## 2 Specim Preparation  
### Material Selection and Specim Preparation  
In order to study the permeation behavior of rock salt with mudstone, we use natural mudstone from the gas storage facility in Jintan from a depth of approximately 934 m-935 m. Relatively pure rock salt is used to develop mud rock/salt specimens with different mudstone content. The intact rock salt has grains that are small and evenly distributed, and the grain boundaries are not strongly apparent. The salt is pale red and has a content of soluble matter over 96.3%. Figure 1: Schematic diagram of a salt cavern gas storage facility. The weakly consolidated clay forming a strongly consolidated rock through moderate epigenetic effects (such as compaction, dehydration, recrystallization and cementation) is called mudstone. The natural mudstone used in this paper is from the gas storage facility in Jintan from a depth of approximately 934 m-935 m. The mudstone we use is hard, brittle and relatively dense and has some secondary structures such as porous structure, pinhole structure and honeycomb structure in locality. Some of the internal cracks of mudstone are filled by glauberite in a later stage and are stellate distributed. X-ray diffraction shows that quartz, dolomite and illite have the largest proportions in the mudstone, and the average quartz content is about 32%. The mudstone has a small amount of interstitial salt, and the main soluble mineral in the mudstone is glauberite, Na\({}_{2}\)Ca(SO\({}_{4}\))\({}_{2}\). The XRD result of mudstone is shown in Figure 2.  
The pure rock salt and mudstone (Figure 3) were pulverized into 0.5-mm and 2-mm particles respectively, uniformly mixed with a small amount of cement and near-saturated brine (\(\sim\)1.18 g/cm\({}^{3}\)). The pulverized rock salt particles are then placed in a drying box for 24 h, and the mudstone particles are put in a sealed bag. We make specimens in accordance with the prescribed method [(23)] with a diameter of 25 mm and an H/D ratio of about two. The rock salt particles and mudstone particles are pressed and formed  
Figure 3: Material selection. (**a**) Mudstone; (**b**) rock salt; (**c**) the particles of mudstone; (**d**) the particles of rock salt. Figure 2: XRD results of mudstone. by a hydraulic pressure tester, with material proportioning shown in Table 1. Specimens are prepared as follows:  
1. Mixed specimens of rock salt with mudstone: 1. Weigh specified amounts of rock salt particles and mudstone particles, as well as cement and brine, then put them in a beaker and stir until homogeneous. 2. Add the mixed material to the pressing mold. 3. Using 120 MPa of compression stress, hold for 2 h.
2. Layered specimens of rock salt with mudstone: 4. Weigh a specified amount of particles of rock salt and mudstone. 5. Place prepared material in the mold in the order of rock salt-mudstone-rock salt. 6. Using 120 MPa of compression stress, hold for 2 h.  
Note that in nature, the prolonged geological process of compaction takes place under higher temperature, but lower stress for millions of years. In the laboratory, we use ultra-high pressure to yield an artificial specimen with similar porosity to the natural specimens, but of course, there is no time for recrystallization through internal mass transfer. The specimens are placed in a drying box for 24 h; the physical parameters are obtained, and then, the permeability test is performed. The density of synthetic pure salt is 2.125 g\(\cdot\)cm\({}^{-3}\), and the density of synthetic pure mudstone is 2.283 g\(\cdot\)cm\({}^{-3}\), values similar to those from the research of [PERSON] et al. [22]. The porosity of synthetic pure rock salt is 3.8% with a permeability of 6.93 \(\times\) 10\({}^{-20}\) m\({}^{2}\), and the porosity of synthetic pure mudstone is 17.8% with a permeability of 2.97 \(\times\) 10\({}^{-18}\) m\({}^{2}\). These values are close to those of natural specimens. The two kinds of synthetic specimens are shown in Figure 4, and the physical parameters are listed in Tables 2 and 3.