solutions for long term zonal isolation_oilfieldreview, 2002

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16 Oilfield Review

Solutions for Long-Term Zonal Isolation

Raafat Abbas

Erick Cunningham

Trevor Munk

Clamart, France

Bente Bjelland

Norsk Hydro

Bergen, Norway

Vincent Chukwueke

Nederlandse Aardolie Maatschappij B.V.

Assen, The Netherlands

Alain Ferri

Aberdeen, Scotland

Greg Garrison

Houston, Texas, USA

Doug Hollies

EnCana Corporation

Calgary, Alberta, Canada

Chris Labat

ChevronTexaco

New Orleans, Louisiana, USA

Omar Moussa

Kuala Lumpur, Malaysia

For help in preparation of this article, thanks to MarioBellabarba, The Hague, The Netherlands; Leo Burdylo,Roger Keese, Bill Miller, Erik Nelson and Don Williamson,Sugar Land, Texas, USA; Ryan Cammarata, Bill Dacres,Laurent Delabroy, J ames J ackson and Randy Tercero,Houston, Texas; Youssef El Marsafawi, Kuala Lumpur,Malaysia; Simon J ames, Clamart, France; Brian Koons,New Orleans, Louisiana, USA; Christian Mueller, Stavanger,

Norway; Ron Schreuder, Coevorden, The Netherlands; andDavid Stiles, Calgary, Alberta, Canada.

CBT (Cement Bond Tool), CemCADE, CemCRETE,CemSTONE, DeepCEM, DeepCRETE, DuraSTONE,FlexSTONE, GASBLOK, GeoMarket, LiteCRETE, MUDPUSH,SCMT (Slim Cement Mapping Tool), USI (UltraSonicImager), Variable Density, VDN (VISION Density Neutron)and WELLCLEAN II are marks of Schlumberger.

Improving long-term wellbore integrity is a growing priority. Exploration and

production companies recognize that achieving excellent zonal isolation requires

superior mud removal and proper cement-system design. New simulation software,

environmentally friendly primary cementing systems and worldwide field support

help companies achieve well-construction goals from the outset while enhancing

environmental protection.

Exploration and production (E&P) companies

have been striving for perfect zonal isolation

since the earliest days of wellbore cementing.

The complex relationships between the geology,

chemistry and physics of each oil- or gas-well

completion present unique challenges that

decades of experience alone may not solve.

These challenges manifest themselves in both

new and existing wells.

The scope of existing zonal-isolation problems

is enormous. Worldwide drilling during 2001 led

to 74,000 new wells, of which 48,000 weredrilled in North America.1 A major concern is that

many existing wells are now experiencing

sustained casing pressure, an indication that a

path exists between a pressure source and an

annulus. In the offshore Gulf of Mexico alone,

11,500 casing annuli in 8000 wells may suffer

sustained casing pressure.2 Remediation costs

could be as high as US $1 million per well,

including the costs of workover rigs and finding

and fixing leaks. In Canada, sustained casing pres-

sure afflicts a wide variety of onshore wells, from

shallow gas wells to heavy-oil wells. Correcting

these problems worldwide may cost as much as

US $2.75 billion over 10 years. Clearly, avoiding

these remediation expenses is preferable.

Sustained casing pressure may result from

many causes, such as poor primary cementing,

inadequate requirements in effect when olderwells were cemented or deterioration of the

cement matrix with time. Regardless of the

cause, industry and regulatory bodies recognize

the need to protect the environment from leaking

reservoir fluids. Poor zonal isolation may result in

a loss of well control or contamination of water

1. World Trends: Industry Pace Should Quicken, WorldOil223, no. 8 (August 2002): 3337.

For more on 2001 drilling data:http://www.worldoil.com/magazine/magazine_link.asp?ART_LINK=02-08_world-abraham_T2.htm#top.

2. Bourgoyne AT J r, Scott SL and Manowski W: A Reviewof Sustained Casing Pressure Occurring on the OCS,

LSU study funded by the Minerals Management Service,US Department of the Interior, Washington, DC, underContract Number 14-35-001-30749.

3. For more on remediating existing wellbores: Barclay I,Pellenbarg J , Tettero F, Pfeiffer J , Slater H, Staal T,Stiles D, Tilling G and Whitney C: The Beginningof the End: A Review of Abandonment andDecommissioning Practices, Oilfield Review 13,no. 4 (Winter 2001/2002): 2841.

Boisnault J M, Guillot D, Bourahla A, Tirlia T, Dahl T,Holmes C, Raiturkar AM, Maroy P, Moffett C,Prez Meja G, Ramrez Martnez I, Revil P andRoemer R: Concrete Developments in Cementing

Technology, Oilfield Review11, no. 1 (Spring 1999): 1629.


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Autumn 2002 17

sources, which can be catastrophic for the environ-

ment and the local population. Improving primarycementing in new wells and repairing leaking

wells are logical steps toward improving zonal

isolation and protecting the environment.3

In this article, we discuss solutions that

improve zonal isolation in primary cementing,

beginning with mud-removal technology.

Examples from operations demonstrate the

impact of new techniques and fluids. Innovative

simulation software and a worldwide network of

field-support laboratories help engineers opti-mize cement-job designs.

Optimizing Mud Removal

Effective drilling-fluid removal is a prerequisite for

primary cementing success. As the cement slurry

sets, mud left in the wellbore can prevent develop-

ment of a hydraulic seal, which can result in

such adverse phenomena as the production

of unwanted fluids, loss of hydrocarbons to

low-pressure zones, sustained casing pressureunderground blowouts or accelerated casing

corrosion. These can lead to additional, and usually

unexpected, expenses to solve these problems. In

addition to the general condition and quality of the

wellbore, factors influencing mud removal include

mud conditioning, displacement procedures, wel

geometry and centralization of the casing.


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In preparing to cement a casing string, a

cementing crew pumps a series of fluids down

the casing to displace the drilling fluid from the

interior of the casing and the casing-formation

annulus. The first fluid pumped usually is a chem-

ical wash or spacer that separates the drilling

fluid from the cement slurry.

Chemical washes have a density and viscos-

ity close to that of water or oil; water, diesel or

oil may be used as base fluids for chemical

washes. When pumped ahead of the cement

slurry, they promote mud removal by diluting,

thinning and dispersing the mud. Chemical

washes can be formulated to remove both

water- and oil-base drilling fluids. Ultimately,

chemical washes improve the quality of the bond

between the casing and the cement, and

between the cement and the formation. Because

chemical washes tend to have a low viscosity,

they are pumped in turbulent flow (right).

