Numerical Analysis of Reservoir Features and Injection Modes on Carbon Exchange Capacity during CO₂-ECBM Processes

CO2-enhanced coalbed methane recovery (CO2-ECBM) has been proposed to realize energy security and low-carbon emission reduction. CO2 injection into coals is a dynamic process involving the evolutions of gas kinetics, formation temperature, and mechanics. To deeply investigate the sensitivities of CO2 injection parameters (pressure Pinj and temperature Tinj) and initial formation parameters (water saturation Sw0 and permeability k0) on carbon exchange recovery efficiency, a thermo-hydro-mechanical coupling model was established, by considering the mutual effects of gas adsorption and migration, temperature, and effective stress on the pore-permeability evolution. Compared to the case of primary recovery, CO2-ECBM could well improve CH4 production because of its superiorities of competitive adsorption, driving force, and expansion stress. Different parameters had various sensitivities on the carbon exchange behaviors; for instance, the injected CO2 with a larger Pinj migrated with a greater gas coverage area, which deteriorated the formation permeability, and the decrease of CH4 content and the increase of CO2 content were positively correlated with Pinj, indicating that the displacement rate of the binary gas was mainly determined by diffusion pressure. The parameter of Tinj was not sensitive to the CH4 production because of the minor impacts of thermal conduction of coals on the displacement efficiency; thus, it is important to optimize Tinj selection to balance the costs and benefits. The efficiencies of CH4 recovery and CO2 storage were negatively affected by larger Sw0, which indicated that the residual water decreased the relative permeability of the gas phase and hindered the migration of the binary gas, and adding surfactants was proposed to decrease the residual water saturation. The coal formation with a larger k0 had more complex gas flow paths and increased the probability of carbon exchange. Finally, the economic evaluation of CO2-ECBM and the investment decision rules were also preliminarily studied. The results could provide some theoretical guidance for the engineering practice.