changing the SiO2/Na2O ratio in the activator,
the initial gelation peak of the slurry can be manipulated to reach the values needed to
plug fractures and stop losses during a lost circulation event. Once the cement plug is in
place, the slurry will start to develop compressive strength. The thickening time of LSS-
activated geopolymer could not be measured due to the high viscosity of the
geopolymer/mud hybrid upon mixing.
98
Figure 5.8 - Effect of adding 20% original SBM on thickening time of (a) LSH-8M and
(b) SSS-0.24 geopolymer slurries (G2) at 125 °F and 3,000 psi
0
10
20
30
40
50
60
70
0:00 12:00 24:00 36:00 48:00 60:00 72:00
Co
nsi
sten
cy (
Bc)
Time (h:min)
Neat
20% SBM
(a)
0
10
20
30
40
50
60
70
0:00 12:00 24:00 36:00 48:00 60:00 72:00
Consi
sten
cy (
Bc)
Time (h:min)
Neat
20% SBM
(b)
99
SUMMARY 5.4
This chapter explored the feasibility of using silicate-based activator for SBM
solidification. Both liquid form and solid form sodium silicate were tested. The properties
examined include rheology, thickening time and compressive strength. The main variable
in designing silicate-activated geopolymer was the SiO2/Na2O ratio. The following
conclusions are drawn from the experimental findings:
1. LSS activation resulted in unfavorable rheology readings and gel strength. The
presence of SBM further increased the viscosity of LSS-activated geopolymer,
leading to an unpumpable slurry with a rapid setting behavior. In contrast, SSS
activation did not affect the rheological behavior of geopolymer in the presence or
absence of SBM.
2. For neat geopolymer slurries, LSS activation accelerated the pumping time. SSS
activation, by contrast, retarded the pumping time in comparison to hydroxide
activation. The presence of SBM significantly accelerated the pumping time for
both silicate and hydroxide activation. The SSS activation showed an initial high
consistency peak that was broken down with continued shearing.
3. Silicate activation greatly improved the compressive strength compared with pure
hydroxide activation, confirming the findings in existing literature. However, the
effectiveness of strength enhancement highly depended on the aluminosilicate
source selected.
Overall, silicate activation affects both the fresh state properties as well as
hardened state properties of geopolymer hybrid cement in comparison to hydroxide
activation. By adjusting the type and dosage of silicate in the activator, the pumping time
and compressive strength of the geopolymer hybrid can be altered.
100
Chapter 6: Mechanical Properties and Self-Healing Capability
To ensure a robust cement design and long term well integrity, this chapter further
studies the mechanical properties of the geopolymer hybrid cement. Specifically, the self-
healing capability of the geopolymer hybrids was evaluated under both uniaxial and
triaxial confinement conditions. The self-healing behavior exhibited from these tests
gives insight into the capability of the geopolymer hybrids to accommodate subsurface
stress variations and regain strength after deformation and failure. It is important to note
here that the term “self-healing” relates to healing of the matrix material itself, not to the
incorporation of e.g. swellable materials to recover hydraulic isolation after the
cementitious material is compromised and invaded by hydrocarbons (Cavanagh et al.,
2007; Reddy et al., 2010; Taoutaou et al., 2011). A true self-healing material is
particularly well-suited for wells that will undergo a lot of geomechanical / tectonic
loads, and can be adopted as a versatile material for temporary / permanent abandonment
of wells.
Furthermore, the cement-to-pipe bond strength of geopolymer hybrids was
measured for the first time in the presence and absence of a layer of drilling mud on the
pipe surface. The hydraulic conductivity, porosity and pore size distribution of
geopolymers were compared with hardened Portland cement and an alkali-activated slag.
CONFINED COMPRESSIVE STRENGTH 6.1
The confined compressive strength tests on geopolymer (G1), geopolymer hybrid
(containing 20% SBM and 80% geopolymer by volume) and uncontaminated Portland
cement slurries were conducted at 100 psi and 500 psi confining pressures. Two samples
were tested for each slurry at each pressure. Figure 6.1, Figure 6.2 and Figure 6.3 showed
101
the differential stress with respect to axial strain for each of these samples. The maximum
differential stress is equal to two times of the maximum shear stress in the sample. As
shown in the figures, the differential stress increased with an increase in the confining
stress. The neat geopolymer sample (Figure 6.1) showed a brittle failure pattern at 100 psi
confining pressure with the post peak differential stress dipping by more than 50% of the
peak stress. When a sample exhibits brittle behavior, the stress decreases with increased
strain past the yield point. At 500 psi confining pressure, the geopolymer started to show
brittle-ductile transition behavior wherein beyond post peak the differential stress
gradually decreased as the axial strain was increased. The geopolymer hybrid (Figure 6.2)
showed ductile behavior at both 100 psi and 500 psi confining pressures. Ductile
behavior is characterized by the ability of the material to deform without losing
toughness; in other terms, the stress continuously increases with increasing strain. Ductile
behavior is advantageous for cement sheath integrity in a well because the set cement will
deform or flow instead of cracking. In comparison (Figure 6.3), hardened Portland
cement slurries were more brittle than the geopolymer hybrid at both confining pressures.
The confined compressive strength is defined as the peak stress the sample was
able to withstand. The values are calculated as the summation of differential stress and
the confining stress. In Table 6.1, as expected, the confined compressive strength was
higher at a higher confining pressure for all three slurries. Note that for the geopolymer
hybrid in particular, the confined compressive strength at 500 psi confining stress was in
excess of 2,800 psi, which will be more than sufficient for most types of primary
cementation applications.
