Modelling Hydraulic Fracturing Propagation in Heterogeneous Reservoirs Using Cohesive Zone Methods

Abstract

The recent success in exploiting low permeability shale reservoirs has heavily relied on hydraulic fracturing to produce hydrocarbons economically in disadvantaged reservoir conditions. Although horizontal drilling significantly increases the contact area between the wellbore and the reservoir, the objective of hydraulic fracturing is set on creating further expanded conductive flow paths into the reservoir. This research uses cohesive zone method to numerically simulate hydraulic fracture propagation in the presence of natural fractures in two- and three-dimensional model. The Cohesive element approach limits fracture propagation to some predefined paths. However, in highly fractured formations since hydraulic fractures are growing through a network of natural fracture by placing cohesive elements through natural fractures it is possible to track the development of a network of induced hydraulic fractures. Moreover, cohesive elements remove stress singularity at the tips of fractures, which improves numerical stability of the model. Additionally, fracture models based on Griffith's criterion cannot predict fracture initiation. A numerical model was developed coupling both fluid flow in fracture network and rock deformations to study the interaction between hydraulic and natural fractures at different scales. The cohesive zone method assumes the existence of a fracture process zone characterized by a traction-separation law rather than an elastic crack tip region. The cohesive finite element method provides an alternate, effective approach for quantitative analysis of fracture behavior through explicit simulation of the fracture process. Activation of natural fractures during fracturing treatment improves the effectiveness of the stimulation tremendously. Here, integrated methodology initiated with laboratory-scale fracturing properties using a semicircular bending test is presented to determine cohesive properties of rock and natural fractures. A cohesive finite element model is used to reproduce laboratory results to verify the numerical model for interaction between the hydraulic fracture and cemented natural fractures. The results suggest that the distribution of pre-existing natural fractures can play a significant role in the final geometry of the induced fracture network. Moreover, understanding of natural fracture distribution in the reservoir will have an economical impact in projects where fracture geometry is better designed according to underground conditions.