Molecular-Level Mechanisms Governing Methane-Induced Changes in Coal Interfacial Wettability

Abstract Understanding the wettability evolution of coal–water interfaces under methane-bearing conditions is of critical importance for coal seam water injection, gas drainage, and coal–gas outburst prevention in deep coal formations. However, the microscopic mechanisms by which methane influences interfacial wettability remain poorly understood. In this study, molecular dynamics simulations were performed to construct a coal–water–methane three-phase system based on the Wiser coal model, and the effects of temperature (298–343 K) and methane pressure (0–9 MPa) on coal–water interfacial wettability were systematically examined. The results show that increasing temperature significantly enhances coal–water wettability, whereas increasing methane pressure produces the opposite effect. From a molecular perspective, elevated temperature strengthens coal–water interaction energy, increases hydrogen-bond density, promotes water accumulation at the interface, and enhances molecular mobility, thereby facilitating droplet spreading behavior. In contrast, increasing methane pressure weakens coal–water interaction energy and hydrogen bonding, reduces water enrichment at the interface, and suppresses water molecular diffusion, thereby leading to reduced wettability. Further analysis reveals that methane molecules competitively adsorb on active coal surface sites, occupying interfacial space and disrupting hydrogen-bond networks, thereby inhibiting water adsorption and spreading processes. Radial distribution function and density profile analyses further confirm that temperature enhances the interfacial ordering of water molecules, whereas methane pressure reduces interfacial structural stability. Overall, wettability evolution is governed by the coupled effects of interfacial interaction energy, hydrogen-bond network, molecular distribution, and diffusion behavior. This work provides a molecular-level understanding of methane-regulated coal wettability and offers theoretical guidance for improving coal seam water injection performance and gas extraction efficiency.