Polymer Flooding: Establishing Specifications for Dissolved Oxygen and Iron in Injection Water

Summary It is now common knowledge among enhanced-oil-recovery (EOR) practitioners that the combination of ferrous iron (Fe2+) and dissolved oxygen (O2) causes severe oxidative degradation to EOR polymers, resulting in a lowering of molecular weight (MW) and hence, a loss of viscosity. During the design of polymer-flooding projects, an important question is thus the acceptable levels of Fe2+ and O2 that can be tolerated in injection-water specifications. Furthermore, we would like to be able to predict the extent of degradation in the case of excess Fe2+or oxygen ingress. However, despite more than 50 years of research and a general understanding of the degradation mechanism involved, quantitative prediction of the extent of degradation has proved elusive and dependent on the measurement protocol. This is likely because of the fastidious experimental protocols required to work under anaerobic or limited-oxygen conditions. We examine existing protocols and demonstrate that experiments in which either Fe2+ or O2 is the limiting reagent yield equivalent results when the stoichiometry of the Fe2+ autoxidation reaction with oxygen is taken into account. On the basis of these findings, a novel, easy approach is proposed to quantify polymer-oxidative degradation as a function of either O2 or Fe2+ content. The limits of 225 ppb Fe2+ and 32 ppb of O2 are fixed for Flopaam 3630S in 6 g/L brine in the concentration range 500–1,500 ppm to ensure that degradation of low-shear plateau viscosity does not exceed 10%. Higher levels will lead to severe polymer degradation. The influence of polymer concentration, temperature, and salinity is also investigated. At last, evolution of redox potential and pH during Fe2+ oxidation is discussed along with the injectivity risk associated with the formation of Fe3+. There is a direct practical application of these findings for the design of surface facilities for polymer dissolution and transport and for the prediction of degradation in case of oxygen ingress. Moreover, a simple and easily performed protocol is proposed for the evaluation of polymer oxidative degradation for any given field conditions.