A research team in China has carried out an in-depth analysis of the long-term structural stability of compressed air energy storage (CAES) systems built in two-butted-well horizontal (TWH) salt caverns located within low-grade, inclined salt formations, a configuration that is increasingly relevant as high-quality salt resources become harder to find.
The researchers noted that TWH caverns already dominate China’s salt cavern landscape, accounting for roughly 85% of existing sites. As demand for underground energy storage grows, salt caverns with ideal geological characteristics are becoming scarce. Many remaining caverns are located in formations with high sediment content, in some cases reaching 90% or even being fully embedded in sedimentary material. This reality makes it essential to better understand how TWH caverns in low-grade, dipping salt layers perform over long operational lifetimes, particularly when used for CAES applications.
The study focused on a representative TWH cavern model based on conditions at the Qianjiang Salt Mine in Hubei Province, in south-central China. For simulation purposes, the salt cavern was simplified into an idealized ellipsoidal geometry. The modeled cavern measured approximately 800 meters in length, 300 meters in width, and 800 meters in thickness, with a height of 200 meters. The long and short radii of the ellipsoid were set at 100 meters and 40 meters, respectively, while the connecting channel between cavern sections had a diameter of 15 meters. The total thickness of the salt-bearing formation and interlayers was 286 meters, of which 190 meters consisted of salt.
To evaluate how geological conditions influence cavern stability, the team examined multiple burial depths and formation dip angles. Roof depths of 500, 800, 1,200, 1,600, and 2,000 meters were analyzed, alongside dip angles of 7.5°, 15°, 20°, 25°, and 30°. The dip angle describes the inclination of the salt layer relative to horizontal; steeper angles mean one side of the cavern lies deeper underground and is subjected to higher stress loads than the other, increasing structural asymmetry.
The simulated CAES system was assumed to operate continuously for 30 years, following a daily cycle of eight hours of compressed air charging and five hours of discharging. Long-term performance was assessed using four key indicators: volume shrinkage rate (VSR), plastic zone volume (PZV), maximum surrounding rock displacement (Dis-Max), and safety factor (SF). VSR reflects the gradual loss of cavern volume caused by salt creep, while PZV represents the portion of surrounding rock that undergoes permanent deformation. Dis-Max captures the largest displacement observed in the rock mass around the cavern, and the safety factor indicates how close the stress conditions are to triggering structural failure.
According to the results, both cavern volume shrinkage and maximum rock displacement increase rapidly as burial depth increases, following an exponential trend. The researchers observed a particularly sharp acceleration once depths exceeded 1,500 meters. Steeper dip angles further intensified deformation, especially in the lower portions of the cavern, increasing asymmetry in both volume loss and displacement. At a depth of 2,000 meters, each additional 10° increase in dip angle led to a 3.53% rise in volume shrinkage and an increase of nearly half a meter in maximum displacement. The study also found that sedimentary material surrounding the cavern played a notable buffering role, reducing deformation and making sediment volume shrinkage less sensitive to changes in dip angle.
Based on these findings, the researchers concluded that cavern design guidelines should vary with depth. For deep salt caverns at depths of 1,500 meters or more, they recommend limiting formation dip angles to no more than 20% to maintain long-term stability. In contrast, shallower caverns located above 1,000 meters could tolerate steeper dip angles of up to 30% without significantly compromising safety. The team emphasized that these conclusions provide practical guidance for future CAES development in inclined salt formations, which are widely distributed across China and increasingly important for large-scale energy storage.
The research was published under the title “Stability evaluation of horizontal connected salt cavern compressed air energy storage in low-grade dipping salt layers” in the journal Earth Energy Science. The study was conducted by scientists from Chongqing University, the SINOPEC Petroleum Exploration and Production Research Institute, the Ministry of Natural Resources’ Key Laboratory of Unconventional Natural Gas Evaluation and Development in Complex Tectonic Areas, and the Guizhou Engineering Research Institute of Oil & Gas Exploration and Development Engineering, contributing valuable insights to ongoing Energy News coverage of underground energy storage technologies.