102
Figure 6.1 - Differential stress vs. axial strain for two samples each for geopolymer
subjected to a confining pressure of 100 psi and 500 psi at 7 days
Figure 6.2 - Differential stress vs. axial strain for two samples each for geopolymer
hybrid subjected to a confining pressure of 100 psi and 500 psi at 7 days
0
2000
4000
6000
8000
0.0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0%
Dif
fere
nti
al S
tres
s (p
si)
Axial Strain
100 psi (#1)100 psi (#2)500 psi (#1)500 psi (#2)
100 psi
500 psi
0
2000
4000
6000
8000
0.0% 0.5% 1.0% 1.5% 2.0%
Dif
fere
nti
al S
tres
s (p
si)
Axial Strain
100 psi (#1)
100 psi (#2)
500 psi (#1)
500 psi (#2)
100 psi
500 psi
103
Figure 6.3 - Differential stress vs. axial strain for two samples each for Portland cement
subjected to a confining pressure of 100 psi and 500 psi at 7 days
Table 6.1 - Confined compressive strength at 7 days
Pc = 100 psi Pc = 500 psi
Geopolymer 3330 psi 5000 psi
Geopolymer Hybrid 2000 psi 2870 psi
Portland Cement 5600 psi 7850 psi
MECHANICAL PROPERTIES 6.2
As one of the goals of this study was to investigate the potential use of
geopolymers in P&A operation, it would be beneficial to study the mechanical properties
0
2000
4000
6000
8000
0.0% 0.5% 1.0% 1.5% 2.0%
Dif
fere
nti
al S
tres
s (p
si)
Axial Strain
100 psi (#1)
100 psi (#2)
500 psi (#1)
500 psi (#2)
100 psi
500 psi
104
of the hardened geopolymer slurries under triaxial stress conditions. Properties such as
Young’s modulus and Poisson’s ratio are important performance parameters as they
reflect the flexibility of a cement system and its ability to withstand the varying
geomechanical / tectonic loads that act on cement and cemented casing string during the
lifetime of an abandoned well. Here, these elastic constants were determined from the
linear portion of the stress vs. strain curve of samples that were cured for 7 days and 28
days.
Young’s modulus characterizes the ductility of a material, with a lower Young’s
modulus value corresponding to a more ductile material. As shown in Table 6.2, the
Young’s modulus of neat geopolymer sample was 0.39 Mpsi by 7 days. The sample
became more brittle by 28 days and the Young’s modulus increased to 0.54 Mpsi. Both
the neat geopolymer and geopolymer hybrid showed lower Young’s modulus values
compared with Portland slurry at 7 days and 28 days, indicating that geopolymer was
more ductile than Portland cement. Similar conclusion was observed by Khalifeh et al. in
their study where they compared the properties of potassium silicate-activated class C fly
ash-based geopolymer and Portland cement (2015).
Measurements also showed higher Poisson’s ratio () for both geopolymer and
geopolymer hybrid cements in comparison to the values of Portland cement (Table 6.2).
However, the Poisson’s ratio values of Portland cement obtained are lower than the
values reported by Iverson et al. (2008). The reported Poisson’s ratio was 0.2 for neat
Portland cement, although the samples were cured and measured at different downhole
temperature and pressure conditions. The lower Poisson’s ratio values could be attributed
to the creeping effect. In other words, the sample deformed viscously in a time-dependent
manner and the measurement was conducted before the creep strain became more
appreciable.
105
Table 6.2 - Mechanical properties of hardened slurries at 500 psi confining pressure
7 days 28 days
E (Mpsi) ν E ( Mpsi) ν
Geopolymer 0.39 0.12 0.54 0.10
Geopolymer Hybrid 0.37 0.09 0.43 0.14
Portland 0.65 0.06 0.66 0.09
SELF-HEALING PROPERTIES 6.3
The self-healing behavior of geopolymer was studied in order to evaluate the
capability of the cement sheath to adapt its structure to the influence surrounding
environment. Of particular interest is the ability of the cement sheath to regain integrity
and hydraulic isolation after it has been compromised by cracking, e.g. under the
influence of a changing geomechanical load in an abandoned well. Self-healing capability
was evaluated both with and without confining pressure. The results for geopolymer and
Portland cement samples without confinement are shown in Figure 6.4. The vertical axis
shows the compressive strength of the samples that were pre-damaged with respect to the
compressive strength of the samples that were not pre-damaged. As shown in Figure 6.4
(a), geopolymer samples that were pre-loaded at 7 days of curing and re-healed for an
additional 21 days consistently retained a higher percentage of compressive strength
compared to Portland cement at all three pre-loading levels. In addition, geopolymer
samples that were pre-loaded developed higher compressive strength than those without
any pre-load, reflected by the data bars that exceed the baseline (100%). This could be
106
attributed to the pre-loading stresses that cracked unreacted particles. More reaction
products could form when these newly cracked surfaces come in contact with pore fluid.
This behavior revealed that geopolymer slurry has intrinsic self-healing capability when
subjected to certain level of damage.
The same test was repeated for samples that were pre-loaded at 28 days of curing
and re-healed for an additional 28 days. As shown in Figure 6.4 (b), for the samples pre-
loaded to 70%, geopolymer retained more than 120% compressive strength, which was
significantly higher than the values for Portland cement. The samples loaded to 30% and
50% of their compressive strength did not show a significant difference between the two
types of slurries. The samples that were pre-loaded to 70% of their compressive strength
were very likely to have microstructure damage, which the geopolymer was able to re-
heal but Portland cement was not. At 30% and 50% pre-loading levels, only little damage
was introduced to the sample structure, which explains why no significant difference
between the two slurries was observed.
107
Figure 6.4 - Self-healing properties of geopolymer and Portland cement pre-loaded at (a)
7 days and (b) 28 days under atmospheric conditions. Cylindrical samples were prepared
at 170 °F
40%
60%
80%
100%
120%
140%
30% 50% 70%
28
-day
Co
mp
ress
ive
Str
ength
Rec
over
y
Pre-loading Level at 7-day (% of 7-day Compressive Strength)
(a)
Geopolymer
Portland cement
Baseline compressive
strength of 28-day samples
without pre-damage
40%
60%
80%
100%
120%
140%
30% 50% 70%
56-d
ay C
om
pre
ssiv
e S
tren
gth
Rec
over
y
Pre-loading Level at 28-day (% of 28-day Compressive Strength)
(b)
Geopolymer
Portland cement
Baseline compressive
strength of 56-day samples
without pre-damage
108
Since the samples were only loaded up to a maximum of 70% of the compressive
strength in the unconfined tests, these tests can only partially reflect the self-healing
capability of the material. In particular, the unconfined test method could not provide
information regarding the self-healing capability past peak stress due to catastrophic
failure of the samples, a situation not representative of material behavior in the downhole,
confined environment. Therefore, a confined self-healing test was designed such that
materials were loaded beyond their yield point to create significant internal damage to
their matrices. The ability of the hardened cement to fully recover - or to develop even
higher - compressive strength would truly reflect any self-healing capability.
