## 6 Conclusions  
(1) During the experiment, it was found that the mudstone is more compressible than rock salt, and the compressibility of rock increased with the increase of porosity.  
(2) The pseudo-pressure seepage equation has extensive application value, so the specimen permeability was calculated by the pseudo-pressure method. The Klinkenberg permeability of the synthetic specimens was obtained by curve-fitting. The permeability of synthetic pure rock salt was 6.93 \(\times\) 10\({}^{-20}\) m\({}^{2}\) and its porosity 3.8%, and the permeability of synthetic pure mudstone was 2.97 \(\times\) 10\({}^{-18}\) m\({}^{2}\), with a porosity 17.8%. The test results were close to the natural specimens, and it shows that this synthetic material permeability model is a reasonable analogue of use in engineering.  
(3) Comparing permeability test results for the two kinds of synthetic specimens, it was found that the permeability of the mixed specimens was about 40% higher than that of the layered rock salt specimens at the same mudstone content. This suggests that in salt cavern gas storage cases, the layered rock salt is tight at the top and bottom of the cavity. The permeability of the transition zone of rock salt and mudstone around the cavity is higher, and the transition zone would be the permeable channel to be concerned with; although the permeabilities are still very low, and there would also be the beneficial effect of capillary blockage of the brine-filled channels in the field case.  
(4) We also found that for both kinds of specimens, there was a strong exponential relationship between Klinkenberg permeability and mudstone content: when the mudstone content is below 40%,  
Figure 16: The relationship between \(K_{\infty}\) and mudstone content. (**a**) Mixed specimen; (**b**) layered specimen.  
the Klinkenberg permeability increases only slightly with mudstone content, whereas above this threshold, the Klinkenberg permeability increases significantly.  
Note: The formation mechanisms of natural rock specimen and synthetic specimen were different. Natural rock was formed in the long-term diagenesis, while the synthetic specimens in this paper were formed in ultra-high pressure. The porosity and permeability of synthetic specimens were similar to the natural specimens, but the natural rock (mudstone/salt) had different pore space structures compared to the synthetic specimens. Therefore, the approach, respectively the outcome (permeability/porosity relationship), can only be a first attempt at investigation.  
The authors wish to acknowledge the financial support of the National Natural Science Foundation of China (Grant Nos. 51774266, 51404241, 41602328), National Natural Science Foundation of China Innovative Research Team [Grant No. 51621006], and Natural Science Foundation for Innovation Group of Hubei Province, China [Grant No. 2016 CFA014]. Moreover, the authors wish to thank the reviewers for constructive comments and suggestions that have helped us improve our manuscript.  
The paper was written by [PERSON] under the guidance of [PERSON]. The experiment scheme, data analysis and pre-literature research were carried out by [PERSON] and [PERSON]. [PERSON] deepened the conclusions and did the English editing of the paper. The experiment was carried out by [PERSON] under the help of [PERSON] and [PERSON].  
The authors declare no conflict of interest.