Effect of Charge Distribution on the Viscosity and Viscoelastic Properties of Partially Hydrolyzed Polyacrylamide

The majority of the polymers used in enhanced oil recovery (EOR) processes are water-soluble, acrylamide-based polymers, typically partially hydrolyzed polyacrylamide (HPAM). Industrial-scale production of HPAM polymers for EOR involves either posthydrolysis of polyacrylamide or copolymerization of acrylamide and sodium acrylate. Although products with the same molecular weight and average degree of hydrolysis can be made, the two processes give rise to products with different distributions of charge between individual polymer molecules. In this work, we show that charge distribution at constant molecular weight and average degree of hydrolysis has a significant effect on HPAM viscosity and viscoelasticity of calcium-containing brines. In EOR processes, viscosity and viscoelastic properties are key performance parameters, and calcium ions are an inherent part of most formation and injection waters. Thus, knowledge of the average degree of hydrolysis is not sufficient for performance prediction. To our knowledge, this is the first comprehensive study of the effect of charge distribution on the rheological behavior of industrial HPAM obtained by copolymerization and posthydrolysis.