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.