Tensile mechanical properties and AE characteristics of shale in triaxial Brazilian splitting tests

Abstract

Under the high temperature and high stress geological environment, how the physical and mechanical properties of deep shale reservoirs will change has become a hot research issue. Based on the Confined Brazilian Test, this paper improves a test method for the tensile strength of shale under triaxial conditions with a simple specimen fixture and convenient specimen preparation and performs a high confining pressure (0–60 MPa) Brazilian splitting test on Longmaxi Formation shales. The stress-displacement curve records the deformation and failure of shale. Acoustic emission technology (AET) detects the evolution process of fractures and further identifies failure modes. 3D optical scanning system reconstructs the fracture morphology. The results show that the maximum tensile strength of the shale specimens in the triaxial condition is increased by 58.5% relative to direct tensile strength. As the confining pressure gradually increases to 60 MPa, the specimen enters the stress state of compressed in three directions, the tensile failure mode gradually transitions to shear failure mode via tensile-shear composite failure mode, and the fracture angle gradually increases. The AE signal in triaxial conditions mainly shows a leap trend. The appearance of AE signal with high peak frequency can be used as a reference for micro-fracture initiation. The presence of shear fractures increases the roughness of the shale main fracture surface, and this effect diminishes as the confining pressure increases. This study can provide a reliable theoretical basis for the optimal design of hydraulic fracturing in deep shale gas reservoirs.