Impacts of relative permeability hysteresis, wettability, and injection/withdrawal schemes on underground hydrogen storage in saline aquifers

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• Hydrogen (H 2 ) - brine relative permeability hysteresis results in a lower H 2 withdrawal factor but a higher H 2 withdrawal purity. • A more water-wet rock leads to a lower H 2 withdrawal factor. • A larger H 2 withdrawal rate results in a higher H 2 withdrawal factor but with increased water production. • Underground hydrogen and carbon dioxide geo-storage demonstrate significant differences. Underground hydrogen storage (UHS) is a key strategy in the implementation of a large-scale hydrogen (H 2 ) economy and promotion of renewable energy development/utilization. For UHS in water-wet saline aquifers, H 2 displaces in-situ brine during injection; during well shut-in and H 2 withdrawal, brine imbibes back into the flow paths where it displaces some H 2 . These processes are influenced by H 2 -brine transport physics, H 2 -brine-rock interactions and injection/withdrawal schemes, which, in turn, determine H 2 storage capacities and injection/withdrawal efficiency. However, these effects are poorly understood. Therefore, this work focuses on the impact of relative permeability hysteresis (RPH), wettability, and H 2 withdrawal rate on UHS performance in a saline aquifer. Furthermore, differences between UHS and CO 2 geo-storage (CGS) are examined. The primary findings include: 1) RPH results in a smaller H 2 withdrawal factor ( H 2 - W F ), but a larger H 2 withdrawal purity ( H 2 - W P ); 2) H 2 - W F increases with rock hydrophobicity, while H 2 - W P is mostly insensitive to rock wettability; 3) under similar storage conditions, H 2 - W F and H 2 - W P are approximately 10% less than CO 2 - W F and CO 2 - W P . These insights demonstrate the significance of RPH and rock wettability on UHS performance and provides guidance on H 2 injection/withdrawal scheme optimization. This study aids in the implementation of an industry-scale hydrogen economy.