## 1 Introduction  
With the increasing severity of global climate change, sustainable development has become a major challenge facing the world today. Low-carbon development to achieve carbon peak and carbon neutrality has become an international consensus for addressing climate change and promoting green and sustainable development [1; 2; 3]. Energy transition is a crucial part of the global response to climate change, with the core being the construction of a new green and low-carbon energy system [4]. The \"Fostering Effective Energy Transition 2024\" report released by the World Economic Forum uses the Energy Transition Index to evaluate the performance of the current energy systems of 120 countries. Europe continues to lead the global Energy Transition Index rankings, with the top 10 completely composed of European countries (Figure 1). This is mainly due to their substantial investment in the research and development of clean energy technologies and the widespread use of clean energy [5; 6]. In recent years, China has also made significant progress in the energy transition. It has not only significantly increased the production capacity of renewable energy but has also further developed and invested in the manufacturing capabilities of clean energy technologies such as electric vehicle batteries, solar panels, and compressed-air energy storage [7]. Among various clean energy technologies, the vigorous development of underground energy storage facilities is considered crucial for China to achieve \"carbon neutrality\" and upgrade its energy structure [8; 9]. Underground energy storage refers to the use of deep underground spaces to store energy or energy substances such as oil, natural gas, hydrogen, compressed air, and carbon dioxide, as well as strategic scarce materials like helium [10]. Salt caverns are considered the most ideal underground energy storage facilities and have attracted wide attention. Figure 1: Energy Transition Index (ETI) scores of the top 20 countries [5]. Since China built its first salt-cavern gas storage facility in 2012, it has successively planned and designed more than ten salt-cavern gas storage facilities in cities like Pingdingshan, Huai'an, Chuzhou, Jianghan, and Zhangxing. In recent years, it has also planned the construction of multiple salt caverns for compressed-air energy storage, hydrogen storage, and helium storage in the cities of Yingcheng, Heze, Feicheng, Jintan, and Yexian, marking that the underground energy storage industry has entered a golden development period. Restricted by the geological conditions of complex layered salt strata, the construction process of salt-cavern underground energy storage facilities in China is slow, severely restricting the development of the energy storage industry [11]. Taking the Jintan salt-cavern gas storage facility as an example, the geological conditions of the Jintan salt mine are relatively good, with an average mineral content of 85%, a depth between 1000 and 1200 m, and a low interlayer content. The construction period of a 300,000-cubic-meter salt cavity can be four years at the shortest and as long as eight years, with a large time span. The salt mines for over 10 upcoming salt-cavern storage facilities in Pingdingshan, Huai, Chuzhou, Zhangxing, and other locations exhibit characteristics of low salt rock quality, high residue content, and deep burial depths. Their geological conditions are worse than those of the Jintan salt mine. Referring to the construction model of the Jintan gas storage facility, the construction period will be longer. In addition, the proposed construction models for two-well and horizontal-well salt caverns and the utilization of residues are still in the exploration stage, and there is still a long way to go for on-site application [12]. Against this backdrop, the utilization of old salt caverns has been put on the agenda and has attracted wide attention [13]. Thanks to China's long-standing salt mining history, salt enterprises have left behind a large number of old salt caverns from salt production.