Combined Effect of Contact Area, Aperture Variation, and Fracture Connectivity on Fluid Flow through Three-Dimensional Rock Fracture Networks

Abstract

In order to investigate the combined effect of contact area, aperture variation, and fracture connectivity on the fluid flow through a fractured medium, a series of flow simulations were implemented on two types of three-dimensional discrete fracture network (3D DFN) models constituting fractures having spatially variable apertures and parallel plates, respectively. The flow tortuosity within the 3D DFN models was examined by changing the density, aperture distribution, and closure of fractures. The results show that compared with the 3D DFN models constituting parallel plates, the model with variable apertures provides more pronounced 3D preferential flow pathways. At the individual fracture scale, the preferential flow pathways mostly converge within the void spaces of large aperture, and at the network scale, they are located in the most transmissive fractures within the connected networks. The permeability of 3D DFNs depends not only on the contact area and aperture variation within individual fractures but also on the fracture connectivity and the contact at fracture intersections within the fracture network. Increasing the fracture connectivity tends to enhance the permeability, while increasing the contact at fracture intersections would significantly reduce the permeability. A correlation between the equivalent permeability of 3D DFNs constituting fractures with spatially variable apertures and parallel plates is proposed incorporating the effect of network-scale topology. A tortuosity factor for 3D DFNs is defined based on the proposed model, and it can account for two competing effects when the model is upscaled from individual fracture to fracture network: the permeability reduction induced by contact obstacles at fracture intersections and permeability enhancement induced by increasing the fracture connectivity.