CIPC 2004-232

Application of X-Ray CT for Investigation of CO2 and WAG

Injection in Fractured Reservoirs

D. Chakravarthy, V. Muralidharan, E. Putra and D.S. Schechter

Texas A&M University

CIPC 2004-232

CO2 for EOR

CO2 Flooding – Both for secondary and tertiary recovery

Water flooding - secondary recovery

Understanding CO2 flow is complex

Extreme heterogeneities like fractures

CIPC 2004-232

Fractured Reservoir

Glasscock Co

Reagan CoUpton Co

Midland Co

Martin Co Borden Co

Spraberry Trend AreaSpraberry Trend Area

An NFR with extensive fractures

OOIP 10B bbls

Ultimate recovery < 12%

CIPC 2004-232

CO2 Flood in Spraberry

Glasscock Co

Reagan CoUpton Co

Midland Co

Martin Co Borden Co

ET O’Daniel CO2 Pilot

CO2 flood not successful in mobilizing oil

Fluid flow through fractures

Early breakthrough, Oil bypass

CIPC 2004-232

SignificanceFracture Studies

Better understand multiphase flow in fractures

Understand physical mechanisms of oil bypass

Improve modeling of tracer studies

Improve prediction of sweep in NFR

Better performance prediction and improved recovery!

CIPC 2004-232

Fluid Flow

Fluid flow in reservoirs

Common heterogeneities

Extreme heterogeneities

Fluid mobilityInjection rates

CIPC 2004-232

Objectives1. How is a highly mobile fluid like CO2 affected by

heterogeneity?

2. What happens in the presence of fractures?

3. How does injection rate affect oil recovery and sweep?

4. Can WAG improve sweep efficiency in a fractured system? What other solutions exist?

X-Ray CT Scanner

CIPC 2004-232

Workstation

3D CT image

Digital detector

Principles of X-Ray Tomography

Object

X-Ray source

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X-ray CT scanner

CT number

Depends on density

Based on X-Ray attenuation

Every material has a characteristic CT number

E.g. CT for air = -1000

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Experimental Setup

I-1

Core Holder

OverburdenPump

OverburdenPressure Line

GasChromatograph

Back PressureRegulator

PressureTransducer

PressureTransducer

Data AcquisitionSystem

GraduatedCylinder

PressureGauge

PressureGauge

Flow ControlValve

Computer

Flow DividingValve

Oil Accumulator

CO2 Accumulator

InjectionPump

Wet test meter

CIPC 2004-232

Experimental Outline

Displaced Fluid - SoltrolTM Refined Oil

Doping agent – 1-iodohecadecane

Displacing Fluid - CO2

Pressure - 800 psig

Temperature - 75° F

CO2 phase – Vapor CO2 Phase diagram

CIPC 2004-232

CO2 Phase diagramP

ress

ure

(N

ot

to s

cale

)

Temperature (Not to scale)

Solid

Liquid

Vapor

Triple Point

-70°F

100 psi

Critical Point

89°F

1070 psi

CIPC 2004-232

CO2 Phase diagramP

ress

ure

(N

ot

to s

cale

)

Temperature (Not to scale)

Solid

Liquid

Vapor

Dense Vapor

Liq

uefi

es

Does n

ot

liq

uefy

CIPC 2004-232

CO2 Phase diagramP

ress

ure

(N

ot

to s

cale

)

Temperature (Not to scale)

Solid

Liquid

Vapor

Immiscible displacement

CIPC 2004-232

Experiments

CO2 injection

Unfractured Cores

Fractured Cores

Injection Rates

Viscosified water

Gel

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Unfractured Core

(a) Dry core scans

(b) Oil saturated core

Low density

High density

Red color indicating regions with higher

CT numbers

CIPC 2004-232

Injection Rate = 1 cc/min

3 minutes of CO2 injection 5 minutes of CO2 injection

15 minutes of CO2 injection 60 minutes of CO2 injection

CIPC 2004-232

Injection Rate = 0.3 cc/min

30 minutes of CO2 injection 60 minutes of CO2 injection

120 minutes of CO2 injection 165 minutes of CO2 injection

Uniform sweep

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Reconstructions (1 cc/min)

Oil saturated core

CO2 injection – 3 minutes

CO2 injection – 5 minutes

CO2 injection –15 minutes

CO2 injection – 45 minutes

CO2 injection – 60 minutes

Heterogeneity

CIPC 2004-232

Reconstructions (0.3 cc/min)

