Cement integrity challenges associated with underground hydrogen storage: A review of geochemistry and microbial activity

Underground hydrogen storage (UHS) offers a scalable solution for balancing renewable energy supply and demand, but maintaining wellbore integrity remains a major challenge. This review critically examines the interactions between cement, hydrogen (H2), carbon dioxide (CO2), and hydrogen sulfide (H2S) under UHS conditions, with particular attention to microbial activity that accelerates cement degradation. The scope covers experimental studies, field observations, and modeling work focusing on Class G, Class H, and ordinary Portland cement (OPC). Results indicate that while H2 gas exhibits negligible direct reactivity with cement, exposure to H2-saturated solutions can improve the mechanical properties and durability of cement. However, indirect microbial and acid gas effects, especially from CO2 and H2S, can deteriorate the mechanical and chemical stability of cement over time. Comparisons between laboratory and simulation data highlight discrepancies between short-term experimental results and long-term predictions, emphasizing the importance of coupled microbial-geochemical modeling. Mitigation strategies such as biogas-resistant cement, supplementary cementitious materials (SCMs), and advanced well monitoring are also discussed. This review points out the need for integrated experimental, numerical, and field-scale studies to advance understanding and develop robust predictive models. Ultimately, this integrated approach should pave the way for the development of AI-enabled monitoring and maintenance systems, facilitating proactive management and improved long-term reliability of UHS operations.