## 2 Overview of the Working Face and Depressurized Gas Extraction Process Methods
### Process Plan for the Extraction of Depressurized Gas Using Directional Long Boreholes  
Four directional long boreholes are designed and constructed on the side of the air return lane of the 1504 working face in East 2. They are constructed in each drilling site, and arranged along the top plate of the 15 coal seam to extract the gas from the present seam and the fracture zone of the surrounding rock. They have a vertical height of 20-30 m, a diameter of 120 mm, and end points 30-120 m away from the horizontal distance of the air return lane. Analysis of the Distribution Pattern of Directional Long Boreholes at Different Horizontal Distances from the Side of the Air Return Lane  
In order to derive the optimal layout parameters for directional long boreholes in terms of horizontal distance from the side of the air return lane, borehole spacing, and negative pressure, the initial borehole parameters with scientific reference values were set according to the borehole parameters of the neighboring mining layer: negative pressure for extraction was 20 kPa, and the spacing of the boreholes was 30 m. A regular analysis was carried out on the horizontal distance between the boreholes and the side of the air return lane, and borehole spacing and negative pressure parameters were optimized according to the derived optimization parameters. 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; borehole spacing is 30 m; the negative pressure of extraction is 20 kPa; and the independent variable X is the horizontal distance of the borehole from the side of the air return lane. The simulation results are shown in Figures 7-11. Figure 7: Gas concentration distribution map of the horizontal distance of 20 m from the borehole to the side of the air return lane. Figure 8: Gas concentration distribution map of the horizontal distance of 25 m from the borehole to the side of the air return lane. Figure 10: Gas concentration distribution map of the horizontal distance of 35 m from the borehole to the side of the air return lane. Figure 9: Gas concentration distribution map of the horizontal distance of 30 m from the borehole to the side of the air return lane. According to the analysis of the simulation results for different values of horizontal distance from the side of the air return lane, it can be seen that with the increase in horizontal distance between the borehole and the side of the air return lane, the gas concentration in the mining air zone first decreases and then increases. The integral analysis of the drilling area using the post-processing module in COMSOL shows that the pure quantity of gas extraction increases from 7.82 m\({}^{3}\)/min to 8.93 m\({}^{3}\)/min when the horizontal distance is extended from 20 to 30 m, and then stabilizes when the distance is increased from 30 m to 50 m. This indicates that the influence of the wind flow is received when it is nearer to the side of the air return lane, which leads to a lower pure quantity of gas extraction. As it gradually moves away from the air return lane side, the influence of the wind flow diminishes and the pure quantity of gas extraction increases and remains stable within certain limits. The average extraction concentration of the borehole before the horizontal distance of 30 m is 45.70%, while after 30 m, the average extraction concentration of the boreholes reaches 50.75%, which is a significant increase in the extraction effect. From 20 m to 30 m, the concentration in the upper corner decreases from 0.55% to 0.39%, and from 30 m onwards, it fluctuates around 0.39%.