Geomechanical integrity and geochemical reactions of shale caprocks for hydrogen storage: A comprehensive review

Hydrogen Underground hydrogen storage (UHS) is a viable solution for growing energy demands while advancing the transition to net-zero carbon emissions. The success of UHS relies heavily on maintaining the geomechanical integrity of caprocks, which act as natural seals to prevent hydrogen migration and ensure long-term storage efficiency. Despite their favorable properties, such as low permeability and high capillary pressure, shale caprocks face significant challenges under UHS conditions, including stress variations, cyclic loading, and potential fault reactivation. This review synthesizes the latest advancements in understanding the geomechanical behavior of shale caprocks, highlighting experimental studies and numerical modeling approaches. It examines the key factors influencing caprock stability, including hydrogen diffusion, adsorption behavior, buoyant transport, and the effects of cyclic loading on mechanical fatigue. Mitigation strategies such as adaptive management, real-time monitoring, and reinforcement techniques, including grouting and synthetic barriers, are also discussed. Although current research suggests that shale caprocks maintain strong integrity under typical hydrogen storage conditions, knowledge gaps remain regarding their long-term performance, particularly under repeated stress cycles and varying operational conditions. Addressing these gaps is critical for ensuring safe and efficient UHS operations. This review offers a comprehensive understanding of the challenges, practical strategies, and future research directions required to enhance the reliability of UHS systems, thereby supporting their pivotal role in sustainable energy storage.