## 5 Analysis of Test Results  
### Comparison of Porosity and Permeability between Synthetic Specimens and Natural Specimens  
With Equation (20), we obtained the specimen gas permeability at four different inlet pressures, and the Klinkenberg permeability and slip factor \(b\) can be determined by the pseudo-pressure method. The results are shown in Figure 10; the indicators a1\(\sim\)i1 in the figure are mixed specimens of mudstone and rock salt, and Specimens a2\(\sim\)g2 are layered rock salt. Figure 10: \(Cont.\) \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**NO.** & **Inlet Pressure (MPa)** & **Reciprocal of Mean Pressure \(1/p\) (MPa\({}^{-1}\))** & **Volume Flow (mL-s\({}^{-1}\))** & \(K_{g}\) (10\({}^{-21}\) m\({}^{2}\))** \\ \hline a1 & 0.333 & 4.74 & 0.008 & \(2.65\times 10^{2}\) \\ a1 & 0.414 & 4.15 & 0.011 & \(2.29\times 10^{2}\) \\ a1 & 0.475 & 3.62 & 0.014 & \(2.18\times 10^{2}\) \\ a1 & 0.545 & 3.19 & 0.017 & \(1.99\times 10^{2}\) \\ a2 & 0.305 & 4.92 & 0.004 & \(1.75\times 10^{2}\) \\ a2 & 0.383 & 4.13 & 0.006 & \(1.60\times 10^{2}\) \\ a2 & 0.450 & 3.63 & 0.008 & \(1.51\times 10^{2}\) \\ a2 & 0.523 & 3.23 & 0.010 & \(1.38\times 10^{2}\) \\ i1 & 0.311 & 4.84 & 0.109 & \(4.43\times 10^{3}\) \\ i1 & 0.377 & 4.18 & 0.158 & \(4.20\times 10^{3}\) \\ i1 & 0.451 & 3.62 & 0.226 & \(4.11\times 10^{3}\) \\ i1 & 0.511 & 3.27 & 0.283 & \(3.96\times 10^{3}\) \\ g2 & 0.322 & 4.77 & 0.093 & \(3.49\times 10^{3}\) \\ g2 & 0.388 & 4.09 & 0.137 & \(3.43\times 10^{3}\) \\ g2 & 0.457 & 3.58 & 0.189 & \(3.34\times 10^{3}\) \\ g2 & 0.518 & 3.23 & 0.242 & \(3.30\times 10^{3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Permeability test results. From Figure 10, we can see that the correlation coefficients are close to one, so the results are consistent and reliable. From the data, the Klinkenberg permeability of synthetic pure rock salt specimens is 6.93 \(\times\) 10\({}^{-20}\) m\({}^{2}\) (the average values of a1 and a2), and that of synthetic pure mudstone specimens is 2.97 \(\times\) 10\({}^{-18}\) m\({}^{2}\) (the average values of i1 and g2). The test results are in agreement with the natural specimens, indicating that the model material is a reasonable analogue of the natural material. At the same time, the permeability of synthetic pure rock salt with a small amount of cement  
Figure 10: Fitting results of \(K_{g}\) and \(1/p\). (**a**) Mudstone content is 20%; (**b**) mudstone content is 40%; (**c**) mudstone content is 50%; (**d**) mudstone content is 60%; (**e**) mudstone content is 80%; (**f**) synthetic pure rock salt and synthetic pure rock salt with binder; (**g**) synthetic pure mudstone and synthetic pure mudstone with binder. and brine (b1) is about 1/3 lower than that of synthetic pure rock salt, and the permeability of synthetic pure mudstone with a small amount of cement and brine (h1) is about 1/3 lower than that of synthetic pure mudstone. The cement and brine in the mudstone and rock salt particles play a role in blocking permeable channels. According to the porosity in Table 3 and the Klinkenberg permeability in Table 6, the effective pore throat radius of the synthetic pure rock salt is 1.445 \(\times\) 10\({}^{-3}\) um calculated by Equation (10) (the average values of a1 and a2), and the effective pore throat radius of the synthetic pure mudstone is 2.059 \(\times\) 10\({}^{-2}\) um (the average values of i1 and g2). From the comparison of the two kinds of specimens in Figure 10, the results show that the permeability of the mixed specimens is about 40% higher than that of the layered mudstone/salt specimens. In a salt cavern gas facility, the permeability of the transition zone of rock salt and mudstone is likely to be higher than that of layered mudstone/salt interbed area. The permeability of the rock salt bands in situ is of course very low because the crystals are large, the grain arrangement is dense and the pore throat equivalent radius is small.