## 5 Discussion
### Operation Pressure Determination of BWSC with Large Height Difference  
The above stability analysis results indicate that under low pressure, there are significant differences in deformation and damage between the deep and shallow cavities of the large-height-difference BWSC, which is significantly different from the small-height-difference case in Reference [46]. This implies that the application of this type of gas storage requires special attention to the design of the operating pressure to ensure its safe operation. For the large-height-difference BWSC, considering an average in situ stress gradient of 0.023 MPa/m, and following the pressure determination method in Section 4.2.1, the upper-limit pressure difference and lower-limit pressure difference under different depth differences between the tops of the two salt caverns are calculated, and the variation curves are plotted as shown in Figure 18. The figure shows that as the depth difference between the tops of the two caverns increases, both the upper-limit pressure difference and the lower-limit pressure difference increase linearly, and the change in the upper-limit pressure difference is more significant than that in the lower-limit pressure difference. When the depth difference reaches 200 m, the upper-limit pressure difference is approximately 3.5 MPa, and the lower-limit pressure difference is approximately 1.4 MPa. This means that the upper-limit pressure of the shallow cavity is about 3.5 MPa lower than that of thedeep cavity, and the lower-limit pressure is about 1.4 MPa lower, indicating a significant operation pressure difference. Under this difference, there is a problem that the operating pressure may meet the requirements of the deep cavity but not those of the shallow cavity. For example, the upper-limit pressure of the deep cavity may be higher than that of the shallow cavity. If the upper-limit pressure of the BWSC is designed according to the casing shoe depth of the deep cavity, there is a risk that the upper-limit pressure of the shallow cavity is too high, which may damage the cavity's sealing. Given the significant structural differences in large-height-difference BWSCs, when the two cavities are connected and share the same injection-production pressure system, the operating pressure design must simultaneously meet the safety and stability requirements of both the deep and shallow cavities. Therefore, the conventional design mode of the upper and lower operating pressures for single-cavity salt caverns is no longer applicable. In addition, when the large-height-difference BWSCs are filled with natural gas using the OIOD method, the wellhead pressure is adjusted to be equal through pressure balance. The gas pressure difference at the casing shoes at the tops of the two caverns can be calculated using Equation (7). For example, with an upper-limit pressure of 25.5 MPa and in combination with the parameters in Table 3, the results of the gas pressure difference at the tops of the two caverns under different height differences are shown in Figure 19. As shown in the figure, the gas pressure difference at the tops of the two caverns increases gradually as the height difference between them grows. When the height difference between the two caverns reaches 200 m, the calculated gas pressure difference at the casing shoes of the cavern tops is approximately 0.4 MPa, which reflects the characteristic of a small gas pressure difference at the casing shoes of the two caverns in large-height-difference BWSCs. This is significantly different from the pressure limit difference at the casing shoes of the two caverns determined by the formation stress gradient. After the large-height-difference BWSCs are filled with gas and the pressure is balanced, although the top depths of the two caverns differ greatly, the gas pressures at the casing shoes are almost the same and can  
Figure 18: Variation curves of upper-/lower-limit pressure differences between two interconnected salt caverns versus the depth difference of cavern roofs.