354 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. Figure 1 – Concept of CO₂ injection/fixation in goafs 2. FIELD OVERVIEW 2.1 Boring investigation At the study site, the goafs of an underground coal mine are present at a depth of approximately 400 m, with a seam dip of about 30°. A borehole was drilled from the ground surface to a depth of 430 m. The drilling results confirmed the presence of a homogeneous mudstone layer exceeding 300 m in thickness, extending from depths of 6 m to 377 m, which functions as a caprock. The goaf was identified at depths around 400 m (Figure 2). An injection test was subsequently conducted to evaluate the permeability of the goaf.
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