Water–Methane Competitive Adsorption and Displacement in Deep Coal: Molecular Insights from the Daji Coalbed Methane Block, Ordos Basin, China

Water introduced during hydraulic fracturing can profoundly modify methane retention and transport in deep coal reservoirs, but the molecular-scale linkage between competitive adsorption and water-driven displacement remains unclear. Deep No. 8 coal from the Daji Block, Ordos Basin, China, was characterized using multiscale pore-structure measurements and molecular-structure analyses, and an experimentally constrained coal model was established for molecular simulations. The coals are high-rank anthracites with vitrinite reflectance (Ro) values ranging from 2.63% to 2.95% (mean 2.76%), and micropores account for 68.44–88.20% of the pore volume. Simulations were conducted to quantify CH4 adsorption at moisture contents of 0–4 wt%, H2O–CH4 competitive adsorption, and pressure-driven water invasion in a 2 nm slit nanopore. Increasing moisture content reduced the maximum simulated CH4 adsorption capacity from approximately 27 to 10 cm3/g. In the binary system, H2O showed a pronounced adsorption preference over CH4 because of its stronger affinity for oxygen-containing polar sites and nanopore walls. Pressure-driven simulations at 5, 10, 15, and 20 MPa showed progressively stronger water invasion and CH4 depletion with increasing driving force. Under a pressure difference of 10 MPa, water preferentially accumulated along the coal walls and displaced CH4 toward the pore center and displacement front, decreasing the CH4 density in the invaded region to approximately 24% of its initial value. These results reveal a dual control of water on methane behavior in deep coal: pre-existing moisture suppresses CH4 adsorption through preferential site occupation, whereas pressure-driven water invasion promotes CH4 desorption, redistribution, and displacement. This study provides a molecular-scale framework for understanding gas–water interactions governing methane retention and mobilization in deep coal nanopores.