Nowadays, a salt cavern, used as underground energy storage (e.g. natural gas, crude oil, hydrogen), is becoming more and more popular in China due to its many advantages, such as low permeability (≤10⁻²¹ m²), good water-soluble mining and the damage-healing characteristic of salt rocks. It not only solves the problem of energy resource supply-demand imbalance in China, but also provides a better, more secure and cost-effective way to store energy compared to aboveground energy storage systems. As for salt cavern storage, permeability is our primary concern in engineering, which is mainly influenced by damage and healing. In this work, some damaged salt specimens were prepared by uniaxial compression tests (the loading rate was 0.033 mm s⁻¹). Then those specimens were immersed in either a saturated brine solution or oil at 50 °C for a few days. Microscopic investigations were carried out by X-ray Computed Tomography (CT), Scanning Electron Microscope (SEM) and Nuclear Magnetic Resonance (NMR) to investigate the change of salt microstructures after healing. Possible micro-healing mechanisms were discussed. It was found that fluids played an important role in the healing process of damaged salt. Healing effectiveness of micro-pores and -cracks with the brine solution was higher than that with oil mainly because of crystal recrystallization. The surface of the grains was smooth and had no visible microcracks after healing in brine, while there were many pits and micro-tunnels with oil. Oil could hinder the healing process by impeding the diffusion effect and restraining grain recrystallization. Meanwhile, intragranular and intergranular water could also work as a lubricant resulting in softening which made salt rock more deformable. NMR results confirmed that damaged salt had a better recovery with brine, displaying lower porosity and lower permeability compared to that with oil. This work provides preliminary microscopic investigations on the healing of damaged salt in order to reveal the salt healing mechanism.

A promising large-scale energy storage is underground compressed air energy storage (CAES) in lined rock caverns. To ensure the safety and stability of storage caverns because of the influence of periodic injection during production, it is crucial to understand the mechanical behavior of lining concrete under different complex stress paths. In this study, three types of uniaxial compressive fatigue test and uniaxial creep test were conducted on concrete. The following conclusions were obtained from the results. 1) The irreversible deformation after the interval was larger than that before the interval in the discontinuous multi-step cyclic loading (DMCL) test. 2) Loading velocity significantly influenced concrete fatigue, and the irreversible deformation in the cycle of low loading velocity was greater than that in the cycle of high loading velocity. 3) The residual strain increased with an increase in stress level. 4) The creep strain increased with an increase in stress level during the multi-step creep loading test; the fractional derivative results were more consistent with the experimental results. 5) The permeability of concrete increased rapidly under the influence of an external force when the stress level exceeded 0.73.

It is well known that the Strategic Petroleum Reserve in salt caverns is an important means to solve the potential petroleum shortage. However, salt mines in China are mainly lacustrine-layered structures with many mudstone interlayers. Meanwhile, the acid value of extracted crude oil has been increasing in recent years. The acid erosion of salt cavern interlayers by crude oil would affect the safety of bedded salt cavern oil storage. Therefore, combined with acoustic emission technology, the multistage creep mechanical behaviors of natural mudstone interlayer samples and the mudstone interlayer samples treated by oil erosion are studied in this paper. The internal structures of interlayer samples before and after oil erosion were systematically analyzed by SEM. The results show that petroleum acid (naphthenic acid) plays a significant deterioration role in the mechanical properties of mudstone interlayers during the petroleum reserve process. Compared with the uncorroded samples, the mudstone interlayers after oil erosion show obvious brittle failure characteristics. At low stress levels, the axial strain grows stepwise and the lateral strain is smaller than the axial strain. At high stress levels, however, the lateral creep is obviously higher than the axial creep. A large number of AE signals were generated at the initial loading stage for different stress levels. After the creep stabilized, the AE signals were significantly reduced. During the process of petroleum erosion, the internal pores in rocks continued to develop with the dissolution of many mineral particles. This suggests that crude oil storage in the bedded salt rocks could accelerate the deterioration of the surrounding rocks via erosion. This paper could provide basic research data and a reference for the construction of oil storage in the bedded salt rocks.