A Dynamic Hybrid Dual Porosity Model to Simulate the Waterflood-Induced Fractures in Low-Permeability Reservoir

Abstract

A new type of fracture which was caused by the waterflooding process in the low-permeability reservoirs has attracted the engineers’ great attention in China recently. This new type of fracture, named the waterflood-induced dynamic fracture, has a significant impact on the swept volume of water flooding and the distribution of remaining oil, and therefore, directly affects the reservoirs development. However, because of the special features of these fractures, such as the morphological changes, the time-varying permeability and vulnerable to the production performance, it is almost impossible to characterize them accurately in the normal reservoir simulation. In order to describe the waterflood-induced dynamic fracture, a new model was established in this paper. By overlapping the dynamic fracture model and the psudo-tenser permeability model to the dual porosity model which consists of a set of disconnected matrix blocks and a network of connected fractures, a new dynamic hybrid dual porosity model was derived. Compared with the conventional dual porosity model, the dynamic hybrid dual porosity model allows the property of the fracture system to change with time, so that the above problems can be solved very well. Through the numerical simulation of a typical low-permeability oilfield by using the established model, the evolution process of the waterflood-induced dynamic fracture including opening, extending and closing, and its impact on the development of the low-permeability reservoir were carefully studied. The simulation results show that the dynamic fracture grows continuously and slowly throughout the entire life of the oilfield development. And it has significantly narrowed the water flooding swept range, and reduced the vertical producing degree of water flooding to a certain extent, which is very consistent with the production performance data of this oilfield. Meanwhile, the precision accuracy of simulation result has been observably improved by using this new model, which indicates that the model can precisely depict the changes of the dynamic fracture both in its morphology and its properties. Furthermore, the established dynamic hybrid dual porosity model could be used to elucidate the mechanism of the evolution of the waterflood-induced dynamic fracture and so forth. Therefore, it provides an important technical means for the study of the effective development and prompt adjustment of water injection policy of low-permeability reservoirs.

An Improved Embedded Discrete Fracture Model with Fracture Growth for Water-Induced Fracture Simulation in Low Permeability Reservoirs

“…According to rock mechanics, when a hydraulic tension fracture is at the point of propagating, the fluid pressure at the fracture tip should overcome the normal stress vertical to the fracture surface and the rock cohesion force; the fracture extension pressure is calculated by Equations ( 15) and ( 16) [32]. In some cases, the simplified form is used as is shown in Equation (17). If the propagate condition is satisfied, a new fracture element is connected to the initial fracture to characterize the propagation of the fracture which is called the dynamic fracture element.…”

“…In recent years, the discrete fracture model (DFM) was used to realize the dynamic changes of fractures [14,15] but was limited to the complexity of grid division. A dynamic fracture model framework was established in which the opening and closure of fractures were reflected by permeability change of grids [16,17]; however, the model oversimplifies the characterization of fractures. So far, analytical fracture models coupling with traditional reservoir numerical simulator were used to simulate water-induced fractures by most scholars; some innovative solutions such as dynamic DFM and dynamic fracture model were proposed in recent years, but shortcomings exist when dealing with fracture propagation.…”