Anisotropic mechanism and characteristics of deformation and failure of Longmaxi shale

Abstract

Although spectacular advances in shale gas reservoir stimulation, i.e., hydraulic fracturing, the deformation behavior and failure mechanism of shale are not understood well enough. The basic reason is that shale contains stratified structure named bedding plane and its complicated petrophysical properties. The mechanical behavior of shale as the loading direction is parallel to the bedding plane is significantly different from that as the loading direction is perpendicular to the bedding plane. This work is devoted to characterization of the Longmaxi shale deformation and failure mechanism through experiments, from which 4 different deformation stages are obtained and volumetric strain deformation behavior is analyzed systematically. Then the effects of confinement as well as bedding layer orientation, which is defined as the angle between the loading direction and normal direction of bedding plane, on the elastic modulus, peak strength, and cohesion of shale are investigated. The analytical results illustrate that the volumetric strain dilatation is a constant, which is independent on the confining pressure and bedding layer orientation; its brittleness is high (0.55−0.85, B 1) and porosity is small (4.7%−5.2%). It is also found that the anisotropic degree of shale is better to be described by Young’s modulus than to be characterized by peak strength. As the confining pressure increases from 0.1 MPa to 60 MPa, the anisotropy degree decreases from 1.36 to 1.16, which declare that the Longmaxi shale belongs to low anisotropy rock according to the standard introduced by Ismael. These results not only enrich experimental database of shale, but also provide references for reservoir hydraulic fracturing design. Moreover, these results will help us to further understand shale laboratory mechanical properties.