Failure mechanism of bedded salt formations surrounding salt caverns for underground gas storage

Understanding the failure mechanism of bedded salt formations surrounding salt caverns is of great importance for underground gas storage. However, laboratory mechanical experiments of cores alone are insufficient to determine the mechanical properties of bedded salt formations, because the stress state of the cores varies spatially, and man-made damage might have occurred during the coring process, especially at the interfaces. Therefore, both physical simulation experiments and numerical analyses are needed to better understand the failure mechanism of bedded salt formations surrounding salt caverns. According to the physical simulation tests, the uniaxial and triaxial compressive strength curves of bedded salt rocks appear to be U-shaped as the dip angle changes, implying that shear failure may occur more easily at the top and bottom haunches of the cavern than elsewhere. Numerical analyses show that plastic zones occur initially at the top and bottom haunches of the cavern, which is accordance with the physical test results and theoretical analyses. For the two simulated models, the plastic zones in the interlayers tend to expand towards the model boundary to induce the instability of the salt cavern, particularly at the middle of the cavern with soft and weak interlayers after years of creep. Conversely, the plastic zones in the rock salt begin to occur at the top and bottom haunches of the cavern and then expand gradually to other places, albeit with a limited scope. The results suggest that the creep of rock salt can lead to the failure of interlayers in bedded salt formations, thereby affecting the stability of salt caverns.