Underground hydrogen storage: a review

: Large-scale underground storage of hydrogen gas is expected to play a key role in the energy transition and in near future renewable energy systems. Despite this potential, experience in underground hydrogenstorage remains limited. This work critically reviews the most important elements of this crucial technology,including hydrogen properties and their significance for subsurface operations, sources for hydrogen and historical hydrogen storage operations, to set the state of the art. The cyclical nature of hydrogen storage operationswill produce pressure and stress changes within the reservoir that could affect the integrity of the well, the reservoir, the caprock and the entire subsurface storage complex. To minimize geomechanical leakage risks andoptimize the storage operation it is crucial to understand the pressure and stress history of the storage site, tooptimize well locations to manage pressure and to identify the reservoir-specific cushion gas to working gasratio. Finally, we outline the major scientific and operational challenges required to ensure the safe and efficientdeployment of underground hydrogen storage at a large scale