ph.d. student ben niu supervisor: ass.prof. alexander a. shapiro ass. prof. wei yan

Phase Identification and Saturation Determination in Carbon Dioxide Flooding of Water Flooded Chalk

Using X-ray Computed Tomography

Ph.D. Student Ben NiuSupervisor: Ass.Prof. Alexander A. Shapiro

Ass. Prof. Wei Yan Prof. Erling H. Stenby

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• Introduction• Aim of the study• Experimental facilities• Experimental procedures• Experimental results• Conclusions• Future plan

Content

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Introduction • CO2 flooding of oil reservoir can potentially enhance oil recovery

significantly. At the same time, 1/3~1/2 injected CO2 could be stored during this process.

• CO2 flooding is characterized by complex phase behavior and geochemical reactions

• CO2 flooding experiments were performed using CT (Computed

Tomography) scanning to determine multiphase in-situ saturations for further understanding the recovery mechanisms.

• This study is a part of the project ‘EOR through CO2 Utilization’ funded by

the Danish National Advanced Technology Foundation and DONG Energy

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Aim of the Study• Long – term goal: Three phase flooding on chalk reservoir

samples at high P and T by using CT scanning, to obtain information on phase saturation.

• Flooding experiments with CT scanning in the literature– Two immiscible phases (many)– Two miscible phases (some)– Three-phase flooding at irreducible water saturation

(immobile water) at high P and T (several)– Three-phase flooding at high P and T (almost none)

• Short–term objectives: – Investigate if different phases could be identified

simultaneously– Find solutions for identifying the various phases in three

phase flooding

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Experimental Facilities CT Scanner

• Fourth generation Siemens SOMATOM scanner

• Three energy levels: 80kV, 120kV, 137kV

• X-ray computer tomography may provide information on density, porosity, mineralogy, hetergeneities, phase saturations……

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Experimental Facilities Core Holder

• Aluminium core holder (Max. 100bar) • Carbon fiber core holder (1000bar,150C)

Carbon fiber wrapped coreholder Shematic of experimental setup

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Experimental Procedures

• Four core flooding experiments were carried out to investigate if different phases could be identified simultaneously.

• Fluids: • Isopar-L, n-decane • Distilled water• CO2

• Dopants needed to increase contrast of CT numbers • Gas and oil phases have similar densities at high pressure

experimental conditions (65bar-100bar); • Oil phase: Iodododecane (C12H25I), Water phase: potassium

iodide (KI), sodium tungstate dihydrate (Na2WO4·2H2O).

• Two energy levels, 80 kV( 250mA·s) and 120 kV (330mA·s) • Reduce the beam-hardening effects;• Protect the machine (the tube life of CT scanner).

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Experimental ProceduresReference scans: dry core, core saturated with each fluid

100% Oil

Water flooding

Gas flooding

Oil at 1-Swi

Gas flooding

Water flooding

Gas flooding

No flooding Oil at 1-Swi

Flooding: inj. rate 0.1 cc/min, scanned at every 2mm

1st Exp. 3rd Exp.2nd Exp. 4th Exp.

Clean and dry the core

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1st Experiment Core Materials

Plug sample properties P and T Fluids and dopant

NameLength(mm)

Diameter (mm)

Porosity(%)

kwater

(md)P

(bar)T

(oC)Oil and dopant

(wt %)Water and dopant

(wt%)Gas

Core#1 44.00 25.69 35.50 0.32 65 15Isopar-L

5% IodododecaneDistilled water

7% Na2WO4·2H2OCO2

Core#2 77.02 25.72 40.00 0.13 65 15Isopar-L without

dopant

Distilled water1 % Na2WO4·2H2O

3 % Na2WO4·2H2OCO2


dopantDistilled water

5 % KICO2

Core#4 74.82 37.04 26.74 0.23 100 15n-decane,


3 % KICO2

• Perfrom CO2 flooding at residual oil saturation with doped water and doped oil.

100% Oil Wat. flooding Gas flooding

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1st Experiment Data Analysis

• CT number at different energy levels

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• Sarutations are not corrected, negative saturation is calculated.

Before water

flooding

Water flooding

CO2 flooding

Oil in the core (cc) 7.9 3.1 1.7

Water in the core(cc) 0 4.8 1.4

Oil produced (cc) — 4.8 1.4

Water produced (cc) — — 3.4

Oil recovery 0 55% 18%

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• Dopant is adsorbed at the inlet of the core.

CT and 3-D reconstruction images (120 kV) of a dry core after the experiment

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2nd Experiment Core Materials


NameLength(mm)

Diameter (mm)

Porosity(%)

kwater

(md)P

(bar)T

(oC)Oil and dopant


(wt%)Gas

Core#1 44.00 25.69 35.50 0.32 65 15Isopar-L


7% Na2WO4·2H2OCO2


dopant


3 % Na2WO4·2H2OCO2



5 % KICO2

Core#4 74.82 37.04 26.74 0.23 100 15n-decane,


3 % KICO2

• Test dopant (C12H25I) with various concentrations

No flooding

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2nd Experiment Data Analysis

Fig.7 CT number at different energy levels (Experiment 2)

• Three-phase can be treated as pseudo-two phase flooding.

