Molecular Simulation Elucidation of Gas Adsorption and Desorption Mechanisms in Coal Micropores: Synergistic Effects of Pore Size and CO₂ Injection on CH₄ Recovery

In order to study the adsorption and desorption mechanisms of gas in micropores, a coal matrix model incorporating microporous structures is proposed in this paper. The adsorption characteristics of CH₄ and CO₂ in 1, 2, and 5 nm pore size models and the effect of CO₂ injection on CH₄ desorption in models containing adsorbed methane were studied. The results show that in different pore size models at the same temperature and pressure the adsorption capacity for CO₂ exceeds that of CH₄. The capacity for gas adsorption diminishes as temperature rises and enhances as pressure increases. With the increase of pore size, the coal's maximum CH₄ adsorption capacity dropped from 5.96 to 5.89 mmol/g, while its maximum CO₂ adsorption capacity declined from 7.39 to 6.28 mmol/g. At low pressure (<2 MPa), CH₄ and CO₂ molecules were mainly adsorbed in coal macromolecules. As pressure rises, the nanopores exhibit a marked increase in their capacity to adsorb gas.CO₂ which mainly drives CH₄ by occupying adsorption sites. With the increase in temperature, the total energy of the CH₄-CO₂ system's initial model rose from -759.98 to -628.44 kcal/mol, while the total energy of the desorption model climbed from -698.37 to -483.67 kcal/mol. After injection of CO₂ into different models, the relative concentration of CH₄ in pores increases, and as the pore size increases, the impact of gas injection desorption is enhanced. The results show that pore size and CO₂ injection have a certain synergistic effect on CH₄ desorption in coal models containing adsorbed methane.