Chemical washes are available for a wide variety

of applications.

Spacer fluids also provide a buffer that ischemically compatible with both the drilling fluid

and the cement slurry during displacement.

Complete displacement of drilling fluid by the

spacer is critical to the establishment of zonal

isolation; incomplete displacement may result in

a continuous mud channel across the zone of

interest, creating communication paths between

zones. Such communication results in the pro-

duction of unwanted fluids, loss of hydrocarbons

and even migration of fluids to surface. For oil-

base muds, the surfactants used in chemical

washes and spacers change the wettability of

the casing and the formation near the wellborefrom oil-wet to water-wet. This contributes to

improved cement bonding.

MUDPUSH II spacers are compatible with

cement slurries, water- and oil-base mud, and

fresh water, seawater or brackish water. These

spacers can be designed at densities from 10 to

20 lbm/gal [1200 to 2400 kg/m3], and at temper-

atures up to 300F [149C]. The spacers have

demonstrated excellent reproducibility between

laboratory design and field performance, and are

easy to mix at the wellsite.

The MUDPUSH II spacer properties are

adjusted to minimize environmental impact,including lower toxicity, improved biodegradation

and lower bioaccumulation.4 A smaller quantity

of chemicals than that required with other

spacers produces the desired performance char-

acteristics. Fewer chemicals are discharged or

handled as fluids return to surface, less storage

space is required and less waste is generated.

Optimizing the rheological properties of a

spacer fluid improves zonal isolation and mud dis-

placement. Rapid selection and adjustment of

additive concentrations in spacer fluids now are

possible with the WELLCLEAN II Engineering

Solution Advisor software. This software reduces

the time and effort required to optimize fluid prop-

erties, whether the fluids are designed for turbu-

lent or laminar flow. Simulation results from

CemCADE cementing design and evaluation soft-

ware, which are based on actual data, can be

imported into WELLCLEAN II Advisor software.5

When used to design the cementing job, the output

from the WELLCLEAN II Advisor software reduces

the risk of error and improves the efficiency.

When designing a MUDPUSH II spacer for

removing mud in turbulent flow, the WELLCLEAN II

Advisor software indicates the optimal additive

concentration to stabilize the spacer so that

weighting agents do not settle out and the rheo-

logical properties do not change. At the same

time, the software sets fluid properties at the

best level to achieve turbulent flow at low pumprates. In some cases, pumping-rate restrictions

require a laminar flow regime; turbulent flow

generally involves higher pumping rates. If the

strategy specifies design of the spacer for lami-

nar flow, the software provides optimized spacer

properties and additive concentrations at the

required density and temperature. The degree of

mud removal and the presence of mud channels

are more commonly linked to wellbore geometry,

rugosity, washouts, fluid viscosity and density

than to flow regime. Multiple simulations demon-

strate the consequences of various additive

concentrations on spacer properties. Spacer prop-erties for a particular job are always designed for

compatibility with the mud and cement.

In addition to a comprehensive database

of laboratory tests, the WELLCLEAN II Advisor

software provides mathematical models and a

reasoning engine that can derive spacer prop-

erties by interpolating results at various

temperatures, densities and additive concentra-

tions (next page). The software incorporates the

Bingham plastic, power-law and Herschel-

Bulkley rheological models.6

Field measurements of spacer properties

designed using the WELLCLEAN II Advisor soft-ware have proved to closely match predicted

design data. For example, on a high-pressure

well in the Middle East, the mud density required

to kill the well was 18.7 lbm/gal [2240 kg/m3].

A fracture gradient close to the pore pressure

resulted in drilling-fluid losses across the weaker

zones. The operator decided to run and cement

18 Oilfield Review

4. Bioaccumulation is the enrichment of a substancein an organism, such as bioconcentration from exposureto the substance in the environment or uptake from thefood chain.

5. For more on CemCADE software: Fraser L, Stanger B,Griffin T, J abri M, Sones G, Steelman M and Valk P:Seamless Fluids Programs: A Key to Better Well

Construction, Oilfield Review8, no. 2 (Summer1996): 4256.

6. A Bingham plastic model is a two-parameter rheologicalmodel widely used in the drilling-fluids industry todescribe flow characteristics of many types of fluids.Fluids obeying this model exhibit a linear shear-stress,shear-rate behavior after an initial shear-stress thresholdhas been reached. A Herschel-Bulkley fluid can bedescribed mathematically by a three-parameter rheologi-cal model. The Herschel-Bulkley equation is preferredto power-law or Bingham relationships because it resultsin more accurate models of rheological behavior whenadequate experimental data are available. A power-lawfluid is described by a two-parameter rheological modelof a pseudoplastic fluid, or a fluid whose viscositydecreases as shear rate increases. Water-base polymermuds, especially those made with XC polymer, fit thepower-law mathematical equation better than theBingham plastic or other two-parameter models.

7. Primary cementing operations may involve as manyas four slurries, but jobs with two slurries, knownas the lead slurry and the tail slurry, are more common.Lead refers to the first slurry pumped during primarycementing operations. Tail refers to the last slurrypumped during primary cementing operations. Typically,the tail slurry covers the pay zone and is denser thanthe lead slurry.

8. Bonett A and Pafitis D: Getting to the Root of GasMigration, Oilfield Review8, no. 1 (Spring 1996): 3649.

9. For more on the Tampen operations: Bjelland B,Hansen K and Abbas R: Tampen Planning Gets ConcreteResults, Harts E&P75, no. 8 (August 2002): 7072.

Static mudlayer

Flowingfluid

Laminar Flow Turbulent Flow

> Mud-removal flow dynamics. In laminar flow(left), flow lines are parallel and individual parti-cles move in parallel paths. Mud particles tend

to accumulate near the borehole wall, makingcomplete mud removal difficult. In turbulent flow(right), the energetic, swirling eddies entrainmore mud particles than laminar flowpathsbefore becoming saturated. The turbulent eddiesalso move surfactants or dispersants in thechemical wash or spacer fluid throughout theborehole to deform and remove the static mudlayer at the borehole wall.


4/14

Autumn 2002 19

a liner, and required an optimized spacer fluid

on short notice.

Typically, designing the spacer for this situa-tion would require a lengthy laboratory process.

