Micro-mechanical analysis of the effects of stress cycles on the dynamic and static mechanical properties of sandstone

For heterogeneous porous sedimentary rocks, the static mechanical properties can differ a lot from the dynamic elastic properties, even within an extremely low-stress difference. We perform uniaxial load-unload tests on one porous and one tight sandstone by setting a series of minor stress cycles to explore how the static properties are associated with their dynamic properties. The experimental results suggest that the dynamic Young's modulus is characteristically higher than the static Young's modulus at almost any stress level, whereas the static Poisson's ratio expresses a lower value at low-stress level and a higher value at high-stress level in contrast to the dynamic Poisson's ratio. Both dynamic and static properties of the porous sandstone exhibit more stress-dependences than those of the tight sandstone. We analyze the nonlinearity and hysteresis of the stress-strain curves of two sandstones and draw a generalized curve with different segments to comparatively investigate the hidden micro-mechanisms behind their dynamic-static behaviors, accompanied by an axial grain contact model and a frictional slip model. Form both experimental and micro-mechanical viewpoints, the static properties determined from minor stress cycles are closer to pure elasticity, which are preferable to making correlations with the dynamic elastic properties.