Mechanism of water adsorption and penetration at the coal interface under methane pressure and temperature

Injecting water into coal seams serves as an effective approach for mitigating coal mine disasters, with its effectiveness largely determined by the wettability of coal seams to aqueous solutions. This study integrates physical experiments with molecular dynamics simulations to investigate the macro and microscopic wettability characteristics of the coal–water–methane interface, focusing on the effects of high gas pressure and temperature in deep coal seams. Physical experiments measured the coal–water contact angle under a methane atmosphere, revealing that increased methane pressure elevates the contact angle, reducing water's wettability on coal, whereas higher temperature decreases the contact angle, enhancing wettability. Molecular dynamics simulations, employing a large-scale rough-wall coal–water–methane model, examined the adsorption and diffusion behaviors of water and methane molecules under varying methane pressures and temperatures. Increased methane pressure led to higher contact angles, decreased coal–water interaction energy, reduced hydrogen bond counts, lower relative concentrations of water molecules, and diminished mean square displacement of water molecules, collectively weakening the interaction between water molecules and coal macromolecules. Conversely, elevated temperature resulted in the opposite trends, promoting adsorption and diffusion of water molecules. Higher methane pressure hampers water molecule adsorption and penetration into coal pores due to methane occupying adsorption sites, thereby weakening water-coal interactions. In contrast, increased temperature suppresses methane adsorption, facilitating methane desorption and diffusion, thus creating space for water molecule adsorption and penetration. This study enhances the understanding of coal–water interface wettability, providing theoretical support for water injection technology in deep coal seams.