355 Figure 2 – Drilling core samples 2.2 Permeability of the goaf In the injection test, the permeability of the goaf was evaluated by gradually increasing the injection pressure using a water pump and measuring the corresponding changes in injection rate. In this test, the injection rate, bottomhole pressure, and bottomhole temperature were measured while increasing the injection pressure from 0.1 to 1.2 MPa. The pressure was increased in increments of 0.1 MPa, and water injection was maintained for 5 minutes at each pressure level. The results of the injection test are shown in Figures 3(a) and (b). As shown in Figure 3(a), the bottomhole pressure was approximately 3.95 MPa prior to the start of the injection test and during periods when water injection was temporarily stopped. Because the water level in the injection well was confirmed to extend nearly to the ground surface, the hydraulic head of the goaf is considered to be equivalent to the hydrostatic pressure. As the injection pressure increased, a corresponding increase in bottomhole pressure was observed. Furthermore, at injection pressures exceeding 0.6 MPa, fluctuations in bottomhole pressure occurred during the 5 minutes injection period. This behavior suggests that, under pressure conditions above 0.6 MPa, fracture opening, mobilization of angular rock fragments, or similar phenomena may have occurred, resulting in changes to the water flow pathways. Similarly, the relationship between injection pressure and injection rate shown in Figure 3(b) indicates a clear change in the trend of injection rate increase with pressure beyond 0.6 MPa. This observation implies the formation of new permeable pathways within the goaf. Based on these results, the permeability coefficient was calculated using Eq. (1). K={Q ∗ρ ∗g∗ln( ) ⁄ }/(2 ∗Ls ∗ ∗104) (1) where, K is the permeability coefficient (cm/s), Qw is the injection flow rate (m³/s), ρw is the density of water (kg/m³), g is the gravitational acceleration (m/s²), Ls is the length of the injection interval (m), r is the boring radius (m), and Pw is the injection pressure (MPa). In this study, the density of water was taken as ρw = 997.06 kg/m³, the gravitational acceleration as g = 9.81 m/s², the injection interval length as Ls = 15 m (the total length of
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