Simulation of complex hydraulic fracture generation in reservoir stimulation

Abstract

In unconventional petroleum and geothermal reservoirs, a complex fracture network is mostly desired in order to enhance the reservoir permeability. In this paper, the mechanism of hydraulic fracture branching and generation of a cluster of fractures is investigated via numerical simulation. Firstly, the reservoir rock is modeled using lattice cell version of the discretized virtual internal bond method. An algorithm is developed to identify the hydraulic fracture trajectory. A sensitivity study is conducted with different pressurization rates, fluid pressures, in-situ stresses and orientations of perforations. It is found that a high fluid pressure and a high pumping rate are the necessary and sufficient conditions, respectively for creating fracture branching and clusters. The pressurization rate plays a critical role in branching fracture, but the role it plays is subjected to its duration as fracture propagates. The fluid pressure, as a controlling factor that integratively accounts for the effects of pressurization rate and duration, determines the degree of fracture branching. The location of fracture network is controllable by adjusting the magnitude of fluid pressure. The direction of propagation and distribution of fractures are determined by the in-situ stress contrast. As the contrast in the in-situ stresses increases, the branched fractures increasingly tend to propagate along the maximum in-situ stress direction. As expected, the effect of perforation direction on fracture growth is local; with hydraulic fracture advancing forward, it gradually turns to and propagates along the maximum in-situ direction. The degree of contrast between the in-situ stresses determines how fast the hydraulic fracture turns to the maximum in-situ stress direction. The simulation study helps to understand the mechanism and dynamics of complex hydraulic fracture generation in unconventional reservoirs.