Abstract:

Geological CO₂ storage is a critical part in carbon capture, utilization, and storage technology for achieving emission reduction goals. As the scale of CO₂ injection expands and its duration prolongs, the risks of leakage from wellbores and caprocks gradually increase due to wellbore integrity failures, fault reactivation, and fracture propagation or fissure development in caprocks. Post-leakage CO₂ will rise vertically and diffuse horizontally into subsurface including groundwater systems and soil environment, where it spreads extensively and undergoes multiple types of transformation, thereby affecting regional ecological and environmental safety. Under the coupled thermo-hydro-mechanical-chemical field within geological formation, such multi-pathway, cross-spatial, large-scale, and long-term migration, diffusion, and transformation processes are extremely complex. Accurate identification and quantitative assessment of leakage safety and environmental risk rely on various migration-transformation simulation methods. This study systematically summarizes the main leakage pathways of geological CO₂ storage (wellbores, caprocks, and faults), explains the leakage mechanisms, and discusses key influencing factors such as temperature, pressure, and geochemical reactions. The migration and transformation mechanisms of CO₂ after leakage in underground environments such as faults, caprocks, groundwater, and soil are analyzed, and the dominant controlling factors and environmental effects of migration and transformation are identified. This study reviews the simulation methods for CO₂ migration and coupled migration and transformation in caprocks, faults, groundwater, and soil, as well as their application in validating leakage mechanisms, identifying migration-transformation patterns, and predicting environmental risks. This study highlights that the current model still faces challenges in accurately simulating the dynamic distribution of gas phase exsolution in faults, cross-formation multi-environment coupling, and microbial transformation process. Future research should focus on coupled modeling of cross-formation multi-environment migration and transformation, achieve full-space and full-process simulations of migration and leakage, and establish an integrated simulation framework of injection-migration-leakage-diffusion systems, thereby enabling the application of optimized leakage monitoring and precise environmental risk prediction.

Key words: geological CO₂ storage, CO₂ leakage, migration and transformation, numerical simulation, environmental impact