However, the WELLCLEAN II Advisor software

quickly optimized the spacer design using actual

well conditions and fluid properties. The liner

was cemented successfully, and the operator

benefited from the efficient process to design an

effective spacer for such difficult conditions.

There were no indications that any cement was

lost during operations.

In ordinary cementing operations, the spacermay be followed by multiple cement slurries.7

The preflush-spacer-cement series must displace

all fluids from the annulus to prevent develop-

ment of mud or spacer channels within the

cement sheath.8 Such channels may allow unde-

sirable formation-fluid migration. The presence

of mud also can induce shrinkage cracks, reduce

compressive strength or increase permeability,

any of which can negatively affect set-cemen

properties. Once the slurry is pumped, a mechan

ical plug is launched into the casing and

displaced to the bottom of the well by anothe

fluid, usually the drilling fluid needed to drill the

next section of hole. At the end of the operation

cement occupies the annular space between the

casing and the penetrated formation from the

bottom of the hole up to the desired level.

Effective mud removal, a crucial part of any

cementing operation, cannot be achieved

without considering the effects of all relevan

parameters. WELLCLEAN II Engineering Solution

technology makes use of innovative products and

tools to improve cement placement. Together

these optimized chemical-wash systems, custom

spacers, innovative software and a testing

methodology to evaluate the effectiveness o

preflushes in displacing drilling fluids all enhance

mud removal and zonal isolation.

Mud Removal in Action

In the mature offshore region of TampenNorway, Norsk Hydro engineers are increasing oi

production by improving zonal isolation with

WELLCLEAN II technology.9 Although the subsi

dence, compaction and high downhole stresses

that are common in this area could cause cemen

failure, engineers suspected that fluid channels

within the cement were their most significan

problems. These fluid channels probably were

caused by poor cement placement in wells tha

were highly deviated to horizontal. WELLCLEAN I

simulator results agreed with the cement logs

indicating intermixing of fluids throughou

the liner length and poor cement coverage othe annulus.

The WELLCLEAN II simulator software is a

powerful, two-dimensional numerical-simulation

tool for showing critical results such as the

percentage of cement coverage, the fluid concen

trations, the risk of having a mud film or channe

at the end of the cement job, and the turbulen

contact timeall as a function of depth and

time. The simulator accounts for such parameters

as well geometry and trajectory, downhole fluid

properties and volumes, pump rates and casing

centralization. The simulator then predicts the

efficiency of mud removal and assesses thelikelihood of a mud channel being left in the

cement. The simulations are presented as two

dimensional maps that show fluid positions and

concentrations, fluid velocity and flow regime

Animations of the simulations show the entire

fluid-displacement process for a job.

> Optimizing spacer designs. WELLCLEAN I I Engineering Solution Advisor software simplifies andspeeds spacer design.


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WELLCLEAN II simulator predictions have

been validated both by physical laboratory

experiments and field performance. Cementing

engineers may use these simulations to alter

their designs to optimize zonal isolation. For

example, the simulator helps engineers choose

between turbulent or laminar flow regimes, ordecide how to avoid detrimental contact

between drilling mud and cement during pump-

ing operations.

To improve zonal isolation in a Tampen area

well during 2001, Norsk Hydro engineers selected

an innovative preflush system with low-toxicity

spacers, more complete biodegradation and

lower bioaccumulation than other systems. The

new well was horizontal, similar to previous prob-

lematic wells. Additional centralizers were placed

to improve standoff and cement distribution

around the casing. Output from the CemCADE

software showed that the additional centralizers

would not lead to excessive running forces,

which are an indication of the potential for thecasing to become stuck as it is run into

the wellbore.

Norsk Hydro also wanted to improve the

mechanical properties of the cement system.

Norsk Hydro engineers selected CemCRETE

concrete-based oilwell cementing technology

a system with a high solid-fraction, high plastic

viscosity, low permeability and low porosity.10

Simulations indicated that no formations would

be fractured using a 14-lbm/gal [1679-kg/m3]

CemCRETE slurry-system blend. Also, 300 bbl

[48 m3] of fresh mud at a relatively lower density

and viscosity would be pumped to dilute the mud

in the hole and reduce its gel strength.

Prior to the operation, simulations predictedthat the modified design would improve mud

removal significantly. In addition to predicting

cement coverage of greater than 95% over the

majority of the production-liner length, simulator

data indicated no risk of leaving mud behind the

cement. The cementing operation was executed

successfully. Cement-bond logs of the 7-in. liner

demonstrated that the results were superior to

20 Oilfield Review

Raw Acoustic Impedance Cement Map with Impedance Classification

Minimum of Acoustic

Impedance

Mrayl 100

Average of Acoustic

Impedance

Mrayl 100

Maximum of Acoustic

Impedance

Liquid

Bonded

Microdebonding

Mrayl 100

-500.00000.31250.62500.93751.25001.56251.87502.18752.50002.81253.12503.43754.06254.37504.68755.0000

-500.00000.31250.62500.93751.25001.56251.87502.18752.50002.81253.12503.43754.06254.37504.68755.0000

Liquid

Gas or DryMicroannulus


BondedRaw Acoustic

Impedance

Cement Mapwith Impedance

Classification

Microdebonding

-1000.0-500.00.32.63.03.54.04.55.05.56.06.57.07.58.0

-1000.0-500.00.32.63.03.54.04.55.05.56.06.57.07.58.0

> Improving bonding in horizontal wells in Norway. Ordinary spacer designs resulted in inadequate cement bonding, as shown in the cement-evaluation logat left. In particular, there is a dearth of yellow, or bonded, section in Track 2. More effective cement-job designs, including more complete mud removalusing MUDPUSH I I spacers, resulted in improved bond, as shown in the abundant yellow section in Track 3 (right).


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Autumn 2002 2

those from previous cementing jobsexcellent

bonding and no indication of any fluid channels

(previous page). Since then, additional wells

have been completed successfully in Tampen

using this improved methodology, and more are

planned during 2002.

In the Gulf of Mexico, achieving effective

zonal isolation was crucial for five costly

extended-reach wells with an oil-water contact in

the hydrocarbon-bearing zone. Adequate cement-

to-casing and cement-to-formation bonding

within these pay sands had proven challenging;

good bonding existed only across shale sections.

