Creep Behaviour of Bischofite, Carnallite and Mixed Bischofite-Carnallite Salt Rock
Abstract:Some salt deposits contain the valuable magnesium and potassium salts bischofite and carnallite, as well as halite, in the form of pure and mixed layers. During extraction of such salts from the subsurface by solution mining, the material in the undissolved walls will flow into the caverns. In order to accurately predict the flow of wall rock material, feasible production rates and related subsidence, a good understanding of the creep behaviour of bischofite, carnallite and mixed salt rocks under in situ conditions is required.
We have conducted conventional triaxial compression tests on polycrystalline bischofite, carnallite and mixed bischofite-carnallite-halite rock samples machined from natural cores. The experiments were carried out at true in situ P-T conditions of 70°C and 40 MPa confining pressure. All experiments consisted of strain rate stepping runs, applying strain rates in the range 10-5 to 10-8 s-1, reaching 2-4% axial strain per step, with individual steps being followed by stress relaxation down to strain rates ~10-9 s-1.
Both bischofite and carnallite reached near steady state creep behaviour within each constant strain rate step. Carnallite was found to be 4-5 times stronger than bischofite. For bischofite as well as carnallite, we observed that during stress relaxation, the conventional power law stress exponent n changed from ~5 at 10-5 to ~1 at 10-9 s-1. The absolute strength of both materials remained higher if the relaxation started at a higher stress, i.e. at a faster rate. We interpret this as indicating a difference in microstructure at the initiation of the relaxation, notably a smaller grain size related to dynamical recrystallization during the constant strain rate step. The data thus suggest that there is gradual change in mechanism with decreasing strain rate, from grain size insensitive dislocation creep to grain size sensitive (pressure solution) creep. The mixed bischofite-carnallite-halite salt rock did not approach steady state creep within individual steps. The overall strength of the mixed material was 1.3-1.5 times that of carnallite at the faster strain rates, increasing to 2.5-3.5 times at the slower rates. The strength of the mixed material is also slightly higher than that of wet synthetic polycrystalline halite under the same conditions.