Adsorption and Diffusion Mechanism of CO 2 /CH 4 in Bituminous Coal Pores: Molecular Dynamics Simulation of Coupling Effects of Multiple Factors on Enhanced Coalbed Methane Recovery Rate

CO 2 -enhanced coalbed methane recovery (CO 2 -ECBM) improves CH 4 recovery and enables geologic CO 2 sequestration. Using giant canonical Monte Carlo (GCMC) and molecule dynamics (MD) simulations, this study reveals nanoscale mechanisms of CO 2 /CH 4 adsorption–diffusion in bituminous coal under temperature–pressure coupling. The research results show that CO 2 adsorption exceeds CH 4 due to stronger electrostatic interactions with coal matrix. One nm pores dominate the low-pressure adsorption process, while 3/5 nm pores are conducive to high-pressure multilayer CO 2 adsorption. CO 2 preferentially occupies low-energy sites, with adsorption selectivity increasing with pore size/pressure. High temperatures weaken adsorption but enhance CO 2 displacement efficiency in macropores. The effect of temperature and pressure coupling enhancement accelerates the diffusion of CO 2 /CH 4, but the strong adsorption between CO 2 and coal matrix hinders its migration. Due to the weak adsorption and CO 2 displacement, the diffusion coefficient of CH 4 is significantly higher than that of CO 2 . CO 2 Strong adsorption, weak diffusion and CH 4 weak adsorption, strong diffusion are the basis of CO 2 -ECBM. Strategically leveraging CO 2 ’s adsorption advantage to displace CH 4, while exploiting CH 4 ’s rapid diffusion through macropores, optimizes methane recovery. The findings provide molecular-scale insights for designing efficient CO 2 -ECBM in coal seams.