Objective With increasing demand for large-scale hydrogen storage and transport, underground hydrogen storage (UHS) has gained prominence due to its high capacity and low construction costs. However, the competitive adsorption mechanisms of hydrogen and cushion gases within clay mineral pores remain inadequately elucidated. This study clarifies these processes at the molecular level, providing a theoretical foundation for optimizing UHS operation strategies and cushion gas selection.

Methods A Na⁺-montmorillonite slit pore model was constructed using Grand Canonical Monte Carlo (GCMC) simulation. Adsorption behaviors were simulated under varying pressures, temperatures, pore sizes, cushion gas types, and mixing ratios. Key parameters—including hydrogen adsorption isotherms, molecular density distributions, and selectivity coefficients—were analyzed alongside visual characterization of equilibrium configurations. The spatial distribution and interaction mechanisms of gas molecules within the pores were directly demonstrated, providing more reliable microscopic explanations.

Results Hydrogen adsorption capacity increased with pressure and decreased with temperature, exhibiting no significant variation across pore sizes of 2.0–6.5 nm. Cushion gases notably reduced hydrogen adsorption, with nitrogen exerting more pronounced inhibitory effect than methane. In a H₂-N₂ mixture (60％:40％ molar fraction), hydrogen adsorption plateaued near 6 MPa due to intense competitive adsorption of nitrogen. Selectivity coefficients of methane and nitrogen relative to hydrogen declined with pressure but remained above 1, with nitrogen’s selectivity coefficient decreasing more rapidly.

Conclusion UHS facilities should adopt high-temperature and high-pressure operation strategies to minimize hydrogen adsorption losses in caprocks while maintaining sealing integrity. Nitrogen is superior to methane as a cushion gas for suppressing excessive hydrogen adsorption. From a selectivity coefficient perspective, increasing pressure may rapidly diminish montmorillonite’s hydrogen selectivity; the performance of different cushion gases at elevated pressures warrants further investigation. Although based on an idealized model, the revealed microscopic competitive adsorption mechanisms provide theoretical guidance for caprock sealing evaluation, cushion gas selection, and operational optimization in UHS engineering. Future research should prioritize developing complex models that better represent real geological conditions and conducting experimental validation to enhance engineering applicability.