The confined self-healing tests were carried out on geopolymer, geopolymer
hybrid and hardened Portland cement slurries. As described in the experimental method
chapter (Chapter 3), a cylindrical sample was first loaded beyond yield point at 7 days
and was allowed to re-heal for an additional 21 days. Two samples were measured for
each slurry. As shown in Figure 6.5 and Figure 6.6, both geopolymer and geopolymer
hybrid samples showed significant self-healing capability with the 28-day peak stress
values largely exceeding the 7-day peak stress values. In comparison, hardened Portland
cement could no longer support the 7-day stress once the sample was yielded (Figure
6.7). Table 6.3 summarized the changes in peak stresses after re-healing period. As can
be seen, the peak stresses of two neat geopolymer samples increased by 35% and 30%
respectively. The geopolymer hybrid samples also showed re-healing and peak stresses
increased by approximately 18%. The peak stresses of Portland cement samples dropped
by more than 20% for both samples.
109
Figure 6.5 - Self-healing capability of geopolymer (G) under 500 psi confining stress, for
two samples (a) and (b). The G-7 sample was loaded beyond its yield point at 7 days, and
the same sample was re-tested to failure at 28 days (G-7-28). Note the evident increase in
peak stress observed for the samples at 28 days
0
1,000
2,000
3,000
4,000
5,000
6,000
0.0% 0.5% 1.0% 1.5% 2.0% 2.5%
Dif
fere
nti
al S
tres
s (p
si)
Axial Strain
(a)
G-7
G-7-28
0
1,000
2,000
3,000
4,000
5,000
6,000
0.0% 0.5% 1.0% 1.5% 2.0% 2.5%
Dif
fere
nti
al S
tres
s (p
si)
Axial Strain
(b)
G-7
G-7-28
110
Figure 6.6 - Self-healing capability of geopolymer hybrid (GH) under 500 psi confining
stress, for two samples (a) and (b). The GH-7 sample was loaded beyond its yield point at
7 days and the same sample was re-tested to failure at 28 days (GH-7-28). Note the
evident increase in peak stress observed for the samples at 28 days
0
1,000
2,000
3,000
4,000
5,000
0.0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0%
Dif
fere
nti
all S
tres
s (p
si)
Axial Strain
(a)
GH-7
GH-7-28
0
1,000
2,000
3,000
4,000
5,000
0.0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0%
Dif
fere
nti
al S
tres
s (p
si)
Axial Strain
(b)
GH-7
GH-7-28
111
Figure 6.7 - Self-healing capability of Portland cement (P) under 500 psi confining stress,
for two samples (a) and (b). The P-7 sample was loaded beyond its yield point at 7 days
and the same sample was re-tested to failure at 28 days (P-7-28). Note the evident
reduction in peak stress observed for the samples at 28 days
0
2,000
4,000
6,000
8,000
0.0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0%
Dif
fere
nti
al S
tres
s (p
si)
Axial Strain
(a)
P-7
P-7-28
0
2,000
4,000
6,000
8,000
0.00 0.01 0.01 0.02 0.02 0.03 0.03
Dif
fere
nti
al S
tres
s (p
si)
Axial Strain
(b)
P-7
P-7-28
112
Table 6.3 - Peak stress values at 7-days and after 21-day waiting period (at 500 psi
confining stress)
Pre-loading
Peak Stress
(7-day) psi
Re-loading
Peak Stress
(28-day) psi
% Change
Geopolymer
Sample (a) 4,010 5,420 35%
Sample (b) 3,960 5,120 30%
Geopolymer
Hybrid
Sample (a) 3,420 4,030 18%
Sample (b) 3,520 4,110 17%
Portland
Cement
Sample (a) 6,630 3,960 -40%
Sample (b) 6,890 5,330 -23%
Both unconfined and confined self-healing measurements revealed that
geopolymer-based cementing materials have intrinsic self-healing property that is well-
suited for supporting long-term zonal isolation, as e.g. required by wells that are
abandoned or decommissioned. This self-healing capability of geopolymer can be
attributed to the formation of extra reaction product when the unreacted particles in its
microstructure get exposure to pore fluids.
The loss of zonal isolation after the cement has set can be attributed to mechanical
failure of the hardened cement or the de-bonding of cement sheath from the casing and/or
formation interfaces. Several previous approaches used to mitigate cement failures from
long-term exposures to stresses and temperatures include: (1) adding fibrous materials
into the cement slurry for improved toughness of the cement matrix (LaPrade and Low,
2003); (2) designing cement slurries with the capability to withstand the physical stresses
113
that might be encountered during the lifetime of the well (Bybee, 2000; Stiles and
Hollies, 2002); (3) using “self-healing” sealant materials to re-seal the leak path if one is
present (Cavanagh et al., 2007; Reddy et al., 2010; Taoutaou et al., 2011). Note that the
first two approaches target improved failure resistance but provide no self-healing
benefits. The sealant method is usually composed of swelling additives that seal the
cracks when exposed to wellbore fluids or heat, in an attempt to re-establish hydraulic
isolation. It is currently unknown if this approach provides a durable solution for the
longer term. Moreover, unlike the latter method, which only provides sealing of cracks,
geopolymer actually closes any micro-cracks by intrinsically developing more reaction
product, thus providing a superior solution for zonal isolation. A material with self-
healing capability is particularly well suited for wells that will undergo a lot of in-situ
stress variation caused by depletion of the reservoir, hydraulic fracturing operation, or
other geomechanical loading. For well abandonment applications, where long-term
cement integrity is critical, the self-healing geopolymer cement presents a viable option
as a permanent sealant that can re-heal if it becomes compromised. Thus the study of
long-term self-healing properties of geopolymer is encouraged in order to access the
capability of geopolymer to provide effective isolation over a long period of time, even
decades.
