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°, 30°, 45°, 60°, 90°) 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 × 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°-sample and 45°-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.