The 3D stress state within typical salt structures

Salt caverns are created during the process of solution mining or built actively for underground storage purposes required for the energy transition. In most cavern-scale numerical models, deviatoric stresses within the salt dome are assumed to be negligible in magnitude. However, as salt structures are typically not homogeneous, this assumption is known to be incorrect. Stress variations may be caused by internal heterogeneities such as the presence of anhydrite layers, or by the large-scale structure and ongoing deformation of the salt dome or pillow as a result of their lower density compared to the overlying rocks. The rheology of the salt itself, a not very well constrained parameter, which varies significantly between different types of salt, may also have a significant effect.

In the scope of the Dutch KEM-17 project (Knowledge Programme on Effects of Mining) on Over-pressured salt solution mining caverns and possible leakage mechanisms, we examined which differential stresses can develop in a typical salt-structure (salt pillow, salt wall, and flat-bedded salt).  In order to make recommendations for avoiding undesired interference effects between caverns and salt dome boundaries, it is crucial to understand better how the stresses caused by salt-deformation vary within the salt dome. Which lower/upper bounds are to be expected for a particular type of structure? Where are such stresses likely to be negligible, and can we safely use existing approaches that neglect the background stress field? To what extent do uncertainties in the model parameters and geometries affect the stress state in the salt dome? To answer these questions, we used 3D thermomechanical models, for which we incorporated the state-of-the-art rheological flow laws of salt and assessed the stress state over approximately 300 kyrs, including the effect of tectonic regimes and glacial (un-)loading.