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Stanford UniversityGlobal Climate & Energy Project

MIT Carbon Sequestration Forum VIIIStanford, CA

November 14, 2007

Flow and Adsorption in Coal Beds

Lynn Orr and Tony Kovscek

with all the real work done byKristian Jessen, Carolyn Seto, Tom Tang, Wenjuan Lin, Sameer Parakh, Tanmay Chaturvedi, and Jichun Zhu

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Why Deep Unmineable Coal Beds?

• Expand the number of geologic settings that might store CO2?

• Recover adsorbed CH4?• Possible co-storage of

other components (SOx, H2S)?

• Coal-fired power plants often near coal beds.

Much better ability to predict the interaction of flow and multicomponent adsorption in a fractured medium is required.

Central issue: where does the injected CO2 go?

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Multicomponent Flow in Coal Beds

• Analytical solutions for simplified models• Experiments to delineate mechanisms and test our ability to

model• More complex models to predict what happens at larger scale

coal bed

CH4

CO2 N2

N2disposal

CO2

CH4

flue gases

N2

N2

CH4

Objective: Predict quantitatively what happens during flow induced by CO2 injection for carbon storage in coal beds.

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Probing flow mechanisms:Sorption, k-reduction, displacement

• Pore pressure: 60~1100 psi• Gas mixtures made in the lab by weight• Add reference cell for sorption measurements

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CO2, CH4, N2 Adsorption/DesorptionDry Powder River Basin (WY) Coal

• Pure components are well fit by Langmuir isotherm

• CO2 adsorbs preferentially

• Adsorption hysteresis for all gases

• Scanning loops are observed

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0

200

400

600

800

1000

1200

1400

1600

0 500 1000 1500 2000

P, psia

Sor

ptio

n, S

CF/

ton

CO2 @Swi=0CO2 @Swi=8.47%CH4 @Swi=0CH4 @Swi=8.67%N2 @Swi=0N2 @Swi=8.54%

0

200

400

600

800

1000

1200

1400

1600

0 500 1000 1500

P, psia

Volu

me

adso

rbed

, SC

F/to

n CH4 at T=22oCCH4 at T=40oCCO2 @Swi=0CO2 at T=40oC

Swi=0

Effect of TemperatureEffect of Moisture

Gas Sorption on Wet Coal at Elevated Temperature

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Simplified mathematical modelComplex interactions of flow, adsorption

Assumptions• 1D flow• local equilibrium• homogeneous porous

medium• no dispersion or diffusion• neglect gravity and

capillary forces( )LiDLgiDGDi

iDLiDLgiDGi

fxfyuH

aSxSyG

ρρφ

φρρ

+=

−++=

1

0=∂∂

+∂∂

ξτii HG

Solution• Recast system of 1st order equations as an eigenvalue problem• Eigenvalues are speeds at which a composition propagates• Eigenvectors give directions (paths) in composition space that satisfy

the molar balance equations• Find the correct path for initial, injection compositions• Solutions include shocks (jumps in composition), continuous variations

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Shocks and rarefactions

• When CO2displaces CH4, a sharp front is predicted and observed

• When N2displaces CH4, the N2 and CH4flow together in predictions and experiments

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CO2 injection to displace adsorbed CH4

More strongly adsorbing component displaces more weakly adsorbing component

• Preferential adsorption of CO2 creates self-sharpening displacement• Local flow velocity decrease due to adsorption of CO2• Separation of CO2 from CH4

0 0.5 1 1.5 20

0.5

1

Sg

0 0.5 1 1.5 20

0.5

1

CH

4

0 0.5 1 1.5 20

0.5

1

CO

20 0.5 1 1.5 2

0

0.5

1

H2O

0 0.5 1 1.5 20.5

1

1.5u

D

mocfd 1000

I

C B

AA

O

λ

CH4

H2O

I

C

B

A

O

CO2

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N2

H2O

O A B

C D

I

N2 injection to displace adsorbed CH4

0 0.5 1 1.5 2 2.5 3 3.5 40

0.5

1

Sg

0 0.5 1 1.5 2 2.5 3 3.5 40

0.5

1

N2

0 0.5 1 1.5 2 2.5 3 3.5 40

0.5

1

CH

40 0.5 1 1.5 2 2.5 3 3.5 4

0

0.5

1

H2O

0 0.5 1 1.5 2 2.5 3 3.5 40.5

1

1.5u

D

mocfd 1000

I

BA

OCC

D

• reduction of partial pressure desorbs CH4• local flow velocity increase due to CH4 desorption• co-production of N2 and CH4 at outlet

CH4

weaker adsorbing component displaces more adsorbing component

λ

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CO2/N2 mixtures displacing CH4: prediction versus experiment

• CO2 retarded behind N2/CH4zone

• CO2 is separated from N2 by adsorption chromatography

• N2 and CH4produced together – requires a separation for CH4recovery

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Flue gas injection with water present

• Inject N2/CO2 mixture to displace H2O and CH4• N2 propagates rapidly, CO2 slowly• Local flow velocity changes due to volume change as

components adsorb/desorb• co-production of CH4 and N2 at outlet

CO2

H2O

CH4

N2I

O

A B

C D E F

0 0.5 1 1.5 2 2.5 3 3.5 40

0.5

1

1.5

Sg,

uD

Sg (moc)