CEMENT-TO-PIPE BOND STRENGTH 6.4
When the cement-to-pipe shear bond strength was tested for geopolymer samples
on a clean steel bar at 7 days, the peak bond strength was above 200 psi for both
replicates (Figure 6.8). Beyond this, the load dropped at a decreasing rate due to the
sliding stress (or skin friction) against the pipe (Nahm et al., 1995). In comparison,
114
Portland cement samples had a peak value less than 100 psi. The 7-day unconfined
compressive strength for geopolymer and Portland cement is 1,640 psi and 4,100 psi
respectively. The shear bond to compressive strength ratio of geopolymer is therefore
13%. This ratio for hardened Portland cement slurry, however, is only 2%.
Figure 6.8 - Cement-to-pipe shear bond strength for two samples of Portland cement and
geopolymer with clean steel pipe at 170 °F on day-7
The cement-to-pipe shear bond strength was also evaluated when the steel bar was
pre-coated with SBM. This simulates the less-than-optimal conditions when the SBM is
not completely displaced from the surface of the casing. As shown in Figure 6.9, the
presence of SBM significantly lowered the bonding capability of both slurries. The pipe
bond strength of geopolymer is around 30 psi, and is slightly higher than the values for
Portland cement (10 psi). The effect of SBM can also be visually observed as shown in
0
50
100
150
200
250
0 50 100 150 200
Cem
ent-
to-P
ipe
Shea
r B
on
d S
tren
gth
(psi
)
Time (s)
Portland (1)
Portland (2)
Geopolymer (1)
Geopolymer (2)
115
Figure 6.10. The color of the hardened slurries close to the pipe surface is darker than the
rest of the samples, indicating the presence of SBM.
Figure 6.9 - Cement-to-pipe shear bond strength for two samples of Portland cement and
geopolymer with SBM coated steel pipe at 170 °F on day-7
(a) (b)
Figure 6.10 - Cross-section view from bottom of the cement-to-pipe shear bond test
samples for (a) Portland cement and (b) geopolymer
0
10
20
30
40
50
0 20 40 60 80 100 120 140 160 180
Cem
ent-
to-P
ipe
Sh
ear
Bo
nd
Str
ength
(psi
)
Time (s)
Portland (1)
Portland (2)
Geopolymer (1)
Geopolymer (2)
116
HYDRAULIC CONDUCTIVITY 6.5
As mentioned in chapter 3, the hydraulic conductivity of Portland cement,
geopolymer and alkali-activated slag samples were measured using the pressure
transmission test.
The pressure transmission curves of hardened Portland cement when injecting
artificial seawater (ASW) in first cycle and light mineral oil (LMO) in the second cycle is
shown in Figure 6.11 (a). As can be seen in the figure, the pressure transmission was
much slower when injecting LMO compared to ASW due to the water-wet nature of the
cement sample. The linear trend lines for both pressure transmission curves were also
plotted. By comparing the slope of the linear fit lines, a delay factor around 10 was
obtained, indicating that the pressure invasion of the oil phase will be 10 times slower
when Portland cement was exposed to LMO. The pressure transmission curves for
hardened geopolymer and alkali-activated slag are shown in Figure 6.11 (b) and (c)
respectively. Compared to Portland cement, both geopolymer and slag showed a much
slower pressure transmission rate when injecting LMO. The delay factors for these two
alkali-activated materials were 35 and 45 respectively, much higher than the value of
Portland cement, indicating that it takes longer time for oil phase pressure to transmit
through geopolymer and slag.
117
Figure 6.11 - Pressure transmission curves, linear fit and the delay factor for (a) Portland
cement, (b) geopolymer, and (c) slag at 28 days
y = 2.0E-03x
y = 2.2E-04x
0
1
2
3
4
5
0 2000 4000 6000 8000 10000 12000 14000 16000
ln((
Pm
-Po
)/(P
m-P
t))
Time (s)
(a) Portland_ASW
Portland_LMO
Delay factor ~ 10
y = 1.6E-03x
y = 4.8E-05x
0
1
2
3
4
5
0 2000 4000 6000 8000 10000 12000 14000 16000
ln((
Pm
-Po)/
(Pm
-Pt)
)
Time (s)
(b) Geopolymer_ASW
Geopolymer_LMO
Delay factor ~ 35
y = 4.0E-03x
y = 8.7E-05x
0
1
2
3
4
5
0 2000 4000 6000 8000 10000 12000 14000 16000
ln((
Pm
-Po
)/(P
m-P
t))
Time (s)
(c) Slag_ASW
Slag_LMO
Delay factor ~ 45
118
To better compare the hydraulic conductivity of these three cementitious
materials, the linear fit trend lines of the ASW pressure transmission curves were
replotted and are shown in Figure 6.12. The hydraulic conductivity values were
calculated based on the slope of the linear fit lines and are indicated in the figure. As can
be seen, the conductivities of geopolymer and Portland cement were comparable and
were both lower than the conductivity of slag. Based on the definition of hydraulic
conductivity (equation 3.4), the permeability is proportional to the hydraulic conductivity
for a specific fluid. Cement with lower permeability is less vulnerable to external fluid
invasion, which is a step in the right direction for ensuring good zonal isolation. In this
study, assuming the LMO has a viscosity of 30 mPa·s and a compressibility of 4.3E-10
Pa-1
at the test pressure and temperature, all three hardened materials had oil permeability
values in the range of 300-1800 μD. It can be speculated that within this permeability
range, hydrocarbon would not migrate through the cement matrix at any detectable rate.
Figure 6.12 - Hydraulic conductivity (HC) and linear fit trend lines of Portland cement,
geopolymer, and slag at 28 days
0
1
2
3
4
5
6
0 500 1000 1500 2000 2500 3000
Ln
((P
m-P
o)/
(Pm
-Pt)
)
Time (s)
Linear (Portland_ASW)
Linear (Geopolymer_ASW)
Linear (Slag_ASW)HC = 2.5E-06 m2/s
HC = 1.2E-06 m2/s
HC = 1.0E-06 m2/s
119
POROSITY 6.6
Porosity is another factor that influences the ability of cement to prevent gas
migration and to provide zonal isolation. Space for gas or hydrocarbon to enter the
annulus includes the inherent porosity of the cement, any cracks or fractures in the
cement sheath, and any gap at the cement bonding interfaces. In the previous sections the
ability of geopolymer to resist cracking under triaxial stresses and to bond towards casing
has been discussed. Here the porosity of geopolymer is studied and compared with
conventional Portland cement.
