Impacts of relative permeability hysteresis, wettability, and injection/withdrawal schemes on underground hydrogen storage in saline aquifers
Underground hydrogen storage (UHS) is a key strategy in the implementation of a large-scale hydrogen (H2) economy and promotion of renewable energy development/utilization. For UHS in water-wet saline aquifers, H2 displaces in-situ brine during injection; during well shut-in and H2 withdrawal, brine imbibes back into the flow paths where it displaces some H2. These processes are influenced by H2-brine transport physics, H2-brine-rock interactions and injection/withdrawal schemes, which, in turn, determine H2 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 H2 withdrawal rate on UHS performance in a saline aquifer. Furthermore, differences between UHS and CO2 geo-storage (CGS) are examined. The primary findings include: 1) RPH results in a smaller H2 withdrawal factor (H2-WF), but a larger H2 withdrawal purity (H2-WP); 2) H2-WF increases with rock hydrophobicity, while H2-WP is mostly insensitive to rock wettability; 3) under similar storage conditions, H2-WF and H2-WP are approximately 10% less than CO2-WF and CO2-WP. These insights demonstrate the significance of RPH and rock wettability on UHS performance and provides guidance on H2 injection/withdrawal scheme optimization. This study aids in the implementation of an industry-scale hydrogen economy.