## 4 Discussion
#### 4.2.3 The Reason for the Permeability Difference under the Sole Influence of Temperature  
The single-factor test of temperature showed that the permeabilities of the four subcategories of sandstone decreased with increasing temperature, but the variation trends were different. The reason were the differences was that the pore structures of the four subcategories of sandstones are different. Subcategory I\({}_{1}\) sandstone had high porosity and a large average pore throat radius (see Table 9). The skeleton grains of this sandstone expanded slowly, resulting in the slow shrinkage of the primary pores and pore throats. Accordingly, the permeability of subcategory I\({}_{1}\) sandstone decreased linearly and slowly with increasing temperature. The porosity and the average pore throat radius of the sandstones in subcategories I\({}_{2}\) and II\({}_{1}\) were moderate (see Table 9). With increasing temperature, the primary pores and pore throats of sandstones slowly shrank, and the pore permeability slowly decreased at the beginning. However, an increasing number of pores and pore throats began to close when the temperature was higher than 45 \({}^{\circ}\)C, and the closure of pores led to a rapid decline in permeability. For this reason, the permeability of subcategory I\({}_{2}\) and II\({}_{1}\) sandstone decreased slowly at the beginning, but the deceleration increased significantly when the temperature was higher than 45 \({}^{\circ}\)C. The porosity of subcategory II\({}_{2}\) sandstone was low, the average pore throat radius was small (see Table 9), and the number of connected pores was very small, and a light pore throat change caused by temperature change led to the plugging of the pores. Therefore, the permeability decreased rapidly at \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sandstone Subcategories** & **I\({}_{1}\)** & **I\({}_{2}\)** & **II\({}_{1}\)** & **II\({}_{2}\)** \\ \hline S\({}_{\text{o}}\) & \textless{}1.00 & 1.00–1.40 & 1.40–1.60 & \textgreater{}1.60 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: The sorting coefficients of four subcategories of sandstone. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sandstone Subcategories** & **I\({}_{1}\)** & **I\({}_{2}\)** & **II\({}_{1}\)** & **II\({}_{2}\)** \\ \hline \(\Phi\) (\%) & \textgreater{}20 & 15–20 & 7–15 & \textless{}7 \\ R\({}_{\text{ave.}}\) (\(\upmu\)m) & \textgreater{}3.7 & 0.7–3.7 & 0.14–0.7 & \textless{}0.14 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: The porosities and average pore throat radii of the four subcategories of sandstone. the beginning of the temperature rise, but when the temperature continued to increase to more than 45 \({}^{\circ}\)C, the deformation of the sandstone pores was increasingly smaller, and the change in the permeability value was increasingly smaller. 2.4 The Reason for the Permeability Difference under the Coupling Action of the \"Three-High\" Factors  
The comparison of the orthogonal test results of subcategory I\({}_{2}\) and subcategory II\({}_{2}\) sandstones showed that different subcategories of deep-mine sandstones behaved differently:  
1. The order of factors influencing the seepage of subcategory I\({}_{2}\) sandstone was confining pressure > water pressure > temperature, while that of subcategory II\({}_{2}\) sandstone was confining pressure > temperature > water pressure. 2. The permeability of subcategory I\({}_{2}\) sandstone first increased and then decreased with increasing water pressure difference, while that of subcategory II\({}_{2}\) sandstone first decreased and then increased with increasing water pressure difference. 3. The permeability of subcategory I\({}_{2}\) sandstone decreased slowly with increasing temperature, while that of subcategory II\({}_{2}\) sandstone decreased rapidly with increasing temperature.