Impact of creep mechanisms on stress and deformation behaviour around salt caverns

Salt caverns are formed by solution mining and may be used for energy storage purposes after the production phase. During the operational lifetime, and in particular during abandonment, the spatial and temporal distribution of stress changes and deformation around caverns leads to convergence of salt around the caverns. In turn, this may lead to surface subsidence and potentially affect cavern integrity. Deformation around salt caverns is governed by different creep mechanisms, encompassing transient and steady-state creep stages. Steady-state creep is governed by a combination of dislocation and diffusion creep mechanisms. Deformation due to dislocation creep is dominant at relatively high stresses, whereas grain size-sensitive diffusion creep, particularly pressure solution, contribute significantly at low stresses. When modelling rock salt behaviour, these mechanisms need to be properly accounted for. Recently, studies have suggested that a threshold differential stress may exist below which pressure solution does not take place, which needs to be accounted for. In addition, dynamic recrystallisation may take place through grain boundary migration, driven by differences in energy stored in neighbouring grains due to dislocation creep strain. The process of grain boundary migration reduces the (work hardening) energy in the system as old grains are consumed by new ones, causing weakening. Furthermore, incorporation of transient creep is generally based on the description given in the Munson and Dawson model.

In this study, we aim to simulate different cavern operation phases, such as leaching, production, and abandonment, to analyse the effect of transient creep, pressure solution creep and its threshold stress on the stress and deformation evolution around the cavern. The coupled effects of these complex creep characteristics on cavern behaviour have not yet been studied in detail. Such, more extensive coupling, are needed to better align laboratory- and field-based observations of salt mechanical behaviour, and apply it to large-scale numerical models. The commercial mechanical simulator FLAC (Fast Lagrangian analysis of continua) has been used to develop a 2D model for a single cavern system, which can be used to examine cavern convergence, subsidence and cavern integrity. An empirical model is used to define the threshold strain limit for dynamic recrystallisation by grain boundary migration, analogous to the Munson-Dawson strain limits for transient creep.

The results show a significant effect of pressure solution creep on stress and deformation behaviour around the cavern. In the production phase, the transient creep does not show any significant effect on cavern behaviour; however, it could be important under varying loading conditions. The extent and magnitude of convergence and subsidence are dependent on the rate of pressure solution creep and its threshold stress. A preliminary analysis of the onset of the dynamic recrystallisation around the cavern suggests that DRX may be active in the lower regions of the cavern.