Characteristic strain amplitude for tight sedimentary rocks: Transition from elasticity to nonelasticity

Abstract

We perform a series of increasing-amplitude triaxial unload cycling tests on three tight sedimentary rocks to explore the strain-dependent mechanical properties. We report both dynamic and static Young’s modulus and Poisson’s ratio as a function of the strain amplitude from ∼10−7 (dynamic test) to ∼4 × 10−3 (static test). With the increasing strain amplitude, static mechanical properties gradually decrease from values approaching dynamic properties. Through the experimental data distribution and the quadratic fitting results, we define a characteristic strain amplitude of about 5 × 10−5 to distinguish elasticity-dominated and non-elasticity-dominated behaviors for three tight rocks. Such transitional behavior in tight sedimentary rocks can be microscopically explained by the gradual activation of frictional slips from the beginning of the cyclic stress unload. These observations provide direct experimental evidence of the transition from elasticity to non-elasticity for tight sedimentary rocks during static measurements.