Mechanistic Study on the Influence of Stratigraphy on the Initiation and Expansion Pattern of Hydraulic Fractures in Shale Reservoirs

The unique depositional characteristics of shale reservoirs lead to the extreme development of shale laminae, natural fractures, and other weak surfaces, which have an important influence on the hydraulic fracture extension behavior and the final fracture geometry. Therefore, because the deep shale laminar development and hydraulic fracture interaction theory and influence mechanism are not clear, this paper focuses on the analysis of the influence law of the ground stress difference, laminar surface cementation characteristics, laminar surface dip angle, and other factors on the hydraulic fracture extension pattern in shale reservoirs based on the cohesive unit method. Numerical calculation results show that (1) the hydraulic cracks are first expanded along the direction perpendicular to the minimum horizontal principal stress after starting to crack from the injection point and then turn to expand along the side of the laminar surface when encountering the laminar surface. (2) Three laminar dips of 5°, 30°, and 60° were compared and found. A certain ground stress difference is conducive to communicating more laminated joints and forming a more complex fracture network, while too large of a ground stress difference reduces the complexity of artificial fractures. (3) The smaller the inclination of the laminate surface is, the greater the positive stress it is subjected to, the more difficult it is for the laminate joints to undergo shear slip, and the more difficult it is to open. (4) Under the same ground stress combination state and laminar surface inclination, laminated cementation strengths of 2 and 0.5 MPa were compared, and it was found that the lower the cementation strength of the laminate is, the more likely the hydraulic fracture will be activated to shear slip damage. At the same time, the reliability of the simulation study on the effect of laminar surfaces on hydraulic fracture extension based on the cohesive unit method is verified by combining with the corresponding indoor hydraulic physical model test, which provides a reference for the accurate description of the interaction mechanism between laminar surfaces and hydraulic fractures.