Hydrogen Geostorage in Organic-Rich Shales: Critical Insights Into the Role of Cushion Gas in Hydrogen Accumulation within Organic Nanoporous Systems

Abstract

Depleted shale formations have the potential for hydrogen geostorage. The storage mechanisms, however, are complex and influenced by several factors including mineralogy, pore size distribution, residual hydrocarbons in place, and the choice of cushion gas. This study aims to investigate hydrogen distribution within this multiscale pore system, with a focus on understanding how hydrogen accumulates in the organic nanoporous network. Such insights are critical for the long-term storage and recovery assessments. Using molecular simulations, representative organic matter comprising nanoporous kerogen and nanopores of different sizes was constructed. Hydrogen intake of the organic system in the presence of residual amount of natural gas was quantified, considering multiple hydrogen injection scenarios. Despite stronger chemical affinity toward natural gas, hydrogen accumulated in all pore sizes, even the smallest, potentially beneficial for long-term storage but hindering rapid recovery. Moreover, the study was extended to investigate the role of cushion gas in the accumulation of hydrogen in organic structures. It was found that introducing cushion gases, such as methane and carbon dioxide, reduces hydrogen intake in the nanopores, with carbon dioxide being the most effective due to its stronger attraction to kerogen. Nitrogen, on the other hand, had relatively lower impact. The results were consistent with the observed trends in the analysis of the nonbonding energy of all systems. The results reported in this study provide critical insights into the factors influencing hydrogen accumulation in the organic constituents of shale formations for an optimized design of hydrogen geostorage in depleted shale gas reservoirs.