Molecular Simulation of H2/CH4 Competitive Adsorption and Diffusion in Representative Reservoir Minerals: Implications for Hydrogen Storage in Depleted Gas Reservoirs

Underground hydrogen storage in depleted gas reservoirs has emerged as a key technology for large-scale hydrogen energy storage. However, the occurrence state of hydrogen is governed by residual methane and reservoir mineral surfaces. This study combines grand canonical Monte Carlo and molecular dynamics simulations to investigate the competitive adsorption and diffusion of hydrogen and methane in quartz, kaolinite, and calcite. The results show that at low methane contents (<0.2), the adsorption capacity of hydrogen is primarily governed by its interaction with the mineral surface, exhibiting the strongest adsorption in calcite and the weakest in quartz, whereas the diffusivity follows the opposite trend. As the methane content increases, the occurrence state of hydrogen becomes increasingly controlled by the distribution of methane. The preferential occupation of surface adsorption sites by methane substantially suppresses hydrogen adsorption in quartz and calcite, whereas the abundant methane remaining in the bulk region of kaolinite imposes the strongest restriction on hydrogen diffusion. Increasing temperature promotes methane desorption, releasing interfacial adsorption sites, restoring the near-wall hydrogen adsorption peak, and enhancing hydrogen diffusivity, whereas increasing pressure shifts hydrogen from interfacial adsorption at low pressures to bulk-phase filling at high pressures while reducing its diffusivity.