Gas adsorption and desorption and displacement has a significant effect on coal deformation and permeability evolution during the primary recovery of coalbed methane (CBM) and enhanced coalbed methane recovery (ECBM). The objectives are to (1) quantify the coal deformation and permeability change caused by methane (CH 4 ) displacement with carbon dioxide (CO 2 ) and ( 2) model the transportation of CH 4 and CO 2 in deformed coalbed. In this study, the gas adsorption and desorption and displacement, coal deformation, and permeability evolution during CBM and ECBM recovery were described by an internally consistent adsorption-strain-permeability model, of which the simplified local density (SLD) adsorption theory, a theoretical strain model, and a matchstick-based permeability model were rigorous coupled. The coupled model was then verified with all of the CH 4 and CO 2 measured gas adsorption and desorption and coal strain data published in the past 60 years. Next, sensitivity analysis was further conducted on the coupled model to highlight and calibrate its performance. Finally, the coupled model was integrated into the Transport of Unsaturated Groundwater and Heat Simulator (TOUGH2) to simulate the ECBM process. The results show that the coupled model can simultaneously describe gas adsorption and desorption and displacement, coal deformation, and permeability evolution during ECBM recovery with only six parameters, including slit width, solid−solid interaction potential energy parameter, surface areas of CH 4 and CO 2 , adsorption expansion modulus, and initial porosity. The coupled model can predict both CH 4 and CO 2 adsorption and the induced coal deformation fairly accurately at a pressure up to 20 MPa, and the average relative errors are within 9.76% and 9.14%, respectively. The results also suggest that the adsorption capacity of CO 2 is 2−5 times as large as that of CH 4 , and the volumetric strain induced by CO 2 adsorption is 2−8 times as large as that caused by CH 4 adsorption. While the stronger adsorption capacity of CO 2 on coal offers an option for CO 2 -ECBM, matrix swelling due to CH 4 displacement with CO 2 may narrow down or even close the cleat, significantly reducing the permeability and thus impacting the injection efficiency. Last but not least, the original TOUGH2 simulator predicts similar results with several other CBM simulators. However, it is impossible that 90% of CH 4 can be displaced within 90 days. Considering the coal deformation and permeability change due to CH 4 displacement with CO 2 , the modified TOUGH2 simulator shows that only 24% of CH 4 is displaced in the first 90 days, and it takes about 1800 days to displace 90% or more. Advances in the understanding of CH 4 displacement by CO 2 and their transportation mechanisms in coal seams suggests that the success of CO 2 -ECBM depends on the optimal management of matrix swelling.

Recovery of coalbed methane (CBM) can trigger a series of coal-gas interactions, including methane desorption, coal deformation, and associated permeability change. These processes may impact each other. A primary objective of this analysis is to simultaneously quantify these interactions and their impacts during CBM recovery. To achieve this and other objectives, a rigorously coupled adsorption–strain–permeability model was developed. Gas adsorption, coal deformation, and cleat permeability characteristics were described using simplified local density (SLD) adsorption theory, a theory-based strain model, and matchstick-based permeability models, respectively. The strain model was verified against measured methane-adsorption-induced coal strain data published during the past 60 years, and the coupled model was tested using well test data measured in the San Juan Basin of New Mexico in the United States. Results suggest that the strain model is very consistent with measured coal deformation data for fluid pressure up to 80 MPa, and the average relative errors between measured and predicted results are all <15.51%. Generally, simulated volumetric strain first increases then decreases with pressure, and the maximum volumetric strain occurs at a pressure of ∼20 MPa, which is strikingly different from the pressure corresponding to maximum Gibbs adsorption. Simulation results also suggest that methane adsorption/desorption, coal deformation, permeability evolution, and their coupled impacts are quantitatively reasonable and consistent with observed data. Several cleat permeability models were coupled and tested, and the improved Palmer and Mansoori model exhibited the best performance among those tested. Since methane sorption-induced volumetric strain of typical San Juan Basin coal is 5–9 orders of magnitude larger than that due to reservoir compaction, matrix shrinkage dominates and leads to a monotonic increase of permeability during CBM recovery.

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Carbon dioxide (CO 2 ) geological sequestration and coal-bed methane (CBM) recovery in deep coal seams are usually operated with a pressure higher than 10 MPa. The adsorption mechanisms of methane (CH 4 ) and CO 2 on coals in such a situation, however, are not yet revealed. With the help of a high-pressure gas adsorption system, CH 4 and CO 2 adsorption isotherms were first conducted on two coal samples. Simplified local density (SLD) theory was then tailored and applied to describe the adsorption characteristics of specific CH 4 and CO 2 on coals. Next, the adsorption mechanisms of high-pressure CH 4 and CO 2 on coal samples were revealed on the basis of adsorbed and bulk density distributions within the matrix pores. The results show that the high-pressure gas adsorption on coals is different from that at low pressure. The excess adsorption capacity first increases and then decreases with pressure, and the maximum value occurs at a specific pressure referring to reverse pressure. The maximum excess adsorption capacity of CH 4 is less than that of CO 2 , while its reverse pressure is greater than that of CO 2 . The reversal of excess adsorption with pressure depends on the relative increase in adsorption and bulk phases. However, the mechanisms of adsorption reversal vary with gas types. The reversal of CH 4 excess adsorption is due to the saturation near the pore wall, while the CO 2 excess adsorption reversal is due to the dramatic bulk density changes near the critical point. Advances in the mechanism of high-pressure gas adsorption on coals suggests that coal seams deeper than 1000 m have more recovery potential, and CO 2 -enhanced coal-bed methane recovery success depends on the optimal management of its injection pressure and the associated coal swelling.

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