Any new cement-job designs would have to

accommodate well deviations up to 77 degrees,

synthetic-base drilling mud and gas influx from

the formation. Failure to isolate the water zones

within the pay sand resulted in lower oil produc-

tion and additional disposal costs for produced

water. ChevronTexaco spent as much as $200,000

per well for squeeze-cementing operations and

rig time to repair flawed primary cement jobs in

three of the five wells.With the aid of the WELLCLEAN II simulator

package, CemCADE software and the while-

drilling acquisition of caliper logs using the VDN

VISION Density Neutron tool, Schlumberger

engineers were able to alter design parameters

to improve mud removal.11 The engineers consid-

ered the acquisition of caliper data particularly

important because previous job designs

depended on assumptions about hole shape and

volumeparameters that significantly influence

mud removal and predictions of the required

slurry volume. CemCADE results led to changes

in centralizer placement, spacer fluids, cement-slurry properties, fluid volumes and pumping rates.

The cement slurry incorporated the GASBLOK

gas migration control cement system and an

expanding agent. These additives eliminate gas

influx from the formation, control fluid loss

to the formation and minimize bulk-volume

reduction during cement placement and setting.

The upgraded job designs and fluids greatly

improved zonal isolation in the next three wells.

Outputs from the WELLCLEAN II simulator pro-

gram were in complete agreement with the

cement-evaluation logs (right). The three new

wells did not produce water and did not requireremedial cementing operations. Elsewhere in the

10. For an introduction to CemCRETE technology: Boisnaultet al, reference 3.

11. The VDN device provides compensated neutron andazimuthal lithodensity measurements while drilling. Theresulting density image also enables structural analysis.

Discriminated

CCL

03 100-1 APIV

Gamma Ray

0 360degrees

SCMT Relative Bearing

0 100mV 200

1.25

2.50

3.75

5.00

6.25

7.50

8.75

10.00

11.25

12.50

13.75

15.00

16.25

17.50

18.75

20.00

1200s Cement Map Image

Min MaxAmplitude

CBT 5-ft Variable DensityCBT 3-ft Amplitude

0 10mV

Amplified CBT 3-ft Amplitude

0 100API

CBT 3-ft Transit Time

11,800

11,900

12,000

12,100

12,200

12,300

Depth,

ft

12,400

12,500

Wide

Mud

Tail

CW100

MUDPUSH WHTO

Narrow

Wide

12,600

12,700

12,800

12,900

11,800

11,900

12,000

12,100

12,200

12,300

Depth,

ft

12,400

12,500

Wide

High

Medium

Low

None

Narrow

Wide

12,600

12,700

12,800

12,900

Fluids Concentration Map Risk of Mud on the Wall

12,000

12,100

> Improving mud removal in the Gulf of Mexico. WELLCLEAN II simulator results (top) indicated thatchanges in centralizer placement, spacer fluids, cement-slurry properties, fluid volumes and pumpingrates would improve mud removal and primary cementing. The simulated cement map at left indicatesa high concentration of cement (gray) around the wellbore; the green area in the simulation indicatescomplete mud removal. The cement map in Track 5 of the SCMT Slim Cement Mapping Tool log ( bottom)confirms excellent cement placement around the casing. The Variable Density display in Track 4 showsgood cement bonding.


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Gulf of Mexico, WELLCLEAN II Engineering

Solution results have improved primary cement-

ing (right).

The Role of Advanced

Cementing Technology

Mud removal plays a crucial role in the success

of cementing operations, but the selection of an

appropriate type of cement also is critical. Since

the first primary cementing operation in 1903,

service companies have developed many types of

oil- and gas-well cements to address the

extremes of subsurface conditions.12 Cement sys-

tems must withstand the effects of subsurface

pressures, temperatures and formation fluids to

provide lasting zonal isolation.

Even when a conventional slurry is properly

placed and initially provides adequate zonal iso-

lation, changes in downhole conditions can

induce stresses that compromise the integrity of

the cement sheath. Tectonic stresses and large

increases in wellbore pressure or temperature

can crack the sheath, and can even reduce it torubble. Radial displacement of casing, caused by

cement bulk shrinkage or temperature or pres-

sure decreases, can cause the cement to debond

from the casing or formation and create a

microannulus.13 Decreases in fluid weight during

drilling and completion also cause debonding.

Routine well-completion operations, including

perforating and hydraulic fracturing, negatively

affect the cement sheath.14

The most recent advance, CemSTONE

advanced cementing technology systems, pro-

vides reliable, long-term zonal isolation despite

changing downhole conditions. These systemshave predictable set-cement properties, such as

flexibility, expansion and impact resistance after

setting, so they can be designed to withstand

stresses that would destroy ordinary cements.

Proprietary additives and proven, engineered-

particle blends in CemSTONE systems meet

specific mechanical-property requirements, such

as elasticity, expandability, compressive and ten-

sile strength, durability and impact resistance.

Like CemCRETE slurries, the particle-size distri-

butions of CemSTONE systems make them easy

to mix and pump.

The cement integrity achieved routinely withCemSTONE systems helps cut maintenance

costs, ensures isolation for stimulation treatments,

reduces the possibility of annular pressure during

the producing life of gas wells, and extends the

productive life of steamflood wells and wells in

tectonically active areas. This high degree of

cement integrity also improves isolation for

multilateral junctions, saves time and reduces

22 Oilfield Review

56

Cement coverage, %

60 64 68 72 76

Original Job Without Optimization

Wide Narrow Wide

Fluidsconcentration

map

Mud

Spacer

Cement

22,000

23,000

24,000

Depth,

ft

Depth,

ft

Wide Narrow Wide

Risk of mudon the wall

High

None

70

Cement coverage, %

80 90 100

22,000

23,000

24,000

Wide Narrow Wide

High

None

Optimized Job Using WELLCLEAN II Simulator Software

Wide Narrow Wide

Mud

Spacer

Cement

Fluidsconcentration

map

Risk of mud

on the wall

Cement coverage, %

60 80 100

Fluidsconcentration

map CBT logVariable

Density log

Wide Narrow Wide

Depth,

ft

23,600

23,540

23,480

23,420

Diesel

Chemical wash

Spacer

Cement

> More mud-removal success in the Gulf of Mexico. In another part of the Gulf of Mexico, simulationof initial design plans with the WELLCLEAN II simulator package indicated a mud channel was likely toform in the cement sheath (top). After optimizing centralizer placement, spacer and slurry properties,and displacement rates and volumes, the simulator demonstrated that the new job design would avoidchannel formation (middle). Cement-evaluation logs confirm improved zonal isolation (bottom).


8/14

Autumn 2002 23

difficulty in setting mechanical whipstocks, and

eliminates cement sloughing behind casing when

opening windows and drilling out in incompetent

formations (see New Aspects of Multilateral

Well Construction, page 52).

