## 3 Results
### Permeability Variation of Coal-Rock Mass during the Mining Process  
We investigated the permeability variation of the coal-rock mass during the mining process in Figure 6. The permeability enhancement appears above the goat and varies with the pressure-relief area induced by the advanced working face. Similar to the stress variation, the permeability enhancement is limited to a small area above the excavated coal-rock mass with a maximum permeability of 140 mD. In contrast, the permeability of other areas remains unchanged as the working face is advanced to 20 m (Figure 6a). It enlarges along the vertical and horizontal directions with the extending working face (Figure 6b-d), gradually covering the area above the goat when the working face advances from 20 m to 100 m (Figure 6e). We noted that the length of the permeability enhancement area still increases along the horizontal direction, while the height of the area remains unchanged with an unobvious variation when the working face advances from 100 m to 140 m (Figure 6f). The differential expansion in direction is consistent with the pressure-relief area, which verifies the reliability of the stimulated pressure-relief zone. It means that the height variation in the pressure-relief area remains unchanged as the advancement of the working face exceeds 100 m, and the gas extraction through the borehole located in the pressure-relief area becomes stable. Thus, some deviation in the gas extraction and the favorable extraction area may appear if the on-site inspection boreholes are drilled before the working face advances to 100 m.  
The permeability of the oil and gas reservoir is divided into five different grades, namely, ultra-low (<10 mD), low (10-100 mD), medium (100-500 mD), high (500-2000 mD), and ultra-high permeability (>2000 mD) in China. Higher permeability of the coal-rock mass leads to better gas extraction performance. Thus, we zone the overlying rock mass based on its permeability for placing the on-site borehole in the high and ultra-high permeability to bring acceptable gas extraction efficiency. Figure 6 presents the variation of the permeability zone during the working face advancement from 20 m to 140 m. The initial permeability of the area above the goat is ultra-low, with a permeability of less than 10 mD, which is not conducive to gas extraction. It increases with the advancement of the working face, presenting obvious zoning characteristics by different permeability. A low-permeability area appears above the goat when the working face advances to 20 m (Figure 6a). It gradually transforms into a medium-permeability area, and an  
Figure 5: Horizontal stress evolution as the working face advances from 20 m to 140 m.  
other low-permeability area exists above it with the advanced working face increasing to 40 m (Figure 6b). Then, these areas convert to the high-permeability area and medium-permeability area with the enhanced permeability of rock mass, respectively, when the working face advances to 60 m and 80 m (Figure 6c,d). Further advancement of the working face leads to additional increase in the rock permeability. The permeability of the adjacent area above the goat exceeds 2000 mD, becoming the ultra-high permeability area as the working face advances to 100 m, which is favorable for gas extraction (Figure 6e,f). Meanwhile, the rock mass above the goat forms three areas in the vertical direction, namely, the medium, high, and ultra-high permeability regions (Figure 6e). The height of these regions becomes unchanged, while the length increases as the working face advances to 140 m (Figure 6f), which verifies the results of [PERSON] et al. [31] indicating that the horizontal permeability is more sensitive to stress variation than the vertical one. In summary, the stress of the overlying rock mass varies as the working face advances, resulting in the variation of permeability and its zonation in the vertical direction.