## 2 Materials and Methods
### Saltstone Stratigraphic Conditions and Cavity Building Plan
Top Plate Position (T): This is the top position of the model, above the cavity. Monitoring displacement changes at this point can help assess the stability of the roof and potential risk of collapse. Left position of the cavity side gang (L): this is the position on the left side of the cavity, and monitoring displacement changes at this point can provide information on lateral stresses and deformations. Right position of the cavity side gang (R): this is the position on the right side of the cavity, corresponding to the left position, and it is equally important to monitor displacement changes at this point to get a full picture of lateral stresses and deformations. Baseline position (B): This is the bottom position of the model, located below the cavity. Monitoring displacement changes at this point helps to assess the stability of the footing and potential subsidence or uplift. Figure 3: Schematic diagram of the gas storage model: (**a**) Six-layer sandwich; (**b**) Eight-layer sandwich; (**c**) Ten-layer sandwich. The data from these observation points can provide a comprehensive understanding of the displacement changes in different parts of the gas storage reservoir during long-term operation, thus providing an important reference for the safe operation and structural optimisation of the gas storage reservoir. Firstly, the construction of the cavity model was carried out, and subsequently, the mechanical parameters were set in FLAC3D to determine the computational parameters for the creep properties of the rock [34; 35], as shown in Table 2. (In the context of salt mines, the aforementioned data are considered standard and are subject to testing at the mine level. The data presented in Table 2 have been sourced from the mine's operational documentation). The initial and boundary conditions of the reservoir cavity have an important influence on the mechanical characterisation of salt cavern reservoirs. According to the research of related scholars, a certain consensus exists on the value of the air pressure cycle in the cavity of the gas storage reservoir, i.e., it is believed that the value of the pressure during the air pressure cycle should be limited to 0.3-0.8 times that of the geopathic stress in the vertical direction. Studies have shown [36] that the main factor affecting the long-term stable operation of gas storage reservoirs is low pressure, so related scholars have proposed that the minimum air pressure is set at 33% or more. Considering multiple factors, the range of circulating gas pressure in the reservoir was finally determined to be 0.3-0.8 times the vertical geopathic stress (10-22 MPa) [37; 38; 39]. In this paper, the range of cyclic air pressure is set to be 12-18 MPa, and the main manifestations of single salt cavern cavity destabilisation include roof failure and cavity deformation, while the stability of the sidewalls needs to be considered as well [40; 41; 42]. In this study, three sets of tests were conducted: a 6-layer mudstone entrapment, an 8-layer mudstone entrapment, and a 10-layer mudstone entrapment. By comparing the displacement changes of the top and bottom plates and sidewalls, as well as the volume shrinkage rate, in these three sets of tests, it is possible to discuss the influence of different numbers of mudstone interlayers on the long-term operation of gas storage reservoirs. The aforementioned test setup can facilitate an understanding of the stability performance of salt caverns under different numbers of entrapment conditions, thereby providing a valuable reference point for the design and operation of gas storage facilities. By monitoring the alterations to the top and sidewalls of the salt caverns, it is possible to ascertain the degree to which varying quantities of mudstone entrapment influence the stability of the salt caverns. This enables the formulation of more logical operational and management strategies.