Sg (fd 5000)

uD

0 0.5 1 1.5 2 2.5 3 3.5 40

0.5

1

λ

com

pone

nt m

olar

co

mpo

sitio

n

N2

CH4

CO2

H2O

O

AB

CC

DD

EF

I

injectionN2 = 0.9CO2 = 0.1

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N2/CO2 mixture displacing H2O/CH4 mixture

• Complex pattern of shocks and rarefactions predicted appears to be present in experimental observations

• Additional experiments needed to confirm details

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0 0.5 1 1.5 2 2.5 30

0.2

0.4

0.6

0.8

1

pore volumes injected

CH

4 rec

over

y 1.0 N2

0.9 N2

0.4 N2

0.2 N2

0.05 N2

0.0 N2

Effect of flue gas composition on CH4 recovery

0 0.5 1 1.5 2 2.5 30.6

0.8

1

1.2

1.4

1.6

λ

u D

1.0 N2

0.9 N2

0.4 N2

0.2 N2

0.05 N2

0.0 N2

• Efficient recovery of CH4 from gas injection into coals

• Faster CH4 recovery for higher N2injection concentrations

• Higher N2concentrations show larger increase in local flow velocity

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Adsorption/Desorption/Transport Modeling

• 1D:

• ai: extended Langmuir vs IAS

• dual porosity– primary and secondary (grain): φ= φ1+φ2

– instantaneous equilibrium– 2%< φ2 <8%

• PR-EOS with

• Finite difference solution

φ∂Ci

∂t+ (1 − φ)

∂ai

∂t+ ∇⋅ (vCi) = qi

Vads = zii∑ bi

coal

& C

H4

CH4 + CO2 + N2

gas analyzer

46/54 CO2 /N2

p = 600 psiSw = 0

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IAS vs Langmuir Adsorption

Selectivity:

s2,1 =( x / y)2( x / y)1

=( x / y)CO2

( x / y)N2

Sele

ctiv

ity, C

O2/N

2

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(25 grid blocks)

(25 grid blocks)

coal

& C

H4

gas analyzer

100% CO2

p = 600 psiSw = 0

no parameter adjustment

CH4 + CO2

CH4 CO2

CH4 CO2

Ideal adsorbed solution vsextended Langmuir adsoprtion

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(25 grid blocks)

(25 grid blocks)

no parameter adjustment

coal

& C

H4

CH4 + CO2 + N2

gas analyzer

46/54 CO2 /N2

p = 600 psiSw = 0

CH4

CO2

N2

CH4

CO2

N2

IAS is better than Langmuir, for multicomponent flow, but not perfect

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0.7

0.8

0.9

1

0 100 200 300 400 500 600

pressure (psi)

fuga

city

coe

ffici

ent

CH4CO2N2

Current Flow Modeling Activitiesdispersion, non-ideal behavior, and hysteresis

0

100

200

300

400

500

600

700

0 200 400 600 800

Pressure (psi)

Sor

ptio

n (s

cf/to

n)

predicted frombounding curves

yiPΦi = xiπiγi Methane sorption

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Is permeability sensitive to gas type?sorption/permeability/gas injection

After grinding Coalpack

net effective stress = 400 psi

poverburden

ppore

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1200Intact composite coal plugs

0Corresponding CT-images

Total length: 21.17 cm Diameter: 2.79 cm

Porosity: 7% Permeability to He: 1.7 md

Composite Coal

CT

Coal plugs

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0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 200 400 600 800 1000

Pore pressure, psia

perm

eabi

lity

to C

H4,

md

Swi=0T=22°C

pressure increases

pressure decreases

Is permeability sensitive to gas type?hysteresis loading/unloading

net effective stress = 300 psi

poverburden

ppore

CH4

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 200 400 600 800 1000

Pore Pressure, psia

Per

mea

bilit

y to

CO

2, m

d

pressure increases

pressure decreases

T=22oCSwi=0

CO2

- There is a permeability hysteresisbetween loading and unloading process

- CO2 adsorption causes greater permeability reduction than CH4

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Conclusions

• Qualitative predictions of simplified analytical model are confirmed by experiments (sharp fronts, continuous composition variations, complex banks of fluids).

• CO2 can be separated from N2 in a coal bed, but compression of lots of N2 and separation of CH4 and N2is then required.

• The ideal adsorbed solution model gives better predictions of the interplay of multicomponent adsorption and flow, but modeling of nonideality and hysteresis will also be required to make fully quantitative predictions.

• Permeability generally declines as the amount of adsorbed gas increases. The reduction for CO2 is greater than that for CH4 at a given pressure. Managing permeability will be an issue for deep coal injection.

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Acknowledgement

• The support of the Global Climate and energy Project is gratefully acknowledged.

• We are also much indebted to Margot Gerritsen and to Jerry Harris and Mark Zoback and their research groups, our collaborators working on geophysical measurements and their applications to CO2 storage in coal beds.

• For more information about this work and other GCEP research, see:– http://gcep.stanford.edu/events/symposium2007/presentations.html– http://gcep.stanford.edu/events/symposium2007/posters.html