Journal of Colloid and Interface Science 234, 269–283 (2001) doi:10.1006/jcis.2000.7306, available online at http://www.idealibrary.com on Unsteady-State Flow of Flexible Polymers in Porous Media Pacelli L. J. Zitha, ∗,1 Guy Chauveteau,† and Liliane L´ eger‡ ∗ Department of Applied Earth Sciences, Delft University of Technology, Mijnbouwstraat 120, 2628 RX Delft, The Netherlands; †Division Gisements, Institut Franc ¸ais du P´ etrole, 1 et 4 Avenue de Bois-Pr´ eau, 92506 Rueil-Malmaison Cedex, France; and ‡Laboratoire de la Mati` ere Condens´ ee, Coll` ege de France, 11, Place Marcellin Bethelot, 75231 Paris Cedex 05, France Received October 28, 1999; accepted October 30, 2000 In this paper we report an investigation of the unsteady-state flow of polymer solutions through granular porous media. The ex- periments were performed using high-molecular-weight nonionic and anionic polyacrylamides dissolved in water containing NaCl and model porous media obtained by packing silicon carbide (SiC) grains having a narrow grain size distribution. Before injection in porous media, the polymer solutions were carefully filtered accord- ing to a method that was proved to be efficient in removing any possibly remaining microgels. The SiC grain surface was passively oxidized by a controlled thermal treatment in order to obtain a sur- face partially covered by a thin silica layer having adsorption prop- erties similarto those of quartzitic sand. By packing SiC grains of different sizes, porous media having identical adsorption proper- ties and well-known pore throats sizes can be obtained with a good reproducibility. Parameters investigated include pore size, velocity gradient, polymerconcentration, and adsorption energy. A striking unsteady-state flow behavior (pressure build-up at constant flow rate) is observed when three conditions are fulfilled:(a) the velocity gradient is largerthan that known to be able to induce a coil–stretch transition, (b) the polymer adsorbs on the pore surfaces, and (c) the length of stretched macromolecules is largerthan the effective pore throat diameter. When one of these conditions is not satisfied the flow remains steady. These observations are interpreted by a mecha- nism involving the adsorption and bridging across pore restrictions of elongated chains. We propose to refer to this peculiar mode of polymeradsorption as bridging adsorption. C  2001 Academic Press Key Words: flexible polymers; polyacrylamides; elongation; ad- sorption; pore bridging; porous media; polymer filtration. I. INTRODUCTION Flexible polymers dissolved in (salted) water exhibit distinc- tive properties in porous media, allying a variety of physical fea- tures (complex rheology, adsorption, depletion near pore walls, permeability reduction, etc.) covering a broad range of useful- ness. Fields where these properties are crucial include hydrocar- bon recovery, water production and treatment, soil stabilization, filtration for separation, and purification in various laboratory 1 To whom correspondence should be addressed. E-mail: p.l.j.zitha@ ta.tudelft.nl. This author was affiliated with IFP during this study. and industrial processes, to mention only a few. As a conse- quence great research efforts have been devoted in the past to investigating the characteristics of polymer flow-through porous media. As has been shown in several reviews (1–4), most previous studies have concerned either the influence of the velocity gra- dient in high-permeability media or the apparent viscosity in low-permeability media maintaining the velocity gradient suf- ficiently low. Studies devoted to pressure build-up phenomena in porous media are scarce in the literature and, to our knowl- edge, only Ref. (5) seems to have noted that pressure build-up occurs when the flow rate exceeds a certain onset value. In these early experiments, high-molecular-weight hydrolyzed poly- acrylamide samples were injected by steps of increasing flow rates in low-permeability (small pores) granular porous media obtained by packing crushed quartz grains. At high flow rates, a slow pressure build-up was observed followed by a leveling off to a plateau. The plateau values increased with the flow rate and were always higher than the values that could be expected from viscosity measurements. However, when the experiments were performed the knowledge of the elongational and adsorp- tion behavior was latent which hindered the interpretation of the results. At the same time, other authors (6, 7) showed that not only adsorption but also retention by mechanical means should be taken into account in the attempts to understand polymer flow in porous media. Several works (8, 9) showed later that polymer aggregates and microgels present in most commercial polymers could be at the origin of pressure build-up. This led to the de- velopment of a method of filtration with a low-velocity gradient to remove the microgels as described in Ref. (8). Since the behavior of polymer solutions at high flow rates is deemed crucial to control a large number of applications (re- duction of water production in oil and gas recovery, industrial and laboratory filtration processes), the study reported below has been carried out. The purpose of this study was to investi- gate unsteady flows under well-controlled conditions in order to provide a reliable interpretation of this phenomenon. This is possible now mainly because a better understanding of the elongational flow and adsorption properties of flexible polymers in porous media has been achieved over the past two decades (3). 269 0021-9797/01 $35.00 Copyright C  2001 by Academic Press All rights of reproduction in any form reserved.