Role of geochemical reactions on caprock integrity during underground hydrogen storage

Underground hydrogen storage in depleted gas reservoirs is a promising and economical option for large-scale renewable energy storage to achieve net-zero carbon emission. While caprock plays an important role in sealing capacity, current knowledge is still limited on the effect of H2-brine-rock geochemical interactions on caprock integrity, raising concerns about the viability of long-term UHS. To address this problem, we developed kinetic batch models to characterize the time-dependent redox-reactions which are unique for underground hydrogen storage. This is combined with analytical estimates for the extent of hydrogen penetration into caprock. Our results show that the dissolution degrees of all tested minerals in three types of shales are <1 % in 30 years, indicating a strong caprock integrity and containment ability during underground hydrogen storage from a geochemical perspective. Reactive transport calculations indicate that hydrogen only affects a few metres of the caprock above the reservoir, so that storage integrity of thick caprocks will be unaffected. Similarly, the overall amount of hydrogen penetrating into caprock is likely to be a tiny fraction of the amount stored, typically much <1 %. Overall, our results suggest that H2-brine-shale geochemical interactions may not compromise caprock integrity during underground hydrogen storage.