Experimental study on crack propagation and failure mode of fissured shale under uniaxial compression

• The crack initiation and propagation law of shale with double fissures are studied. • The failure mode of fissured shale includes tensile failure and tensile-shear failure. • σ imc of fissured shale increases with the increase of the inclination angle of fissures. • The crack network of inclination angle of fissured shale of 45° is the most complex. Analyzing the crack propagation evolution mechanism and monitoring the failure mode characteristics are the keys to understanding the hydraulic fracturing of shale reservoirs, especially in shale formations with significant bedding and brittleness characteristics. Apart from bedding, there are many primary fractures in shale. To better study the crack initiation and propagation law and final failure mode of shale with primary fractures of different inclination angles, a series of uniaxial compression tests of shale with double fissures of different inclination angles (0°, 30°, 45°, 60° and 90°) were conducted based on acoustic emission (AE) and digital image correlation (DIC) techniques. The results show that: 1) The peak strength and peak strain of shale with double fissures both increase with the increase of the inclination angle of fissures, while the elastic modulus generally remains stable. 2) Crack initiation stress of shale with double fissures increases with the increase of the inclination angle of fissures, while the initiation angle decreases. 3) Tensile cracks dominate overall, but more shear cracks are generated and developed at the final buckling failure, and the proportion of shear cracks at 30° and 60° is much higher than that of the other three angles. In addition, fractal dimension and connectivity analysis show that the crack network of 45° is the most complex. 4) As the inclination angle of fissures increases, the failure mode of the fissured shale specimen gradually changes from tensile failure to tensile-shear failure. Hence, this study will contribute to the development of shale gas and provide theoretical guidance for exploring fracture propagation laws under shale reservoir stimulation.