Enhanced Gas Recovery and CO2Storage in Coal Bed Methane

Reservoirs: Optimized Injected Gas p jComposition for Mature Basins

of Various Coal Rankof Various Coal RankSPE 139723 SPE 139723

Karine Schepers and Anne Oudinot , Advanced Resources International, Inc. Nino Ripepi, Virginia Center for Coal and Energy Research

ObjectivesObjectives

• Identify key parameters driving ECBM and y y p gsequestration processes in deep unminable coal seams.

• Better understand cleat permeability changes in response to injected gas composition during ECBM process, by coal rank.process, by coal rank.

• Optimize injected gas composition to maintain injectivity while sequestering carbon dioxide.

SPE 139723 SPE 139723 2

OutlinesOutlines

• Objectivesj• Key reservoir parameters driving CBM and CO2

sequestration• Cleat permeability changes during ECBM• Optimizing enhanced gas recovery while sequestering

carbon dioxidecarbon dioxide• Summary and Conclusions

SPE 139723 SPE 139723 3

Gas Storage vs Coal RankGas Storage vs. Coal Rank

)

nt (s

cf/t)

ent (

cm/g

)

Coal rank

as C

onte

n

Gas

Con

te

Ga G

SPE 139723 SPE 139723 4

Cleat Permeability vs. Coal Ranky

• Higher the coal rank, the lower the cleat permeability (C C R vs pressure)permeability (Cp, Cm, Rk vs. pressure)

• Low rank coals: – high moisture content, high matrix porosity hence high g g p y g

pore compressibility: maximum loss of fracture permeability

– Early stage coalification, lower matrix compressibility: minimum fracture permeability improvementminimum fracture permeability improvement

(mD

)rm

eabi

lity

SPE 139723 SPE 139723 5

a) Low Rank Coals b) Medium Rank Coals c) High Rank Coals

Pressure (psia) Pressure (psia) Pressure (psia)

Per

Most Commercial CBM Projects M di R k C lare Medium Rank Coals

• High Rank Coal: high gas content but low injectivityg g g j y• Low Rank Coal: low gas content but high injectivity• Good compromise are medium rank coals with average

gas content and initial permeability

CPU dPipeline Termination Point

CPU and Telemetry Station

CO2 Line Heater

Flow Control Valve

Tracer Injection

SECARB Appalachian SECARB Appalachian ––COCO22 Pilot in Pratt coalPilot in Pratt coal

WellheadPressure and Temperature Sensors

NETL’s Perfluorocarbon Tracer Trailer

jPoint

SWP SWP –– COCO22 Injection in Fruitland coalInjection in Fruitland coalSPE 139723 SPE 139723 6

Cleat Permeability during CO2-ECBMCleat Permeability during CO2 ECBM

40

45 Preferential adsorption of

20

25

30

35

onte

nt, m

3/to

nne

Methaneonte

nt (m

3 /m3 )

InjectionCO2: high storage capacity

0

5

10

15

2 000 4 000 6 000 8 000 10 000 12 000 14 000 16 000

Gas

Co Methane

Carbon DioxideNitrogen

Gas

Co

Preferential adsorption 3.5

4

4.5

5

D

CH4 CO2 N2

D)

- 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

Pressure, kPaPressure (kPa)

of CO2 induces coal swelling resulting in an injectivity loss

1

1.5

2

2.5

3

Perm

eabi

lity,

mD

Start

rmea

bilit

y (m

Injectionj y

SPE 139723 SPE 139723 7

0

0.5

1

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

Pressure, kPaPressure (kPa)

Pe

Cleat Permeability during N2-ECBMCleat Permeability during N2 ECBMLower adsorptivity of N2 :40

45

- N2 stays in fractures while CH4is stripped out of matrix sites

- Rapid N2 breakthrough20

25

30

35

onte

nt, m

3/to

nne

Methanenten

t (m

3 /m3 )

0

5

10

15

2 000 4 000 6 000 8 000 10 000 12 000 14 000 16 000

Gas

Co Methane

Carbon DioxideNitrogen

Gas

Co

Injection

Injection3.5

4

4.5

5

D

CH4 CO2 N2

mD

)

- 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

Pressure, kPaPressure (kPa)

Preferential adsorption

1

1.5

2

2.5

3

Perm

eabi

lity,

mD

Start

DepletionDisplace with N2

Continued Injection

erm

eabi

lity

(m

Injection

of CH4 induces coal matrix shrinkage resulting in injectivity

SPE 139723 SPE 139723 8

Pressure

0

0.5

1

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

Pressure, kPa

P

Pressure (kPa)

g j yimprovement

ECBM Optimization ProcessECBM Optimization Process

• How can injected gas composition be optimizedHow can injected gas composition be optimized to maintain injectivity while sequestering CO2for different coal ranks?

