## 4 Results and Discussion  
### Gas extraction Effectiveness  
To analyze the effectiveness of extraction, the gas flow and gas concentration in the main extraction pipe were recorded each day. The changes in gas flow and gas concentration are shown in Figure 6.  
The gas concentration curves shown in Figure 6 are wavy. The results of the W2708N working face decrease with time, whereas the peaks of the W2708S working face decrease first and then increase mainly because the extraction boreholes are drilled and connected to the main extraction pipe by subareas. The working faces are divided into five (W2708S) or six (W2708N) subareas. The extraction boreholes in a subarea are connected to the main extraction pipe at the same time after they are all drilled. For each subarea, the gas concentration will gradually decrease as the gas content and gas desorption rate decrease, and the rate of decline becomes increasingly lower. For the entire working face, the gas concentration in the main extraction pipe will increase when a new subarea (with a high gas concentration) is connected. The peaks in concentration become increasingly smaller because the gas flow in the pipe becomes increasingly larger in the first 3 months. The curve for the W2708S working face is a \"U\" type curve because HFT has poor effectiveness in tectonic regions but good effectiveness in nontectonic regions. The gas concentrations of the W2708N and W2708S working faces were quite different in the first 45 days because SDHFT was used in the tectonic region of the W2708S working face. When a hydraulic fracture meets a structural plane, the fracture may not be able to pass through the structural plane and will extend along the plane (Figure 7), leaving some areas unfractured. Additionally, the hydraulic fractures that extend along the plane may connect to the roadway and draw in air during the extraction period, which may lead to low gas concentrations. HOST was effective and resulted in high gas concentrations in the tectonic region of W2708N. SDHFT was used in the nontectonic regions of both working faces. Therefore, the gas concentration curves are similar. However, the gas concentration for W2708N was approximately 3 grades higher than that of W2708S over the final 6 months. The average gas concentration from W2708N over 9 months was 24.94%, which was 47.1% higher than that of W2708S (16.96%). This result occurred because of the reduced effectiveness of the employed technology in the W2708S tectonic region. The gas flow curves in Figure 6 are also wavy. The curves have multiple peaks that became higher with time. This phenomenon likely occurred because the subareas are progressively connected. For each subarea, the gas flow rapidly increased to peak values within one day. The gas flow then started to decline after several days. The rate of decline decreased over time. For the entire working  
Figure 6: Concentration and flow curves of the extracted gas. face, the gas flow increased when each new subarea was connected. The gas flow then slowly decreased after all of the subareas were connected. When the extraction boreholes were connected, the gas content and gas pressure were relatively high, and the fracture morphology was good. These conditions are beneficial for gas migration, desorption and extraction. The gas content and gas pressure gradually declined as extraction progressed and ultimately produced a negative impact on gas extraction. The average mixed and pure gas flows for the W2708S working face were 22.01 m\({}^{3}\)/min and 4.24 m\({}^{3}\)/min, respectively, and those for the W2708N working face were 22.16 m\({}^{3}\)/min and 6.13 m\({}^{3}\)/min, respectively. The mixed gas flows were similar. However, the pure gas flow of W2708N was 44.6% higher than that of W2708S because some air may have been drawn into the W2708S tectonic region during the extraction period. The W2708N working face had a better permeability enhancing effect and a higher gas concentration than the W2708S working face.