From micro to macro: A comprehensive review for underground hydrogen storage technologies and challenges

Hydrogen has attracted much public attention amid global energy transitions and renewable energy utilization. Underground hydrogen storage (UHS), offering large-scale storage capacity at low cost, plays a key role in enhancing hydrogen production and utilization efficiency. This review systematically examines UHS from micro to macro perspectives. It begins with comparing the properties of hydrogen, CO2 and CH4, highlighting key differences between hydrogen and other gases stored in underground media. Low molecular weight of hydrogen increases leakage risks, presenting unique storage challenges. The review then outlines common storage structures and existing global UHS sites. Five primary storage mechanisms are identified, including structural trapping, residual trapping, dissolution trapping, mineral trapping and adsorption. Among these, structural trapping emerges as the dominant mechanism. Site screening methods and potential sites in China are also discussed. Using established evaluation criteria, the review examines multiscale influencing factors, such as solid-fluid interaction, geochemical reactions, mechanical analysis, multiphase flow, storage capacity, and injection and withdrawal schemes, across micro, mesoscopic, and macro levels. These factors interact dynamically. It was observed that geochemical reactions may alter rock geo-mechanical properties, potentially compromising storage integrity. Injection and withdrawal schemes must account for complex fluid-solid interactions. Despite advancements, unresolved challenges persist due to the complexity of underground systems. Accordingly, the review concludes with targeted recommendations to address research gaps, providing a theoretical foundation for optimizing underground systems.