Underground hydrogen storage in depleted reservoirs: A critical review of research evolution and governing subsurface processes

Underground hydrogen storage (UHS) in depleted hydrocarbon reservoirs is a study of finding its way into the large-scale integration of renewable energy sources with energy decarbonization plans. The core of this review is to integrate bibliometric analysis and mechanics insights to advance the understanding of UHS and support its progress towards technical maturity. A Scopus-based review of 1543 publications published between 2009 and 2025 indicates a marked increase in UHS research output, with annual publication counts showing clear acceleration after 2015 and stronger growth after 2020. This growth reflects expanding scholarly activity across the UHS research community, although it should not be interpreted as evidence that UHS has reached the technological maturity of established underground gas storage (UGS) systems. Three interdisciplinary themes have emerged in the bibliometric landscape: geochemical interactions between hydrogen, rock, and brine; pathways of microbial consumption and conversion; and the wellbore and caprock geomechanical integrity. Mechanically, the characteristics of hydrogen, namely, high diffusivity, low viscosity and redox reactivity, present challenges in depleted reservoirs, where fluids, mineralogical heterogeneity, and active microbes affect the storage performance, retention of purity, and cyclic efficiency. Pilot studies emphasize the need to optimize cushion gas, conduct predictive compositional modelling, and conclude extensive characterization to guarantee operational and recovery fidelity. Despite these developments, critical knowledge gaps still exist, especially in geochemical andgeo-mechanical coupling over the long term, hydrogen-induced material degradation, pore-scale transport dynamics, and quantitative microbial metabolism in reservoir conditions. Future research should focus on long cyclic field testing, the development of hydrogen-incompatible wellbore materials, microbiologically guided management of reservoirs, and sophisticated reactive-transport-geo-mechanical operating models. This review provides a consistent and practical basis for improving the reliability, scalability, and security of UHS in depleted reservoirs. Despite this progress, UHS remains at a lower technology readiness level than conventional UGS, with limited large-scale deployment and ongoing uncertainties regarding subsurface behaviour and long-term storage integrity.