There are two types of CemSTONE technol-

ogy in use now, DuraSTONE advanced durable

cement technology and FlexSTONE advanced

flexible cement technology systems. Each system

combines specially sized particles with special

material, resulting in a cement system that offers

enhanced flexibility and durability. As of

September 2002, more than 90 FlexSTONE sys-

tems and 25 DuraSTONE systems have been

pumped worldwide to combat a wide variety of

operational challenges.

Designed for multilateral junctions, kickoff

plugs and wells requiring impact-resistant

cement, DuraSTONE systems blend engineered

particles of different sizes with high-strength

metallic microribbon technology. The result is a

set cement that is two to three times tougher and

has impact resistance up to 20 times higher thanordinary Portland cements. In Abu Dhabi, UAE,

DuraSTONE cement plugs have substantially

improved kickoff success rates in more than

20 jobs to date.15 The improvements included

reduced time required to achieve kickoff and an

increase in the success rate of kickoff plugs

because DuraSTONE plugs are so difficult to drill.

FlexSTONE systems combine the engineered

particle-size distribution of CemCRETE systems

with flexible particles that accommodate wide

ranges of temperature, pressure and fluid den-

sity. These special particles lower the Youngs

modulus, increasing the flexibility of the setcement (right).16 When expansion is desirable,

these systems can be designed to produce as

much as 3% linear expansion after full cement

hydration; ordinary expanding cement systems

allow less than 1% linear expansion. Improved

mechanical properties make FlexSTONE systems

ideal for wells in steamfloods and tectonically

active regions.

Formation properties play a critical role in the

performance of wellbore cements. Optimizing the

relationship between the mechanical properties

of the formation and the mechanical properties

of the cement sheath is a requirement for long-term cement integrity during pressure changes,

temperature changes or cement expansion.

Because it is not possible to alter the properties

of the formation, engineers instead must manipu-

late the mechanical properties of the set cement

to achieve the correct combination of flexibility

and expansion. FlexSTONE technology and care-

ful cement-job design make this possible.

A new two-dimensional modeling package

helps engineers simulate the behavior of the

cement sheath in different pressure and tem-

perature regimes and wellbore configurations.

The software inputs include well configuration,

points of interest, cement properties, formation

properties and casing properties. The software

combines this information with a database ofcement properties to generate an optimized

cement design. Known as the Stress Analysis

Model (SAM), the software calculates the

properties necessary for the cement to main-

tain integrity and detects risks of cracking in

tension, rupture in compression or formation

of a microannulus.

12. Smith RC: Preface, in Nelson EB: Well Cementing.Sugar Land, Texas, USA: Schlumberger Dowell(1990): 16.

13. For more on cement bulk shrinkage: Thiercelin M,Baumgarte C and Guillot D: A Soil Mechanics Approachto Predict Cement Sheath Behavior, paper SPE/ISRM47375, presented at the 1998 SPE/ISRM Eurock,

Trondheim, Norway, J uly 810, 1998.

14. For more on cement mechanical response to downholestress: Thiercelin MJ , Dargaud B, Baret J F andRodriguez WJ : Cement Design Based on CementMechanical Response, paper SPE 38598, presentedat the 1997 SPE Annual Technical Conference andExhibition, San Antonio, Texas, USA, October 58, 1997.

15. For more on DuraSTONE kickoff plugs: Al-Suwaidi A,Hun C, Babasheikh A and Cunningham E: Cement AidsChallenging Sidetracks, Harts E&P75, no. 2 (February2002): 5153.

16. Le Roy-Delage S, Baumgarte C, Thiercelin M and Vidick BNew Cement Systems for Durable Zonal Isolation,paper IADC/SPE 59132, presented at the 2000 IADC/SPEDrilling Conference, New Orleans, Louisiana, USA,February 2325, 2000.

F

F

F

F

r

l

l

r

l

l

Poissons ratio v= lr

Axia

lstressF(tension)

Axial strain l (tension)

Youngs modulus (e)

Tensile strength

Axial strain

Axial stresse=

> Cement properties. Youngs modulus describes the relationship betweenstress and strain in a uniaxial stress test (top). For cement, the lower the

Youngs modulus, the more flexible the cement is. Poissons ratio is the ratioof transverse strain (r) to axial strain (l) (bottom). Tensile deformation isconsidered positive, and compressive deformation is considered negative. Thedefinition of Poissons ratio contains a minus sign so that ordinary materialshave a positive ratio, ordinarily between 0.15 and 0.25 for cement. Tensilestrength refers to the ability of material to be stretched before rupturing.


9/14

Isolating Depleted from

Productive Formations

Isolating formations with widely varying pres-

sures is challenging but imperative. In The

Netherlands, Nederlandse Aardolie Maatschappij

B.V. (NAM, a joint venture of Shell and Exxon)

wrestled with the difficulty of isolating the

depleted Zechstein 2 carbonate formation

from the producing Limburg formation. Fracture

stimulation of the Limburg presents an additional

challenge to maintaining the integrity of the

cement sheath.

The Coevorden-57 well, located onshore in

the northeastern Netherlands, deviates as much

as 64 degrees from vertical on its way to a mea-

sured depth of 3998 m [13,117 ft] in the Limburg

sandstone (below left). Critical elements for suc-

cessful cementing of the 412-in. liner included

complete mud removal and a cement system that

would withstand the pressure increase during a

hydraulic-fracturing operation while maintaining

long-term isolation of formations with distinct

reservoir pressures.

Casing centralizers were placed such that

there were three centralizers for every two

casing joints from 3386 to 3923 m [11,109 to

12,871 ft] and two centralizers per joint of casing

from 3924 to 3998 m [12,874 to 13,117 ft]. This

centralizer placement ensured that cement

would surround the casing even in highly devi-

ated sections where casing tends to lie on the

low side of the borehole. The cement would

extend from total depth to 150 m [492 ft] above

the top of the liner hanger located at a depth of

3372 m [11,063 ft].

Cementing engineers used the WELLCLEAN II

simulator and CemCADE software to optimize the

job design (next page). At the beginning of the

operation, 20 bbl [3.18 m3] of fresh mud would be

pumped to begin cleaning the hole. MUDPUSH

spacer would follow the mud to provide addi-

tional hole cleaning and to prevent the drilling

fluid from contaminating the cement slurry. This

displacement train would be followed by 1.63

g/cm3 [13.6 lbm/gal] FlexSTONE slurry, the first

application of FlexSTONE technology in Europe.

