## 2 Overview of the Working Face and Depressurized Gas Extraction Process Methods
### Physical Modeling and Meshing of Unloaded Gas Extraction in Mining Air Zones  
According to the actual situation of the 1504 working face in East 2 of Dalong Coal Mine, COMSOL numerical simulation software was used to establish the model of the working face and the mining air zone. In the model, the length of the working face is 235 m, the width is 4 m, and the height is 2.35 m; the length of the mining air zone is 300 m, the width is 235 m, and the height is 35 m; and the length of the air intake and return lane is 20 m, the width is 4 m, and the height is 2.35 m. The geometric model and model meshing are shown in Figures 2 and 3. In the model, the mining air zone portion is considered to exhibit porous media flow, and the air intake and return lane, the working face, and the high-level oblique boreholes are free-flowing. The porous medium is divided into a natural accumulation area, a load-influenced area, and a compaction stabilization area according to the collapse of the overlying rock layer at different locations of the mining air zone. The porosity and permeability are different due to the different distribution characteristics of the fracture development, and the specific regional porosity and permeability are shown in Table 1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Area** & **Porosity/\(n\)** & **Permeability/\(K\)** \\ \hline fracture zone & 0.038 & 1.2807 \(\times\) 10\({}^{-8}\) \\ natural accumulation area & 0.281 & 9.2708 \(\times\) 10\({}^{-8}\) \\ load-influenced area & 0.186 & 2.0977 \(\times\) 10\({}^{-8}\) \\ compaction stabilization area & 0.091 & 7.4575 \(\times\) 10\({}^{-9}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameter settings for each area of the mining air zone. Figure 3: Geometry meshing diagram of the goaf. Figure 2: Geometry model diagram of the goaf. In the physical field module of the free-flow area and the porous media flow, the working face of the air intake and return lane are considered as free-flow areas, and the mining air zone is a porous media area. The initial value of the model is one standard atmospheric pressure, i.e., 101,325 Pa. The air intake lane is a velocity entrance boundary, based on the actual air supply to the working face, with a velocity of 1.6 m/s. The air return lane was set with an exit boundary at a pressure of \(-\)2 kPa, the borehole exit pressure was set to a negative extraction pressure, and the rest of the solid boundary was set to the wall. Simulation Analysis of the Impact of the Effect of High-Level Oblique Boreholes on the Process of Extracting Unloaded Gas  
The following arrangement parameters of the high-level oblique boreholes in the test mine were set according to the theoretical analysis and numerical calculation: the diameter of the boreholes is 94 mm; the vertical heights of the boreholes from the bottom plate are 15 m, 17 m, 19 m, 21 m, 23 m, and 25 m; the horizontal distances from the side of the air return lane are 20 m, 35 m, 50 m, 65 m, 80 m, and 95 m; the negative pressure of extraction is 20 kPa; and borehole spacing is 15 m. Taking the independent variable S as the advancing distance of the working face, the simulation results shown in Figures 4-6 were obtained. Figure 4: Diagram showing the concentration of gas at the working face advancing 10 m.  
Figure 5: Diagram showing the concentration of gas at the working face advancing 20 m.  
According to the above analysis of the simulation results with different advancing distances of the independent variable S working face, the area of low gas concentration in the mining air zone gradually increases with the change in the advancing distance of the independent variable S working face. When S increases from 10 m to 30 m, the gas concentration decreases and the area of low concentration expands accordingly, which is due to the imperfect development of the fracture zone in the area where the borehole is located when S is 10 m, resulting in a lower gas extraction mix.