# Double Unloading Gas Control Technology for Fracturing Soft Coal Seams in Overlying Key Strata
## 3 Analysis of Simulation Results
### Drilling and Extraction Rules before and after Stress Relief of Key Strata  
#### 3.3.1. Gas Pressure Distribution in Extraction Boreholes  
Gas extraction is a long-term process; usually coal mines use 180 d as an extraction cycle, so the number of days of extraction for the coal seam was monitored for 30 d, 90 d, and 180 d, and the gas pressure distribution cloud map was plotted when the stress of the overlying rock layer on the coal seam was gradually depressurized from 13 MPa to 10 MPa and 8 MPa as shown in Figure 10. Subfigures a-c illustrate the cloud maps of the extraction pressure for ordinary boreholes over time, while subfigures d-f depict the cloud maps of the extraction pressure for hole drilling over time. The results demonstrate that the pressure relief effect of the overlying strata of the coal seam can enhance the gas extraction range of the coal seam drilling with increasing extraction time. Furthermore, within the same extraction time, the gas extraction pressure reduction range of the hole drilling was significantly greater than that of the ordinary extraction drilling. As shown in Figure 10d, the rate of decrease in gas pressure during the extraction time from 30 d to 90 d is significantly larger than that from 90 d to 180 d, and it can be predicted that the rate of decrease of gas pressure after 180 d is even slower.  
Figure 9: Variation in coal seam permeability.  
According to the difference in stress of the overlying rock layer, the gas pressure extraction curves of hole drilling and ordinary drilling at 180 d of extraction were plotted, as shown in Figure 11. There is a difference in the gas pressure drop rate under different stress levels of the overlying rock layer, and the gas pressure drop rate of coal seam with overburden stress of 8 MPa was faster than that of coal seam with overburden stress of 13 MPa; this indicates that the greater the degree of decompression of overburden, the more obvious the effect of gas pressure drop of the coal seam is. As shown in Figure 11, with the overburden stress of 8 MPa, the gas pressure at 25 m away from the extraction borehole is 1.29 MPa, which is lower than that of the ordinary borehole of 1.35 MPa. Therefore, under the same stress of the overlying rock layer, the range and speed of gas pressure decrease of the hole drilling is larger than that of the ordinary drilling. As shown in Figure 12, when the stress of the overlying rock layer is 13 MPa, with the increase in the number of days of extraction, the decrease in gas pressure in the hole drilling is larger than that in the ordinary drilling; the gas pressure in the hole making drilling decreases from 1.6 MPa to 1.33 MPa, while the gas pressure in the ordinary drilling decreases from 1.6 MPa to 1.4 MPa. In the same period of time of extraction, the decrease in the gas pressure of the two sides of the hole drilling is greater than that of the ordinary drilling.  
Figure 11: Variation curve of overlying rock stress and gas pressure overlying rock layers. **(a)** Hole drilling. **(b)** Ordinary drilling.  
Figure 10: Gas pressure distribution under different overburden stresses.