Oil saturated core

CO2 injection – 30 minutes

CO2 injection – 60 minutes

CO2 injection – 120 minutes

CO2 injection – 150 minutes

CO2 injection – 180 minutes

CO2 injection – 300 minutes

Heterogeneity

Uniform sweep

CIPC 2004-232

CT Number Plots100% Oil CO2

Decrease in CT number

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CO2 Saturation

CO2 Saturation – 3 Minutes CO2 Saturation – 5 Minutes

CO2 Saturation – 15 Minutes CO2 Saturation – 45 Minutes

CIPC 2004-232

Porosity and Saturation

Porosity equation

Saturation equation

1000,

,,,

Sampleyx

dryyx

SampleyxSample

yx CT

CTCTS

1000,,

,

Sample

dryyx

SampleyxSample

yx CT

CTCT

CIPC 2004-232

Spatial variation of saturation

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

4 14 24 34 44 54 64 74 84 94

Distance, mm

CO

2 S

atur

atio

n

0.3 PV 0.5 PV 1.5 PV 3 PV 5.4 PV

Early breakthrough

Injector ProducerAverage saturation is 95%

CIPC 2004-232

Spatial variation of saturation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

4 14 24 34 44 54 64 74 84 94

Distance, mm

CO

2 S

atu

rati

on

0.11 PV 0.22 PV 0.45 PV 0.57 PV 0.68 PV 1.1 PV

Average saturation is

96.7%

Late Breakthrough

CIPC 2004-232

Oil recovery

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6

Pore Volumes Injected

Oil

Rec

ove

ry, %

0.03 cc/min 1 cc/min

≈ 60 minutes

≈ 300 minutes

CIPC 2004-232

Fractured Core

Fluid flows through fracture simultaneously

displacing oil from the matrix

Counter current exchange mechanism Fracture

Matrix

Oil + gas

Gas

CIPC 2004-232

Fractured Core

Oil saturated core

Fracture filled with oil

CIPC 2004-232

Breakthrough Scans

Increase in CO2 saturation in

fracture

Irregular CO2

saturation due to heterogeneity

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Final CO2 saturation

2.1 PV of injection

Final recovery of about 58%

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Water Alternating Gas (WAG)

Alternate injection of water and gas

Fairly homogeneous reservoirs

In the presence of extreme heterogeneities?

CIPC 2004-232

Brine preparation

Brine iodated with sodium iodide and potassium iodide

High mobility, early breakthrough

Add Xanthan to increase viscosity and reduce mobility

High injectivity

Viscosity insensitive to salinity

Objective: Delay CO2 breakthrough!

CIPC 2004-232

Oil saturated core

Oil Saturated Core

Higher density regions

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Water Injection

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Water Breakthrough

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CO2 injection (0.4 pv)

Large amount of CO2 is diverted into

the matrix!

CIPC 2004-232

Overall recovery ≈ 85% Incremental recovery ≈ 4.5%

Analysis

Reason? Core used was strongly water wet

Xanthan still in gelant form

Considerable amount of water “leakoff” into the matrix

Increase in viscosity gave similar results

CIPC 2004-232

Experiment with gel

Injected directly into fracture

Seen as a yellow streak in the fracture

Gel

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CO2 injection

CO2 in matrix

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CO2 Injection

Block in grooves

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Final recovery

Final Recovery obtained ≈ 95%

Gel effective in diverting CO2 into matrix

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Oil recovery

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5

Pore volumes injected

Oil

re

cov

ery,

%

Viscosified water injection CO2 injection following water

Continuous CO2 Injection CO2 injection in the presence of gel

Highest Recovery

CIPC 2004-232

Better Model

Match saturation and flow profile apart from other parameters

Reliable upscaling

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ConclusionsAre low injection rates better?

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6

Pore Volumes Injected

Oil

Re

co

ve

ry,

%

0.03 cc/min 1 cc/min

Lower rates give better sweep and lesser utilization of CO2 but time taken is higher.

Rates need to be optimized

CIPC 2004-232

Conclusions

Can WAG delay CO2 breakthrough from fractures?

Viscosified water can delay breakthrough but “leak off” prevention is required (particulate matter)

Performance might be better in oil-wet reservoirs

“Washout” problems

CIPC 2004-232

ConclusionsIs gel treatment an effective solution?

Gels can prevent breakthrough and improve recovery

No “wash out” problems

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5

Pore volumes injected

Oil

rec

ov

ery,

%

Viscosified water injection CO2 injection following water

Continuous CO2 Injection CO2 injection in the presence of gel

CIPC 2004-232

Issues

Gels for conformance control

Gel type

Injection Pressure

Resistance factor and “screen out”

Gel placement

CIPC 2004-232

Problems

Fractured System

Low Matrix k

High fracture k

Early breakthrough, oil bypass

Fluid flow through fractures

CIPC 2004-232

Alternate injection of specific pore volumes of water and gas to reduce gas mobility.

Proven to be mostly effective in fairly homogeneous reservoirs.

Performance in the presence of extreme heterogeneities like fractures has not been adequately investigated.