## 1 Introduction
The application of salt cavities in underground energy storage is becoming increasingly widespread. An underground salt cavern is mainly built by injecting fresh water into a drilled well from the ground to dissolve the underground salt rock and form cave with a suitable volume. Affected by geological conditions, the salt-bearing strata in China are characterized by a low salt rock grade, multiple interlayers, and thin salt layers, resulting in a large amount of insoluble residue occupying the bottom of the cavity. Moreover, due to the limit of brine reception concentration, it usually takes 4-8 years to build a salt cavern with the conventional single-well construction technology, which seriously hinders the construction of large-scale salt cavern underground storage (SCUG). With the increasing complexity of geological conditions and the growth of storage facilities demand, various cavity construction methods such as two-well cavern construction with small spacing and a horizontal well salt cavern have been proposed [26; 27; 28; 29] (as in Figure 1), but they still have technical bottlenecks [30]. How to efficiently and quickly construct underground salt cavern storage has become a major challenge at present. In this situation, the old cavities left by salt mining companies have attracted people's attention. China has a long history of salt production, and a large number of old salt cavities have been left behind, most of which are butted well salt caverns (BWSC) [31; 32] (as in Figure 2). According to statistics, the space volume of old salt cavities in Jiangsu and Henan provinces in China has reached more than 200 million cubic meters, and it is still growing at a rate of 20 million cubic meters per year. If these old cavities are fully utilized to transformed into underground storage facilities, this can not only eliminate potential risks, but also save a lot of time in cavern construction and quickly form large-scale underground storage capacity [33]. In Europe and North America, the conditions of salt bearing formation are good, and SCUG mainly relies on new cavity construction, without examples of old cavity utilization. In China, the transformation and utilization of old salt cavities is still in its early stage. Some domestic scholars have conducted exploratory research on this technology. [PERSON] et al. [34] introduced the production mode of salt dissolution and brine extraction, and preliminarily explored the basic idea of converting convection well cavities into gas storage facilities. [PERSON] et al. [35] proposed the old cavity renovation technology by plugging old wells and drilling new wells, as well as intermediate brine drainage wells. [PERSON] et al. [36] put out the testing methods and technical requirements for wellbore renovation during the old cavern transformation process. [PERSON] et al. [37] established a three-dimensional geological model of butted well old cavities and explored the cavern stability under the conditions of gas storage and compressed air energy storage. [PERSON] et al. [38] conducted a simulation experiment of gas injection and brine drainage by butted wells, and studied the variation law of operation parameters under different gas injection and brine drainage methods. [PERSON] et al. [39] analyzed the feasibility of transforming old cavities into gas storage from the aspects of geology, engineering, and the economy. Current research mainly focuses on the BWSC transformation and utilization methods such as gas injection and brine removal, stability, and sediment utilization. In fact, the complete morphology of the cavity is the key factor affecting the transformation process. Due to a part of the cavity being occupied by insoluble residue, the instrument is unable to measure its shape, resulting in uncertainty regarding the stability of the cavity itself and cavity groups, the method of gas injection and brine removal, the volume of residue used for gas storage, etc., which affects the utilization of the old cavity.