Impact of heterogeneity, rock-fluid interactions, and cyclic loading on reservoir rock mechanical integrity during underground hydrogen storage

Green hydrogen production from renewable energy sources and its subsequent storage in depleted hydrocarbon reservoirs is considered a sustainable solution to carbon emission-based environmental issues. Among many challenges associated with this process, multi-scale heterogeneity in the reservoir rocks, ranging from microscopic grains to macroscopic faults, emerges as a pressing concern. Heterogeneities are weak zones, and their presence reduces the reservoir rock's strength while intensifying its mechanical degradation from biogeochemical reactions and cyclic loading during underground hydrogen storage (UHS). Different accessory minerals introduce mineralogical heterogeneity to the reservoir rock. Their geochemical reactions with injected hydrogen cause mineral dissolution/precipitation, altering the rock structure and affecting its mechanical integrity. Concurrently, cyclic hydrogen injection and production periodically vary the reservoir rock's effective stress, creating local stress concentrations in its heterogeneous structures and accelerating rock failure. Faults, a key structural heterogeneity in the reservoirs, can be reactivated by increased pore pressure from hydrogen injection, inducing seismicity. Fault slip risk is also increased by water from microbial reactions in UHS, which lubricates the faults, reducing the fault's friction and shear strength. Beyond amplifying the mechanical weakening, heterogeneities make the reservoir rock's mechanical response complex and unpredictable. The mechanical integrity of the reservoir rock during UHS, concerning its heterogeneous nature, remains under-explored. Even the related studies present limited replication of the subsurface storage conditions. Considering this knowledge gap, the review aims to synthesize existing knowledge on the mechanical integrity of heterogeneous reservoir rocks during UHS in depleted hydrocarbon reservoirs with recommendations for improving future experimental studies.