One of the understudied problems of hydraulic fracturing are patterns and mechanisms of cracks propagation in the media containing plastic enclaves and layers. The importance of this problem is related to the development of unconventional reservoirs, which may contain a significant amount of clays and kerogen. Apparently, under specific conditions, such formations manifest plastic properties and failure mechanism is of viscous nature and is attended by remarkable plastic deformations. Thus, the question about crack propagation pattern in the layered formations with great contrast in strength and elastoplastic properties is arisen. Bazhen shale formation could be referred to such unconventional reservoirs. The formation covers an area of almost one million square kilometers and contains about 18 trillion tons of organic matter. The reservoir is built up of interstratification of clay, siliceous and carbonate layers with kerogen, organic matter and other minerals particles, which results in the heterogeneous distribution of mentioned above characteristics of massive. The goal of this research is the investigation of specific of cracks propagation in the media, containing plastic layers and layers with contrast strength properties. The study was carried out by using the numerical simulation of deformation and failure process of medium with the initial crack of a given length. Calculations were performed using software package where the system of equations of continuum mechanics including equations of motion and consistency and constitutive relations of material behavior were solved numerically. Material behavior is defined in terms of modified Drucker-Prager model with unassociated flaw law. The emphasis is on the investigation of crack propagation character in the medium with various properties and revealing the conditions under which branching of cracks may occur. As leading factors affecting the cracks propagation character, strength and elastoplastic properties, thickness and relative position of layers and initial stress distribution are considered. Further significant factor defining character of cracks propagation is rate of load application in terms of pressure increasing rate and its changing on the tip of the crack, which as a result determine distribution of pressure in the growing crack.
