## 6 Conclusions  
The numerous old BWSCs in China serve as crucial carriers for accelerating the large-scale construction of underground gas storage facilities, spurring the rapid development of new energy fields like energy storage and hydrogen storage, and achieving the \"dual carbon\" goals. Among them, the old large-height-difference BWSCs encounter significant challenges regarding the feasibility of their transformation and utilization. Relying on the geological conditions and data of BWSCs in Pingdingshan, Henan province, this paper takes the old large-height-difference BWSCs as the research object. It establishes relevant theoretical and geological models; focuses on issues such as the gas injection and brine discharge methods, technical parameters, and operating pressures of large-height-difference BWSCs; and explores the technical feasibility and safety stability of the transformation and utilization of this type of salt cavity. The main conclusions are as follows:  
1. The gas injection and brine drainage method for the butted-well salt cavity depends on its residual content and cavity structure. A lower residual content makes the TITD method more suitable. When the residual content is high and the height difference between the residual top surfaces of the two-sided salt cavities is large, the OIDD method is preferable. If the residual content is high but the height difference of the residual top surfaces is small, the TIDD method is a better choice. For butted-well salt cavities with a large height difference, the residuals inside the two-sided salt cavities  
Figure 19: Gas pressure difference between the tops of two caverns with butted wells under different height differences. The red dashed line highlights the gas pressure difference when the elevation difference is 200 m.  
also have a significant height difference due to their mining mode, clearly indicating that the OIOD method is the most appropriate. 2. A set of operation parameter models for brine injection and discharge in salt caverns using an OIOD method has been established. These models have been verified through on-site testing and demonstrate good adaptability. They can provide predictive templates for wellhead gas injection pressure, gas injection rate, and casing shoe gas pressure during the GIBD process in butted-well salt caverns, showcasing the technical feasibility of the OIOD method for BWSCs with height differences. 3. The special structural characteristics of large-height-difference butted-well salt caverns lead to significant deformation and damage differences between the two connected caverns. The deeper cavern exhibits greater deformation damage compared to the shallower one, making it the weak point in such systems. Differing from conventional operating pressure determination methods for single-well single-cavity systems, a design model is proposed specifically for the large-height-difference interconnected cavern: adopting the lower limit of operating pressure for the deeper cavern and the upper limit for the shallower cavern. Stability evaluation has confirmed the feasibility of this model, effectively ensuring the operational safety of salt cavern systems. 4. The transformation and utilization of butted-well salt caverns with large height differences plays a significant role in rapidly expanding the scale of underground energy storage. This study demonstrates the feasibility of such transformations and utilizations from aspects of gas injection and brine discharge methods, operational parameters, and stability. However, research in this area is still in its early stages, with challenges that urgently need to be addressed, including the morphological characterization of salt cavities covered by residues, the distribution patterns and temporal effects of real residues within the salt cavities, the flow patterns of gas-driven brine displacement within the residues, and the efficiency of gas storage. These issues represent key areas for future research focus. Conceptualization, D.Z. and Q.W. ; methodology, H.L. and K.L.