Coupled thermal-hydrological-mechanical modeling of CO2-enhanced coalbed methane recovery

CO2-enhanced coalbed methane recovery, also known as CO2-ECBM, is a potential win-win approach for enhanced methane production while simultaneously sequestering injected anthropogenic CO2 to decrease CO2 emissions into the atmosphere. In this paper, CO2-ECBM is simulated using a coupled thermal-hydrological-mechanical (THM) numerical model that considers multiphase (gas and water) flow and solubility, multi component (CO2 and CH4) diffusion and adsorption, heat transfer and coal deformation. The coupled model is based on the TOUGH-FLAC simulator, which is applied here for the first time to model CO2-ECBM. The capacity of the simulator for modeling methane production is verified by a code-to-code comparison with the general-purpose finite-element solver COMSOL. Then, the TOUGH-FLAG simulator is applied in an isothermal simulation to study the variations in permeability evolution during a CO2-ECBM operation while considering four different stress-dependent permeability models that have been implemented into the simulator. Finally, the TOUGH-FLAC simulator is applied in non-isothermal simulations to model THM responses during a CO2-ECBM operation. The simulations show that the permeability evolution, mechanical stress, and deformation are all affected by changes in pressure, temperature and adsorption swelling, with adsorption swelling having the largest effect. The calculated stress changes do not induce any mechanical failure in the coal seam, except near the injection well in one case of a very unfavorable stress field.