The flow thickens: predicting anomalous flow resistance behavior of viscoelastic fluid flow in porous media

Flows of viscoelastic polymer solutions in porous media are ubiquitous in industrial, energy, and environmental applications. Despite their often shear-thinning nature, these fluids can exhibit anomalous “flow thickening” above a threshold flow rate in porous media, marked by a drastic increase in flow resistance. The precise mechanisms generating flow thickening have remained a puzzle since the 1960s due to an inability to directly access pore-scale flow behavior. Previous studies have suggested the roles of extensional viscosity, elastic flow instabilities, and chemical adsorption or clogging of the polymer; however, direct experimental quantification of these mechanisms remains lacking. Here, we show that a mechanical energy balance incorporating the added viscous dissipation due to chaotic fluctuations generated by an elastic flow instability coupled with resistance from polymeric extensional stresses can capture the macroscopic flow resistance of polymer solution flow in 2D and 3D porous media with defined geometries. Our model directly links in situ pore-scale flow fields obtained using confocal microscopy to the macroscopic flow resistance and highlights the importance of pore geometry in mediating the different flow resistance contributions. Our work sheds light onto the fluid rheology and pore geometry-dependent physical mechanisms generating flow thickening in porous media and provides guidelines towards controlling macroscopic flow resistance in practical applications.