In literature, multiple approaches have been adapted to characterize the porosity
and the pore size distribution of the hardened cement matrix, however, an exact pore
structure characterization remains difficult to achieve. A hardened cement sample
contains pores that have various pore sizes, shapes, interconnectivity, accessibility and
surface roughness. In general, the pore structure of cementitious materials contains air
voids, capillary pores, and gel pores. The pores in hardened cement slurries can be
classified into two categories, gel pores (< 10 nm) that are formed within hydration
products, and capillary pores (10 nm – 10000 nm) that are inter-connected and dominates
diffusivity (Mindess et al., 2003).
The porosity of geopolymer characterized with transmission electron microscopy
(TEM) and gas adsorption porosimetry has been reported in the literature (Kriven et al.,
2006; Metroke et al., 2010). The porosity of geopolymer forms when considerable
amount of water is forced out during the polymerization process and resides between the
precipitates. The pores have a length scale of 5-20 nm. Studies showed that these pores
are filled with free water (Bell et al., 2010, 2009).
The porosity and total pore area obtained from the MIP measurement were
reported in Table 6.4. As shown in the table, the porosity values of all three hardened
120
cement slurries were similar at approximately 15%. It can also be seen that the slag
sample showed smaller pore diameter and larger pore area, indicating the slag had
smaller pores compared to the other two cement slurries. The pore size distribution plot
also confirmed this observation. As shown in Figure 6.13, the pore size distribution of
Portland cement and geopolymer were very similar with a normal distribution-like curve
with a single peak at 30 nm on a log scale. The slag sample, on the other hand, was a
bimodal distribution with peak values at 9 nm and 14 nm.
Table 6.4 - Porosity and pore size evaluation of Portland, geopolymer and slag from MIP
measurement at 7 days
MIP-Porosity
(%)
Total Pore Area
(m2/g)
Avg. Pore
Diameter (µm)
Portland Cement 17 8.5 0.048
Geopolymer 14 8.3 0.044
Slag 16 24 0.017
121
Figure 6.13 - Pore size distribution of Portland cement, geopolymer and slag obtained
from MIP test, samples were cured at 170 °F for 28 days
It is important to mention that mercury intrusion porosimetry (MIP) test has its
own limitations. MIP cannot provide a true pore size distribution because mercury cannot
pass through the narrowest pores or the isolated pores in the pore network. The total
porosity estimated from MIP test will also differ from the values obtained by other
techniques. The MIP porosity will usually be smaller than the true porosity values due to
inaccessible pores. On the other hand, there are studies that believe the MIP porosity can
be closer to the true value where mercury pressures can collapse small pores so that the
isolated pores become accessible (Beaudoin, 1979; Diamond, 1971).
0
0.005
0.01
0.015
0.02
0.025
0.03
1 10 100 1,000
Incr
emen
tal
Intr
usi
on
(m
L/g
)
Pore Size (nm )
Portland cement
Geopolymer
Slag
122
ULTRASONIC CEMENT STRENGTH 6.7
As mentioned in the background chapter, cement strength can be tested via
destructive method, i.e., hardened cement cubes or cylinders of a particular age can be
crushed for its strength. Non-destructive strength testing with ultrasonic cement analyzer
(UCA) is a method that can continuously measure the compressive strength over a period
of time. The strength values are obtained based on empirical correlations between sonic
wave transit time and UCS that were pre-programmed in the UCA. The main drawback
of UCA is the use of a single set of empirical correlations for different formulations.
Regardless of the limitations with UCA test, it is still a popular method to evaluate the
compressive strength of Portland cement as it allows a continuous measurement of the
strength development profile. API recommended practice (API RP 10B-2, 2010) has
included both destructive and non-destructive test methods as means to quantify the
strength values of hardened cement.
In an effort to develop geopolymer for well cementing purposes, the present study
attempted to run geopolymer in UCA following API standard (API RP 10B-2, 2010).
Geopolymer sample was formed with 8 M NaOH activation and fly ash FA1. The
pressure was stepped up to 5,000 psi and the temperature was ramped from room
temperature to 189 °F in 2 hours. Figure 6.14 shows the compressive strength and transit
time data of geopolymer directly obtained from built-in UCA correlations that were
developed for Portland cement. As can be seen in the figure, for geopolymer sample the
transit time varied between 9.5 and 11.5 sec/in and the value reached a peak at
approximately 2 hours before starting to ramp down. For a typical Portland cement
slurry, on the other hand, the transit time started with a higher value around 17 sec/in
and ramped down once the cement started to develop compressive strength (Figure 6.15).
The main issue can be noticed by observing the compressive strength curve. The built-in
123
correlations from UCA resulted in negative compressive strength values for the
geopolymer sample for the first 19 hours. The 24 hour compressive strength eventually
reached 235 psi, far from the actual UCS value measured by crushing geopolymer cubes
cured for 24 hours (1,280 psi). Clearly the existing correlations that were developed for
Portland cement do not apply to geopolymer chemistries. This could be attributed to the
fact that fly ash particles are spherical and a lot of times hollow. The reflective and
refractive indices for fly ash differ significantly from Portland cement particles which
leads to a change in the sonic wave propagation path and the transit time.
Figure 6.14 - Geopolymer sonic compressive strength and transit time obtained from
UCA test with built-in empirical correlations developed for Portland cement, test was
conducted at 189 °F and 5,000 psi
9
9.5
10
10.5
11
11.5
12
-600
-400
-200
0
200
400
600
0:00:00 6:00:00 12:00:00 18:00:00 24:00:00
Tra
nsi
t T
ime
(use
c/in
)
Com
pre
ssiv
e S
tren
gth
(psi
)
Time (hh:mm:ss)
Compressive Strength Transit Time
124
Figure 6.15 - Portland cement sonic compressive strength and transit time obtained from
UCA test with built-in empirical correlations developed for Portland cement, test was
conducted at 170 °F and 3,000 psi
As discussed in the literature review section, Khalifeh et al. (Khalifeh et al., 2014)
developed a set of UCA correlations for geopolymer (Table 2.1). Here the applicability of
these correlations was tested for their effectiveness in evaluating geopolymer
compressive strength. Notice that geopolymer slurry was formed with class F fly ash as
opposed to the class C fly ash in their study. The same testing temperature and pressure
conditions were used in this study for a direct comparison. Figure 6.16 shows the
compressive strength curve when the published correlations were applied. As shown in
the figure, the negative compressive strength problem was fixed after 2.5 hours.
