357 3. FIELD INJECTION TRIAL In this study, injection trials were conducted in the goaf with the aim of CO₂ injection and mineralization, using CO₂ MNB water and BFS slurry. Furthermore, to evaluate the effectiveness of the reactions of the injected BFS slurry, the permeability of the goaf was assessed through post-injection injection tests, and the solidified slurry recovered two years after the injection trials was analyzed. 3.1 Injection trials with CO₂ MNB water and BFS slurry The injection trials were conducted using the equipment configuration illustrated schematically in Figure 4. CO₂ MNB water was prepared by installing a fine-bubble generator (TH-10; Taisei Kogyo Co., Ltd.) in a 3 m³ water tank and circulating the water in the tank for more than 3 hours. The CO₂ supply rate was 45 L/min. BFS slurry was prepared by adjusting the mixture ratio of water, BFS, and admixtures to a W/B ratio of 1.0–2.0 prior to injection. The overall injection volumes for the experiments are summarized in Table 2. In total, 84 m³ of water, 23 m³ of CO₂ MNB water, and 78 m³ of BFS slurry were injected, confirming that the injectability into the goaf was adequate. Figure 4 – Experiment facilities of field injection trial 3.2 Change in permeability of the goaf BFS slurry injected into the goaf was expected to reduce permeability by blocking fractures through solidification in the subsurface. To evaluate the post-injection permeability of the goaf, injection tests were conducted immediately after the injection trials with CO₂ MNB water and BFS slurry, and again two months later. The results of the injection tests are shown in Figures 5(a) and (b). As shown in Figure 5(a), prior to the experiments, the injection flow rate under an injection pressure of 1.0 MPa was approximately 100 L/min. Immediately after the experiments, the flow rate decreased to 24 L/min, and two months after the experiments, it further decreased to 12 L/min. The permeability coefficient calculated under the 1.0 MPa injection pressure condition indicated that the permeability of the goaf had decreased after the experiments as shown in Figure 5(b). This reduction is attributed to the blockage of
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