Long-Term Reaction Characteristics of CO₂–Water–Rock Interaction: Insight into the Potential Groundwater Contamination Risk from Underground CO₂ Storage

CO₂ sequestration into saline aquifers is considered to be one of the most promising options for reducing industrial CO₂ emissions to the atmosphere. However, there are still many uncertainties regarding the storage of CO₂ in the subsurface because of a lack of knowledge about CO₂–water–rock interaction within CO₂ reservoirs and the potential risk of CO₂ leakage. In this study, we construct a semi-open type experimental system that can reproduce the interactions under conditions close to those of actual CO₂ reservoirs. Using the system, we conduct CO₂–water–rock interaction experiments for 8 months to monitor the long-term reaction and the mobilization of harmful metal elements. Altered tuffaceous rock is used in the experiment because these tuffaceous rock formations (called “Green Tuff”) are a potential candidate for CO₂ storage in Japan. The results show that the major-element water composition will converge to the point where host rock dissolution and secondary mineral precipitation are balanced; then, the interaction will proceed under a certain groundwater composition. In addition, we found that groundwater contamination by some metal elements (Ni, Ba, and Mn) may reach unsafe levels for drinking water as a result of CO₂-water–rock interaction.