Technological path and geological guarantee for energy storage in underground space formed by coal mining

Coal resource is still in the main position of China’s energy structure, but the development of coal industry is facing the new challenge of “carbon peaking and carbon neutrality”. Actively developing coal underground space energy storage technology is an effective means to promote low-carbon and clean energy utilization, and also a key measure to ensure China’s energy strategic security. This study discussed the potential utilization of underground space formed by coal mining based on the current energy storage technologies. Focusing on new energy storage technologies such as underground pumped storage hydropower plants in coal mines (UPSHCM), thermal energy storage (TES), compressed air energy storage (CAES), electrochemical energy storage (EES), coal underground space biomass energy storage (CUBES), and other storage based on underground space of coal mining, the concept and mode of energy storage for different types of energy in abandoned mines are mainly elaborated, and the key technical problems of geological guarantee faced in the process of energy storage are systematically analyzed. The general idea of the technology of energy storage in underground space formed by coal mining is: using the underground space formed by coal mining with low potential energy difference as a cascade reservoir (UPSHCM), or as a media and energy storage space (TES, CAES, EES, CUBES, etc.), which can not only improve the utilization rate of underground space formed by coal mining, avoid the waste of land resources, but also minimize the disturbance to the ecological environment. Although the underground space formed by coal mining can be used as a large-scale energy storage reservoir, there are still some geological problems and geological guarantee techniques to be solved urgently in its development and utilization process. The followings are mainly included: (1) Analysis and safety evaluation of geological conditions and siting suitability. Specifically, this study should be conducted on the geological factors of energy storage space in terms of geotechnical engineering properties and environmental geological conditions, to identify the main control factors of energy storage space stability and their weights, as well as establish the site selection index system and evaluation methods. Thereby, the regional structure, strength of surrounding rock and development of holes and fissures can be found out to ensure the safety of the site. (2) Construction of energy storage geological body and research on key technologies. That is, the multi-scale performance evolution model of the geological body under the coupling conditions of thermal (T), hydrological (H), mechanical (M) and chemistry (C) of the energy storage geological body should be constructed according to the actual conditions of underground space and relevant parameters. The isolation wall construction of underground space, the establishment of energy storage space system, the evaluation of energy storage potential, and the innovation in surrounding rock reinforcement and improvement, seepage prevention and other safety measures of underground space should be conducted in combination with numerical analysis and simulation technology. In addition, geological guarantee should be provided for the long-term safe and efficient development of underground space energy storage. (3) Research on the performance evolution of energy storage geological body. Definitely, the thermal damage characteristics of surrounding rock at multiple scales (microscopic, mesoscopic and macroscopic) and the deterioration mechanism of surrounding rock fatigue strength should be revealed based on the coupling model of multi-phase (solid and liquid) and multi-field (temperature, stress, seepage and chemical fields), so as to obtain the key parameters, including the physical parameters of energy storage medium, injection volume, injection-production frequency, energy storage time and energy storage pressure, etc. Besides, a reasonable surrounding rock reinforcement process and stability control technology should be developed, and the long-term settlement law of overburden rock should be analyzed, thus providing basis for the leakage prevention project of the reservoir. (4) Assessment of long-term safety and stability of underground space. The long-term and multi-scale health monitoring and dynamic evaluation of geological conditions, safety conditions and environmental conditions of underground space formed by coal mining should be carried out, a safety early warning system for the geological environment of storage should be built, and the operation process and geological environment evolution should be analyzed to provide all-round safety monitoring and early warning for energy storage in underground space formed by coal mining. This study could provide an idea for the utilization of underground space resource formed by coal mining in China, and promote the transformation and development of China’s coal industry to be green, clean and efficient under the “dual carbon” goal.