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2nd Experiment Data Analysis

• Adsorption happens even at low concentration of dopant.

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3rd Experiment Core Materials


NameLength(mm)

Diameter (mm)

Porosity(%)

kwater

(md)P

(bar)T

(oC)Oil and dopant


(wt%)Gas

Core#1 44.00 25.69 35.50 0.32 65 15Isopar-L


7% Na2WO4·2H2OCO2


dopant


3 % Na2WO4·2H2OCO2



5 % KICO2

Core#4 74.82 37.04 26.74 0.23 100 15n-decane,


3 % KICO2

• Determine water saturation under three phases conditions

Oil at 1-Swi Gas flooding

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3rd Experiment Data analysis


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3rd Experiment Data analysis

• Water saturations from mass balnce and CT scanning agree well

Original core At Swi CO2 Flooding

Isopar-Lin the core (cc)

0 4.5 –Water

In the core(cc)5.2 0.7 –

Isopar-L produced (cc) – – –

Water produced (cc) – 4.5 –

Water saturation

1.0 0.13 –Isopar-L

saturation0 0.87 –

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Challenges in Data Analysis

• In a homogenous chalk, the CT number of the pixels follows a normal distribution. The mean value and standard deviation obtained from different software are correct (FPImage and Image J).

CT number distribution in the region of interest of the dry core

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• Software Challenges:

1. Inhomogeneity of the core

• High density objects in the core, give high CT numbers

• Mean value and standard deviation are affected by the noise

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Challenges in Data Analysis2. Maximum CT number CTmax

• The maximum CT number is 3071, and any pixels with a higher value will be set to 3071.

• By using dopant, decent contrast of CT numbers can be obtained at 120kV. But CT numbers at 80kV may exceed the upper limit.

• Can we estimate the correct mean value if some of the CT values are higher than CTmax?

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Challenges in Data Analysis• An optimization method (nonlinear Huber estimation) is used to meet the

two challenges, correcting the CT number, thus avoid the influence from inhomogeneity of the core and compensate for overflow of CT values.

Challenge 1

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Challenges in Data Analysis• Optimization changes the mean value by 25 for Challenge 1 and 101 for Challenge 2 • ΔCT between oil and water is around 100

Challenge 2

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• An example for the corrected data at 80kV (3rd Experiment), in which ImageJ does not work well.

• Saturation calculated based on optimization is better than that from ImageJ. (Results form 120kV as reference)

• When oil and water both are doped, the accuracy of saturation calculation could be enhanced

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4th Experiment Core Materials


NameLength(mm)

Diameter (mm)

Porosity(%)

kwater

(md)P

(bar)T

(oC)Oil and dopant


(wt%)Gas

Core#1 44.00 25.69 35.50 0.32 65 15Isopar-L


7% Na2WO4·2H2OCO2


dopant


3 % Na2WO4·2H2OCO2



5 % KICO2

Core#4 74.82 37.04 26.74 0.23 100 15n-decane,


3 % KICO2

• Identify three phases saturation with application of optimization method

Wat. flooding Gas floodingOil at 1-Swi

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4th Experiment Data analysis


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• Three phase saturation and mass balance

Swi

conditionWater

floodingGas

flooding

Water in the core

(cc) 6.63 10.81 5.45

Oil in the core(cc) 14.92 10.74 5.08

CO2 in the core (cc) 0.00 0.00 11.02

Oil Recovery – 28% 38%

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• Calculation of three-phase saturatons1

1 11 1

1 1 1 113 3

2 2 2 2

2 2 2 2

1

1

wog wo w

g w g wg

o o w wog w

g w g w

CT CTCT CT

CT CT CT CTS

S CT CT CT CT

CT CT CT CT

A B

Subscripts o oilphase w water phase g gas phase wog three phases 1, 2 energy level 1, 2

• The CT numbers of different phases increase similarly when the energy level changes from 80 to 120 kV, making the coefficient matrix A3 ill-conditioned (the smallest determinant is 0.018) and calculated saturation extremely sensitive to CT numbers (1 unit error in CTo1 leads to a maximum deviation of 0.127 in the calculated So)

• If the entries in the second column of A3 have opposite signs, A3 can be well-conditioned. Possible dopant with Kedge between 120 kV and 80 kV could be the choice (rarely used).

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Conclusions• Comparison of the dry core images before and after

experiment is important to reveal the problem of dopant adsorption

• High pressure three-phase flooding can be visualized as pseudo two-phase, where the most interesting phase can be identified at a single energy level.

• CT numbers in a homogeneous chalk sample follow normal distribution at 80 kV and 120 kV. The nonlinear Huber estimation is a useful tool to correct the average CT number at 80 kV.

• The major reason for the failure in simultaneous determination of the three-phase saturations is the is ill-conditioned saturation equation.

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Future Plans

• The concept of ‘Pseudo two-phase’ flooding will be applied in the future experiments with the Danish chalk cores.

• More dopants, with suitable properties, will be tested under the two scanning energies.

ph.d. student ben niu supervisor: ass.prof. alexander a. shapiro ass. prof. wei yan

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