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.