SPE 139723 SPE 139723 9

Model ConstructionModel Construction

• Simulation performed using COMET3Simulation performed using COMET3• 5-spot injection pattern, vertical wells• 10 years of primary production followed by 1510 years of primary production followed by 15

years of injection• Well spacing determined based on CO2Well spacing determined based on CO2

breakthrough occurring after 8 to 10 years of injection (100% CO2 injection scenario)

SPE 139723 SPE 139723 10

Model ConstructionModel Construction

d

Depth 3,000 ft

Thickness 30 ftProducer Thickness 30 ft

Pressure Gradient 0.433 psi/ft

Initial Sw 0.99 fraction

Pd/Pi 1 fraction

Fracture Spacing 1 inch

Sorption Time 10 days

Gas Gravity 0.6 air = 1.0

Injector

Methane Content 1 fraction

Temperature 126˚ ˚F

Water Density 62.4 lbm/ft3

W t Vi it 0 6 cpWater Viscosity 0.6 cp

Producer Skin 0 n/a

Injector Skin -2 n/a

FBHP 25 psi

Corey Constants Swirr 0.1Krw-max 1.0 Krw-shape 2.0Sgr 0.05 Krg-max 0.85 Krg-shape 2.0

Pre-Injection Time (Primary Production) 10 yrs

Max Injection Pressure 1,800 psi

Max Injection Rate 5,000 Mscfd

g p

SPE 139723 SPE 139723

Parametric Study: Porosity and Permeability per Coal RankPermeability per Coal Rank

Parameter High Rank Medium Rank Low Rank Unit

Average Permeability 1 25 100 mD

Permeability Anisotropy 1:2 1:2 1:2Permeability Anisotropy 1:2 1:2 1:2 ‐

Porosity 0.25 0.5 1.5 %

Pore Compressibility 2.00E‐04 4.00E‐04 5.00E‐04 1/psi

Permeability Exponent 3 3 3 ‐

Matrix Compressibility 5.00E‐06 1.00E‐06 5.00E‐07 1/psi

CO2 S lli F 1 25 2 3CO2 Swelling Factor 1.25 2 3 ‐

N2 Swelling Factor 0.5 0.5 0.5 ‐

SPE 139723 SPE 139723 12

Injection ScenariosInjection Scenarios

• Gas mixturesGas mixtures– 100% CO2

– 75% CO2/25% N22 2

– 50% CO2/50% N2

– 25% CO2/75% N2

– 100% N2

• Injection rate constraint: 5MMcfd max• Injection pressure constraint: 1,800psia

maximum bottom-hole pressure

SPE 139723 SPE 139723 13

Low Rank CoalsLow Rank Coals

• 240 acres spacing to reach CO2 breakthrough after240 acres spacing to reach CO2 breakthrough after 10 years of injection

• At end of primary production, permeability was p y p , p ydown from 100 mD to 11 mD: factor of 10 due to high pore compressibility.

• Can we maintain or improve this permeability?

14SPE 139723 SPE 139723

Low Rank Coals – 100% CO2 Injection% 2 j2,500,000

Cum  C1  Production,  MSCF 

C G P d ti MSCF

6000

N2  Injection   Rate,   MSCFD 

CO2 I j ti R t MSCFD

1 500 000

2,000,000

on, M

cf

Cum  Gas  Production,  MSCF 

Cum C1 Prod, Mscf, No Injection Cumulative Injected CO2: 5.2BcfIncremental CH4 production: 167MMcf

