Meso-fracture mechanism and its fracture toughness analysis of Longmaxi shale including different angles by means of M-SENB tests

Abstract

Researches on fracture toughness and crack initiation, propagation influenced by bedding at mesoscale are quite beneficial to the understanding of hydraulic fracturing and wellbore stability analyses in shale gas exploitation. Three-point bending tests with Scanning Electron Microscopy (SEM) were applied on 29 Longmaxi shale samples with different pre-notch angle at mesoscale. Peak load, failure mechanism and fracture toughness of shale were analyzed and discussed in detail. Experimental results show that the peak load is directly proportional to the angle β between the bedding plane and the pre-notch. In addition, the crack propagation path is very zigzag with the increase of β. It is obvious that the bedding weak surface seriously affects the crack initiation and propagation under the action of the principal stress field. Mode I crack is normally initiated at a relatively large curvature radius of notch edge. However, mixed mode crack I/II is easily initiated at the location where the principal stress is severely concentrated. Two typical facture modes have been found in Longmaxi shale. At the beginning of the expansion, mode I crack perpendicularly passes through the weak bedding plane and then extends into the weak layer. Mode I/II mixed slant-crack directly passes through the bedding plane or goes zigzag on the bedding plane or become zigzag before reached to weak bedding plane, and then passes through the weak plane perpendicularly or just deflects perpendicularly. The branch crack occurs near the main crack propagates along or through the mineral particles. Meso-cracks are mainly propagated around the crack tip, natural fissures, crack turning point, and large mineral particles. Cracking direction changes during the propagation process and eventually parallel to the main crack and then stops, or it intersects with the main crack and forms exfoliated particles. By utilizing both FEM software and test results, the I/II mixed mode fracture toughness of shale has been calculated, and the pure mode I fracture toughness has been obtained as 0.7359 MPa m0.5 approximately. Furthermore, both stress intensity factor and the T stress change law of I/II mixed mode cracks were also obtained and discussed in detail.