Influence of crystal plastic deformation on dilatancy and permeability development in synthetic salt rock

Abstract

While the fluid transport properties of rocks are well understood under hydrostatic conditions, little is known regarding these properties in rocks undergoing plastic deformation. In this study the influence of macroscopic plastic deformation on permeability has been investigated experimentally using synthetic salt rock. Dilatometric triaxial deformation experiments performed on this material, at room temperature, confining pressures (Pc) in the range 5–20 MPa and strain rates of ∼ 4 × 10−5 s−1 to total strains of ∼ 15%, exhibited work hardening behaviour with minor amounts of dilatancy at Pc < 18 MPa. Microstructural observations confirmed that deformation occurred by dislocation glide, with grain boundary microcracking in the dilatant field. At the lowest pressures, deformation-induced dilatancy of only 0.1–0.2 vol.% produced extremely rapid initial increases in permeability (from ≤ 10−21 m2 to ∼ 2 × 10−16 m2), suggesting critical behaviour of the type described by percolation theory. This rapid permeability development with dilatancy is well described by crack linkage models based on percolation theory, provided a broad range of fluid conductance is incorporated. The study has shown that minor dilatancy (< 0.2 vol.%) during plastic deformation of salt rock can lead to very large increases in permeability. This is of direct interest with regard to the behaviour of salt rock in waste disposal and storage systems, and may have important implications for the transport of fluids through, and the interaction of fluids with, crystalline rocks undergoing crystal plastic deformation in nature.