Non-Newtonian Fluid Transport in Geological Fracture Network

Abstract: Transport of non-Newtonian fluids through natural fracture networks is relevant to many geophysical phenomena and frequently prompted by subsurface engineering activities. Yet, rheologically non-Newtonian behaviour and its effects are overlooked by fracture-flow studies that rely on the Newtonian viscosity assumption. We contrast and compare the distribution of network flow produced using the conventional Newtonian viscosity with that obtained by injecting viscoplastic non-Newtonian fluid over a broad range of fracture influxes . Fluid rheology, characterised by shear-thinning and yield stress of a polymer solution, is modelled using the Herschel-Bulkley-Papanastasiou (HBP) approach. We numerically solve the Navier-Stokes equation for the polymer flow in the realistic metre-wide network geometry of BED1, comprising thousands of millimetre-width fractures mapped in the Bristol Channel coast, UK. Our new results show that fluid rheology always alters the flow distribution regardless of any network influxes applied, reinforcing the flow in straight fractures aligned with the far-field pressure gradient. The Newtonian assumption fails to capture local viscosity variations and the formation of highly viscous solid-like flow regions, thus misestimating the network flow velocities and the effective fracture volume. Darcy's law does not apply to the polymer fluid flow within the investigated influx ranges. Keywords: Discrete fracture model, polymer injection, fracture intersection, fluid rheology, non-Newtonian fluid