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 (H 2 ) and carbon dioxide (CO 2 ) storage, aiding the energy transition and decarbonization goals. Effective underground hydrogen storage (UHS) depends on selecting an appropriate cushion gas, typically CO 2 or methane (CH 4 ), to control pressure, minimize H 2 loss, and enhance injectivity and productivity. This study uses molecular dynamics simulations to analyze the adsorption and diffusion behaviors of pure H 2 and H 2 /CH 4 and H 2 /CO 2 mixtures in slit nanopores of kaolinite and silica with different surface morphologies under subsurface conditions. Kaolinite exhibited the lowest H 2 diffusivity due to higher surface adsorption and interfacial gas density on its hydrophobic surface. In contrast, the Q 2 silica morphology (ionized with Na and OH) showed reduced interfacial density and adsorption, enabling higher H 2 diffusivity. CO 2 had the strongest surface affinity and served as a more effective diffusion barrier than CH 4 with 25 % cushion gas significantly reduced H 2 mobility. Overall, kaolinite-rich caprocks offer better sealing efficiency, while silica-rich formations (resembling Q 4 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. • Geological surface chemistry controls H 2 , CH 4 , and CO 2 adsorption in rock nanopores. • CO 2 shows highest affinity and displaces H 2 more than CH 4 in all rock formations. • 25 % cushion gas reduces H 2 diffusion; higher ratios show minimal change. • H 2 diffusion is lowest in kaolinite, linked to surface adsorption and retention. • Findings guide UHS optimization and cushion gas selection in shale formations.