Abstract
The lattice bond cell (LBC) is a newly developed lattice model for dynamic fracture simulation. It represents a rock with a discrete structure composed of lattice bond cells. Each lattice bond cell can take any geometry with any number of bonds. The interactions between particles in a cell are characterized by an interatomic potential. In this study, the LBC is used to simulate dynamic hydraulic fracture. A special technique is developed to impose fluid pressure to fracture faces. The simulation examples suggest that the LBC can simulate the initiation, propagation and branching behaviors of fracture driven by the internal high fluid pressure without any separate fracture criterion. The fracture branching behavior is analyzed. The branching degree of hydraulic fracture is governed by the fluid pressure level while the branched fracture growth direction is governed by the ratio of in-situ stress. It is suggested that the LBC is a feasible approach to simulating dynamic hydraulic fracture.
1. INTRODUCTION
Hydraulic fracture (HF) is an important technique to stimulate oil production in the unconventional reservoir. So far many methods have been developed to simulate HF process. However, most of them focus on the quasistatic HF while few on the dynamic ones. In reality, some hydraulic fractures branch due to the internal high fluid pressure. Branching is a kind of unstable fracture response to rapid loading. It cannot be captured by the static simulation method. Hence, it is necessary to develop an effective method to simulate the dynamic HF. The dynamic HF is different from the usual dynamic fracture in that it is driven by the internal fluid pressure. The pressure boundary condition varies as the fracture develops. Thus, the dynamic HF behavior becomes more complicated. In the dynamic fracture simulation, the discrete element method presents obvious advantages over the continuous one. It has been extensively used to simulate the quasi-static HF, e.g. Torres and Castano [1] and Marina et al [2], but few to the dynamic HF. Recently, Zhang[3] has developed a lattice bond cell method to model the continuum. This lattice bond cell method is different from the conventional lattice method in that its discrete structure is composed of lattice bond cells. Each lattice bond cell can take any geometry and can have any number of bonds. A hyperelastic interatomic potential is introduced to describe the interactions between particles. This makes the lattice bond cell method quite suitable to simulate the dynamic fracture for the hyperelasticity governs the instability of dynamic fracture[4, 5]. In this paper, the lattice bond cell model is used to simulate the dynamic HF. Due to the complicated variable pressure boundary condition, the fracture direction is subjected to many factors. Da Silva and Einstein[6] have studied the fracture initiation and possible propagation direction of HF under quasi-static load. Zhang and Chen[7] suggests that the dynamic re- HF always tends to develop along the maximum in-situ stress direction. But in these researches, no dynamic fracture branching behavior is involved. If the HF is subjected to the dynamic fluid pressure, the fracture propagation direction becomes more complicated. The dynamic HF simulation becomes especially important to determine the HF direction.
