## 4 Gas Measurement and Analysis under Field Stress Action  
To investigate the influence of high-pressure fluid stress borne by coal on gas desorption, the hydraulic presplitting process test was implemented in the haulage roadway of the 1166 working face in Sijichun Coal Mine, Guizhou Province. There was no fault geological structure in the test area, and the coal seam was stable. Fracturing holes and observation holes (1#-5#) were drilled to investigate the gas pressure, gas content, gas concentration, and single-hole pure gas extraction, as displayed in Figure 8. The observation holes were used to measure the gas pressure. Figure 8: Layout plan of drill holes in hydraulic presplitting test area. Figure 7: Changes to gas channels under high-pressure fluid stress. Before the hydraulic presplitting process was implemented, the fracturing holes and observation holes (1#-5#) were constructed first, and the observation holes were simultaneously used for the measurement of the gas pressure and gas extraction. During this period, the actual pressure range of hydraulic fracturing was 0-34 MPa, and the pressure was stabilized at 32-34 MPa after pressure was initially applied. When the process measure was completed, the total amount of water injected was 198.6 m\({}^{3}\). According to the previous hydraulic presplitting test in this mine, the radius of the effective fracture influence was 22 m. The previous test area belonged to the same geological unit and coal seam with the existing test area, and the coal seam occurrence was steady without geological structures, so the radius of effective fracture influence was set to 22 m during hydraulic presplitting in this test area. Beyond this distance was a zone where the extension of the hydraulic fracture ended and the high-pressure fluid stress exerted its action. The gas measurement in the stress-affected zone was analyzed as follows. Analysis of gas pressure in the stress-affected zone  
The observation holes (1#-5#) were constructed simultaneously with fracturing holes. After that, the gas pressure changes before and after hydraulic presplitting were observed, as shown in Figure 9.  
Because observation hole 1# was located at the radius of the effective fracture influence of 22 m and on the edge of the effective fracture influence, a big gap was observed between observation hole 1# and observation holes 2#-5# in terms of gas pressure. As can be seen from the change curve in Figure 9, the gas pressure in the observation holes (1#-5#) before the stress affected the test area was 9.8-10.5 MPa, and the gas pressure changed only by 0.7 MPa. Under the stress action, the gas pressure was 6.46-18.2 MPa, and the maximum gas pressure exceeded 7.7 MPa before the stress action. Concentration analysis of gas extracted in the stress-affected zone  
The gas concentration changes in the observation holes (1#-5#) in the stress-affected zone are exhibited in Figure 10.  
Observation hole 1# was located on the edge of the radius of the effective fracture influence, and the gas concentration ranged from 35.1% to 40.6%. The gas concentration in observation holes 2#-5# was 77.4-90.4%. Therein, the highest gas concentration in observation hole 2# was 86.6-90.4%, always being higher than that in observation hole 1#. Analysis of pure gas extraction in the stress-affected zone  
The changes in the pure gas extraction in the observation holes (1#-5#) in the stress-affected zone are displayed in Figure 11.  
Figure 9: Variation curves of gas pressure before and after hydraulic presplitting. The pure gas extraction from observation hole 1# was 0.51-0.55 m\({}^{3}\)/min, which was higher than that from the other observation holes. As the distance from the observation hole to the fracturing hole increases, the pure gas extraction presents a progressive declining trend.