Deep underground energy storage is the use of deep underground spaces for large-scale energy storage, which is an important way to provide a stable supply of clean energy, enable a strategic petroleum reserve, and promote the peak shaving of natural gas. Rock salt formations are ideal geological media for large-scale energy storage, and China is rich in salt rock resources and has a major shortage of energy storage space. Compared with the salt domes in other countries, the salt rock formations in China are typical lacustrine bedded salt rocks characterized by thin beds, high impurity content, and many interlayers. The development of large-scale energy storage in such salt formations presents scientific and technical challenges, including: ① developing a multiscale progressive failure and characterization method for the rock mass around an energy storage cavern, considering the effects of multifield and multiphase coupling; ② understanding the leakage evolution of large-scale deep underground energy storage caverns; ③ understanding the long-term performance evolution of large-scale deep underground energy storage caverns; ④ developing intelligent construction technologies for the deep underground salt caverns used for energy storage; and ⑤ ensuring the long-term function of deep underground energy storage spaces. The solution to these key scientific and technological problems lies in establishing a theoretical and technical foundation for the development of large-scale deep underground energy storage in China.

Subsurface geothermal energy storage has greater potential than other energy storage strategies in terms of capacity scale and time duration. Carbon dioxide (CO₂) is regarded as a potential medium for energy storage due to its superior thermal properties. Moreover, the use of CO₂ plumes for geothermal energy storage mitigates the greenhouse effect by storing CO₂ in geological bodies. In this work, an integrated framework is proposed for synergistic geothermal energy storage and CO₂ sequestration and utilization. Within this framework, CO₂ is first injected into geothermal layers for energy accumulation. The resultant high-energy CO₂ is then introduced into a target oil reservoir for CO₂ utilization and geothermal energy storage. As a result, CO₂ is sequestrated in the geological oil reservoir body. The results show that, as high-energy CO₂ is injected, the average temperature of the whole target reservoir is greatly increased. With the assistance of geothermal energy, the geological utilization efficiency of CO₂ is higher, resulting in a 10.1% increase in oil displacement efficiency. According to a storage-potential assessment of the simulated CO₂ site, 110 years after the CO₂ injection, the utilization efficiency of the geological body will be as high as 91.2%, and the final injection quantity of the CO₂ in the site will be as high as 9.529 × 10⁸ tonnes. After 1000 years sequestration, the supercritical phase dominates in CO₂ sequestration, followed by the liquid phase and then the mineralized phase. In addition, CO₂ sequestration accounting for dissolution trapping increases significantly due to the presence of residual oil. More importantly, CO₂ exhibits excellent performance in storing geothermal energy on a large scale; for example, the total energy stored in the studied geological body can provide the yearly energy supply for over 3.5×10⁷ normal households. Application of this integrated approach holds great significance for large-scale geothermal energy storage and the achievement of carbon neutrality by 2050.