The SCMT device was used to evaluate the

cement bond following the job and confirmed

excellent cement-casing and cement-formation

bonding (below right). The SCMT device is a mul-

tisensor cement-logging tool that produces a

360 map image of the cement.

24 Oilfield Review

SurfaceDepth, m

North SeaGroup

Chalk Group

Rijnland

Altena

Keuper

Zechstein

Limburg

0

500

1000

1500

2000

2500

3000

13 38-in. casing

9 58-in. casing

4 12-in. liner

7-in. liner

Coevorden

THE NETHERLANDS

BELGIUM

GERMANY

FRANCE

N or t

hS

ea

0 50 100 150 miles

0 80 160 240 km

> Location of the Coevorden-57 well, TheNetherlands. The Coevorden-57 well penetratesdepleted Zechstein carbonate rocks and produc-tive Limburg reservoir, as shown in the crosssection. Permanently isolating these formationsfrom each other in a deviated, fracture-stimulatedwell is challenging.

> FlexSTONE evaluation in The Netherlands. The SCMT log shows excellentresults, with map amplitudes (long dashes in Track 3) ranging from 3 to 10 mV,which indicates excellent bonding between the cement and casing. The Vari-able Density display (Track 4) shows strong formation arrivals, which demon-strate good bond between the formation and cement.

Amplitude

Variable Density Cement Map Image

mVs

Min Max

200 1200

Minimum Map Amplitude

mV0 100

Amplitude

Gamma RayDiscriminated

CCL

V3 -1 API0 150

CBT Amplitude

mV0 100

Relative Bearing

degrees0 360

Maximum Map Amplitude

mV0 1000

CBT 3-ft. Transit Time

s100 600

Average Map Amplitude

mV0 100

Minimum Map Transit Time

s100 600

CBT Amplitude

mV0 10

Maximum Map Transit Time

s100 600

1.2

5

2.5

0

3.7

5

5.0

0

6.2

5

7.5

0

8.7

5

10.0

0

11.2

5

12.5

0

13.7

5

15.0

0

16.2

5

17.5

0

18.7

5

20.0

0


10/14

Autumn 2002 25

1 2

3 4

5 6

> Simulation of fluid-pumping sequence for the Coevorden-57 well. WELLCLEAN II simulator runshelped optimize fluid selections and the pumping schedule for the Coevorden-57 well. In all illustra-tions, each pair of illustrations from the simulator shows the effectiveness of mud removal (right) andpredicts cement distribution (left). In all illustrations of fluid concentrations, brown represents drillingfluid, blue is chemical wash, green is MUDPUSH spacer and gray is FlexSTONE slurry. In the finalillustration (6), the simulator predicts that all other drilling fluids will have been removed and thatFlexSTONE slurry will completely cover the wellbore wall, results that were confirmed by the well logs.


11/14

Cementing Wells for Steamflooding

In northeastern Alberta, Canada, EnCana

Corporation is constructing the first phase of

its Christina Lake Thermal Project (right).

This venture currently is expected to produce

more than 70,000 barrels per day [11,123 m3/d]

of bitumen from the Athabasca oil sands in the

Cretaceous-age McMurray formation using

steam-assisted gravity drainage (SAGD). The first

phase of the project will allow the company to

evaluate SAGD performance and flow barriers

within the reservoir. This information will help

the company optimize well designs, well place-

ment and fluid recovery in subsequent phases

of the project.17

SAGD is an emerging technology that is sim-

ple in concept but complex in application (see

Heavy-Oil Reservoirs, page 30). SAGD wells

are drilled in pairs. The wells are parallel to each

other, with anywhere from 20 to 200 m [66 to

656 ft] of separation between the horizontal

reservoir sections of the wellbores (below right).

The upper horizontal well is used to inject steam.The heat from the injected steam allows the thick

crude to flow more freely, with the assistance of

gravity, to the lower producing well.

A flawless primary cement sheath is critical

for SAGD success. Annular gas influx as the

cement sets can result in steam breakthrough.

SAGD wells typically experience thermal expan-

sion and contraction of the wellbore, which can

lead to cement failure. When cement failure

occurs, the operator must choose between

expensive remedial efforts with unpredictable

results or well abandonment. EnCana sought to

improve the quality of the primary cement so thatit would not confront either steam breakthrough

or cement failure.

26 Oilfield Review

17. For more on the Christina Lake Thermal Project:Suggett J , Gittins S and Youn S: Christina Lake ThermalProject, paper SPE/Petroleum Society of CIM 65520,presented at the 2000 SPE/Petroleum Society of CIMInternational Conference on Horizontal Well Technology,Calgary, Alberta, Canada, November 68, 2000.

18. For more on SAM simulations: Stiles D and Hollies D:Implementation of Advanced Cementing Techniques toImprove Long Term Zonal Isolation in Steam AssistedGravity Drainage Wells, paper SPE/Petroleum Societyof CIM/CHOA 78950, presented at the 2002 SPEInternational Thermal Operations and Heavy OilSymposium and International Horizontal Well

Technology Conference, Calgary, Alberta, Canada,November 47, 2002.

19. GASBLOK systems control annular gas migration whilecementing. These systems include a nonretarding liquidthat provides fluid-loss control and gas-migration controlproperties for cement slurries at temperatures from 80 to160F [27 to 71C] across a wide range of densities aslow as 10.5 lbm/gal [1258 kg/m3]. The GASBLOK additiveis a suspension of polymeric microgels that act as fluid-loss reducers by rapidly plugging the pore throats of thecement filter cake. The microgels in the interstitial waterof the cement matrix reduce cement-matrix permeabilityand decrease the continuity between pores during thecritical liquid-to-solid transition phase, further limitinggas migration.

20. Although CSLs support both cementing and stimulation,this article focuses on their role in cementing.

C A N A D A

ALBERTAChristina Lake

0

0 400 800 1200 1600 km

200 400 600 800 1000 miles

> Location of Christina Lake Thermal Project, Alberta, Canada.

