Adsorption and diffusion behavior of pure H₂, and H₂/CO₂, H₂/CH₄ mixtures in sandstone-rich clay shale reservoirs: Insights from molecular dynamics simulations

Underground porous formations offer promising potential for hydrogen (H₂) and carbon dioxide (CO₂) storage, aiding the energy transition and decarbonization goals. Effective underground hydrogen storage (UHS) depends on selecting an appropriate cushion gas, typically CO₂ or methane (CH₄), to control pressure, minimize H₂ loss, and enhance injectivity and productivity. This study uses molecular dynamics simulations to analyze the adsorption and diffusion behaviors of pure H₂ and H₂/CH₄ and H₂/CO₂ mixtures in slit nanopores of kaolinite and silica with different surface morphologies under subsurface conditions. Kaolinite exhibited the lowest H₂ diffusivity due to higher surface adsorption and interfacial gas density on its hydrophobic surface. In contrast, the Q² silica morphology (ionized with Na and OH) showed reduced interfacial density and adsorption, enabling higher H₂ diffusivity. CO₂ had the strongest surface affinity and served as a more effective diffusion barrier than CH₄ with 25 % cushion gas significantly reduced H₂ mobility. Overall, kaolinite-rich caprocks offer better sealing efficiency, while silica-rich formations (resembling Q⁴ topology) may favor hydrogen accessibility and recovery during injection and production phases. These results underscore the influence of mineral type and cushion gas on hydrogen transport and confinement. Integrating these molecular-level insights with pore- and reservoir-scale models is essential for optimizing hydrogen recovery and ensuring long-term sealing in UHS operations.