Mechanisms of Water Effects on Methane Adsorption and Diffusion in Low-Rank Coal Nanopores: Experimental Characterization and Molecular Simulation

During coalbed methane production, water plays a crucial role in methane adsorption and diffusion; however, the adsorption and diffusion of methane in water-bearing low-rank coal remain difficult to characterize synergistically within a unified molecular-scale framework, leaving the mechanism by which water affects methane poorly understood. To address this issue, this study focuses on low-rank coal from the Anjialing Mine in the Ningwu Coalfield, China, and integrates experimentally constrained molecular modeling with analyses of adsorption energy, density distribution, and mean square displacement (MSD). Low-pressure CO2 adsorption and low-temperature N2 adsorption were used to characterize the pore structure, while XPS, FTIR, and solid-state 13C NMR were employed to constrain the two-dimensional molecular structure, based on which a three-dimensional macromolecular model and a 1 nm nanopore model were constructed. At 303.15 K and 0–50 MPa, a dry methane single-component system and a methane-water competitive system were compared in terms of adsorption capacity, adsorption-energy probability distribution, spatial density distribution, and MSD. The results show that micropores smaller than 1 nm contribute more than 75% of the pore volume and 91% of the specific surface area of AJL coal. In the dry system, methane adsorption increases rapidly below 10 MPa and then slows gradually; the adsorption energy is mainly concentrated around −10 kcal/mol, and the methane density distribution within the pores is relatively ordered. After water is introduced, methane adsorption decreases by approximately two-thirds, the adsorption-energy distribution broadens to −35 to 0 kcal/mol, and the heterogeneity of the density distribution increases; with increasing pressure, methane adsorption increases whereas water adsorption decreases. Short-time MD comparisons show that water molecules preferentially associate with polar oxygen-containing functional groups and form low-mobility clusters. The resulting decrease in methane MSD, particularly in the 5 MPa high-water-loading configuration, indicates reduced accessible migration space. By linking characterization of the same AJL coal sample with GCMC–MD analysis, this study connects water-induced changes in methane uptake, adsorption-energy distribution, spatial distribution, and short-time mobility within the modeled 1 nm pore, providing a molecular-scale interpretation of methane–water competition.