Impact of pressure solution creep and threshold stress on deformation and stress state around a salt cavern

Abstract

Time-dependent deformation (creep) of rock salt leads to cavern convergence during salt solution mining, and causes subsidence at the surface. Typically, models analyse time-dependent deformation by considering the steady-state creep of rock salt governed by both dislocation and pressure solution creep mechanisms. Deformation due to dislocation mechanisms (nonlinear creep) is dominant at relatively high stress, whereas pressure solution (linear creep) contributes significantly at low stress. A threshold stress may exist below which pressure solution does not occur. Incorporating this threshold stress in simulations has already significantly improved the accuracy of cavern convergence and surface subsidence predictions compared to field observations. The current study aims to build upon this, extending current modelling capability by incorporating state-of-the-art constitutive equations for steady-state creep as well as a realistic threshold stress for pressure solution. We developed a 2D axi-symmetrical finite difference, computation-based geo-mechanical model to investigate the deformation behaviour and stress distribution around a cavern during different stages of solution mining. Preliminary benchmark results show the impact of creep characteristics on the evolution of cavern volume reduction, and surface subsidence, comparable to previous studies. Further, the extent and magnitude of stresses around the cavern are highly impacted by the inclusion of pressuresolution creep and its threshold stress.