## 3 Selection of Gas Injection and Brine Discharge Method  
### Proposition of Different GIBD Methods  
Gas injection and brine drainage is the final step in transforming old salt caverns into energy storage facilities and is a pivotal determinant of their storage capacity. In China, the layered salt formations often contain insoluble interlayers. As a result, a substantial amount of insoluble residue accumulates at the bottom of salt caverns, notably diminishing their gas storage volume. In recent years, the idea of utilizing the residue space at the cavern bottom for gas storage has been proposed and explored [18; 19]. This initiative aims to boost the gas storage capacity of salt caverns and fully capitalize on their potential. The utilization of the residue space for gas storage is closely related to the GIBD method. In foreign salt dome formation, the salt rock boasts high purity, and there is little residue at the bottom of the salt caverns. Therefore, the GIBD processes do not account for residue utilization. However, in China, especially in the measured old caverns formed by butted wells, the residue can take up as much as 90% of the cavern volume. Disregarding this residue volume would lead to a significant waste of gas storage space [20]. The implementation of the GIBD method hinges on the overall shape of the salt cavern. For interconnected-well salt caverns, the shape of the cavern occupied by the residue and the connecting channels cannot be measured by instruments, which restricts the transformation process. Currently, an estimation method (Equation (1)) based on brine extraction volume, well logging data, etc., has been proposed to calculate the volume of the bottom residue, providing key parameters for the economical utilization of old caverns. Moreover, based on the wellbore trajectory, the shape of the upper cavity, the residue volume, and practical drilling knowledge [21; 22], the depth of the  
Figure 3: Schematic diagram of the formation process of butted-well salt caverns with a height difference from salt mining enterprises. (**a**) Drilling a vertical well to perform cavity dissolution; (**b**) Drilling a directional well to establish connection, injecting water through the vertical well, and extracting brine via the directional well; (**c**) Injecting water through the directional well and extracting brine via the vertical well; (**d**) Continuation of water injection through the directional well and brine extraction via the vertical well. The arrow indicates the fluid flow direction.  
top interface of the bottom residue and the connecting channels can be roughly described, which provides a basis for making decisions regarding the GIBD method.  
\[V_{se}=\frac{V_{b}\cdot c\cdot a\cdot\beta}{\rho_{s}\cdot(1-\alpha)} \tag{1}\]  
For the butted-well salt caverns, different GIBD methods result in different utilization rates of the residual space volume and involve different construction techniques. Based on this, three gas injection and brine drainage methods are proposed--\"one injection and one discharge (OIOD)\", \"two injection and one discharge (TIOD)\", and \"two injection and two discharge (TITD)\"--as illustrated in Figure 4. The OIOD method refers to a process where a brine drainage pipe string is lowered into only one well. Gas is injected through one well, while brine is drained from the other well. After gas injection on one side is completed, independent gas injection and brine drainage operations are carried out on the side of the brine-draining well. The TIOD method requires drilling a new well in the middle of the two existing wells to get into the connecting channel. Gas is simultaneously injected through the two side wells, and brine is drained from the new drilling well. The TITD method involves lowering brine drainage pipe strings into both wells down to the top surface of the residue, and each well undergoes independent GIBD operations. The advantages and disadvantages of the three GIBD methods are shown in Table 1.