Adsorption and Diffusion Mechanism of CO₂/CH₄ in Bituminous Coal Pores: Molecular Dynamics Simulation of Coupling Effects of Multiple Factors on Enhanced Coalbed Methane Recovery Rate

CO2-enhanced coalbed methane recovery (CO2-ECBM) improves CH4 recovery and enables geologic CO2 sequestration. Using giant canonical Monte Carlo (GCMC) and molecule dynamics (MD) simulations, this study reveals nanoscale mechanisms of CO2/CH4 adsorption-diffusion in bituminous coal under temperature-pressure coupling. The research results show that CO2 adsorption exceeds CH4 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 CO2 adsorption. CO2 preferentially occupies low-energy sites, with adsorption selectivity increasing with pore size/pressure. High temperatures weaken adsorption but enhance CO2 displacement efficiency in macropores. The effect of temperature and pressure coupling enhancement accelerates the diffusion of CO2/CH4, but the strong adsorption between CO2 and coal matrix hinders its migration. Due to the weak adsorption and CO2 displacement, the diffusion coefficient of CH4 is significantly higher than that of CO2. CO2 Strong adsorption, weak diffusion and CH4 weak adsorption, strong diffusion are the basis of CO2-ECBM. Strategically leveraging CO2's adsorption advantage to displace CH4, while exploiting CH4's rapid diffusion through macropores, optimizes methane recovery. The findings provide molecular-scale insights for designing efficient CO2-ECBM in coal seams.