Influence of adsorbed water on diffusion coefficient and permeability of coal

Understanding the influence of water adsorption on gas transport properties is critical for optimizing coalbed methane (CBM) extraction. This study investigates the influence of adsorbed water on methane diffusion and apparent permeability in three coals representing sub-bituminous, bituminous, and anthracite ranks. The samples were selected as geologically representative end-members from major coalbed methane basins in China. Adsorbed water was introduced via humidity equilibration at 97% relative humidity, simulating residual moisture conditions typical of partially dewatered reservoirs. Pressure decay experiments show that under dry conditions, apparent permeability exhibits a non-monotonic variation with coal rank, with the medium-rank coal displaying the highest value among the three tested samples. Upon exposure to adsorbed water, apparent permeability decreases by 39% to 70% across all ranks, with greater suppression observed in low-rank and high-rank coals compared to the medium-rank sample. This rank-dependent response is attributed to differences in pore structure and water distribution associated with coalification history. While the findings are based on single samples per rank and reflect a simplified moisture condition, they provide mechanistic insight into how coal maturity modulates the sensitivity of gas transport to adsorbed water, offering implications for permeability modeling during the dewatering phase of coalbed methane recovery.