## 2 Overview of the Working Face and Depressurized Gas Extraction Process Methods
### Analysis of Reasonable Spacing Distribution Patterns for Directional Long Boreholes  
The design parameters of the drilling arrangement are as follows: the diameter of the borehole is 120 mm; the vertical heights of the borehole from the bottom plate are 20 m, 23 m, 26 m, and 30 m; the distances from the air return lane are 30 m, 60 m, 90 m, and 120 m; the negative pressure of extraction is 20 kPa; and the independent variable D is the borehole spacing. The simulation results are shown in Figures 12-14.  
Figure 11: Gas concentration distribution map of the horizontal distance of 50 m from the borehole to the side of the air return lane.  
Figure 14: Gas concentration distribution map with borehole spacing of 35 m.  
Figure 12: Gas concentration distribution map with borehole spacing of 25 m.  
Figure 13: Gas concentration distribution map with borehole spacing of 30 m.  
According to the above analysis of the simulation results for different values of the independent variable D (borehole spacing), it can be seen that the area of low gas concentration in the quarry shows the phenomenon of increasing and then decreasing with the change in D. When D increases from 25 m to 30 m, the gas extraction concentration decreases and the area of low concentration expands accordingly, which is due to the stronger interactions between the boreholes at a D of 25 m, leading to a decrease in the amount of gas extraction. When D is 30 m, the influence between boreholes is weaker, the extraction radius can be utilized effectively, and the extraction capacity of each borehole is increased, thus improving the extraction effect. However, when D is increased from 30 m to 35 m, the area of low gas concentration decreases due the boreholes being further away from the air return lane and located in an area with a lower gas concentration, which in turn affects the effectiveness of gas extraction.  
The integration analysis of the borehole area using the COMSOL post-processing module yielded a gas extraction mix of 14.96 m\({}^{3}\)/min, an average extraction concentration of the boreholes of 49.14%, and a pure quantity of gas extraction of 7.35 m\({}^{3}\)/min at the borehole spacing of 25 m. At 30 m, the gas extraction mix is 17.48 m\({}^{3}\)/min, the average extraction concentration of the boreholes is 51.09%, and the pure quantity of gas extraction is 8.93 m\({}^{3}\)/min. At 35 m, the gas extraction mix is 17.31 m\({}^{3}\)/min, the average extraction concentration of the boreholes is 45.69%, and the pure quantity of gas extraction is 7.91 m\({}^{3}\)/min. The reason for this phenomenon is that as the borehole spacing increases, the interaction between the boreholes decreases, and the extraction mix increases and stabilizes. However, the pure quantity of gas extraction fluctuates in a trend of increasing and then decreasing. When the borehole spacing is 25 m, the boreholes have not yet reached the effective extraction radius, resulting in relatively low pure extraction. When the borehole spacing is increased to 30 m, the interaction between the boreholes is reduced, the extraction radius of the boreholes is reached, and the pure amount of extraction is thus increased. However, at 35 m, the boreholes cannot play a joint role due to the excessive spacing, which in turn decreases the pure amount of pumping. The upper corner gas concentrations were 0.53%, 0.39%, and 0.47% at borehole spacings of 25 m, 30 m, and 35 m, respectively, showing a trend of first decreasing and then increasing. The best gas extraction effect was achieved at the borehole spacing of 30 m.