## 5 Analysis of Test Results
### Comparison of Porosity and Permeability between Synthetic Specimens and Natural Specimens
[PERSON] (1993) found that the interface of rock salt and mudstone is not weak, and the rock salt crystals and mudstone particles form a zig-zag interface. Because the intact rock salt permeability is low, the interface of the rock salt with the mudstone plays a leading role in overall permeability. Note that the equivalent pore throat size of mixed specimens formed by two kinds of particles is larger than that of layered rock salt, so the permeability of the mixed specimens is slightly higher than the layered rock salt. The fitting results of Klinkenberg permeability are shown in Table 6.  
We tested the porosity and permeability of natural specimens in order to compare with synthetic specimens. The porosity of natural pure rock salt is 3.1%, and the porosity of synthetic pure mudstone is 16.9%. The porosity of synthetic pure rock salt is 3.8%, and the porosity of synthetic pure mudstone is 17.8%. The porosity results show that the synthetic specimen porosity is in good agreement with the natural specimen. The picture of the natural specimens and the permeability results can be seen in Figures 11 and 12. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & \(\mathbf{K_{\infty}}\) **(10\({}^{-21}\) m\({}^{2}\))** & \(\mathbf{b}\) **(MPa)** & \(\mathbf{R^{2}}\) \\ \hline a1 & 67.1 & 0.61 & 0.963 \\ b1 & 52.0 & 0.33 & 0.932 \\ c1 & 5.60 \(\times\) 10\({}^{2}\) & 0.45 & 0.999 \\ d1 & 8.68 \(\times\) 10\({}^{2}\) & 0.26 & 0.951 \\ e1 & 1.58 \(\times\) 10\({}^{3}\) & 0.19 & 0.999 \\ f1 & 1.74 \(\times\) 10\({}^{3}\) & 0.17 & 0.968 \\ g1 & 1.95 \(\times\) 10\({}^{3}\) & 0.19 & 0.970 \\ h1 & 2.12 \(\times\) 10\({}^{3}\) & 0.06 & 0.966 \\ i1 & 3.06 \(\times\) 10\({}^{3}\) & 0.09 & 0.979 \\ a2 & 71.6 & 0.30 & 0.983 \\ b2 & 3.76 \(\times\) 10\({}^{2}\) & 0.22 & 0.982 \\ c2 & 5.74 \(\times\) 10\({}^{2}\) & 0.24 & 0.924 \\ d2 & 1.03 \(\times\) 10\({}^{3}\) & 0.29 & 0.981 \\ e2 & 1.26 \(\times\) 10\({}^{3}\) & 0.23 & 0.986 \\ f2 & 1.52 \(\times\) 10\({}^{3}\) & 0.28 & 0.961 \\ g2 & 2.87 \(\times\) 10\({}^{3}\) & 0.05 & 0.980 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The fitting results. From Figure 12 and Table 7, we can see that the permeability of natural pure salt rock is 4.23 \(\times\) 10\({}^{-20}\) m\({}^{2}\), and the permeability of natural mudstone is 2.22 \(\times\) 10\({}^{-18}\) m\({}^{2}\). From Figure 10, we know that the permeability of synthetic pure salt rock is 6.93 \(\times\) 10\({}^{-20}\) m\({}^{2}\), and the permeability of synthetic mudstone is 2.97 \(\times\) 10\({}^{-18}\) m\({}^{2}\). The permeability results show that the synthetic specimens are in good agreement with the natural specimen. The test results of porosity and permeability of synthetic specimens are close to the natural specimens, which shows that the synthetic material model for permeability testing is a reasonable analogue for the ranges we addressed to emulate the natural cavern conditions. \begin{table}
\begin{tabular}{c c c c} \hline
**No.** & **Specimen/Specimen Type** & **Porosity (\%)** & \(\mathbf{K_{\infty}}\) (10\({}^{-21}\) m\({}^{2}\))** \\ \hline a1 & Synthetic pure rock salt & 4.9 & 67.1 \\ a2 & Synthetic pure rock salt & 2.6 & 71.6 \\ A1 & Natural pure rock salt & 3.1 & 42.3 \\ i1 & Synthetic pure mudstone & 19.3 & 3.06 \(\times\) 10\({}^{3}\) \\ g2 & Synthetic pure mudstone & 16.4 & 2.87 \(\times\) 10\({}^{3}\) \\ M1 & Natural pure mudstone & 16.9 & 2.22 \(\times\) 10\({}^{3}\) \\ \hline \end{tabular}
\end{table} 
Table 7: The porosity and permeability of the synthetic specimens and natural rock specimens. Figure 11: Natuural specimens. Figure 12: Natural specimen permeability results. (**a**) Natural pure rock salt; (**b**) natural pure rock salt. Note: We define the salt rock with high salt content as pure salt rock (>95%, both for the natural rock salt and synthetic rock salt).