Influence of natural fractures on hydraulic fracture propagation behaviour

Abstract

The propagation behaviour of hydraulic fractures is affected by natural fractures in unconventional reservoirs and determines the effect of reservoir reconstruction to a certain extent. Using the splitting mode of mesh nodes and zero-thickness cohesive elements, we established a hydraulic fracture random propagation method. The KGD model and Blanton’s experiment were used to verify the reliability of this method, and the cohesive element parameters of the natural shale core were obtained using three-point bending and digital image correlation methods. Finally, the model was used to simulate the propagation behaviour of hydraulic fractures in natural fracture–developed formations. The results show that even if the hydraulic fracture does not come into contact with the natural fracture, the natural fracture will open, accompanied by the shear slip of the natural fracture surface. The propagation of hydraulic fractures generates a stress shadow effect, causing an increase in the local formation pore pressure and minimum horizontal stress. When the horizontal stress difference was more than 2 MPa, the hydraulic fracture expanded along the direction of the maximum horizontal stress, and the fracture morphology was simple. A formation with a high elastic modulus and high injection rate can contribute to increasing the hydraulic fracture complexity. Hydraulic fracture propagation can be captured using natural fractures. Simultaneously, owing to the stress shadow effect, the stress state at the tip of the hydraulic fractures may change from tensile stress to compressive stress, eventually leading to the stop propagation of hydraulic fractures and the formation of asymmetric hydraulic fractures. The research results are helpful for improving the understanding of the geometric shape of the propagation behaviour of hydraulic fractures in natural fractures that develop formations.