Stress-Dependent Unstable Dynamic Propagation of Three-Dimensional Multiple Hydraulic Fractures with Improved Fracturing Sequences in Heterogeneous Reservoirs: Numerical Cases Study via Poroelastic Effective Medium Model
Yongliang Wang, Xuguang Liu
Abstract
Multistage hydrofracturing of horizontal wells is one of the key technologies for deep tight oil and gas resource exploitation. The stable and parallel propagation of multiple fractures formed by multistage perforation cluster fracturing is conducive to the formation of a developed fracture network. However, in the actual process of rock fracturing in deep tight reservoirs, owing to the influence of many factors, such as fracturing sequence, perforation cluster space, reservoir heterogeneity, and stress shadow effect, the three-dimensional (3D) fracture network that propagates unsteadily of multiple fractures is often formed. This has become an important problem limiting artificial transformation and in optimizing fracture networks. In view of the above difficulties, a 3D finite element–discrete element model considering fluid–solid coupling and fracturing fluid leak-off effect is proposed to simulate hydrofracturing, and an effective medium model to describe heterogeneous reservoir rock mass is established. Several typical fracturing sequences (sequential, alternate, simultaneous, and two improved alternate scenarios), perforation cluster spaces (100, 75, 50, and 25 m), and heterogeneous reservoir conditions were analyzed. The spatial unstable propagation, stress field interference, and evolution behavior of the pressure fracture network were simulated, and the quantitative results of the dynamic propagation shape, fracture propagation area, and spatial volume of the pressure fracture network were obtained. The numerical results show that when the perforation cluster space decreases, the stress shadow effect between hydraulic fractures intensifies, and the 3D fracture exhibits unstable propagation and spatial deflection. Compared with sequential and simultaneous fracturing, all types of improved alternate fracturing can effectively reduce the stress interference between fractures, fracture deflection, and form the developed fracture network area. Compared with the homogeneous reservoir model, the heterogeneity of the reservoir rock mass becomes an important factor that limits the propagation of the fracture network. These results can provide a basis for understanding the mechanisms of stress-dependent unstable dynamic propagation of 3D multiple hydraulic fractures of improved fracturing sequences in heterogeneous reservoirs.