Storage integrity during underground hydrogen storage in depleted gas reservoirs

The transition from fossil fuels to renewable energy sources, particularly hydrogen, has emerged as a central strategy for decarbonization and the pursuit of net-zero carbon emissions. Meeting the demand for large-scale hydrogen storage, a crucial component of the hydrogen supply chain, has led to the exploration of under-ground hydrogen storage as an economically viable solution to global energy needs. In contrast to other sub-surface storage options such as salt caverns and aquifers, which are geographically limited, depleted gas reservoirs have garnered increasing attention due to their broader distribution and higher storage capacity. However, the safe storage and cycling of hydrogen in depleted gas reservoirs require the preservation of high stability and integrity in the caprock, reservoir, and wellbore. Nevertheless, there exists a significant gap in the current research concerning storage integrity in underground hydrogen storage within depleted gas reservoirs, and a systematic approach is lacking. This paper aims to address this gap by reviewing the primary challenges associated with storage integrity, including geochemical reactions, microbial activities, faults and fractures, and perspectives on hydrogen cycling. The study comprehensively reviews the processes and impacts, such as abiotic and biotic mineral dissolution/precipitation, reactivation and propagation of faults and fractures in caprock and host-rock, wellbore instability due to cement degradation and casing corrosion, and stress changes during hydrogen cycling. To provide a practical solution, a technical screening tool has been developed, considering controlling variables, risks, and consequences affecting storage integrity. Finally, this paper highlights knowl-edge gaps and suggests feasible methods and pathways to mitigate these risks, facilitating the development of large-scale underground hydrogen storage in depleted gas reservoirs.