Molecular Simulation of CH4 Desorption and CO2 Storage in Coal after Free CO2 Injection

Exploring the effect of varying temperatures, confining pressures, and slit pore sizes on the mechanism of CO2 replacing CH4 in coal reservoirs is critical for CO2 storage and coalbed methane (CBM) exploitation. Molecular simulation was used to construct the model of free CO2 injection into coal containing CH4. The mechanism of different functional group structures in coal affecting CO2 replacement of CH(4 )was analyzed. The CH4 reserve in coal along with the replacement of CH4 in coal by CO2 injection under various temperatures, confining pressures, and pore size conditions was comprehensively studied. The results indicated that the effect on the energy of CO2 replacing CH(4 )was greatest for the carboxyl group, followed by the hydroxyl, ether bond, amino, pyrrole, pyridine, sulfhydryl, thiophene and benzene ring groups. The reserve of CH4 and the ability of CO2 to replace CH4 were found to be limited by temperature and confining pressure. As the pore size exceeded 5 nm, the CH(4 )reserves and replacement ratio gradually decreased. The diffusion coefficient was proportional to temperature and slit pore size for adsorbed and free CH4 and CO2. The higher temperature enhanced the CO2 adsorption stability in the coal matrix. CO2 preferred to be stored in smaller slit pore sizes and was not sensitive to temperature and confining pressure. The findings furnish a theoretical foundation for enhancing CBM recovery and CO2 storage.