Insights into Underground Hydrogen Storage Challenges: A Review on Hydrodynamic and Biogeochemical Experiments in Porous Media

Porous geologic reservoirs, including saline aquifers and depleted oil and gas reservoirs, are gaining attention as solutions to underground hydrogen storage (UHS). While porous reservoirs offer large capacities and are widely available, technical questions surround their ability to retain hydrogen (H2) at high purity through injection-withdrawal cycles. This review centers on recent experimental advancements in understanding critical hydrodynamic and biogeochemical processes that govern the feasibility of UHS in porous reservoirs. Hydrodynamic and biogeochemical processes are key to optimizing UHS performance, since they control hydrogen transport in porous media and interactions with the host rock, fluids, and microbes, respectively. Hydrodynamic experiments to date confirm that capillary fingering dominates in multiphase H2-brine systems typifying UHS in saline aquifers, which can contribute to loss of H2 through residual trapping. Mixing is particularly relevant for UHS in depleted oil/gas reservoirs, where the choice of cushion gas is key for minimizing H2 purity loss. While experiments indicate that geochemistry alone will likely have negligible influence on reservoir properties governing H2 retention and purity, coupled geochemical interactions and microbial activity (i.e., biogeochemistry) that can influence UHS operations yet remains underexplored. Recent experimental studies demonstrate that microbial activity in UHS environments can lead to biofilm growth, which can affect hydrogen recovery through pore clogging and/or wettability changes at pore surfaces. In all, the feasibility of UHS in porous reservoirs requires a deeper understanding of these factors and interconnections among them across conditions anticipated throughout H2 injection, storage, and withdrawal in heterogeneous porous reservoirs. We emphasize critical research directions toward closing these knowledge gaps, where experimental insights are needed both to understand how coupled hydrodynamic and biogeochemical effects influence H2 recoverability and purity as well as to improve accuracy in upscaled reservoir simulations that will be critical to ascertaining the long-term viability of prospective UHS sites.