Lead top of cement at surface

13 38-in. surface casingto +/ 175 m TVD

Flexible tail topof cement at+/ 250 m MD

Kickoffpoint at+/ 200 m

7-in. slotted liner+/ 750 m horizontal

9 58-in. intermediatecasing to +/ 590 m TVD

>

Typical SAGD well pair. Conventional cement protects the 133

8-in. surfacecasing, which is set 175 m [574 ft] deep. The intermediate section, where thewell builds angle from vertical to horizontal, extends to 590 m [1936 ft] andpresents mud-removal and cementing challenges. The 958-in. casing iscemented with a LiteCRETE lead slurry that provides a low density and highcompressive strength. A FlexSTONE tail slurry provides flexibility across mostof the openhole and maintains zonal isolation across the 958-in. casing shoe.

The FlexSTONE systems pumped at Christina Lake were mixed on sitethefirst location in the world to do so. Below the 958-in. shoe, the 834-in. horizontalsection extends 750 m [2461 ft] across the reservoir, which is 20 to 58 m [66 to190 ft] thick. The wells are then completed with an uncemented, slotted linerfor sand control.


12/14

Autumn 2002 27

Engineers used the SAM software to

evaluate potential for cement failure from com-

pression, tension or formation of a microannulus,

and to design the best possible slurry for the

Christina Lake SAGD wells. SAM simulations

indicated that thermally stabilized Class GPortland cement would suffer tensile failure

when exposed to the 14 to 260C [57 to 500F]

temperatures predicted for the SAGD wells.18

Another SAM simulation, incorporating the same

temperature, pressure, time and other conditions

used in the previous simulation, demonstrated

that a flexible cement system would not experi-

ence tensile failure.

In the Christina Lake area, three pairs of wells

have been cemented using a combination of

LiteCRETE lead slurry and FlexSTONE tail slurry.

EnCana selected FlexSTONE systems because of

their superior mechanical properties, particularlytheir capability to survive the thermal expansion

of the casing and the cement sheath. To counter

shallow-gas migration problems typical of west-

ern Canada, the LiteCRETE and FlexSTONE

systems incorporated GASBLOK technology.19

The LiteCRETE lead system typically is placed

from 250 m [820 ft] to surface. The FlexSTONE

tail system is pumped from approximately 590 m

[1936 ft] up to 250 m.

Cement logs of the six SAGD wells indicated

good bond across the zones considered critical

for reservoir isolation. During a workover opera-

tion on one of the wells, the slotted liner was

removed and the USI UltraSonic Imager tool and

the CBT Cement Bond Tool device were run toevaluate the isolation quality of the cement

following steam exposure (above). The log indi-

cates no degradation of cement quality and

strong formation-cement and cement-casing

bonds. There are no indications of gas migration

or casing-vent flows to surface.

Another SAGD well pair is planned during the

last quarter of 2002 to complete Phase I of the

Christina Lake project.

The Role of Client Support Laboratories

in Well Cementing

Successful implementation of new cementingtechnology depends heavily on an international

network of cementing specialists. Schlumberger

operates Client Support Laboratories (CSLs) in

Houston, Texas, USA; Aberdeen, Scotland; and

Kuala Lumpur, Malaysia.20 The CSLs form an

essential link between product development

and field operations by supporting the intro-

duction of new technology, assisting in training

field personnel and providing feedback during

product development. Whenever possible, CSLs

undertake short-term projects so that the produc

centers can concentrate on longer term activities

CSL staff work with the Schlumberge

GeoMarket organization to support bid or tendeprocesses for E&P companies and collaborative

product-development opportunities. All this

results in higher quality products and services in

addition to fit-for-purpose innovations.

Typical CSL projects involve low-cost develop

ments, but some projects have been established

to support alliances. Customers often initiate

short-term projects to meet their specific techni

cal or environmental requirements. Many o

these involve collaboration with regional produc

suppliers to match readily available products

with specific operator requirements.

To support field operations, all CSLs areequipped to follow standard procedures outlined

by the American Petroleum Institute (API) to

conduct extensive formulation studies. These

procedures include quality assurance and quality

control of cement, fluid-compatibility tests, and

measurements such as thickening time, compres

sive strength, fluid loss and free water under the

pressure and temperature conditions found in the

DiscriminatedBond Index

Sonic Variable Density Curve

Amplitude

01Mrayl 100mm 12292

mm 92122

mm

ToolEccentricity

Internal RadiusAverage

mm 92122

Internal RadiusMaximum

mm 92122

External RadiusAverage

mm 92122

Internal RadiusMinimum

mm 12292

Internal RadiusAverage

mm 12292

Internal RadiusMaximum

mm 12292

Internal RadiusMinimum

External RadiusAverage

Amplitude ofEcho Minus Max

Average ofAcoustic

Impedance

dB/m 500

mm 133

dB/m 500

DiscriminatedAttenuation

Cement Mapwith Impedance

Classification

-01 s 1200200

MaxMin

-500.0 -1000.0-500.00.32.63.03.54.04.55.05.56.06.57.07.58.0

0.5-0.4-0.8-1.2-1.6-2.0-2.4-2.8

-4.8-5.2-5.6-6.0

-3.2-3.6-4.0-4.4

API

Gamma Ray

1000

mm

Casing CollarLocator

Process Flags

101

0.50001.50002.50003.50006.5000

dB/m 500

Near Pseudo-Attenuation

Short Pseudo-Attenuation

Average ofThickness

Microdebonding


Liquid

Bonded

< Cement evaluation ina steamflood well. TheUSI UltraSonic Imagertool and the CBTCement Bond Tooldevice were run toevaluate the isolationquality of the cementfollowing steam expo-sure. The log indicatesno degradation ofcement quality andstrong formation-cement and cement-casing bonds.


13/14

subsurface (above left). Equipment exists to sim-

ulate virtually all downhole cementing conditions

by applying high temperatures and pressures tocement slurries. Evaluation of slurry performance

under dynamic, downhole conditions ensures

that the slurry will remain fluid until properly

placed in the wellbore (above right).

Technology transfer is a key function for the

CSLs because disseminating the lessons they

learn also helps the worldwide network of

approximately 100 area and district laboratories

improve their operations. While many courses

offered by the CSLs are designed for

Schlumberger engineers, some courses are

offered to customers. CSLs also evaluate new

equipment and techniques before these aredeployed locally. For example, the Houston CSL

participates in the design of all FlexSTONE sys-

tems pumped in North and South America.

