Macro-fracture mode and micro-fracture mechanism of shale

Through the study of fabric characteristics, mechanical characteristics, crack and microstructure characteristics of shale, the micro-mechanism, controlling factors and fracture modes of shale macro-fracture are revealed. The macro-fracture of shale is affected by fabric characteristics, coring direction, confining pressure etc, and occurs when the pores and micro-cracks extend, merge and link up. Macroscopic failure modes include shear fracture, tensile fracture and slip rupture. Microscopically, the micro-crack failure mainly includes shear slip type (type II) and tensile type (type I). In essence, the shear rupture and slip fracture of shale are all caused by shear slip rupture of micro-cracks, but due to different shale structure, the failure takes on different modes. The tensile fracture is caused by the tensile extension of micro-cracks. In addition, during the macro-fracture, the shear and tensile fracture of micro-cracks co-exist, but make different contribution to the macro-fracture. Finally, shale fracture patterns, namely shear, tensile rupture and slip burst are set up, and the microscopic mechanism of macro-fracture is explained from the microscopic viewpoint. Key words: shale, macro-fracture mode, micro-fracture mechanism, microcrack, shear fracture, tension fractur