## 2 Overview of the Working Face and Depressurized Gas Extraction Process Methods
### Analysis of Negative Pressure Distribution Patterns for Directional Long Borehole Extraction  
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 borehole spacing is 30 m; and the independent variable P is taken to be the negative pressure of borehole extraction. The simulation results are shown in Figures 15-17.  
According to the simulation results of negative pressure for the extraction of different boreholes, it can be seen that under the effect of directional long borehole extraction, the high gas concentration accumulated close to the air return lane test is extracted, which effectively stops the influx of gas into the upper corner of the working face. The borehole extraction capacity is increased accordingly during the process of gradually increasing the negative pressure. When the negative pressure is increased from 15 kPa to 20 kPa, the rate of reduction in gas extraction concentration in the mining air zone is significantly accelerated. As the negative pressure continues to increases from 20 kPa to 25 kPa, the rate of reduction in gas concentration in the mining air zone gradually slows down.  
Figure 16: Gas concentration distribution map at a negative borehole extraction pressure of 2 kPa.  
Figure 17: Gas concentration distribution map at a negative borehole extraction pressure of 25 kPa.  
Figure 15: Gas concentration distribution map at a negative borehole extraction pressure of 15 kPa.  
The integration analysis of the borehole area using the COMSOL post-processing module shows an increase in the gas extraction mix from 14.64 m\({}^{3}\)/min to 19.5 m\({}^{3}\)/min as the negative pressure of extraction increases. The borehole extraction concentration decreases slowly from 53.51% to 51.09% and then rapidly from 51.09% to 46.76%. When the negative pressure of extraction increases from 15 kPa to 20 kPa, the pure quantity of gas extraction increases from 7.83 m\({}^{3}\)/min to 8.93 m\({}^{3}\)/min. When it increases from 20 kPa to 25 kPa, the pure quantity of gas extraction increases from 8.93 m\({}^{3}\)/min to 9.05 m\({}^{3}\)/min. Accordingly, it can be seen that when the negative pressure of extraction increases from 15 kPa to 20 kPa, the increase in the pure quantity of gas extraction from the borehole is faster. This is because the boreholes are close to the air return lane, the surrounding gas extraction concentration is higher, and the increase in the negative pressure of extraction can significantly improve the effect of extraction from the borehole. However, when the negative pressure of extraction is increased to 25 kPa, the gas concentration at the working face side is reduced and the borehole extracts gas from the deeper part of the airspace, resulting in an increase in the weight of the gas extraction mix over the pure quantity of gas extraction and thus a decrease in the pure quantity of gas extraction. Meanwhile, the upper corner gas concentration decreases rapidly from 0.57% to 0.39% and then slowly to 0.36%. It can be concluded that the higher the negative pressure of extraction, the better the extraction effect, but the slower the increase in extraction efficiency, resulting in serious air leakage in the mining air zone. Therefore, it is optimal to choose a negative pressure of about 20 kPa for borehole extraction.