## 4 Stability Evaluation
### Geomechanical Model Establishment  
Based on the geological conditions of the Haolong area in the Pingdingshan Salt Mine, Henan province, the strata information is analyzed. The Pingdingshan Salt Mine features 21 sets of salt groups developed sequentially from top to bottom, with the uppermost being the No. 0 salt group and the lowermost the No. 20 salt group. According to the statistical data from drilled wells in the Haolong area, the final drilling formations of all the wells operated by the salt mining companies are the first member of the Hetaoyuan Formation. The salt-mining intervals span from the No. 14 to the No. 18 salt groups, and there are multiple mudstone interlayers of varying thicknesses within the salt layers. The salt cavern in Figure 2 reparents an old cavern mined by butted wells in the Haolong area. According to the sonar cavity-measuring data, the shallow cavity has a top depth of 1527 m and a bottom depth of 1585 m, while the deep cavity has a top depth of 1722 m and a bottom depth of 1780 m. The total volume of the two cavities is approximately 243,000 cubic meters. To investigate the influence of height difference on the deformation and damage of the BWSC, a regular salt cavern model was used to substitute the real one, and the shape of the residue was simplified, taking into account the height-difference data of the salt cavern in Figure 2, wellbore trajectory data, and stratigraphic conditions. A three-dimensional geomechanical model containing a large-height-difference BWSC was then established, as depicted in Figure 9. The model has dimensions of 740 m in length, 300 m in width, and 1000 m in height. The spacing of the two wells of the large-height-difference BWSC is 260 m. Each cavity has a height of 60 m and a diameter of 80 m. The top height difference between the two cavities is 195 m, and the height of the residue channel is 15 m. To simplify the calculation, an overlying pressure with a value of 25.46 MPa is applied on the top surface of the model, calculated based on the actual depth and the average density of the overburden strata (\(\rho=2300\) kg/m\({}^{3}\)). Normal displacement constraints are applied to the four lateral surfaces of the model, meaning they are just allowed to produce displacement in the vertical direction. Considering the stress release in a long-term rheological environment, the in situ stress of the rock salt formation is applied using equal principal stresses in three directions, with an in situ stress gradient of 0.023 MPa/m, and its redistribution is firstly calculated before creep analysis during the numerical simulation. The gas pressure is applied on the inner surface of the cavity. The entire model is meshed with tetrahedral elements. The mesh quality meets the accuracy requirements, consisting of a total of 755,780 meshes and 1,029,697 nodes. The areas around the BWSC model are set with encrypted meshes, and the meshes gradually become sparser as the distance from the salt-cavern model increases. The Flac3D v6.0 software is used to build the whole model and carry out numerical simulation analysis. The mechanical constitutive model uses the Cpower model in the software, which integrates the Mohr-Coulomb elastoplastic model and the Norton Power creep model. The rock mechanical parameters are shown in Table 5, and the mechanical parameters of the residue are set as 10% of those of the interlayers. The simulation results are post-processedusing the Tecplot 360 v2015 R1 software. The maximum unbalanced force was monitored to check the convergence of the model during the calculation.