New insights into hydrogen sorption mechanism in shale nanocomposites and the role of cushion gas in geological hydrogen storage

Depleted shale gas reservoirs represent a promising site for large-scale underground H₂ storage. However, the mechanism of H₂ sorption in the nanopores of depleted shale gas reservoirs and the influence of cushion gas on H₂ sorption remain unclear. In this study, a molecular model of the composite nanopore was constructed to consider the competitive interactions between different shale rock constituents. The grand canonical Monte Carlo method was utilized to investigate the sorption behaviors of pure H₂ as well as H₂–CH₄ and H₂–CO₂ mixtures. The microscopic mechanism governing pure H₂ sorption in the shale nanocomposite was elucidated, and the influence of CH₄ and CO₂ as cushion gases on H₂ sorption was elaborated. The results show that despite the relatively low sorption strength of H₂, the absolute sorption amount of H₂ is significant due to its small molecular size, linear shape, and large adsorption layer thickness. The presence of CH₄/CO₂ facilitates the formation of a transition adsorption layer for H₂, which inhibit H₂ adsorption on the shale pore surfaces. CO₂ can markedly reduce the H₂ sorption amount in the kerogen matrix and on the quartz surface, exhibiting a more pronounced impact on H₂ sorption than that of CH₄. CH₄ as a cushion gas is more conducive to H₂ storage in deep shale reservoirs, while CO₂ is more appropriate for H₂ storage in shallow shale reservoirs. A high concentration of cushion gas can reduce the preferential sorption capacity of CH₄/CO₂ over H₂, while also reducing the purity of the recovered H₂, thus hindering the H₂ storage performance of shale gas reservoirs. This study advances the understanding of the role of cushion gas in H₂ geological storage and further refines the theory for H₂ storage in depleted shale gas reservoirs.