## 2 Materials and Methods
### Test Methods  
Soft rock analogues and water solutions are synthesized as shown in Table 1. When synthesizing soft rock analogues, the total mass of clay minerals and quartz is 100 g and the mass of water is 5 g. Place the mixture of the above materials evenly in a cylindrical mold (diameter 50 mm) and hold the pressure for 10 min with an uniaxial compression testing machine according to the vertical stress shown in Table 1 (Figure 2a). Take the sample out of the mold and put it into an oven heated to 105 \({}^{\circ}\)C until its mass reaches a constant value. The sample is then cooled to ambient temperature in a desiccator. Finally, in order to prevent the sample turning muddy during water absorption, it is sealed with porous stones (diameter 50 mm, thickness 5 mm) and heat shrinkable tubes. The middle part of the bottom surface is made into a circle (diameter 38 mm) for the water absorption tests (Figure 2b). In this study, 18 samples are synthesized for tests. In order to determine the porosity of the samples by a nondestructive monitoring method, the P-wave velocity \(V_{p}\) (km/s) of synthetic soft rock analogues is measured by C61 non-metallic ultrasonic detector. The porosity of the test sample could be obtained by the relationship between the P-wave velocity and the rock porosity \(n\) (%) (Equation (1), Figure 3) established by [PERSON] [27]. \[n=-5.19V_{p}+27.1, \tag{1}\]  
Then, weigh a certain mass of salt according to the columns of CTs, ATs and CCs in Table 1. Put the salt in deionized water and add HCl and NaOH (considering the contents of Na\({}^{+}\) and Cl\({}^{-}\)) to prepare solutions with certain pH values and mix evenly. Finally, 100 mL of solution supernatant is used as the test solution. In this study, 18 water solutions are synthesized for tests. The water absorption test system is composed of a container, sample, water solution, high-precision electronic scale (Figure 2c), porous stone, etc. The sample is not in direct contact with the solution, but through contact with porous stone to reduce the saturation velocity of the sample (Figure 2d). Each sample is tested for regular intervals. A total of 32 tests are carried out for a total time of 4980 min (The samples are saturated or nearly saturated at 4980 min, and the extension of the test time has no effect on the main conclusions of this study). In each test, the total mass of the water absorption container including the sample should be weighed first and then the remaining mass of the water absorption container without the sample should be weighed. The change in \begin{table}
\begin{tabular}{l l l l l l l l} \hline
**Exp.** & **CMT** & **CMC (wt\%)** & **VSs (MPa)** & **CTs** & **ATs** & **CCs (mol/L)** & **pH** \\ \hline  
a\({}^{+}\) & SO4\({}^{2-}\) & SAT & 4 \\ \hline \end{tabular} Note: Exp.—experiment number; CMT—clay mineral types; CMC—clay mineral contents; VS—vertical stresses; CTs—cation types; ATs—anion types; CCs—concentrations of water solutions; MMT—montmorillonite; ILL—illite; KAO—kaolinite; SAT—saturated state of concentrations.  
\end{table} 
Table 1: Experimental design. water absorption in each test is calculated according to Equation (2), so as to eliminate the experimental error caused by solution evaporation. After the test, remove the heat shrinkable tube and porous stones on the surface of the sample and cut them from the middle. It can be seen that the water absorption of the sample is very uniform (Figure 2e). \[\Delta w_{i}=\frac{(m_{i-1}-m_{i})-(M_{i-1}-M_{i})}{M_{s}}\times 100\%, \tag{2}\]  
where, \(\Delta w_{i}\) (%) is the change in water absorption when the test is conducted to \(i\), \(i\) and \(i\)-1 are the test numbers, \(M\) (g) is the total mass of the water absorption system including the sample, \(m\) (g) is the residual mass of the water absorption system without the sample and \(M_{s}\) (g) is the initial mass of the sample. Figure 3: Relationships between P-wave velocity (\(V_{p}\)) and porosity (\(n\)) with vertical stress (VSs).