# Experimental Investigation on Mechanics and
## 3 Results and Discussion
### Permeability Sensitive Effect during Gas Depletion  
Coal permeability is an important reservoir property during gas drainage which controls the gas drainage rate; thus, it is of significance to study the law of coal permeability during gas depletion. Figure 5 shows the relationships between effective stress, gas flow rate, and gas pressure during gas depletion. It is obvious that with gas depletion, the effective stress increases linearly with the decrease of gas pressure because the axial stress and confining stress are fixed. However, the gas flow rate decreases rapidly first and then slowly with the decrease of gas pressure, and the gas flow rate of tectonic coal is about one order of magnitude higher than that of intact coal. The fitting of the experimental data shows that he relationship between gas flow rate and gas pressure obeys a quadratic polynomial function, no matter the tectonic coal or intact coal, as shown in Table 5.  
The permeability of coal is mainly controlled by its fracture system and affected by stress conditions and gas pressure [33]. Considering that the linear correlation between stress and gas pressure, only the curve of permeability evolution with gas pressure during gas depletion is plotted, as shown in Figure 6. It can be found that the permeability of tectonic coal has the maximum value of \(5.409\times 10^{-16}\) m\({}^{2}\) at 4.0 MPa, then reduces to \(2.480\times 10^{-16}\) m\({}^{2}\) at 1.0 MPa and recovers to \(2.263\times 10^{-16}\) m\({}^{2}\) at 0.5 MPa eventually. Accordingly, the permeability of intact has the maximum value of \(0.564\times 10^{-16}\) m\({}^{2}\) at 4.0 MPa, then reduces to \(0.522\times 10^{-16}\) m\({}^{2}\) at 2.0 MPa and recovers to \(0.652\times 10^{-16}\) m\({}^{2}\) at 0.5 MPa eventually. There are three differences between the permeability evolution of tectonic and intact coal. Firstly, the permeability drop ratio of tectonic coal from the maximum value to the minimum value (54.15%) is bigger than that of intact coal (7.45%), and the permeability recovery ratio of tectonic coal from the minimum value to the final value (5.36%) is smaller than that of intact coal (24.9%). Secondly, the rebound gas pressure of tectonic coal (1.0 MPa) is smaller than that of intact coal (2.0 MPa). Moreover, the permeability of tectonic coal is one order of magnitude larger than that of intact coal before the gas pressure drops to 2.0 MPa, and they become closer after that.  
As we know, the effective stress increases continuously leading to a drop in coal permeability with gas depletion. Meanwhile, the shrinkage of coal matrix due to gas desorption will open the seepage pores and improve the coal permeability. At the same time, the Klinkenberg effect has an advantageous impact on gas seepage in gas reservoirs, especially in low permeability materials at finite gas pressure [34]. Considering the effective stress effects for permeability drop of tectonic and intact coal are close. It can be concluded that the matrix shrinkage effect and Klinkenberg effect for permeability rise of intact coal are more significant than that of tectonic coal. The low permeability of intact coal and the broken of tectonic coal may be the two principal reasons. Therefore, in tectonic  
\begin{table}
\begin{tabular}{c c c} \hline
**Coal Sample** & **Fitting Equation** & \(\mathbf{R^{2}}\) \\ \hline Tectonic coal & \(q=0.4634p^{2}-0.8142p+0.4154\) & 0.9979 \\ Intact coal & \(q=0.038p^{2}-0.0465p+0.0524\) & 0.9999 \\ \hline \end{tabular}
\end{table}
Table 5: Relationship between gas flow rate and gas pressure during gas depletion.  
coal area, we should try to stabilize the gas extraction time at high gas pressure stage. While in intact coal area, we should increase the gas extraction time at low gas pressure stage and take permeability enhancement measures to reach the high and stable production of coalbed methane.  
Figure 5: Effective stress-gas flow rate-gas pressure curves of coal during gas depletion. (**a**) tectonic coal; (**b**) intact coal.