Study of the relationship between surface subsidence and internal pressure in salt caverns

Because of the ultra-low permeability and favorable rheological properties of rock salt, salt caverns are globally considered to be an ideal medium for energy storage. The prediction and control of surface subsidence above storage caverns are the key problems that require steady attention to ensure their long-term safety and stable operation. For salt caverns already in existence factors such as depth, shape and geological conditions cannot be changed. Therefore, the operating condition (i.e., internal pressure) becomes a key factor in influencing surface subsidence. However, cyclic pressure change during the operation phase has not been seriously considered by analytical investigation yet. In this paper, theoretical models for the volume convergence rates in spherical and cylindrical caverns have been derived, thereby developing a new concept equivalent internal pressure that of rock salt is considered also takes cyclic internal pressure into consideration. The analytic solution for surface subsidence was then derived from a combination of transfer and distribution functions. Analytical and numerical solutions for different conditions were compared and verified, while the FDM code, FLAC3D, was used for numerical simulations. This comparison reveals that the use of equivalent internal pressure is suitable for predicting surface subsidence in the long-term cyclic operation conditions.