Numerical Study of the Mechanism of Fracture Height Extension in Shale Reservoirs

Shale oil and gas resources are abundant, and weak surfaces such as stratification and lithological interfaces are developed in the longitudinal direction of the reservoir. Hydraulic fractures are prone to extend along weak surfaces leading to restricted longitudinal transformation, which seriously affects the fracturing effect. However, the mechanism of artificial fracture extension in shale reservoirs with complex weak surface distribution is still unclear. Based on the cohesive pore pressure unit method, a seepage flow-stress-damage finite element fracture extension model was established, and the finite element software was secondarily developed to study the influence law of each main influencing factor on hydraulic fracture penetration under the single bedding and the influence law of multi-bedding spacing on the hydraulic fracture height, and the main controlling factors affecting the hydraulic fracture height of bedding shale were analyzed by the orthogonal experiment method. The results show that the larger the vertical stress difference, the dimensionless strength of the bedding plane, Young's modulus, fluid injection displacement, and fracturing fluid viscosity, the smaller the bedding dip angle, the easier it is to achieve penetration. Tensile damage occurs in the main seam and the initial damage to the bedding surface is mainly shear damage. The main controlling factors affecting the seam height are, in descending order, the cementation strength of the bedding surface, the vertical principal stress difference, the bedding inclination angle, the fracturing fluid viscosity, the injection displacement, the bedding spacing, and Young's modulus. The influence of all factors on fracture height and longitudinal complexity is relatively consistent. The easier it is to achieve longitudinal penetration, the lower the longitudinal complexity is. This study reveals the hydraulic fracture height extension mechanism of single-layer and multilayer shale, which is potentially instructive for the optimal design and field application of fracturing in shale reservoirs.