Abstract
Hydraulic fracturing is a useful tool for enhancing rock mass permeability for shale gas development, enhanced geothermal systems, and geological carbon sequestration by the high-pressure injection of a fracturing fluid into tight reservoir rocks. Although significant advances have been made in hydraulic fracturing theory, experiments, and numerical modeling, when it comes to the complexity of geological conditions knowledge is still limited. Mechanisms of fluid injection-induced fracture initiation and propagation should be better understood to take full advantage of hydraulic fracturing. This paper presents the development and application of discrete particle modeling based on two-dimensional particle flow code (PFC2D). Firstly, it is shown that the modeled value of the breakdown pressure for the hydraulic fracturing process is approximately equal to analytically calculated values under varied in situ stress conditions. Furthermore, a series of simulations for hydraulic fracturing in competent rock was performed to examine the influence of the in situ stress ratio, fluid injection rate, and fluid viscosity on the borehole pressure history, the geometry of hydraulic fractures, and the pore-pressure field, respectively. It was found that the hydraulic fractures in an isotropic medium always propagate parallel to the orientation of the maximum principal stress. When a high fluid injection rate is used, higher breakdown pressure is needed for fracture propagation and complex geometries of fractures can develop. When a low viscosity fluid is used, fluid can more easily penetrate from the borehole into the surrounding rock, which causes a reduction of the effective stress and leads to a lower breakdown pressure. Moreover, the geometry of the fractures is not particularly sensitive to the fluid viscosity in the approximate isotropic model.
Highlights
Hydraulic fracturing can be broadly defined as the process by which a fracture is initiated, which subsequently propagates due to hydraulic loading applied by a fluid inside the fracture
A bonded particle model based on particle flow code (PFC) [32] was used to model hydraulic fracturing [33,34], and the results showed that the model can truly reproduce the physics of injection into low-permeability formations
Our study focuses on investigating hydraulic fracture initiation and propagation by the PFC with modified fluid-mechanical couple mechanism
Summary
Hydraulic fracturing can be broadly defined as the process by which a fracture is initiated, which subsequently propagates due to hydraulic loading (or hydraulic pressure) applied by a fluid inside the fracture. The drawback of laboratory experiments is the issue of proper scaling to ensure the applicability of results to field operations Numerical simulations are another method for analyzing the propagation behavior of hydraulic fractures in naturally fractured reservoirs. Shimizu et al [36] conducted a series of hydraulic fracturing simulations in competent rock by using flow-mechanically coupled PFC2D code, and investigated the influence of the fluid viscosity and particle size distribution Their results show that tensile cracks are dominantly generated in hydraulic fracturing process, while the energy from shear type acoustic emission is larger than tensile type’s. UDEC can be used to model progressive failure associated with crack propagation and fault slip by simulating breakage of pre-existing contacts between pre-defined joint bonded deformable but intact blocks It is capable of modeling fluid flow through a defined fracture network. The influence of fluid viscosity, in-situ stress ratio and fluid injection rate on hydraulic fracturing are analyzed using our numerical model, respectively
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