Experimental study on the characteristics of anisotropic cracking behavior of shales under compression and tension

An in-depth understanding of the anisotropic characteristics of shale is the basis for improving the production capability of shale gas. Therefore, the cracking behavior of shales under compression and tension was investigated experimentally from macroscopic and microscopic perspectives. Uniaxial compression tests were carried out, and the effects of bedding on the generation of microcracks were analyzed through nuclear magnetic resonance (NMR). Then, Brazilian splitting and confocal laser scanning microscopy (CLSM) tests were performed, and the corresponding relationship between the failure mode and the fracture surface morphology was discussed. The results reveal that the mechanical properties and failure modes of shales are affected significantly by the existence of bedding planes. The total volume and size of microflaws after failure increase, and the highest porosity of newly generated microcracks is that of the 0° specimen. The microflaws are classified into two categories, including capillary and seepage microflaws. The newly generated microflaws in the 0°, 30°, and 90° specimens are mainly due to seepage microflaws, while those in the 60° specimen are mainly due to capillary microflaws. In the Brazilian splitting test, the fracture surface morphology of the specimen is closely related to the failure mode and mechanism.