4000

5000

scfd

CO2  Injection  Rate,   MSCFD 

Cum Injection: 5.2 BcfInjectivity loss

100

120

D

1,000,000

1,500,000

Cumulative Prod

uctio

2000

3000

Injection Ra

te, M

60

80

ermeability, mD

1

0

500,000

C

0

1000

1- High pore compressibility 20

40Average Pe

20

0 2000 4000 6000 8000 10000

Days

0 2000 4000 6000 8000 10000

Days

1 High pore compressibility2- CO2 differential swelling

0

20

0 2,000 4,000 6,000 8,000 10,000

Days

Low Rank Coals – 100% N2 Injection2 j30,000,000

Cum  C1  Production,  MSCF 6000

25,000,000

Cum  Gas   Production,  MSCF 

Cum C1 Prod, Mscf, No  Injection

177 MMcf incremental CH4100

120

5000

6000

N2  Injection   Rate,   MSCFD 

CO2  Injection  Rate,   MSCFD 

15,000,000

20,000,000

rodu

ction, M

cf

Nitrogen breakthrough60

80

y, mD

14000

ate, M

scfd

Maximum injection rate reached due to permeability

10,000,000

15,000,000

Cumulative Pr Nitrogen breakthrough

20

40age Pe

rmeability

2

1- High pore compressibility2000

3000

Injection Ra increase: matrix shrinkage and

reservoir pressurization

5,000,0000

0 2,000 4,000 6,000 8,000 10,000

Aver

Days

2- Matrix shrinkage under N2differential swelling1000

0

0 2000 4000 6000 8000 10000

Days16

0

0 2000 4000 6000 8000 10000

Days

Low Rank Coals - Mixtures

1201- Permeability

Low Rank Coals Mixtures

100

D 50%N2/ 50%CO2

increase at beginning of

injection if ~50% nitrogen is injected

60

80

rmeability, m 75%N2/ 25% CO2

25%N2/75% CO2

(high Cp)

2- Permeability decreases due to

40

Average Pe

r

1

2

coal swelling as N2departs from

fractures and CO2is adsorbed

0

20

0 2,000 4,000 6,000 8,000 10,000

Days17SPE 139723 SPE 139723

Low Rank Coals - Summaryy

• CO2 sequestration optimum with 100% CO2 butCO2 sequestration optimum with 100% CO2 but lowest incremental CH4

• Trial and error found the best mixture at 40% N2/ % 260% CO2– 269 MMcf incremental methane– 3.8 Bcf sequestered CO2

– Above 40% N2, no additional incremental recovery

100% CO2 25%N2/75% CO2 50%N2/ 50%CO2 75%N2/ 25% CO2 100% N2

Incremental CH4, MMcf 167 249 260 177 177

Breakthrough Time, years 10 3.5 4 Not reached (25%  n/aBreakthrough Time, years 10 3.5 4@ 15 years)

n/a

Sequestered CO2 at breakthrough time, Bcf 5.2 4.4 3.3 2.1 n/a

18SPE 139723 SPE 139723

Medium Rank CoalsMedium Rank Coals

• 60 acres spacing necessary to achieve60 acres spacing necessary to achieve breakthrough after 10 years of injection

• At end of primary production, permeability down p y p , p yfrom 25 mD to 13 mD: factor of 2 (average Cp and Cm).

• Can we maintain or improve this permeability?

19SPE 139723 SPE 139723

Medium Rank CoalsMedium Rank Coals40

1- Coal swelling

Optimum solution30

35

mD 3

1- Coal swelling2- No permeability increase as for low rank (average Cp)

100% CO2 25%N2/75% CO2 50%N2/ 50%CO2 75%N2/ 25% CO2 100% N220

25

Perm

eability, 

50%N2/ 50%CO2

25%N2/ 75% CO2

75%N2/25%CO2

3- Rapid reservoir re-pressurization

/ / /

Incremental CH4, MMcf 92 127 144 99 99

Breakthrough Time, years 10 4 2 Not reached 25% @ 15 years

n/a10

15

Average  75%N2/25%CO2

100% CO2

100%N2

2

1

Sequestered CO2 at breakthrough time, Bcf 1.2 1.4 1.3 1.0 n/a

0

5

0 2,000 4,000 6,000 8,000 10,000

20SPE 139723 SPE 139723

Days

High Rank CoalsHigh Rank Coals

• Due to their high matrix compressibilityDue to their high matrix compressibility, high rank coals were difficult to model

