## 4 Field Application
### Determination of Drilling Parameters  
We drilled four groups of test boreholes in the grouting roadway of the 204 working face with different inclination angles from 15\({}^{\circ}\) to 28\({}^{\circ}\) (Figures 7 and 8). Thus, the borehole ends were in the different permeability layers with varying vertical distances from the coal seam roof to the borehole ends (\(D_{y}\)). There are six boreholes in every group of gas extraction boreholes with a diameter of 153 mm and a drilling spacing of 1.50 m. The deviation angles of each drill hole are 90\({}^{\circ}\), 94\({}^{\circ}\), 98\({}^{\circ}\), 105\({}^{\circ}\), 112\({}^{\circ}\), and 119\({}^{\circ}\), respectively. Each borehole is sealed by the polyurethane sealing method with a 6 m sealed length. Table 2 details the drilling parameters of the test boreholes. increased from \(-5\) m to \(45\) m (Figure 9). The average gas concentration of each group of boreholes is minimum when \(D_{h}\) is \(-5\) m, ranging from \(1\%\) to \(5\%\). It gradually grows as \(D_{h}\) increases, separately reaching the peak at different \(D_{h}\), then rapidly declining until the borehole collapses. Gas concentration in the \(1\#\) group of boreholes with a maximum vertical distance of \(8.5\) m is the lowest, presenting an average gas concentration of \(4.01\%\) during mining. Although the borehole end is located in the ultra-high permeability area, a large quantity of air is extracted into the borehole under negative pressure due to its connection with the goal. Meanwhile, the borehole in the caved zone collapses with the advancement of the working face, substantially weakening its gas extraction performance. Gas concentration in the caved zone decreases to an unacceptable level. Average gas concentration gradually grows to \(11.61\%\) as \(D_{v}\) rises to \(13\) m, presenting pronounced zoning with the \(D_{v}\) variation, namely, the growth, stable, and descent sections. Gas concentration increases from \(4.67\%\) to \(12.56\%\) in the growth section, with \(D_{v}\) ranging from \(-5\) m to \(25\) m. It stabilizes at an acceptable concentration, fluctuating between \(13.55\%\) and \(21.44\%\) in the stable section, and decreases to \(8.6\%\) in the descent section. The \(2\#\) group of boreholes is located in the high-permeability area of the fractured zone, connected with the caved zone by numerous vertical fractures. These fractures offer channels for gas migration for an acceptable gas concentration. A further increase in \(D_{v}\) leads to a higher gas concentration; however, the growth rate decreases significantly, showing that gas concentration rises from \(11.61\%\) to \(15.80\%\) as \(D_{v}\) increases from \(13\) m to \(19\) m. The \(3\#\) group of boreholes are located in the separate area of the fractured zone, where the bedding tension fractures induced by separated layers provide numerous gas flow channels. Free gas constantly migrates to the separate area through these fractures and accumulates at the boundary between the upper fractured zone and the continuous zone. Thus, gas concentration in the \(3\#\) group of boreholes is more significant than that of the \(2\#\) group of boreholes, although they are all in the high-permeability zone. Meanwhile, the gas concentration distribution in this group of boreholes also shows similar zoning characteristics to those of the \(2\#\) group. With the advancement of the working face, the three zones of the overlying rock mass gradually evolve, followed by an increased rock permeability. It enhances the gas migration to this area, increasing gas concentration in the boreholes as the working face advances from \(-5\) m to \(40\) m. Due to continuous supply, high gas concentrations last for the advanced distance from \(20\) m to \(40\) m. Then, the gas supply weakens as the working face advances over \(40\) m, and the gas concentration declines gradually. Figure 9: Gas concentration variation in boreholes with the advancement of the working face.