Shear-thinning suppresses multistability of unstable flow states for polymer solution flow in porous media

Polymer solution flow in porous media is widely used in applications such as groundwater remediation and enhanced oil recovery. Rheology affects several processes during chemical flooding, from feasibility of fluid injection to pore-scale displacement efficacy; however, bulk rheology measurements are often not translatable to in situ fluid rheology within a porous medium. Our previous work studying flow of a near-Boger fluid in a 1-D pore array found that an elastic instability arises above a threshold flow rate, characterized by multistability of eddy-dominated and eddy-free pores. Multistability arises when the advection timescale between pores is smaller than the polymer relaxation time: Deborah number (De) greater than 1. Here, we examine how shear thinning impacts this multistability. Interestingly, we find that shear thinning solutions become unstable for sufficient fluid elasticity, but multistability is drastically suppressed. We rationalize this behavior by accounting for shear rate-dependent rheology where De is effectively reduced for a shear thinning fluid compared to a Boger fluid at equivalent flow conditions. Our work thus provides key insight into how rheology impacts pore-scale flow features and can be used to guide formulation development for targeted flow behavior.