A comprehensive review of underground CO2 storage and associated risks

Geological carbon dioxide (CO 2 ) storage has emerged as one of the most promising technologies for mitigating greenhouse gas emissions and achieving global carbon neutrality. This review provides a comprehensive overview of CO 2 geological storage with emphasis on storage formations, trapping mechanisms, storage capacity assessment methods, monitoring techniques, risk assessment, mitigation strategies, and the key factors governing long-term storage security. Major geological storage formations—deep saline aquifers, depleted oil and gas reservoirs, unmineable coal seams, and basalt formations—are compared in terms of storage capacity, injectivity, containment, and technological maturity; and the fundamental CO 2 trapping mechanisms (structural, residual, solubility, and mineral trapping) are examined together with their contributions to long-term storage permanence. Storage-capacity assessment methods are critically evaluated within the Carbon Sequestration Leadership Forum (CSLF) Theoretical–Effective–Practical–Matched Capacity framework, including the volumetric equations used for saline aquifers and depleted hydrocarbon reservoirs and their associated sources of uncertainty. The discussion is grounded in seven representative operating and demonstration CCS projects (Sleipner, In Salah, Quest, Illinois Basin–Decatur, Otway, Carbfix, and Gorgon), illustrating how storage capacity, monitoring, and risk-management decisions have been implemented in practice. Storage risks are organized within an integrated Source–Pathway–Receptor (SPR) and Hazard–Failure–Consequence (HFC) framework, and their evolution and coupling are traced across the injection, operational, and post-injection stages of a storage project. Corresponding monitoring technologies and preventive and corrective mitigation strategies are reviewed and explicitly linked to the specific risks they are designed to detect or control. Finally, recent advances in coupled thermo-hydro-mechanical-chemical (THMC) modeling, digital twins, machine-learning surrogates, and distributed fiber-optic and AI-based monitoring are discussed as emerging tools for reducing uncertainty and improving real-time risk management. Overall, this review offers an integrated, stage-aware framework for understanding and managing geological CO 2 storage risk, together with perspectives for improving storage reliability and accelerating the large-scale deployment of carbon capture and storage (CCS).