Visualization of Polymer Retention Mechanisms in Porous Media Using Microfluidics
Authors: Sugar Antonia , Serag Maged F. , Torrealba Victor A. , Buttner Ulrich , Habuchi Satoshi , Hoteit Hussein 
Published: June 2020

Abstract
Understanding polymer transport through porous media is key to successful field implementations, including well conformance control and EOR processes. Polymer retention is typically assessed indirectly through its effect on pressure drops and effluent concentrations. Microfluidic techniques represent convenient tools to observe and quantify polymer retention in porous media. In this paper, we demonstrate how a soft-lithography microfluidics protocol can be used to gain insights into polymer transport mechanisms through rocks.
The design of the microfluidic chips honors typical pore-size distributions of oil-bearing conventional reservoir rocks, with pore-throats ranging from 2 to 10 μm. The fabrication technology enables the design transfer on a silicon wafer substrate using photolithography. The etched wafer holding the negative pattern of the pore-network served as a mold for building the microfluidics chip body out of polydimethylsiloxane (PDMS). The oxygen plasma bonding of the PDMS to a thin glass slide resulted in a sealed microfluidic chip, conceptually referred to as "Reservoir-on-a-Chip". We conduct single-phase polymer flooding experiments on the designed chips to understand how polymer-rock interactions impact polymer transport behavior in rocks. These experiments allow for polymer transport visualization at the molecule-scale owing to the use of polymer tagging and single-molecule tracking techniques.
This study presents, for the first time, a direct visualization of polymer retention mechanisms in porous media. We identified three mechanisms leading to polymer retention: adsorption, mechanical entrapment, and hydrodynamic retention. Polymer adsorption on the chip surfaces resulted in flow conductivity reduction in specific pathways and complete blockage in others, inducing alterations in the flowpaths. This mechanism occurred almost instantaneously during the first minutes of flow then, dramatically diminished as adsorption was satisfied. In addition to static adsorption, flow-induced adsorption (entrapment) was also distinguished from the binding of flowing polymer molecules to the already adsorbed polymer layer. Evidence of polymer desorption was observed, which consents with the presumed reversibility character of polymer retention mechanisms. The narrowest channels along with the reduced area due to adsorption, created favorable conditions for polymer entrapment. Both mechanical and hydrodynamic trapped polymers were successfully imaged. These phenomena led to polymer clogging of the porous network, which is one of the major concerns for operational aspects of polymer flooding processes.
Better understanding and quantification of polymer retention in porous media can help to make better decisions related to field-scale implementations of polymer-based processes in the subsurface. In this study, we used a soft-lithography fabrication technique and single-molecule imaging, to show, for the first time, polymer transport insights at the molecule- and pore-scales. This approach opens a new avenue to improve our understanding of the first principals of polymer retention while flowing through porous media.