However, the compressive strength values were above 500 psi for the first two hours,
which does not match the UCS values. In addition, 24 hour cube strength was 1,280 psi,
0
4
8
12
16
20
24
0
1000
2000
3000
4000
5000
6000
0:00:00 12:00:00 24:00:00 36:00:00 48:00:00 60:00:00 72:00:00
Tra
nsi
t T
ime
(use
c/in
)
Co
mp
ress
ive
Str
ength
(p
si)
Time (hh:mm:ss)
Compressive Strength Transit Time
125
whereas the 24-hour strength value based on the correlations was more than 6,000 psi.
This result indicated that the published correlations for class C fly ash do not apply to
class F fly ash. More factors including differences in source materials, types of activator
(hydroxide vs. silicate activation), and activator-to-fly ash ratios could all influence the
ultrasonic wave transmission, thus should be considered when developing UCA
correlations for geopolymers.
Figure 6.16 - Geopolymer sonic compressive strength and transit time obtained from
UCA test with empirical correlations developed for geopolymer, test was conducted at
189 °F and 5,000 psi
SUMMARY 6.8
In this chapter, several critical hardened state properties of geopolymer were
evaluated in detail, including mechanical properties, self-healing properties, cement-to-
9
9.5
10
10.5
11
11.5
12
-2000
0
2000
4000
6000
8000
10000
0:00:00 6:00:00 12:00:00 18:00:00 24:00:00
Tra
nsi
t T
ime
(use
c/in
)
Com
pre
ssiv
e S
tren
gth
(psi
)
Time (hh:mm:ss)
Compressive Strength Transit Time
126
pipe bond strength, hydraulic conductivity and porosity. The experimental results
indicated that geopolymers can serve as a viable alternative to Portland cement for
primary cementation and as potentially superior candidates for improving barrier integrity
in abandoned wells. The key findings of this work include:
1. This work shows that geopolymers and geopolymer hybrid cements have more
than sufficient compressive strength when subjected to confining stress for most
cementing purposes.
2. Under triaxial loading, both geopolymer and geopolymer hybrid samples
exhibited more ductile behavior in comparison to Portland cement, showcasing
the ability of geopolymer materials to better deal with surrounding stress
fluctuations.
3. The self-healing study demonstrated that geopolymers have great potential to be
used as superior alternatives to Portland cement in well abandonment /
decommissioning applications. The post-failure self-healing capability of
geopolymer was verified under conditions of uniaxial and triaxial loading to
failure. Geopolymers exhibited excellent self-healing of their matrix, with the
material being able to withstand higher stress after they were pre-loaded. Such
behavior was not observed for Portland cement.
4. The cement-to-pipe bonding properties of geopolymer were also studied. When
the steel bar was either clean or was coated with SBM, geopolymers showed
consistently higher shear bond strength values compared with Portland cement.
5. The hydraulic conductivity of geopolymer was compared with hardened Portland
cement slurry and an alkali-activated slag. When injecting ASW, the conductivity
values of geopolymer and Portland were comparable, and they were both lower
than the conductivity of slag. When injecting LMO, the geopolymer and slag
127
samples showed higher delay factor than Portland cement, indicating a lower
diffusivity to oil phase.
6. By conducting MIP tests, the porosity of geopolymer was evaluated. The porosity
and pore size distribution of hardened geopolymer and Portland cement were
comparable, whereas the alkali-activated slag showed similar total porosity but
smaller pore sizes.
7. Existing UCA correlations that were developed for Portland cement cannot be
applied to geopolymer slurries because the equations generated negative
compressive strength values. The correlations developed for class C fly ash
geopolymer from a recently published paper yielded positive compressive
strength values, however, the values failed to match the UCS values obtained by
crushing cement cubes made from class F fly ash.
128
Chapter 7: Conclusions and future work
The main goal of this research was to investigate and develop a versatile mud
solidification method that is applicable to any type of NAF. In this dissertation a
solidification method was successfully developed by blending SBM / OBM with an
appropriate volume of geopolymer. The resulting hybrid cement delivered many of the
necessary characteristics of an oilwell cement slurry. Through detailed evaluation and
verification, it was shown that the geopolymer hybrid cement could be potentially used
for various applications including primary cementing, lost circulation control, well
abandonment and decommissioning, etc.
In this dissertation, background information about the geopolymer design and
mud solidification method was provided in Chapter 2. Chapter 3 described the materials
and experimental methods that were used in this research. Chapter 4 thoroughly
evaluated the effectiveness of geopolymer to solidify SBM, and characterized the
compressive strength, rheological properties and thickening time behavior of the
geopolymer hybrid cement system. Factors including aluminosilicate source, molarity of
alkaline activator, presence of seawater, composition of SBM were all considered.
Chapter 5 investigated the effectiveness of silicate activation on solidifying SBM.
Chapter 6 investigated the hardened state properties such as mechanical properties,
confined compressive strength, self-healing capability, porosity and hydraulic
conductivity that are crucial for well integrity. Finally this chapter provides key
conclusions from this study along with suggestions for future work.
129
CONCLUSIONS 7.1
It has been shown that geopolymers have superior compatibility with NAFs
compared to traditional Portland cement. Geopolymers not only develop sufficient
compressive strength at high levels of mud contamination but also, by incorporating
SBM into geopolymer, their rheological properties can be improved remarkably, allowing
for a pumpable slurry for well cementing operations. By varying the volume ratio of
geopolymer and mud, the properties of the geopolymer hybrid cement can be custom
tailored to the target application. The key findings and conclusions of this research study
are as follows:
Geopolymers showed much better compatibility with NAF-based drilling muds
compared to Portland cement slurries. It was found that geopolymer could solidify
up to 40% of SBM and still develop measurable compressive strength, while
Portland cement slurry lost the ability to harden at the same level of mud
contamination. This observation was the foundation for the development of the
mud solidification technique presented here.