Nominal spacing of 40 acres was used– Nominal spacing of 40 acres was used• At end of primary production, permeability

was up from 1 mD to 9 mDwas up from 1 mD to 9 mD

High matrix Low poreHigh matrix compressibility

Low pore compressibility

21SPE 139723 SPE 139723

High Rank CoalsHigh Rank Coals1- Dramatic loss in   20

Best mixture

permeability with CO2

14

16

18

ty, m

D

A trend seems to appear: as coal rank increases, more nitrogen ist i ECBM f 40% f l k l t 75% f hi h

100% CO2 25%N2/75% CO2 50%N2/ 50%CO2 75%N2/ 25% CO2 100% N28

10

12

ge Permeabilit

50%N2/ 50%CO2

100%N2

75%N2/ 25%CO2necessary to improve ECBM, from 40% for low rank coals to 75% for highrank coals.

Incremental CH4, MMcf ‐ 35 90 214 214

Breakthrough Time, years Not reached Not reached Not reached Not reached 24% @ 15 years

n/a

S t d CO2 t b kth h ti B f 0 01 0 08 0 1 0 9 /2

4

6

Averag

25%N2/75%CO2

100% CO21

Sequestered CO2 at breakthrough time, Bcf 0.01 0.08 0.1 0.9 n/a0

0 2,000 4,000 6,000 8,000 10,000

Days

22SPE 139723 SPE 139723

Summary: Optimum CompositionSummary: Optimum Composition100% CO2 25%N2/75% CO2 50%N2/ 50%CO2 75%N2/ 25% CO2 100% N2LOW RANK 40%N2/60% CO2

Incremental CH4, MMcf 167 249 260 177 177

Breakthrough Time, years 10 3.5 4 Not reached (25% @ 15 years)

n/a

S t d CO2 t b kth h ti B f 5 2 4 4 3 3 2 1 /3 8

269

100% CO2 25%N2/75% CO2 50%N2/ 50%CO2 75%N2/ 25% CO2 100% N2

I l CH4 MM f 92 127 144 99 99

Sequestered CO2 at breakthrough time, Bcf 5.2 4.4 3.3 2.1 n/a3.8

Incremental CH4, MMcf 92 127 144 99 99

Breakthrough Time, years 10 4 2 Not reached 25% @ 15 years

n/a

Sequestered CO2 at breakthrough time Bcf 1 2 1 4 1 3 1 0 n/aSequestered CO2 at breakthrough time, Bcf 1.2 1.4 1.3 1.0 n/a

100% CO2 25%N2/75% CO2 50%N2/ 50%CO2 75%N2/ 25% CO2 100% N2

Incremental CH4, MMcf ‐ 35 90 214 214

HIGH RANK

23SPE 139723 SPE 139723

Incremental CH4, MMcf 35 90 214 214

Breakthrough Time, years Not reached Not reached Not reached Not reached 24% @ 15 years

n/a

Sequestered CO2 at breakthrough time, Bcf 0.01 0.08 0.1 0.9 n/a

ConclusionsConclusions

• For a specific coal rank, ECBM can drastically improvep y pby increasing N2 content in the injected gas stream.

• ECBM due to high N2 content will increase up to amaximum N2 concentration, or threshold: besides thismaximum N2 concentration, or threshold: besides thisthreshold, breakthrough occurs too rapidly to generateadditional methane recovery.

• This N threshold varies between coal ranks as• This N2 threshold varies between coal ranks, aspressure dependant parameters also vary relative to therank.A t d t l k i• A trend seems to appear: as coal rank increases,more nitrogen is necessary to improve ECBM, from40% for low rank coals to 75% for high rank coals.

24SPE 139723 SPE 139723

ConclusionsConclusions• By injecting a 100% nitrogen mixture in medium rank

l l t bilit i i d b d i iti lcoals, cleat permeability is improved beyond initialconditions due to reservoir pressurization. Sameconclusion can be drawn from high rank coals.Laboratory measurement necessary to better understandLaboratory measurement necessary to better understandphysical phenomenon behind it.

25SPE 139723 SPE 139723