Experimental study on the mechanical properties of bedding planes in shale

Abstract

Multi-stage fracturing of horizontal wells to recover shale gas has attracted substantial renewed interest in the physical and mechanical characteristics of shale. The mechanical characteristics, typically the strong anisotropy, significantly affect the nucleation and propagation of hydraulic fractures, as the nucleation mechanisms and propagation pathways primarily depend on the interaction between the actual in situ stress conditions and the anisotropic mechanical properties. However, there remains a lack of effective experimental data on the mechanical properties of the rock matrix and bedding planes. To investigate the mechanical properties, a series of tests, including Brazilian, direct shear and three-point-bending (TPB) tests, were performed on variously shaped Longmaxi shale samples in distinct bedding orientations relative to the loading directions. The results showed that the tensile strength, cohesion, internal friction angle and mode-I fracture toughness of the bedding planes are 4.713 MPa, 8.93 MPa, 31.216° and 0.566 MPa·m1/2, respectively, which are significantly lower than the rock matrix, corresponding to values of 13.164 MPa, 16.175 MPa, 36.222° and 0.957 MPa·m1/2, respectively. This finding demonstrated that the bedding layers are weakness planes on tensile strength, shear strength and fracture toughness in a quantitative manner. However, the values for the rock matrix and Arrester orientation are generally very similar; hence, the mechanical parameters of the rock matrix, especially the fracture toughness and tensile strength, can be approximated by the values determined in the Arrester orientation. For fractures propagating in the direction normal or oblique to bedding, a complex fracture geometry with tortuous propagation pathways is usually generated by bedding cracking and/or fracture deviation towards the bedding-parallel orientation. The mechanical characteristics of the bedding layers play a vitally important part in shale gas development, including the fracture-initiation pressure (FIP) prediction, borehole stability analysis, hydraulic fracture propagation pathways, and complex fracture network generation.