Grain size effects in salt rheology

Abstract

As different microphysical processes controlling salt rheology are grain size dependent, a proper representation of grain size is essential to obtain accurate creep rates. The conventional approach is to incorporate grain size as a constant single (mean) value. This study analyzes grain size effects on salt rheology by considering full grain size distributions. Existing experimental data of synthetic wet rocksalt samples, deformed in axial compression at different strain rates, temperatures of 75–240°C and 50 MPa confining pressure is used. The analysis indicates that (1) the evolution of grain size distributions and creep stress can be explained by a combination of dislocation and pressure solution creep, and the interaction of recovery, work hardening and fluid-assisted grain boundary migration, which can operate around converging salt caverns, (2) grain size distributions need to be represented by a mean (e.g., median) and shape (e.g., width) rather than an arithmetic mean value alone to accurately describe the mechanical behavior of rocksalt, and (3) incorporation of lognormal grain size distributions in creep equations lead to a wider range of deformation conditions where both grain size sensitive pressure solution creep and grain size insensitive (dislocation) creep operate and a wider transition between mechanisms.