## 1 Introduction
Their results also showed large effects of increased stiffness (>50%) and unconfined compressive strength (>15%) in specimens that had been re-stressed to their in situ stress (annealed) for 72 h. Indeed, sampling damage and slow grain boundary annealing likely account for a substantial amount of the experimental scatter in laboratory measurements of permeability and transient creep in salt and can be taken as evidence that properties are altered in the EDZ in the ground. In summary, during the investigation of the physical and mechanical properties of interlayers in bedded salt deposits and their effects on storage caverns, it is found that the porosity and permeability of salt and shale are minuscule and of the same order, and values for anhydrite may be somewhat greater. Specim damage is a significant issue and must be recognized during test programs. In bedded salt storage caverns, anhydrite interlayers may present a greater risk of being a leakage path than shale interlayers for several reasons--creep incompatibility, stiffness and higher intrinsic permeability--so evaluation and testing are needed. [PERSON] and [PERSON] [16] used a pressure pulse method for rock salt with a permeability below \(10^{-17}\) m\({}^{2}\) and found that the permeability within the EDZ is \(10^{-16}\)-\(10^{-20}\) m\({}^{2}\), whereas the permeability of intact salt is less than \(10^{-21}\) m\({}^{2}\). [PERSON] et al. [17] tested the permeability of rock salt under different osmotic pressures and compared results using the Klinkenberg effect and the quasi-static pressure method. [PERSON] et al. [18] analysed carbonate strata storage permeability and established relative models for excavation radius, permeability and porosity. [PERSON] et al. [19] used the equivalent boundary gas percolation model to study the gas pressure distribution in the surrounding rock within five yearsunder different injection pressure conditions in a salt cavern natural gas storage and found that the permeability of bedding surfaces between rock salt and non-salt interlayers has an important influence on the reservoir pressure distribution. Other similar cases involving materials from salt cavern gas storage core holes have been studied in China to generate models that can allow some generalization of the results. Taking the rock salt underground oil and gas storage facility in Jintan, Jiangsu Province, as a prototype, [PERSON] et al. [20] developed a reservoir medium geomechanical model for permeability. [PERSON] et al. [21] developed similar materials for cavity experiments, and based on similarity, [PERSON] et al. [22] developed an artificial model material of rock salt with interlayers. There are few studies on the permeability characteristics of mudstone-rock salt mixes in the laboratory or in the field, so it is difficult to provide reliable guidance for the construction and maintenance of the pressure integrity for caverns in salt strata with non-salt interlayers. A representative stratigraphy of salt caverns in China is shown in Figure 1.  
In this paper, we report on the development of a model material of rock salt with mud and then study and analyze its permeability. We took a storage cavern mudstone interlayer from Jintan and pure rock salt as our basic ingredients and made two kinds of synthetic specimens--mixed and layered--and then carried out permeability tests. These synthetic specimens can be made with any mudstone content, and once a specimen is made, it is straightforward to study the relationships between permeability and mudstone content. Indeed, the permeability testing reveals a reasonably regular relationship between the permeability of rock salt with mudstone and mudstone content, and we then explained the experimental phenomena and results. These results may help provide some methods and guidance for the study of the tightness of layered rock salt gas storage facilities in China.