Mesoscale fracture behavior of Longmaxi outcrop shale with different bedding angles: Experimental and numerical investigations

Abstract

The mechanical properties and fracturing mechanism of shale containing beddings are critically important in shale gas exploitation and wellbore stability. To investigate the effects of shale bedding on crack behavior and fracturing mechanism, scanning electron microscope (SEM) with a loading system was employed to carry out three-point bending tests on Longmaxi outcrop shale. The crack initiation and propagation of Longmaxi shale were observed and recorded by taking photos during loading. The cracking paths were extracted to calculate the crack length through a MATLAB program. The peak load, fracture toughness and fracture energy all increase with the bedding angle from 0° to 90°. The crack length and energy were also found to increase with the bedding angle in the range of 0°–60° and then drop slightly. The fracturing mechanism of shale includes the main crack affected by the bedding angle and disturbed by randomly distributed particles. The main cracking path was accompanied by several microcrack branches which could form an interconnected crack system. When the main crack encounters larger sedimentary particles, it will deflect around the particles and then restore to the initial direction. A numerical technique using extended finite element method (XFEM) coupled with anisotropic cohesive damage criteria was developed, which is able to capture the dependence of crack propagations on bedding angle and sedimentary particles. This study sheds light on understanding and predicting mesoscale fracture behavior of shale with different bedding angles. Keywords: Longmaxi shale, Three-point bending test, Fracture toughness, Fracture energy, Crack propagation, Anisotropic cohesive damage, Extended finite element method (XFEM)