# Study on Gas Extraction Technology for Goal Using L-Shaped Borehole on the Ground
## 4 Numerical Simulation Study on Gas Extraction of L-Shaped Borehole in Mining
### Physical Model Building  
According to the actual situation of the working face, a physical model of the L-shaped well gas extraction in the mining fissure zone was established (Figure 4). The size of the goat model was 300 m \(\times\) 220 m \(\times\) 60 m, the size of the working face was 5 m \(\times\) 220 m \(\times\) 3.5 m, and the dimensions of the two tunnels were 20 m \(\times\) 4.5 m \(\times\) 3.5 m. The diameter of the L-shaped borehole was 0.1714 m. To analyze the extraction effect of the L-shaped borehole at different positions, the drilling positions were set at 23 m, 33 m, 43 m, and 53 m from the roof of the coal seam, respectively.  
According to the above research on the stress and damage law of the mining overburden, the overburden strata in the goat caved and an O-type rupture occurred while the working face advanced. The mining fissures and free caving rocks in the middle of the goat were basically compacted, and the coal-pillar-side separation fissures and caving fissures around the goat remained. The distribution of the caving dilatation coefficient in the goat is described by the following expression (the initial caving dilatation coefficient of the roof is taken to be 1.6, and the compacted dilatation coeffici\[K_{(x,y)}=K_{\rm min}+(K_{\rm max}-K_{\rm min})\times\exp(-m_{0}d_{0}\times(1-\exp( -\xi m_{1}(d_{1}+\phi)))) \tag{4}\]  
where \(K_{\rm(x,y)}\) is the caving and fragmentation coefficient of the goaf; \(K_{\rm max}\) is the crushing expansion coefficient of the initial caving; \(K_{\rm min}\) is the crushing expansion coefficient after compaction; \(m_{0}\) and \(m_{1}\) are the attenuation rates from the solid wall and the working face, respectively; \(d_{0}\) and \(d_{1}\) are the distances from the point (x, y) to the working face boundary and solid wall, m.  
The specific values can be obtained through a trial-and-error calculation. Assuming \(m_{1}=0.0368\), \(m_{0}=0.268\), and \(\xi=0.233\), the \(K_{\rm(x,y)}\) distribution results are illustrated in Figure 4.  
The position of the air inlet was set as the inlet boundary, and the inlet boundary contained the wind speed and gas concentration values. According to the actual situation of the working face, the airflow speed of the air inlet was set to 2.2 m/s, and the gas concentration was 0, that is, the incoming fresh airflow was considered to not contain any gases. According to the calculation process of the field test data of Tunlan Mine, the gas emission from the coal wall of the working face was set at 7.48 \(\times\) 10\({}^{-5}\) kg/(m\({}^{3}\cdot\)s), and the gas emission from the residual coal in the goaf was set at 7.48 \(\times\) 10\({}^{-5}\) kg/(m\({}^{3}\cdot\)s); the suction pressure of L-shaped borehole drainage was 50 kPa.  
As shown in Figure 5, as the vertical distance increases, the gas concentration in the upper corners first decreases, and then, increases, indicating that when the vertical distance parameter of the ground L-shaped borehole is within a certain range, the ground L-shaped borehole is extracted from the goaf. A gas with a higher concentration was generated, and the gas concentration at the points investigated in the goaf was reduced to varying degrees. When the vertical distance of the local L-shaped borehole is too high, the ground L-shaped borehole has a weakened ability to control the gas concentration in the lower goaf. Although the ground L-shaped borehole can extract the gas with a higher concentration in the goaf, it has little impact on the change in the gas concentration near the upper corner. When the vertical distance reaches 53 m, the gas concentration in the upper corner reaches 1.2%. In summary, the L-shaped borehole in the mining fissure zone of the 12507 working face of Tunlan Mine should be located at about 43 m from the roof of the coal seam.  
Figure 4: The physical model of the L-shaped well gas extraction.  
Figure 5: Cont.