Collaborative Research: Use of Novel True Triaxial Tests and Shear Band Theory to Determine Failure Properties of Compactive Porous Sandstones

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A research team from the University of Wisconsin-Madison and Northwestern University is combining experimental observations and shear band analysis in order to investigate the failure properties of high porosity rock. Shear band analysis is a mathematical description of the observed behavior of materials such as rocks by which their failure under applied stresses is concentrated in a narrow band rather than being uniformly distributed. True triaxial tests, in which test samples are subjected to three independent and typically unequal principal stresses simulating realistic field conditions, are being conducted on high-porosity compactive sandstones. The tests employ novel loading paths for each of the principal stresses. These paths, unprecedented in laboratory experiments, are designed to facilitate formulation of constitutive relations and comparison with the theoretical predictions of shear band analysis. The results of this research will provide the first systematic test of the applicability of shear band theory for the full range of stress conditions.A better understanding of rock mechanical behavior and its relation to failure of compactive porous sandstones is essential for interpreting field observations and as input for numerical simulations of complex geomechanical and geophysical problems. These include earthquake mechanics, design of underground storage facilities, energy recovery and sequestration of carbon dioxide. Failure in these materials tends to occur in narrow bands and therefore can drastically alter the ability of the rock formation to trap or transmit fluids. These narrow zones of failure may reduce flow across them and interfere with fluid circulation in applications involving fluid injection (e.g. for carbon dioxide sequestration or liquid waste isolation) and fluid extraction (e.g. for oil field operations). These failure zones can also provide conduits for enhanced flow along them. Such flow can disrupt rock formations intended to trap fluids and prevent them from reaching the surface or interacting with shallow groundwater.