Novel Insights into Gas Sorption Mechanisms in Multiscale Coal Nanopores via Molecular Simulation

The sorption and diffusion dynamics of gases in nanoscale coal pores are crucial for understanding and optimizing coalbed methane recovery. This study developed four nanoscale molecular models with pore sizes of 1, 2, 5, and 10 nm, based on X-ray photoelectron spectroscopy (XPS) and 13C nuclear magnetic resonance (13C-NMR) analyses of target coal samples. The research systematically explored how temperature, pressure, and moisture content affect the transport of CO2, CH4, and N2 in multiscale nanopores. Findings reveal that the overlapping sorption potential fields significantly enhance gas sorption on nanopore surfaces, accounting for the predominant storage of gases in these pores, and this effect weakens as pore size increases. Within the temperature range of 273.15 to 313.15 K, variations in pore size exert a more pronounced influence on gas sorption capacity than temperature itself. Furthermore, the presence of water in coal nanopores leads to capillary condensation, which obstructs pore channels and reduces gas sorption rates. This inhibitory effect is particularly significant in micropores smaller than 2 nm, while it has a minimal impact on mesopores. The study highlights that modifying nanoscale pore structures and effectively removing water from micropores could substantially enhance coalbed methane production efficiency, providing valuable insights for optimizing gas recovery strategies.