Influence law of pore water storage characteristics on the gas adsorption characteristics of coal

This study mainly investigates the influence of pore water characteristics on the adsorption properties of coalbed methane through integrated low field nuclear magnetic resonance (LF-NMR), adsorption experiments, and molecular dynamics (MD) simulations. Pore water states in three coal ranks were characterized during progressive hydration. Multi-scale analysis revealed how pore water evolution regulates methane adsorption processes. During the diffusion-dominated stage (M2–M3), adsorbed water penetrates into the micropores. In the highly wettable brown coal (L1), the adsorbed water content reaches 2.12 g while in the anthracite (A1), it is only 0.29 g. During the active water injection stage (M4–M6), non-adsorbed water dominates in anthracite (over 85% of the total water content of 4.01 g), while adsorbed water remains dominant in lignite (over 60% of the total water content of 3.52 g). Water content plays a key role in methane adsorption in coal. During the water addition phase, the influence of methane adsorption on medium-to-low-rank coal is relatively weak, while the methane adsorption capacity of high-rank coal A1 shows a significant decrease during both the water diffusion and water addition phases, corresponding to a reduction in Langmuir volume of 21.22 cm3/g. Molecular dynamics (MD) results further show that the free energy between molecules on the surface of hydroxyl-modified coal increases, with hydroxyl groups driving electrostatic interactions between coal and water molecules. Increased steric hindrance inhibits hydrogen bond formation and reduces the rate of hydrogen bond growth. There is a significant correlation between pore water content and coal-water molecular interaction energy, which cross-scale validates the results of LF-NMR testing and MD simulations.