ABSTRACT
This paper aims at providing a better understanding of the fracturing behavior of shales due to shearing in relation to stress level, anisotropy and brittle-ductile transition, and the effects of fracturing to the changes in permeability and seismic velocities in shales. Triaxial compression tests were performed using a High-Pressure and High- Temperature (HPHT) triaxial cell on samples of Mancos shale cored horizontally parallel to the bedding plane. During triaxial shearing, the continuous changes in sample permeability, and the P and S wave velocities were simultaneously monitored during loading. The formation of fractures in the test samples during shearing was characterized. Structural anisotropy of shale is evaluated in terms of the shear wave splitting parameter. The results show complicated interactions between initial heterogeneity, brittle-to-ductile transition and stress level on the fracturing and the ensuing permeability changes during shearing of shales. An interesting observation was that tensile fracturing can occur in a compressional regime in shales due to shearing along pre-existing undulating bedding plane laminations. Notwithstanding these complex interactions, it was found that the shear wave splitting parameter can be a useful index for monitoring fracturing and permeability changes of shales during to shearing.
