Role of natural fractures with topology structure for hydraulic fracture propagation in continental shale reservoir

Abstract

Natural fractures tend to further promote fracture network which is initiated by hydraulic fracturing. However, how natural fractures with various topology structures impact hydraulic fracture propagation in continental shale reservoir is still unclear. With Fullbore Microscan Imager (FMI) and geo-statistics of Chang 7 reservoir in Yanchang Formation, typical natural fractures are classified into three topology structures, which are natural fractures with type I, type II and type III nodes respectively, and the average occurrence strikes of natural fractures with type I, type II and type III nodes are SW260°, SW255° and SW255°. Coupling with seepage field and stress field, a model simulating fracture initiation and propagation is established considering the transversely isotropic property of continental shale reservoir. On this basis, the role of typical natural fractures with topology structure for hydraulic fracture propagation is studied qualitatively and quantitatively by Finite Element Method (FEM) with globally embedded cohesive element. Natural fractures with type II nodes have largest effect on inducing hydraulic fracture propagation, hydraulic fractures can communicate most easily with them, and also the length and velocity of hydraulic fracture propagation are the longest and fastest, so fracture network is the most complex after fracturing in naturally fractured continental shale reservoir with type II nodes, which follows by the effect of hydraulic fracture propagation in naturally fractured continental shale reservoir with type III nodes. In comparison, natural fractures with type I nodes have insignificant impacts on hydraulic fracture propagation. The research results are conductive to fracturing design in continental shale reservoir.