The incorporation of SBM into geopolymer slurries significantly improved the
rheological properties of geopolymer at both room temperature and elevated
temperature, allowing for a pumpable slurry for well cementing applications. In
fact, unlike Portland cement slurries, which become more viscous at elevated
temperatures, geopolymer and geopolymer hybrids showed lower rheological
readings with increasing temperature.
Geopolymer successfully solidified two types of SBMs and two types of OBMs.
The composition of SBM showed limited effects on the strength and rheological
properties of the geopolymer hybrids, meaning that the mud-to-cement conversion
is a versatile method that can be adapted to work with various drilling muds. This
130
study further proved that changing synthetic/water ratio or calcium chloride
concentration of SBM had no significant effect on the compressive strength or
rheology of geopolymer hybrid. Changing the synthetic/water ratio of SBM from
70/30 to 60/40 did not alter the thickening time, but lowering the calcium
concentration of the invert phase slightly accelerated the thickening time.
By comparing three different types of fly ashes, it was found that this mud-to-
cement conversion technique is not limited to a single source material, an
important consideration for practical field application which cannot rely on just a
single source. All three types of fly ashes tested in this study showed potential for
formulating geopolymers that are suitable for mud-to-cement conversion. Varying
compositions of fly ashes could lead to varying rheological properties and
pumping time. Therefore, standard API tests have to be conducted for each source
material, e.g. during routine testing in preparation for field application, similar to
Portland cement slurry design.
Triaxial compressive strength testing showed that geopolymer and geopolymer
hybrid cements had more than sufficient compressive strength when subjected to
confinement for most cementing operations. Neat geopolymer and geopolymer
hybrids exhibited more ductile behavior in comparison to Portland cement,
indicating the ability of geopolymer-based material to better accommodate and
deal with surrounding stress fluctuations.
Both unconfined and confined self-healing tests revealed the self-healing
capability of geopolymer and geopolymer hybrids. This was confirmed by the re-
healed peak stress of the cement matrix exceeding the maximum pre-loading
stress. The intrinsic self-healing characteristic is distinctly different from - and is
superior to -current mechanisms for creating self-healing Portland cements, which
131
rely on the re-sealing of micro fractures with chemical sealants or with the use of
fibrous materials to reduce crack growth.
The rheological properties, short term and long term compressive strength of the
geopolymer hybrid can be tailored to the target application with the use of an
appropriate volume of SBM, or by altering the alkaline activator composition.
1. Increasing the amount of SBM in the geopolymer hybrids resulted in
lower rheological readings, lower compressive strength values and longer
thickening time.
2. Activation with liquid sodium silicate (LSS) resulted in unfavorable
rheology readings and gel strength values for both geopolymer and
geopolymer hybrids. By contrast, activation with solid sodium silicate
(SSS) did not affect the rheological behavior of geopolymer in the
presence or absence of SBM.
3. For neat geopolymer slurries, LSS activation accelerated the pumping
time. By comparison, SSS activation retarded the pumping time in
comparison to hydroxide activation. The presence of SBM significantly
accelerated the pumping time for both silicate and hydroxide activation.
The SSS activation showed an initial high consistency peak that broke
down with continued shearing.
4. Silicate activation greatly improved the compressive strength compared
with pure hydroxide activation. However, the effectiveness of strength
enhancement highly depended on the aluminosilicate source selected.
Other properties that are critical to oil / gas well cement slurry design were also
investigated. It was found out that:
132
1. Similar to Portland cement slurries, increased pressure shortened the
thickening time of geopolymer hybrids.
2. The hydraulic conductivity and porosity values obtained for geopolymer
were comparable to Portland cement values.
3. It was found that using seawater did not affect the rheological properties
or compressive strength of geopolymer hybrid slurries.
Based on the key findings listed above, Figure 7.1 illustrates the slurry design
philosophy that can be adapted to formulate geopolymer hybrid cements. In simple terms,
a geopolymer hybrid cement can be formed when an aluminosilicate source, an alkaline
activator, and SBM or OBM are combined. The fresh state properties including surface
and downhole rheological properties and mixture stability should be first evaluated,
followed by the thickening time test, in order to confirm the pumpability of the slurry.
Next, the hardened state properties including compressive strength, mechanical properties
and bonding characteristics should be evaluated. When any of the properties fail to meet
the requirements of the target application, the slurry design parameters should be
reconsidered. Either property-modifying additives need to be added, or the main
components have to be adjusted. It is very important that key properties are evaluated at
representative downhole conditions, particularly using appropriate pressure and
temperature and their ramp-up history representative of the downhole circulation of the
cement. By performing this iteration, a geopolymer hybrid slurry with desired properties
can be achieved.
133
Figure 7.1 - Circular flow diagram showing the design philosophy of geopolymer/mud
hybrid cement
In conclusion, with this new geopolymer-based mud solidification technology, a
wide range of well construction applications open up, including the use of SBM/OBM
compatible lost circulation treatments and plug cementations, the use of geopolymers in
spacer and scavenger fluids prior to primary cementation, the use of geopolymer slurries
in primary cementation, all the way to well abandonment and decommissioning
Fresh-state
Properties
Hardened-state Properties
Slurry Design
• Source material
• Alkali activator
• Admixtures
• Geopolymer/mud ratio
• Mud composition
• Rheological properties
• Thickening time
• Dynamic stability
• Compressive strength
• Compatibility with
various mud
• Mechanical properties
• Bonding properties
• Self-healing properties
• Porosity
• Permeability
134
operations. The enhanced compatibility with SBM and OBM may lead to step-change
improvements in deepwater, narrow-margin cementations in particular.
FUTURE WORK 7.2
One of the main limitations with the proposed mud solidification method is the
variation in performance with different source materials. This has been observed with
compressive strength test, thickening time test and rheological property measurements. It
remains unclear which component(s) of the aluminosilicate source affects each one of the
cement properties. It would be valuable to correlate the type and quantity of reactive
contents in the source material to the performance of the resulting slurry.