The Houston CSL provides technical training,

new-technology implementation and cementing

support for international field operations and

clients in North and South America. The labora-

tory completes short-term engineering projects

on request from clients and develops specific

solutions for local problems. Recently, cementing

experts from the Houston CSL and the

Schlumberger Riboud Product Centre, Clamart,France, developed the DeepCEM deepwater

cementing liquid-additive package for worldwide

application. This technology, which includes a

nonretarding dispersant and a cement-set

enhancer, has proven to be an optimal solution

for cementing shallow strings in deep water. It

has been implemented successfully in deepwater

markets around the world. The CSL offers a basic

laboratory course, an advanced service module

for cementing, new technology introductions at

training courses in the Kellyville Training Center,

Oklahoma, USA, and training for nonspecialists.

Due to its location, equipment and proximity tooilfield product suppliers, the Houston CSL plays

a vital role supporting international operations for

Houston-based companies.

The Houston CSL also has specialized labora-

tory equipment for cement-slurry design and

performance evaluation over a wide range of

temperatures and pressures35 to 600F [2 to

316C] and up to 40,000 psi [276 MPa]. Recently,

the Houston CSL acquired a twin-cell ultrasonic

cement analyzer (next page, left). Other recent

equipment acquisitions include the fluid-migrationanalyzer and the high-pressure, high-temperature

(HPHT) rheometer (next page, right). The HPHT

rheometer helps evaluate fluid performance over a

wide range of downhole conditions to ensure opti-

mal mud removal under extreme conditions.

Supporting operations in a region extending

through eastern Africa, the Middle East and Asia,

the CSL in Kuala Lumpur covers the broadest terri-

tory of the three CSLs. Typical projects at the Kuala

Lumpur facility include high- and low-temperature

cementing-fluids testing, compatibility testing and

fluid mixingall to API specifications. Recent pro-

jects include FlexSTONE system designs for theMiddle East, high-pressure LiteCRETE systems for

China, geothermal well cementing and a low-

density, saline DeepCRETE deepwater cementing

system used in India, with a density of 10 lbm/gal

[1198 kg/m3]. Because the Kuala Lumpur CSL sup-

ports more than 23 field laboratories serving

35 countries, the training of laboratory engineers

and technicians is a significant activity.

28 Oilfield Review

> Predicting cement performance. The consis-tometer shown here has a wide pressure andtemperature operating rangeup to 22,000 psi[150 MPa] and 400F [204C]. This unit, locatedin the Houston CSL, can be attached to a chillerfor controlled cooling. Other consistometersat this facility can achieve higher temperaturesand pressures. The CSLs in Aberdeen and KualaLumpur also have consistometers.

> Analyzing fluid migration. Liquid or gas migra-tion through hydrating cement slurries is a majorcause of well-completion failures. The fluid-migration analyzer shown here offers state-of-the art data acquisition and analysis and can runthe test cell at any angle to simulate wellboredeviation. This device, located in the HoustonCSL, can measure fluid loss through standardscreens or actual rock core samples. Data dis-play and analyses are enhanced by capture ofmore than a dozen data channels, includingabsolute and differential pressures, gas andliquid flows, and temperature.


14/14

The Aberdeen CSL offers client support and

testing similar to the Houston and Kuala Lumpur

facilities, and participates extensively in trainingactivities. The Aberdeen facility supports Europe,

the CIS and western Africa. Its work in the North

Sea offers many challenges and opportunities to

achieve environmentally friendly solutions in the

worlds most demanding regulatory environment.

In Norway, for example, all oilfield chemicals

must be biodegradable, so the Aberdeen CSL

helped develop biodegradable antifoam agents,

surfactants and retarders. For other areas, this

center has supported introduction of specialized

cementing technology, such as FlexSTONE,

LiteCRETE and DeepCRETE technologies. Quality

control of cement-slurry additives, novel blend-performance optimization and development of

new, customized cement additives for local cus-

tomers are key functions of the Aberdeen CSL.

The Aberdeen CSL also offers training in proper

use and calibration of equipment and testing pro-

cedures. Basic and advanced courses are offered

several times each year, and personalized training

is offered when appropriate, particularly for the

introduction of new technology. For example, field

engineers might be offered specialized training

when their district acquires new equipment. The

Aberdeen CSL also performs regular audits oftesting procedures and results. For these audits,

each district performs specific tests, which are

checked for consistency of results.

While the three CSLs have different capabili-

ties and slightly different focuses, they share the

goals of continually improving service quality,

transferring and supporting technology, and

training personnel to better serve customers. The

CSL leaders meet twice each year with the

cementing product-development group at the

Schlumberger Riboud Product Centre. These

meetings allow cementing practitioners from

around the world to present current field pro-jects, discuss pressing needs, exchange ideas

about implementation of new technology and

provide input to current and future research and

development projects.

Improving Zonal Isolation at the Outset

In a future dominated by development of mature,

or brown, fields, operating companies will need

to produce oil and gas more efficiently and with

better economic returns than ever before. Each

well plays a crucial role in this business environ

ment. Each operation, whether it is the actuadrilling of the well, mud removal, cementing

stimulation or any other, plays a key role in wel

performance: every operation must be successfu

at the outset to avoid costly remediation.

As the examples in this article demonstrate

judicious implementation of new technology solves

problems that were too expensive or technically

demanding to overcome with older technology

Operators are committed to eliminating problems

such as sustained casing pressure whenever pos

sible, in many cases by devoting more attention to

mud removal and cementing-system optimization

in the earliest stages of well design.Schlumberger continues to support technology

development to assure unprecedented produc

effectiveness and competence in field operations

With innovations that complement existing prod

ucts and services, ultraefficient technologies wil

abound to tackle the tough brown-field reservoirs

of the coming years. GMG

> Evaluating cement-strength development. Thistwin-cell ultrasonic cement analyzer providesnondestructive determination of cement-strengthdevelopment as samples cure under downholetemperature and pressure conditions. The devicemeasures the change in velocity of ultrasonicsignals transmitted through the cement specimensas they harden. As the strength of the cementspecimen increases, the transit time of the ultra-sonic signal through the sample decreases. Therelative strength of the cement is then calcu-lated using proprietary empirical algorithms.

> Measuring rheological properties. This HPHTrheometer, located in the Houston CSL, offersbroad viscosity measurement capability dueto the wide range of the torque (shear stress)transducer and the extended motor speed(shear rate). This allows the measurement of therheological properties of a variety of differentfluids with pressure and temperature limits of20,000 psi [138 MPa] and 450F [232C]. For wellcementing, it is crucial to understand the rheo-logical properties at downhole pressure andtemperature conditions to correctly place thecement without jeopardizing well integrity.

solutions for long term zonal isolation_oilfieldreview, 2002

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