A holistic review of the risks during underground hydrogen storage

Green hydrogen is increasingly recognised as a key energy carrier for supporting net-zero energy systems, but its large-scale deployment requires safe, efficient, and flexible storage solutions. Underground hydrogen storage (UHS) has emerged as a promising option due to its large capacity and potential cost advantages; however, it introduces a range of geological, operational, and material-related risks that remain incompletely understood. This study presents a comprehensive review of the risks associated with UHS, with particular emphasis on hydrogen leakage, loss of hydrogen purity, and hydrogen entrapment within the subsurface. Existing literature is synthesised and organised using bowtie analysis to systematically link initiating hazards, escalation pathways, potential consequences, and available preventive and mitigative controls. The results highlight that dominant risks vary significantly with storage type and operational stage. Salt caverns are primarily affected by mechanical and operational risks, whereas saline aquifers and depleted oil and gas reservoirs exhibit greater uncertainty due to diffusion, microbial activity, geochemical reactions, and legacy infrastructure. By comparing risk characteristics across storage options, this work provides a structured basis for interpreting how geological setting and operational decisions influence UHS performance and safety. While the analysis is qualitative, it clarifies key risk drivers and control measures relevant to early-stage site screening, storage selection, and operational planning. The findings underline the importance of tailoring risk management strategies to specific storage concepts and identify priorities for future site-specific and quantitative assessments of underground hydrogen storage.