Experimental Study on Hydraulic Fracturing of Bedding Shale Considering Anisotropy Effects

The bedding plane formed by sedimentation makes shale anisotropic. To clarify the influence of bedding on the hydraulic fracturing of shale, the fracture characteristics of bedding shale were first clarified by conducting a hydraulic fracturing experiment on large-scale shale samples with different bedding angles. Subsequently, combined with the experimental results, based on the theory of elasticity, a new fracture initiation criterion for shale hydraulic fracturing considering its anisotropic characteristics was established. The influence of the bedding angle on the hydraulic fracture initiation pressure and initiation angle was analyzed. The results showed that the pump pressure curve during hydraulic fracturing can be divided into four stages: continuous pressurization, internal pressure drop, internal pressure attenuation, and internal pressure equilibrium stage. Corresponding to the four stages of the pump pressure curve, the evolution of hydraulic fracture has four processes: microfracture development, fracture initiation, fracture propagation, and fracture network equilibrium process. When the direction of the maximum principal stress is perpendicular to the bedding, a complex fracture network is easily formed. Depending on whether the bedding plane is open or not, the modes of interaction between the hydraulic fractures and bedding plane could be divided into eight types. Hydraulic fractures initiate in two ways: from the matrix and from natural fractures. During fracturing, with the increase in the bedding angle, the initiation pressure decreases gradually and the initiation angle decreases first and then increases. The knowledge gained in this study can provide data and theoretical support for drilling direction design and fracture pressure evaluation in the field of hydraulic fracturing.