Gas Adsorption, Permeability Reduction, and Gas Flow in CoalW. Lin, G. Tang*, K. Jessen** and A. R. Kovscek, Energy Resources Engineering Department, Stanford University

Adsorption Isotherm

Measurement and Modeling

Electronic balance

Coreholder

Pressure gauge

Pressure gauge

Reference cell

Gas Cylinder

Vacuum Pump

bpbpV

V m

+=

1

Langmuir and Extended Langmuir Langmuir and Extended Langmuir IsothermsIsotherms

Multi-component Adsorption Calculation

V = amount of gas adsorption, SCF/ton;P = pressure;Vm = saturation adsorption constant;b = Langmuir constant;Vi = adsorption volume of component i;i, j = components;n = number of components.

1. The adsorption of pure methane, nitrogen, and carbon dioxide on crushed Wyoming Powder River Basin coal is well represented by the Langmuir isotherm. The adsorption capacity of coal to carbon dioxide is about 3 times of methane and 7 times of nitrogen.

2. Among many isotherms in the literature for gas adsorption, the question remains open as to which isotherm gives better prediction for multi-component adsorption. The extended Langmuir isotherm and the ideal adsorbate solution model gave different predictions in terms of the total amount of adsorption, and amount of adsorption for different gas species. IAS model was considered thermodynamically superior to the extended Langmuir isotherm.

Gas Flow in Coal

Enhanced coalbed methane (ECBM) recovery by injection of CO2 or mixtures of CO2 and N2 is an attractive method to recover additional natural gas resources while at the same time sequestering CO2 in the subsurface. The permeability of coal and the dynamics of ECBM recovery processes are determined in large part by the sorption behavior of mixtures of CH4, CO2 and N2 on the coal surface. Pure gas adsorption is represented well by the Langmuir isotherm. For multi-component adsorption, however, the extended isotherm is not sufficient to represent adsorption behavior faithfully.The ideal adsorbate solution (IAS) model was used to model multi-component adsorption during gas flow through coal. IAS is proved superior to the extended Langmuir isotherm.

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Permeability Evolution

Experiments on Coal Pack

1. Permeability of coal is sensitive to pressure and the type of the injection gas. At the same pore pressure, with an increase of the fraction of CO2 in the injected gas, permeability reduction is more significant. Permeability may be preserved by having some amount of N2 (10 ~ 20% by mole fraction) in the injection mixture.

2. Permeability change due to gas sorption displays hysteretic behavior.3. The change of porosity (calculated based on IAS adsorption model) and change of

permeability for various gas composition do not have a unique correlation.

N2 and CO2 Mixtures: 25%, 50%, 75%, 85%

Pure Gases: CH4, N2 and CO2

N2 and CO2 Mixtures: 25%, 50%, 75%, 85%

Pure Gases: CH4, N2 and CO2

Coal Holder

Flow-MeterBack Pressure Regulator

Gas Container

poverburdenp2

pi

To Vent

�p

Coal Holder

Flow-MeterBack Pressure Regulator

Gas Container

poverburdenp2

pi

To Vent

�p

Net overburden pressure: 400 psi, Pore pressure: 60~1100 psi (6 points)

)])([(2000

2121 ppppA

Lpqk ggg

g −+=

µ

Effect of Injected Gas Composition (Dry Coal)

Injection gas composition: (a) 24% CO2+76% N2, (b) 46% CO2+54% N2, (c) 85% CO2+15% N2, (d) 100% CO2. Results show that coal beds effectively separate mixtures of CO2 and N2. There is opportunity for mixed gas and contaminant gas injection.

Mechanisms

a) Permeability change of coal due to change of effective stress;

b) Permeability change of coal due to sorption of gases.

The coalholder system.

Permeability of the coal pack vs. pressure, injecting different gas at room temperature, net overburden pressure = 400 psi.

Total amount of adsorption calculated using the extended Langmuir isotherm and the ideal adsorbed solution model.

The calculated separation factor of CO2 over N2using the extended Langmuir isotherm and the ideal adsorbed solution model. Extended Langmuir shows no sensitivity to gas composition or pressure.

Correlation between the (calculated based on IAS adsorption model) change of effective porosity and the (measured) change of permeability.

Sorption characteristics of Powder River Basin (Wyoming) coal. Adsorption indicated with open symbols and desorption with closed symbols. M e a s u r e m e n t c o n d u c t e d a t 2 2 º C .

Adsorption Isotherms

Schematic of experimental components for measuring gas adsorption on coal.

�=

+= n

jjj

iimii

pb

pbVV

1

1)tan()(0 Ttconsxppy iii Π=

0 0.5 1 1.5 2 2.5 3 3.5 40

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Con

cent

ratio

n (%

)

0 0.5 1 1.5 2 2.5 3 3.5 40

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Pore volumes injected (PVI)

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cent

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Predicted effluent concentration: Pure CO2 displaces CH4 at 600 psia and 22 ºC.Left panel: Langmuir model including hysteresis (dotted lines) and without hysteresis (full lines);Right panel: IAS model including hysteresis (dotted lines) and without hysteresis (full lines);Symbols represent experimental observations.

Ternary displacement: CO2 / N2 mixtures displaces pure CH4 at 600 psia and 22 ºC.Left panels: Langmuir model including hysteresis (dotted lines) and without hysteresis (full lines);Right panels: IAS model including hysteresis (dotted lines) and without hysteresis (full lines);Symbols represent experimental observations.

bpbpV

V m

+=

1

Ideal Ideal AdsorbateAdsorbate Solution (IAS) ModelSolution (IAS) Model

Schematic of experimental components for permeability experiments.

(Assembled Holder)

Coalholder

Aluminum sleeve

Experiments on Composite Coal Core

Permeability to CO2 vs. pressure at room temperature, and net overburden pressure = 300 psi.

Schematic of experimental components for permeability experiments on a composite coal core.

Modeling Gas Displacement in Coal

1. The match between experimental and modeling results improves as physically realistic complexity is added to the model.

2. Sorption hysteresis must be included in displacement calculations to improve the predictive capabilities of both the extended Langmuir and the IAS approach.

3. For all ternary displacement calculations, the IAS model predicts more accurately the timing of composition fronts arriving at the outlet.

0

0.2

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Pore Pressure, psia

pressure increases

pressure decreases

T=22oCSwi=0

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cent

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• one dimensional, dual-porosity formulation;• gas sorption, transport, and dispersion;• gas sorption, including hysteresis among adsorption and desorption;• implementing extended Langmuir and IAS models.

0 0.5 1 1.5 2 2.5 3 3.5 40

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cent

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n (%

)

0 0.5 1 1.5 2 2.5 3 3.5 40

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ratio

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* Now at Chevron Corporation** Now at the University of Southern California

jj

iiji yx

yxs

//

, =jmj

imiji Bv

Bvs

Langmiur

=,

:

0

0

,

:

i

jji p

ps

IAS

=

02

02

0 φπφπ

φφ

Lr

VLr adsorption−=

24%CO2 / 76 %N2

46%CO2 / 54 %N2