Methane occurrence and diffusion behavior in coal nanopores under deep burial conditions: Effects of pore size and water content

The occurrence and transport behavior of methane (CH4) in coal nanopores under deep burial conditions remain insufficiently understood at the molecular scale, which limits the accurate evaluation and efficient development of deep coalbed methane (CBM) reservoirs. In this study, molecular simulations were employed to investigate CH4 occurrence and diffusion in bituminous coal nanopores, focusing on the coupled effects of burial depth, pore size, and water content. A coal nanopore model based on the Wiser structure was constructed, and grand canonical Monte Carlo and molecular dynamics simulations were performed. The results show that deep-seam conditions significantly enhance CH4 adsorption despite elevated temperature, as the promoting effect of pressure outweighs thermal weakening. Meanwhile, CH4 redistributes from wall-dominated adsorption layers toward the pore interior, forming a more bulk-like state with enhanced mobility. Increasing pore size further promotes CH4 accumulation in the pore center, improving both storage capacity and potential deliverability. Water suppresses CH4 adsorption through competitive occupation of adsorption sites and reduction of accessible pore volume. At low water content, water slightly enhances CH4 mobility, whereas higher water content inhibits both adsorption and diffusion. These inhibitory effects are partially weakened under deep-seam conditions. This study provides molecular-scale insights into CH4 occurrence and transport in deep coal nanopores, supporting the assessment of deep CBM storage capacity.