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.

Carbonated water injection (CWI) is a promising enhanced oil recovery (EOR) technology that has received much attention in co-optimizing CO₂ storage and oil recovery. This study provides a comprehensive review of the fluid system properties and the underlying changes in rock–fluid interactions that drive the CWI-EOR mechanisms. Previous research has indicated that CWI can enhance oil recovery by shifting reservoir wettability towards a more water-wet state and reducing interfacial tension (IFT). However, this study reveals that there is still room for discussion in this area. Notably, the potential of CWI to alter reservoir permeability has not yet been explored. The varying operational conditions of the CWI process, namely temperature, pressure, injection rate, salinity, and ionic composition, lead to different levels of oil recovery factors. Herein, we aim to meticulously analyze their impact on oil recovery performance and outline the optimal operational conditions. Pressure, for instance, positively influences oil recovery rate and CWI efficiency. On one hand, higher operating pressures enhance the effectiveness of CW due to increased CO₂ solubility. On the other hand, gas exsolution events in depleted reservoirs provide additional energy for oil movement along gas growth pathways. However, CWI at high carbonation levels does not offer significant benefits over lower carbonation levels. Additionally, lower temperatures and injection rates correlate with higher recovery rates. Further optimization of solution chemistry is necessary to determine the maximum recovery rates under optimal conditions. Moreover, this review comprehensively covers laboratory experiments, numerical simulations, and field applications involving the CWI process. However, challenges such as pipeline corrosion, potential reservoir damage, and produced water treatment impact the further application of CWI in EOR technologies. These issues can affect the expected oil recovery rates, thereby reducing the economic returns of EOR projects. Finally, this review introduces current research trends and future development prospects based on recently published studies in the field of CWI. The conclusions of this study aid readers in better understanding the latest advancements in CWI technology and the strengths and limitations of the techniques used, providing directions for further development and application of CWI.

Gas and water migration through the hydrate-bearing sediment are characteristic features in marine gas hydrate reservoirs worldwide. However, there are few experimental investigations on the effect of water-gas flow on the gas hydrate reservoir. In this study, gas-water migration in gas hydrate stability zone (GHSZ) was investigated visually employing a high-resolution magnetic resonance imaging (MRI) apparatus, and the formation of hydrate seal was experimentally investigated. Results revealed that normal flow of gas-water at the low flow rate of 1–0.25 mL/min will induce the hydrate reformation. Conversely, higher gas-water flow rates (at 2–0.5 and 4–1 mL/min) need higher reservoir pressure to induce the hydrate reformation. In addition, the hydrate reformation during the gas-water flow process produced the hydrate seal, which can withstand an over 9.0 MPa overpressure. This high overpressure provides the development condition for the underlying gas and/or water reservoir. A composite MRI image of the whole hydrate seal was obtained through the MRI. The pore difference between hydrate zone and coexistence zone produces a capillary sealing effect for hydrate seal. The hydrate saturation of hydrate seal was more than 51.6%, and the water saturation was more than 19.3%. However, the hydrate seal can be broken through when the overpressure exceeded the capillary pressure of the hydrate seal, which induced the sudden drop of reservoir pressure. This study provides a scientific explanation for the existence of high-pressure underlying gas below the hydrate layer and is significant for the safe exploitation of these common typical marine hydrate reservoirs.