An experimental investigation of dynamic recrystallisation processes and their influence on the mechanical properties of natural rock salt samples

Salt caverns are formed in the subsurface during solution mining of salt. After the end of salt production, caverns need to be safely abandoned or may be repurposed for storage of energy carriers such as hydrogen. Salt caverns locally disturb subsurface stresses, leading to creep of the surrounding rock salt. Creep can cause cavern convergence at depth and may result in surface subsidence, with consequences for infrastructure and public safety. Accurate forecasting of cavern stability during abandonment or assessment of suitability for storage requires a deep understanding of the grain scale deformation mechanisms and processes controlling rock salt strength and creep rate. For rock salt, important deformation mechanisms are dislocation creep and pressure solution creep. Laboratory experiments have shown that dynamic recrystallization (DRX) associated with dislocation creep can be activated and contribute to mechanical weakening. However, the weakening effect of DRX is not included in engineering constitutive laws used in salt cavern numerical models. These laws are commonly based on low-strain laboratory experiments, where the influence of DRX is limited, and microstructural data are relatively rarely reported. The aim of this study is to experimentally investigate the dominant DRX process in deforming natural rock salt and its effect on the mechanical behaviour.

Lab experiments have been carried out on natural wet salt samples from the Zechstein formation. We conducted constant strain rate experiments using a triaxial compression apparatus. Experiments were performed at a confining pressure of 20 MPa and a temperature of 125 °C, using constant displacement rates corresponding to strain rates of approximately 5 × 10⁻⁵ s⁻¹ and 5 × 10⁻⁷ s⁻¹, up to 30–40% axial strain. After the experiment, all samples were studied using optical microscopy. Electron backscatter diffraction (EBSD) analysis was performed on the starting material and on two deformed samples, one from each strain-rate condition.

For all samples, we observed an initial transient creep stage followed by a quasi-steady state stage. The transition to quasi-steady occurred at a strain of about 10% for samples deformed at a strain rate of ~5 × 10^-7 s^-1. For samples deformed at the faster strain rate of ~5 × 10^-5 s^-1, continuous hardening occurred up to axial strains of 30%, with a gradually decreasing hardening rate approaching steady state. Light optical and EBSD microstructural analysis revealed grains with a dense substructure including subgrain walls, euhedral shape grains with low to no substructure, and grains with irregular shaped grain boundaries including bulges. We infer that the dominant deformation mechanism in the tested natural samples was dislocation creep, providing sufficient local differences in dislocation density to activate DRX dominated by grain boundary migration processes. DRX led to rheological weakening and quasi-steady deformation. We are working on robust understanding of the parameters controlling DRX as this is essential to evaluate the zones prone to weakening by DRX around salt caverns.