1595 4. CONCLUSION Injection trials using CO₂ MNB water and BFS slurry were successfully conducted in a goaf of an underground coal mine in Mikasa City, Hokkaido, aiming at CO₂ storage and mineralization. The main findings are as follows: • Borehole drilling confirmed the presence of a goaf around depths of 400 m. • The goaf exhibited permeability ranging from 8.86 × 10⁻⁵ to 1.04 × 10⁻⁴ cm/s, indicating good injectivity. • The injection trials confirmed that CO₂ MNB water and BFS slurry could be effectively injected into the goaf. • The permeability of the goaf decreased over time after injection, likely due to fracture closure from solidification of BFS slurry, suggesting that reactions occurred between CO₂ MNB water and BFS slurry in situ. • The strength and CO₂ fixation of the solidified slurry are influenced by subsurface pressure conditions, indicating that high-pressure environments may enhance the hydration and carbonation reactions of the slag, promoting long-term CO₂ immobilization. Overall, these results demonstrate the potential of using CO₂ MNB water combined with BFS slurry for effective CO₂ storage and mineralization in underground mined-out areas. As a next step, further investigation is required to evaluate the CO₂ storage capacity of such environments and to assess the feasibility of large-scale, economically viable injection operations. ACKNOWLEDGEMENTS This study was supported by JSPS KAKENHI Grants JP 25K01715; Module Research Program of the Kyushu University Institute for Energy Research and Education (Q-PIT); and the Asia–Oceania Research and Education Organization (Q-AOS). The authors would like to express their sincere gratitude to Mikasa City, Hokkaido, and all personnel from the related companies and organizations for their valuable cooperation and assistance. REFERENCES Akcin, H., Kutoglu, H.S., Kemaldere, H., Deguchi, T., Koksal, E., 2010. Monitoring subsidence effects in the urban area of Zonguldak Hardcoal Basin of Turkey by InSAR-GIS integration. Nat. Hazards Earth Syst. Sci. 10, 1807-1814. Bajpai, S., Shreyash, N., Singh, S., Memon, A.R., Sonker, M., Tiwary, S.K., Biswas, S., 2022. Opportunities, challenges and the way ahead for carbon capture, utilization and sequestration (CCUS) by the hydrocarbon industry: Towards a sustainable future. Energy Reports 8, 15595-15616. De Silva, P.N.K., Ranjith, P.G., 2012. A study of methodologies for CO2 storage capacity estimation of saline aquifers. Fuel 93, 13-27. Ehlig-Economides, C., Economides, M.J., 2010. Sequestering carbon dioxide in a closed underground volume. Journal of Petroleum Science and Engineering 70, 123-130. Ji, Y., Madhav, D., Vandeginste, V., 2022. Kinetics of enhanced magnesium carbonate formation for CO2 storage via mineralization at 200 °C. Int. J. Greenh. Gas Control 121, 103777.
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