The other main limitation associated with the current mud solidification method is
that rheology control is mainly achieved by changing the proportion of mud in the
geopolymer hybrid, at the cost of compressive strength reduction. Future work has to
focus on identifying effective superplastisizers / rheology modifying additives that do not
negatively affect other key properties of the slurry.
In terms of thickening time control, the current study was focused on
understanding the changes in the thickening time of geopolymer hybrids with sodium
hydroxide and sodium silicate activation. It has been shown that the fresh state properties
of geopolymer hybrids were significantly affected by the types and dosage of the
activator and by the presence of drilling mud. Future work should focus on further
investigation of the interaction between the mud and geopolymers formed with different
activators and mixed activators. Moreover, further laboratory investigation is necessary to
identify other accelerators and retarders for thickening time control.
135
In the present study, the ability of geopolymer hybrid cement to resist excessive
loads triggering yielding in a well has been investigated by conducting confined
compressive strength test and self-healing test. Future research should also look at the
tensile strength and toughness of the hardened geopolymer hybrids, which are all critical
indicators of a cement system’s ability to provide zonal isolation throughout the lifetime
of a well and after abandonment. The present study has attempted to measure the tensile
strength of geopolymer by conducting Brazilian splitting tensile strength tests. However,
the experimental error of this measurement was too large to obtain a good average
strength value. Future work can be directed towards bending or flexural tests. When the
goal of the cement slurry design is for well abandonment and decommissioning, other
properties that need to be evaluated for geopolymer-based cement include:
(1) Volumetric and bulk shrinkage and expansion.
(2) Permeability recovery after yielding / cracking and re-healing, with the ability
to withstand pressure loads associated with hydrocarbon fluid and gas columns.
(3) Long term durability and chemical resistance to wellbore and reservoir fluids
(brine or hydrocarbons).
(4) Static and dynamic stirred fluid loss under differential pressure.
(5) Static gel strength during setting for gas migration control.
(6) Stability control additives for use in blended geopolymer slurries without
significantly affecting the consistency.
(7) Free fluid control.
Low-calcium fly ash-based geopolymers generally requires elevated temperature
curing. The present study only focused on downhole temperature regimes above 125 °F.
In case of deepwater drilling or arctic drilling, where the temperature approaches freezing
point, the slurry compositions presented here would not harden at a practical rate. Future
136
work should develop slurry compositions for applications at temperatures below 125 °F.
Several approaches that could enable effective geopolymer activation for low temperature
curing include: (1) utilization of ground fly ash or ultra-fine fly ash with larger surface
area for higher reaction rate; (2) using high-calcium fly ash or blending in calcium
silicate (BFS or Portland cement) to incorporate C-S-H gel in the aluminosilicate
structure.
137
List of Abbreviations
AAM Alkali-activated material
AAAS Aqueous alkali alumino silicate
AAS Alkali-activated slag
API American Petroleum Institute
ASTM American Society for Testing and Materials
ASW Artificial seawater
BHCT Bottom hole circulating temperature
BHP Bottom hole pressure
Bwoc By wait of cement
Bwos By wait of slag
BFS Blast furnace slag
DI Deionized
EDS Energy dispersive spectroscopy
EPA Environmental Protection Agency
FA Fly ash
GH Geopolymer hybrid
HC Hydraulic conductivity
HPHT High pressure high temperature
IPA Isopropyl alcohol
LCM Lost circulation material
LMO Light mineral oil
LSS Liquid-form sodium silicate
MIP Mercury intrusion porosimetry
138
NAF Non-aqueous drilling fluid
OBM Oil-based mud
OPC Ordinary Portland cement
P&A Plug and abandonment
PSD Particle size distribution
PTT Pressure transmission test
RC Resistor-capacitor
RP Recommended practice
SBM Synthetic-based mud
SEM Scanning electron microscope
S / S Solidification / stabilization
SSS Solid-form sodium silicate
SWR Synthetic / water ratio
TEM Transmission electron microscopy
UCA Ultrasonic cement analyzer
UCS Unconfined compressive strength
UF Universal fluid
WBM Water-based mud
XRD X-ray diffraction
139
List of Key Symbols
σ1 Total axial stress
σ3 Confining stress
σ1- σ3 Differential stress
σa Axial stress
σHg Air-mercury interfacial tension
A Sample cross-sectional area
E Young’s Modulus
Pc Confining pressure
PHg Mercury injection pressure
Pm Upstream fluid pressure
P(l,t) Downstream pressure transmission at sample end as a function of time
Po Initial pore pressure
V Volume of downstream reservoir
l sample length
rp Pore throat radius
β Fluid compressibility
μ Fluid viscosity
ν Poisson’s ratio
k Relative permeability
ϕ porosity
θHg Mercury contact angle
εa Axial strain
εr Radial strain
140
List of Publications
1. Liu, X., Aughenbaugh, K., Lee, H., Nair, S., & Oort, E. van., 2017. Geopolymer -
Synthetic Based Mud Hybrid Cements for Primary Cementing and Lost
Circulation Control. SPE International Conference on Oilfield Chemistry.
doi:10.2118/184558-MS
2. Liu, X., Ramos, M. J., Nair, S. D., Lee, H., Espinoza, D. N., & van Oort, E., 2017.
True Self-Healing Geopolymer Cements for Improved Zonal Isolation and Well
Abandonment. SPE/IADC Drilling Conference and Exhibition.
doi:10.2118/184675-MS
3. Liu, X., Aughenbaugh, K., Nair, S., Shuck, M., & van Oort, E., 2016.
Solidification of Synthetic-Based Drilling Mud Using Geopolymers. SPE
Deepwater Drilling and Completions Conference. doi:10.2118/180325-MS
4. van Oort, E., Aughenbaugh, K., Nair, S.D. & Liu, X., 2016. “Cementitious
Compositions Comprising a non-aqueous fluid and an alkali-activated material”,
Application No. 15/355,586
5. Liu, X., Nair, S. D., Cowan, M., & van Oort, E., 2015. A Novel Method to
Evaluate Cement-Shale Bond Strength. SPE International Symposium on Oilfield
Chemistry. doi:10.2118/173802-MS
141
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