## 3 Selection of Gas Injection and Brine Discharge Method
### Parameter Determination for BWSCs with a Large Height Difference  
In the case of large-height-difference BWSCs, through an analysis of their construction mechanism, it is found that there is a positive correlation between the cavern height and the height of the residue accumulated at the cavern bottom. Under the optimized OIOD mode, only the residue volume of one side cavern can be used for gas storage. The entire GIBD process can be divided into two distinct stages, as illustrated in Figure 6. In the first stage, gas injection is preferably carried out from the side of the cavern with a high residue level, while brine is discharged from the side of the cavern with a low residue level. The process continues until the wellhead of the salt cavern with a low residue height indicates an increase in pressure and the presence of gas, which was observed in the experiment described in Reference [25]. At this point, the gas injection valve at the wellhead of the cavern with a high residue level is closed. In the second stage, gas injection is switched to the annulus of the side of the cavern with a low residue level, and brine is discharged through its own brine discharge pipe until the gas--liquid interface moves to an appropriate position above the residue top surface. Throughout the GIBD process, the precise control of technology parameters is of crucial importance. Therefore, a set of calculation methods for GIBD parameters has been established. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{3}{*}{**Method**} & \multicolumn{3}{c}{**Gas Injection and Brine Discharge Operation Engineering Quantity and Cost Differences**} & \multicolumn{2}{c}{**Residue**} \\ \cline{2-9}  & \begin{tabular}{c} **Brine Discharge** \\ **and Control** \\ **Device/Sets No.** \\ \end{tabular} & \begin{tabular}{c} **Cost** \\ **10\({}^{\text{4}}\)/CNY** \\ \end{tabular} & \begin{tabular}{c} **Snubbing** \\ **Operation** \\ **Times** \\ \end{tabular} & \begin{tabular}{c} **Cost** \\ **Brine Discharge** \\ **Wells No.** \\ \end{tabular} & \begin{tabular}{c} **Newly Drilled** \\ **Brine Discharge** \\ **10\({}^{\text{4}}\)/CNY** \\ \end{tabular} & \
\begin{tabular}{c} **Cost** \\ **10\({}^{\text{4}}\)/CNY** \\ \end{tabular} &
\begin{tabular}{c} **Total** \\ **Utilization** \\ **Rate** \\ \end{tabular} \\ \hline OIOD & 1 & 850 & 1 & 120 & / & / & 970 & 50–70\% \\ TID & 1 & 850 & / & / & 1 & 1200 & 2050 & 90\% \\ TITD & 2 & 1700 & 2 & 240 & / & / & 1940 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of costs and residue volume utilization rates among the three GIBD methods. Figure 5: Variation law of increased residual space utilization percentage in height-differential cavities between TIOD and OIOD methods with key parameters. The entire OIOD system is analogous to a U-shaped tube. The parameters of gas injection and brine drainage can be determined by establishing system equations based on the U-shaped tube principle. During the GIBD process, the following pressure balance equation exists at any gas-liquid interface within the cavity:  
\[P_{i}=\rho_{b}gh+P_{ft}+P_{fs}+P_{b} \tag{2}\]  
The flow of brine in the discharge pipe column generates flow frictional resistance, which can be calculated using the frictional resistance formula (Equation (3)). The magnitude of this resistance depends on fluid properties, pipe roughness, and flow regime. The flow regime can be determined by the Reynolds number (Equation (4)), while the pipe roughness satisfies Equation (5). The hydraulic friction coefficient is closely related to the flow regime, with empirical formulas provided in Table 3. In 1944, American engineer [PERSON] developed the well-known Moody diagram based on a vast amount of experimental data on pipe friction [26]. Starting from the Darcy-Weisbach equation, this diagram quantitatively correlates fluid property parameters and pipe parameters, with the equivalent roughness serving as a crucial variable.