Experimental study of brittleness anisotropy of shale in triaxial compression

Brittleness of rock plays a key role in petroleum related rock mechanics. Compression brittle failure of shale is more relevance to directional wellbore stability. The bedding angle (0 degrees, 30 degrees, 45 degrees, 60 degrees, 90 degrees) dependent brittleness of anisotropic Longmaxi shale samples was studied by conducting triaxial experiments under various confining pressure (15, 25,40 MPa). A servo-controlled loading system was used to keep axial loading strain rate as constant (8.33 x 10(-5)/s) at room temperature. The stress and strain was synchronously and independently recorded with time. We used five definition of brittleness indices that were based on laboratory stress-strain curves to characterize the anisotropy of brittleness. The results show that for 30 degrees -sample and 45 degrees -sample under relative low confining pressure (15 MPa), the stress-strain curves deviate from stress-time relations during instant rupture of post-failure. Under confining pressure of 25 MPa, both pre-failure and post-failure indices of brittleness remain highly consistent along bedding angle as a sine curve. When the confining pressure is increased to 40 MPa, variation patterns of brittleness indices clearly differentiate into pre-failure or post-failure type. It seems that the unstable recording of servo system during explosive failure of samples can be considered as a relative brittleness indicator. The results indicate that the brittleness of samples in some certain bedding angle don't decrease with increasing confining pressure, instead the post-failure type indices may increase. The brittleness anisotropy of Longmaxi shale samples is bedding angle (structure) dependent, and it decreases with increasing confining pressure. (C) 2016 Elsevier B.V. All rights reserved.