Numerical simulation of fracture initiation, propagation and fracture complexity in the presence of multiple perforations

Perforation is the key to the successful implementation of hydraulic fracturing. The research on perforation mechanism is of great significance to perforation planning, completion design, hydraulic fracturing design, and proppant migration analysis. In this paper, a finite element model for simulating crack initiation and propagation in the presence of multiple perforations is established based on the global embedded cohesive zone model (CZM). The reservoir is considered as a dense, low-permeability porous elastic medium, and the coupling between fluid flow and geomechanics, as well as the back-stress effect, are considered. The Blanton's (1982) criteria were used to verify the accuracy of the global embedded CZM. Then five cases are used to discuss the effects of perforation density, horizontal stress difference (HSD) on fracture initiation, propagation, and fracture complexity. The results show: There are four competitive fracture initiation modes for multiple perforations: First, fracture initiation at an early stage and keeping on; Second, hole deformed but not cracked; Third, fracture initiation first and then close; Fourth, at the beginning, there is no fracture initiation, initiation later. With the increase of perforation density, affected by the decrease of flow distribution, increase of friction and stress interference, the initiation rate of perforations gradually decreases, and the fracture pressure shows a tendency of decreasing first and then increasing. When multiple perforations exist, the crack propagation changes from complex to simple, and the number of cracks changes from more to less. Finally, the expansion mode is formed with 2–3 main cracks as the main and micro cracks as the auxiliary. The increase of perforation density can increase the complexity of fractures around the wellbore, and is beneficial to the diversion of fractures and the expansion of the impact range of fractures. However, it can also cause distortion and deformation of the fracture wall and decrease of fracture opening near the wellbore, which can lead to difficulty in proppant migration. Under low HSD, under the influence of stress inversion, connected horizontal cracks are easy to form between holes, and the fractures are easier to turn and more complicated. Under high HSD, disconnected horizontal micro-cracks can be formed between holes, and the main fractures tend to develop into simple straight fractures. The results of our research have important practical guiding significance for the development of perforation plan, hydraulic fracturing design and proppant migration analysis.