Tightness and stability evaluation of salt cavern underground storage with a new fluid–solid coupling seepage model

The effects of operating pressure and ground stress on the seepage parameters (permeability and porosity) of surrounding rock and stability and tightness of salt cavern have been investigated, and a method is proposed for safe operating pressure classification. In previous works, the analysis of stability and tightness of salt caverns were mostly carried out independently, and few studies have considered the interaction between them. However, when used as a storage space for natural gas, petroleum and brine, there are many fluid–solid coupling problems in salt cavern underground storages. To address these problems, we establish a fluid–solid coupling seepage model based on the relationships between pressure, rock deformation and seepage parameters, and apply it to actual engineering. The coupling analysis results show that the relationship between shrinkage displacement of salt cavern and operating pressure is an exponential function with negative correlation, while the relationship between seepage range of a salt cavern and the operating pressure is a power function with positive correlation. Therefore, the effects of operating pressure on the stability and tightness of salt caverns are diametrically opposite. The seepage range of gas in surrounding formations gradually increases with time, and eventually tends to become stable. Under an operating pressure of 12 MPa and after operating for 30 years, the maximum seepage range and shrinkage displacement of the studied salt cavern are 99.72 m and 1.86 m, respectively. Hence the stability and tightness of this salt cavern meet the requirements of the regulations in China, and a gas leakage accident will not occur in this storage during operation. The seepage parameters of the surrounding formation increase with operating time of gas storage, and the increase in permeability is larger than that of porosity. The research results can provide reference and guidance for safe operation and parameter design of salt cavern underground storage.