Experimental study on the brittle characteristics of Longmaxi formation shale

Two evaluation indexes of brittleness ( Bd1 and Bd2 ),which can synthetically reflect the ability of resisting inelastic deformation and the situation of losing bearing capacity,are established based on stress-strain curve,in which the brittleness-drop coefficient R can reflect the difficulty of brittle failure,the stress landing coefficient P and soften- ing modulus M can stand for the strength of brittleness. This paper verifies the applicability of Bd1 and Bd2 by doing u- niaxial and triaxial experiments. In the end,corresponding to failure mode,the authors discuss the brittle anisotropy properties of Longmaxi formation shale under the angle of 0°,30°,60°and 90°and obtain the results as follows:① For Bd1 ,the related parameters can be selected freely according to different purposes. Bd2 is comprehensive and sensitive to the confining pressure. The fitting relationship between Bd2 and confining pressure is quadratic parabola. ② The brittle- ness index of shale decreases with the increase of confining pressure and it drops more quickly under low confining pressure than under high confining pressure. It is bigger under the angle of 90° than any other angles and it is relatively small under the angle of 30°,while the confining pressure is same. ③ Under the uniaxial compression test,the brit- tleness index is closely related to failure mode. The index accesses to maximum while the angle is 0° or 90° and rea- ches minimum while the angle is 30°. Under the confining pressure of 2-20 MPa,the lowering speed of index is su- preme while the angle is 0°. Under the confining pressure of 20-30 MPa,the brittleness index all show a rising trend except 90°. ④ The confining pressure affects the brittleness index mainly by limiting the radial strain and compacting bedding plane. Under the low confining pressure,the brittleness index of shale decreases for the former is dominant un- der the high confining pressure,the case is contrary.