Selection and Screening of Polymers for Enhanced-Oil Recovery

Abstract

Abstract A number of commercially available polymers have been tested for enhanced oil recovery based upon viscosity, filterability, and surfactant compatibility, and chemical and thermal stability testing has been carried out with some of these as well. Several high molecular weight polymers exhibited high viscosities at salinities up to 170,000 ppm NaCl and with greater than 17,000 ppm CaCl2 present. Polyacrylamide polymers hydrolyze at high temperatures and beyond a certain point are subject to precipitation by calcium. If calcium concentrations can be kept below about 200 ppm, the use of polyacrylamide polymers is feasible up to reservoir temperatures of at least 100 °C. For higher concentrations of calcium, copolymers including AMPS moieties should be considered. Calcium tolerance can be improved with sodium metaborate or by using copolymers of acrylamide and sodium 2-acrylamido-2-methylpropane sulfonate (AMPS). The stability problems at elevated temperatures in the presence of iron can be mitigated by the use of chemicals such as sodium dithionite and sodium carbonate. The polymers tested did not lose viscosity after 220 days of aging at 100 °C with dithionite present. Introduction Following Muller's (1981) terminology, we will use "chemical degradation" when referring to the hydrolysis of polymer functional groups and "thermal degradation" when describing the free radical induced breakdown of the acrylic backbone, resulting in molecular weight reduction. Chemical degradation leads to a higher degree of hydrolysis and can only be prevented by the inclusion of more chemically stable monomers, but does not necessarily limit application, and in fact, often results in higher viscosity. Thermal degradation results in a reduction of molecular weight and a loss in viscosity, but this degradation can be prevented in most situations. Chemical Degradation; Hydrolysis and Precipitation of Polymers. Solutions of non-hydrolyzed polyacrylamide (PAM) are nonionic, and hence the viscosity is essentially insensitive to salinity. At elevated temperature and/or pH, the amide moiety undergoes hydrolysis, resulting in a acrylate moiety and the evolution of ammonium ion, as illustrated in Figure 1. The anionic charges of the acrylate moieties results in intramolecular repulsions that increase the hydrodynamic radius of the polymer molecules and hence the solution viscosity. Because of this benefit, commercial polyacrylamide for EOR is usually either post-hydrolyzed by addition of alkali or produced as a copolymer of acrylamide (AM) and acrylic acid or its salt (AA). In either case, the molar fraction of the acrylate moiety is referred to as the degree of hydrolysis (t), and is typically between 0.15 and 0.40 for commercial hydrolyzed polyacrylamide (HPAM) polymers used for enhanced oil recovery. During its residence in the reservoir at elevated temperature and/or pH, t of polyacrylamide polymers increases. At high salinity, the acrylate moieties on HPAM are strongly associated with cations, and the viscosity approaches that of non-hydrolyzed polyacrylamide. Multivalent cations have a much stronger effect than monovalent cations. If t exceeds approximately 0.33 (Zatouin and Potie, 1983), then precipitation is possible if excessive amounts of multivalent cations are present. The critical amount of calcium necessary to precipitate hydrolyzed polyacrylamide decreases with temperature, t, and decreasing monovalent cation concentration. At a high degree of hydrolysis, this has been described as a site fixation phenomenon, and occurs at close to the stochiometric equivalence point between acrylate moieties and cations (around 200 ppm of calcium for a 1000 ppm polymer solution). At lower t, the precipitation phenomenon is due to poor solvation (theta-type precipitation) (Ikagami, 1962).