1587 investment without generating direct economic returns. Technically, uncertainties remain regarding longterm CO₂ leakage rates, which raise concerns about potential environmental and societal impacts, as largescale CO₂ leakage may pose serious risks to human health and ecosystems (Khoo et al., 2011). These cost- and safety-related barriers have limited the widespread implementation of CCS to date (Ehlig-Economides and Economides, 2010). In recent years, carbon capture, utilization, and storage (CCUS) has emerged as an alternative and increasingly attractive approach (Zhao and Itakura, 2023). Unlike CCS, CCUS incorporates the utilization of captured CO₂, which can create economic value through its application in various industrial processes, rather than relying solely on long-term storage (Bajpai et al., 2022; Roh et al., 2018). Among the available utilization pathways, mineral carbonation—whereby CO₂ is converted into stable carbonate minerals such as CaCO₃ and MgCO₃—has received particular attention. A key advantage of mineral carbonation lies in its capacity to immobilize CO₂ in a chemically stable form over geological timescales, ranging from decades to millennia (Kandji et al., 2017; Li et al., 2018; Pang et al., 2012), thereby substantially reducing the risk of CO₂ leakage associated with conventional CCS approaches. Geological formations offer significant potential for long-term CO₂ storage (De Silva and Ranjith, 2012; Ji et al., 2022; Shukla et al., 2010). In particular, abandoned or closed underground coal mines represent promising candidates for safe CO₂ injection and storage, owing to their existing void spaces and wellcharacterized geological conditions. In underground mining–based resource development, mined-out voids are often left unfilled with waste rock or other materials after extraction, and the mine is subsequently closed. In such cases, the overlying strata above the excavated voids naturally collapse, leading to the closure of underground cavities and the formation of goaf areas. Consequently, when the mining area extends over a wide region, ground subsidence can occur over extensive surface areas above the goaf, resulting in problems such as road cracking and building tilting in some regions (Akcin et al., 2010). On the other hand, goaf zones contain numerous fractures, indicating that void spaces are considered to have substantial potential as CO₂ storage reservoirs. In addition, their relatively high permeability allows fluids to be injected without the need for high injection pressures. However, when CO₂ is injected into areas with abundant fractures and voids, such as goafs, there is a significant risk of leakage to the ground surface through fracture networks. To address this issue, the present study investigates the application of micro-nanobubble (MNB) technology, which enables CO₂ to remain stable in water over extended periods and allows injection of CO₂ into the subsurface in quantities exceeding its solubility limit. In this study, CO₂ MNB water and blast furnace slag (BFS) were injected into the goafs, with BFS serving as a CO₂ solidification material. As illustrated in Figure 1, the injected CO₂ can react underground with calcium-rich materials to form carbonate minerals, thereby enabling the safe and long-term sequestration of large amounts of CO₂. Accordingly, this study reports on borehole drilling conducted into a mined-out area (goaf) at depths exceeding 400 m in Mikasa City, Hokkaido. The characteristics of the goaf were evaluated through injection tests, followed by the injection of CO₂ MNB water and BFS slurry. The mechanical properties and CO2 fixation of the solidification materials were also examined.
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