The transition from dislocation creep to pressure solution in natural rock salt and constraints on the threshold stress for pressure solution

Understanding salt creep is crucial for safe salt cavern operation and abandonment.
Key creep mechanisms operating in salt are power law dislocation creep (stress sensitivity
n 5) and grain size-sensitive pressure solution (n 1), with the latter dominating in
natural salt at differential stresses below 3-5 MPa. The factors controlling the stress range
for the transition between these mechanisms are not fully understood, largely because of
limited experimental data at low stress. Moreover, thermodynamic theory suggests that
a threshold stress exists below which pressure solution halts, though it has not been experimentally
verified. We performed triaxial creep experiments (at ~125°C, stress-stepping in the
range 25 to 0.4 MPa) and microstructural analysis on natural salt samples with a mean grain
size of 372 μm. Our results showed that I) measured creep rates were faster than previously
reported for natural rock salt at similar conditions, ii) n gradually decreased with decreasing
− differential stress, and iii) at 0.4 MPa stress, the creep rate was ~2·10 10 s-1 with n 1.8.
Microstructures show evidence of active dislocation mechanisms and fluid-assisted grain
boundary migration. We infer a broad transition zone with combined dislocation creep and
pressure solution, and a threshold stress for pressure solution below 0.4 MPa.