Polymer solutions are often injected in porous media to improve enhanced oil recovery or groundwater remediation efforts, but these applications are limited by an incomplete understanding of the underlying physics. In particular, the removal of oil or contaminants from an aquifer is often limited by the formation of preferential flow pathways that bypass large regions, leaving them untreated. These bypassed regions can potentially be treated using polymers, which are believed to dynamically redirect the flow.<br/><br/>In a tortuous pore space, polymers are elongated dynamically by the extensional and shear fields. At sufficiently high flow rates, polymer elongation cannot keep up with the changing flow field in the tortuous pore space, producing an unstable flow with strong spatial and temporal fluctuations. These flow fluctuations are surprising, because the flows have negligible inertia (Re&lt;&lt;1) and hence the flow is typically assumed laminar. In particular, it is still poorly understood how the spatio-temporal characteristics of this flow state depend on pore geometry. We shed light on this question by systematically varying the spacing between pores. We find that when the pore spacing is large, unstable eddies form upstream of each pore, similar to the case of an isolated pore. By contrast, when the pore spacing is sufficiently small, the flow exhibits a surprising bistability, stochastically switching between two distinct flow states. We hypothesize that this unusual behavior arises from the interplay between the retention of polymer strain between pores, hysteresis in polymer conformations, and fluctuations in the flow. Our results help to elucidate the rich array of flow behaviors that can arise in polymer solution flow through porous media.

Bistability in the flow of polymer solutions in porous media

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.