Shear Viscosities of Ionic Polyacrylamide Solutions

Abstract

Abstract Solutions of ionic polyacrylamide polymers behave pseudoplastic in purely viscometric flow. Flow rate, polymer molecular weight and electrolytes affect solution viscosities to a large extent. Equations are given for the viscosity-shear rate relations in a form that can be used conveniently to account for the effect of viscosity on mobility. Introduction Polymers are being used increasingly in oil recovery operations, and therefore, an understanding of their flow behavior is gaining pragmatic importance. Past studies have shown that in the flow of polymeric fluids through porous media, the increase in solution viscosity, decrease in permeability, and viscoelastic deformations cause permeability, and viscoelastic deformations cause the fluid mobility to be greatly reduced. In general, viscoelasticity, i.e., extensional flow, is not so important because, for the largest part of a reservoir, polymer solution moves at very low and fairly steady polymer solution moves at very low and fairly steady velocities. Jennings et al. have concluded this for the specific polymers that they studied. Permeability reduction plays an important role in Permeability reduction plays an important role in the mobility control, particularly in porous media having low permeabilities initially. Reductions ranging from 25 to 70 times have been reported. However, the alterations that take place in a porous medium during polymer flow, the coupling between the geometry of the porous medium and the properties of the flowing fluid, and the influence of the flow regime on permeability have not been looked into in sufficient detail. A separate study, directed to the understanding of these important phenomena is required. In the present work, the purely viscous behavior of solutions of three partially hydrolized polyacrylamide polymers was obtained under experimental conditions far polymers was obtained under experimental conditions far more extensive than any reported in the literature. Some data have been available in the past for two of the polymers, but the third is a new polymer for which no data have been reported before. Using a Weissenberg rheogoniometer, Cannon-Fenske viscometers, and various capillary cubes, viscosities were measured over 8 decades of shear rate, ranging from 10 to to 10 (5) sec-1. These are the limits of measurable rates of shear and cover those that may apply to flow in reservoirs. Distilled water and various NaCl solutions were used as solvents to afford comparison of the rheological properties between fresh and saline solutions. Measurements were also made with solutions containing calcium' and magnesium to study the effect of divalent cations. EXPERIMENTAL The three polymers, Nos. 500, 700 and NC 1870, are partially hydrolized polyacrylamides manufactured by The Dow Chemical Co., and were from lots 8085, 52 and 87-8100E, respectively. Polymer NC 1870 is currently at a developmental stage and can be obtained in limited quantities; the other two have been available commercially for some time, have been used in the laboratory and in the field. All three are hydrolized to the same extent, containing approximately 25 percent polyacrylate, with the remainder being polyacrylamide. The molecular weights of Nos. 500 and 700 are 2 to 3 and 3 to 7 million, respectively. That of the NC 1870 is higher, but has not been measured due to the usual difficulties in measuring such high molecular weights. Polymer and salt concentrations are given on a weight-parts per million basis. Reagent grade chemicals and double-distilled deaerated water, having a pH of 6.5, were used in all solutions. Formaldehyde was added as a bactericide. To the extent possible, air was kept out of the solutions to avoid oxidation-type degradation of the polymers. Polymer solutions were mixed using magnetic Polymer solutions were mixed using magnetic stirrers and carefully avoiding any mechanical degradation. Solutions of desired concentrations were prepared from stock solutions by dilution. The latter had been passed through 1-micron millipore filters, were optically clear, containing no fish-eyes. The polymer concentration of stock solutions was determined by turbidimetry and nitrogen analysis, the two methods usually agreeing within a few percent. percent. SPEJ P. 469