## 4 Model Validation and Results  
To validate the model described in Section 2, directional hydraulic fracturing method was applied to the NO.8 coal seam at Fengchun mine, Chongqing, Southwest China (Figure 4a). The average thickness of the seam was 2 m and the coal seam dip angle was 24-28\({}^{\circ}\). The coal seam permeability was less than 0.001 mD, and its roof and floor were silty sandstone that had a strong sealing capacity. The NO.8 coal seam was prone to coal and gas outbursts. Several gas accidents causing casualties and huge property losses occurred since the seam was opened. The mine operator usually waits 2-3 years to eliminate the seam outburst risk using conventional boreholes drainage method. Thus, the directional hydraulic fracturing method was utilized to improve the NO.8 coal seam permeability directionally and reduce extraction time. The BRW200/56 high-pressure pump and intelligent control device used in field experimental site are shown in Figure 4b. Field results were compared with our simulation results. Such simulation geometry used for gas extraction after directional hydraulic fracturing is shown Figure 1. The length and width of the slot was 2.1 m and 0.231 m, respectively. The directional distance was 10 m. The main parameters for the THM coupling simulation are given in Table 1. They were taken from experimental results or literatures [26; 37; 38]. The comparison between field data and simulation results are shown in Figure 5. It can be clearly seen that the simulations results were in good agreement with the field data for gas volume. The gas pressure distribution in the matrix at different times is presented in Figure 6. Note that pressure \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Model dimension (m) & \(30\times 15\) \\ Overburden stress F\({}_{\text{y}}\) (MPa) & 8 \\ Maximum horizontal stress F\({}_{\text{x}}\) (MPa) & 12 \\ Young’s modulus of coal \(E\) (MPa) & 2100 \\ Young’s modulus of coal grains \(E_{\text{s}}\) (MPa) & 8469 \\ Poisson’s ratio of coal \(\
u\) & 0.35 \\ Density of coal \(\rho_{\text{c}}\) (kg/m\({}^{3}\)) & 1380 \\ CH\({}_{4}\) Langmuir volume constant \(V_{L}\) (m\({}^{3}\)/t) & 34.47 \\ CH\({}_{4}\) Langmuir pressure constant \(P_{L}\) (MPa) & 0.768 \\ Dynamic viscosity of CH\({}_{4}\)\(\mu\) (Pa-s) & \(1.84\times 10^{-5}\) \\ Temperature of boreholes (K) & 293.15 \\ Initial permeability of the fracture system \(k_{\text{0}}\) (mD) & \(8.75\times 10^{-4}\) \\ Initial matrix permeability \(k_{\text{m}0}\) (mD) & \(1\times 10^{-4}\) \\ Initial matrix porosity \(\phi_{m}\) & 0.0479 \\ Initial hydraulic fracture aperture \(w_{\text{0}}\) at the stress-free & 0.3 \\
\begin{tabular}{c} Initial Temperature of coal seam \(T\) (K) \\ Maximum volume strain \(\varepsilon_{L}\) \\ Thermal expansion coefficient of matrix \(\alpha_{T}\) (\(K^{-1}\)) \\ Specific heat capacity of coal skeleton \(C_{\text{s}}\) (J/(kg \(\cdot\) K)) \\ Specific heat capacity of gas \(C_{\text{g}}\) (J/(kg \(\cdot\) K)) & \(1.625\times 10^{3}\) \\ Thermal conductivity of coal skeleton \(\lambda_{\text{s}}\) (J/(kg \(\cdot\) K)) & 0.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main parameters for the thermo-hydro-mechanical (THM) coupling simulation. Figure 4: Field experimental site of directional hydraulic fracturing: (**a**) location of Fengchun mine in Chongqing, Southwest China and (**b**) the BRW200/56 high-pressure pump and intelligent control device. around the slotting boreholes and hydraulic fractures declined most rapidly. The area of reduced gas pressure expanded gradually with increasing extraction time. In China, the government specifies that a gas pressure of 0.74 MPa is the criterion for the completion of extraction before mining operations are permitted [39]. The pre-mining extraction was equivalent to the outburst elimination time. As shown in Figure 7, the maximum residual gas pressure after 10 days was 2.49 MPa between the fracturing and the directional boreholes. This was far above the 0.74 MPa limit.