Anisotropy of tensile strength and fracture toughness of an organic-rich continental shale: A case study of the Chang 7 shale in Ordos basin, China

Tensile strength and fracture toughness are critical mechanical properties governing fracturing behaviors of shale. However, the mechanical properties of continental shales remain poorly understood. Mechanical property measurements on an organic-rich continental shale from the Yanchang Formation were conducted using Brazilian tests (BT) and semi-circular three-point bending tests (SCBT), combined with acoustic emission (AE) tests. Variations in tensile strength and fracture toughness versus inclination angles θ are analyzed. The results show that both tensile strength and fracture toughness exhibit pronounced anisotropy. Tensile strength varies from 1.09 MPa at θ = 60° to 7.50 MPa at θ = 0°, whereas fracture toughness ranges from 0.056 MPa·m 0.5 for cracks propagating parallel to bedding to 0.659 MPa·m 0.5 for cracks propagating perpendicular to bedding. The maximum anisotropy ratios for tensile strength and fracture toughness are 6.89 and 11.78, respectively. The Chang 7 shale exhibits stronger anisotropy than those reported for marine shales, including the Longmaxi, Marcellus, and Mancos shales. These differences are closely associated with its bedding structure and high organic content. The experimental data provide a basis for characterizing the mechanical properties of the shale, and the findings also offer useful insights for hydraulic fracturing design in continental shale reservoirs.