Experimental study on mechanical properties of shale rock and its strength criterion

Abstract

The mechanical properties of shale rock have a great significance on drilling engineering and the establishment of safety protection facilities in the process of shale gas exploitation. Hence, in this article, the deformation characteristics and the strength criterion of the Longmaxi formation shale rock are examined through uniaxial direct tension tests, uniaxial compression tests, and triaxial compression tests. The results show that shale rock experienced linear elastic stage, yield stage, and failure stage, under the direct tensile state and triaxial compression state. The shale rock experienced initial compaction stage, linear elastic stage, yield stage, and failure stage under uniaxial compression. The fracture surface of shale rock is smooth horizontal section under uniaxial tension. Under uniaxial compression, it is mainly manifested as splitting failure with shear failure in part. Under low confining pressure of triaxial compression, the shale rock show shear-dilatancy failure, and under high confining pressure, the shale rock show local shear failure along the primary fracture. The six strength criteria are used to describe the strength characteristics of shale rock. The results show that the Mohr-Coulomb criterion and Hoek-Brown strength criterion can well describe the shale rock under relative low confining pressure. It is found that both the generalized Hoke-Brown criterion and the power function strength criterion can better describe the strength characteristics of shale rock under different confining pressures than the other four strength criteria. The research results can provide experimental basis and mechanism understanding for the analysis of mechanical properties and engineering application of shale rock.