CO₂ geological storage is a pivotal technology for achieving the global targets of carbon peaking and carbon neutrality. However, the potential risks of CO₂ leakage to environmental safety and long-term storage efficacy are significant, thereby making the establishment of robust and reliable monitoring systems indispensable. This review systematically explores the potential leakage pathways and key monitoring parameters, including wellbore integrity, CO₂ plume migration, and caprock stability. In addition, the mechanisms and influencing factors associated with the three primary CO₂ leakage pathways are systematically summarized. This approach provides a critical assessment of the advantages, applicability and limitations of prevalent geophysical and geochemical monitoring methods. A special focus is placed on optical fiber sensing technology, whose research progress and application feasibility in laboratory settings are summarized in terms of monitoring targets, measurement accuracy and sensing range. Furthermore, this review highlights several global carbon capture and storage demonstration projects to illustrate the integration and performance of various monitoring technologies in practical engineering. To ensure the efficiency and safety of CO₂ geological storage in the future, it is necessary to develop advanced monitoring technologies, such as optical fiber sensing and promoting the integrated deployment of multi-modal monitoring systems. These efforts are considered essential for supporting the large-scale deployment of carbon capture, utilization and storage engineering, particularly in the context of China.

Hydrogen is emerging as a clean energy carrier in the global transition toward decarbonized energy systems. Leveraging established subsurface engineering expertise, underground hydrogen storage can be realized in salt caverns, depleted hydrocarbon reservoirs, and deep saline aquifers. However, the physicochemical characteristics of hydrogen including low viscosity, high diffusivity and strong chemical reactivity create unique challenges for its containment, transport and recovery from porous media. This review systematically analyzes the known interfacial and pore-scale mechanisms governing hydrogen migration, trapping and loss in heterogeneous reservoirs. The key processes comprise capillary trapping, molecular diffusion, interfacial reactions, and microbial activity. Interactions among hydrogen, brine and mineral surfaces are evaluated in terms of wettability, interfacial tension and pore connectivity, all of which directly influence storage efficiency and recovery performance. Advanced experimental methods such as nuclear magnetic resonance, microfluidics models, and X-ray computed tomography, combined with pore-scale simulations, are assessed for their ability to characterize multiphase flow and reactive transport behavior. Furthermore, the impact of operational factors like cushion gas composition, pressure cycling and injection-production strategies on storage integrity is discussed. Addressing these multi-physics and multi-scale challenges is essential for the safe and efficient implementation of underground hydrogen storage. Finally, this review identifies priority research directions aimed at improving mechanistic predictions and optimizing the operational management of hydrogen behavior in subsurface environments.