## 3 Selection of Gas Injection and Brine Discharge Method
### GIBD Method for BWSCs with a Large Height Difference  
For large-height-difference butted-well salt caverns, both the OIOD and TIOD methods are applicable from the perspective of residue utilization. The large-height-difference salt cavern in Figure 2 is simplified into the cavern model in Figure 5. The two side caverns are cylindrical with semi-ellipsoidal tops, and some height-difference parameters remain the same. The height of the cavities on both sides and the diameter of the cylinders are kept consistent. By using the single-variable method and fixing the total height of each of the two caverns, the variation law of the percentage increase in the residue utilization  
Figure 4: Schematic diagram of the three gas injection and brine drainage methods. (**a**) OIOD; (**b**) TIOD; (**c**) TITD. The arrow indicates the fluid flow direction. The yellow area represents the region occupied by gas, and the blue area represents the brine region. volume of the TIOD method compared to the OIOD method in large-height-difference BWSCs with various parameters is analyzed, as shown in Figure 5. It can be seen from the figure that the percentage increase in the residue volume decreases with the increase in the height of the connecting channel and the height of the brine cavity, while it increases as the height of the residue in the deep cavern increases. It basically does not change with the synchronous change in the diameters of the two caverns. Compared to the OIOD method, the TIOD method primarily enhances the residue volume on the side of the deep cavern. When the height of the residue in the deep cavern is small or the height difference between the residues in the two caverns is large, the increase in the residue volume is also limited. Furthermore, the gas storage capacity of the residue depends on the gas storage efficiency of the void space within the residue. Some scholars' research indicates that the gas storage efficiency of the void space in the residue formed by the dissolution of typical layered salt rocks ranges from 8.2% to 13.6% [23; 24]. If calculated at 10%, an increase of 300,000 cubic meters in the residue volume by the TIOD method compared to the OIOD method only corresponds to an increase of about 30,000 cubic meters in the gas storage volume. From a technological and economic perspective, the TIOD method necessitates drilling a new well, which elevates the drilling cost compared to the OIOD method. Additionally, it is challenging to drill into the residue channel without a full understanding of the height of the connecting channel and the shape of the residue. For the purpose of facilitating understanding, taking the example of a butted-well salt cavern, a comparative list of the cost and residue volume utilization rate differences among the three gas injection and brine discharge methods is shown in Table 2. As can be seen from the table, apart from some common expenses, the cost of the TIOD method is much higher than that of OIOD method. After a comprehensive analysis of economic and technological feasibility, the OIOD method is recommended as the optimal GIBD method for large-height-difference BWSCs. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Method** & **Advantage** & **Disadvantage** & **Applicable Objects** \\ \hline \multirow{6}{*}{OIOD} & \multirow{6}{*}{ \begin{tabular}{c} A set of conventional simplified \\ brine drainage equipment \\ Capable of utilizing residual \\ slag space at the bottom of one \\ side cavity \\ Requiring only one conventional \\ snubbing operation \\ \end{tabular}  } & \multirow{6}{*}{ \begin{tabular}{c} Long brine drainage time \\ High pressure control \\ requirements \\ \end{tabular}  } & \multirow{6}{*}{ \begin{tabular}{c} The residual volume is \\ comparable to the cavity \\ capacity, or a significant height \\ difference exists in residues \\ between the two caverns. \\ \end{tabular}  } \\ \hline \multirow{6}{*}{TIOD} & \multirow{6}{*}{