Proposal of Risk Intensity Index for the Impact of CO<sub>2</sub> Leakage from CO<sub>2</sub> Geological Storage Reservoir

Carbon capture and geological storage (CCS) has been widely recognized as an effective strategy that contributes to mitigate the global climate change. However, many reports and papers have pointed out that a large-scale CO2 leak from the reservoir could pose negative health, safety and environmental risks. Dissolution and swelling due to geochemical reactions between the injected CO2 and the rocks in the formation are potential factors that can induce leakage from the reservoir, and it is thought that CO2 may leak along physical leakage pathways such as imperfect cementing on the injection well or cracks in the formation. There is concern that the business risks associated with geological storage will make the evaluation of the economic feasibility of the entire CCS project unclear, hindering the active commercialization of CCS, and delaying the reduction of CO2 atmospheric emissions through CCS. However, to the authors’ knowledge, although there are many studies on risk assessment that are complex and include many factors, no objective and clear indicators to express risks of CO2 geological storage have yet been proposed. This article reviews CCS projects consisting of CO2 capture and transportation, and geological storage. It concludes that it is possible to estimate the project risks of CO2 capture and separation on land from examples of similar process industries. However, for offshore and onshore CO2 geological storage, it is necessary to take into account the risks of sealing failures around the wells that could become leakage routes near the injection wells due to increases in reservoir pressure during the injection and storage process, and the risk of reaching geological defects such as geological faults and cracks as the storage area expands. In this way, it is possible to propose an objective and quantitative risk index based on the cumulative injection volume and verify its validity.