## 2 Samples and Methods
### Experimental Methods and Procedures
#### 2.6.1 Initial Parameters Setup  
The saturated samples of sandstone with different pore structures were placed into the rock sample holder of the Rock 600-50-VHT multifield coupling tester. The confining pressure was applied at a rate of 0.1 MPa/min until the minimum confining pressure was 1 MPa, and \(\sigma\)1 = \(\sigma\)2 = \(\sigma\)3 was realized with the mechanical self-balancing system of the device. The thermostat was heated to a minimum temperature of 25 \({}^{\circ}\)C with the heating system, and the temperature was maintained at a predetermined temperature level in real time by the temperature control system. The seepage pressure \(P_{3}\) at the lower end of the sandstone sample and the seepage pressure \(P_{4}\) at the upper end of the rock sample were increased to the minimum water pressure of 0.1 MPa. 6.2 Permeability Test of Sandstones with Different Pore Structures under the Sole Influence of Confining Pressure  
Because the confining pressure should always be greater than the water pressure during this test, this test is divided into a low confining pressure stage and a high confining pressure stage to prevent seepage around the sample. In the low confining pressure stage, first, the water pressure of the inlet (\(P_{3}\)) was increased to 1.3 MPa, and the water pressure \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline \hline
**The Focus** & **The Experimental Scheme** \\ \hline Influence of confining pressure (single-factor analysis) & Fix the temperature at 25 \({}^{\circ}\)C, the water pressure of the inlet at 1.3 MPa or 6 MPa, and the water pressure of the outlet at 1 MPa; observe the variation in the permeability of sandstones with different pore structures when the confining pressure is 1.5, 5, 10, 15, 20, 30 and 40 MPa. \\ \hline Influence of water pressure (single-factor analysis) & Fix the temperature at 25 \({}^{\circ}\)C and the confining pressure at 20 MPa, 30 MPa and 40 MPa; observe the variation in the permeability of sandstones with different pore structures when the water pressure of the outlet is 1 MPa and the water pressure of the inlet is 2, 4, 6, 8, 10 and 12 MPa. \\ \hline Influence of temperature (single-factor analysis) & Fix the confining pressure at 20 MPa, the water pressure of the inlet at 6 MPa, and the water pressure of the outlet at 1 MPa; observe the variation in the permeability of sandstones with different pore structures when the temperature is 25 \({}^{\circ}\)C, 35 \({}^{\circ}\)C, 45 \({}^{\circ}\)C, 55 \({}^{\circ}\)C and 65 \({}^{\circ}\)C. \\ \hline Coupled influence of three factors: confining pressure, water pressure and temperature (three-factors analysis) & As per the L9 (3 \(\times\) 3) orthogonal table, design a orthogonal test with 3 factors and 3 levels to analyze the permeability evolution of deep-mine sandstones with different pore structures under the coupled action of a high confining pressure, high water pressure and high temperature \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The experimental scheme for investigating the seepage of the deep-mine sandstone. of the outlet (\(P_{4}\)) was 1 MPa. Next, keeping the water pressure stable at both ends, the sandstone permeability was measured under a confining pressure of 1.5 MPa and 5 MPa, respectively. In the high confining pressure stage, first, the water pressure of the inlet (\(P_{3}\)) was increased to 6 MPa. Then, keeping the water pressure stable at both ends, the sandstone permeability was measured under confining pressures of 10 MPa, 15 MPa, 20 MPa, 30 MPa and 40 MPa. After the test of the first sample was completed, the next sample was placed and the above steps were repeated until all samples were tested.