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ABSTRACT: The purpose of this work is to study the evolution of non-elastic deformations and their effect on static stiffness prior to reaching plastic yield. To this end, a series of geomechanical laboratory experiments on preserved rock cores of various lithologies is performed. The experiments include probing of static stiffness on saturated core samples in loading-unloading undrained cycles. Test results are used to obtain parameters that describe the non-elastic response of stress-strain relation. Further, the non-elastic parameters are implemented in an empirical model to evaluate the dependence of stiffness on applied triaxial stress amplitude. Results show that for almost all tested rocks, Young’s modulus and Poisson’s ratio are dependent on stress change during triaxial loading/unloading well before reaching plastic yield. In order to study the factors influencing non-elastic processes, correlations between the computed non-elastic parameters and rock properties such as porosity, mineralogy and stiffness were established.
1. INTRODUCTION It is known that rocks are not linear elastic media and deviate from purely elastic response already at micro-strain level (Winkler et al. 1979). However, in practical geomechanical applications, linear elasticity is often assumed until the strain reaches plastic yield. This assumption may lead to false predictions of stress-strain changes in the subsurface during e.g., hydrocarbon reservoir depletion or injection, geotechnical operations, or long-term nuclear waste storage. In laboratory rock-mechanical test results, stiffness is usually reported as constant values, independent of stress/strain amplitude changes. However, using only a few additional parameters, non-linearity can be addressed by linearizing the compliance function versus stress for static loading/unloading as described below.
In soil mechanics, Kondner (1963) demonstrated that a hyperbolic function can approximate non-linear stress-strain curves for both sand and clay with a high degree of accuracy. On transformed axes, hyperbolae can be plotted as a linear function. Therefore, non-elastic behavior can be described by only two coefficients a and b. a is the inverse of the initial tangent modulus, and b is the inverse of the asymptotic value of the stress difference, which the stress-strain curve approaches at infinite strain.
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