Universal viscosifying behavior of acrylamide-based polymers used in enhanced oil recovery

Conventional polymers used in enhanced oil recovery (EOR) are acrylamide-based copolymers of very high molecular weight. Their viscosity in aqueous solution depends on various physicochemical parameters such as monomer composition, concentration, average molecular weight, polydispersity, salinity level and ionic composition, temperature, etc. Moreover, solutions are non-Newtonian; they exhibit low-shear Newtonian plateau viscosity at a low-shear rate followed by a shear thinning region at a higher shear rate. In the absence of a predictive model, for any new polymer grade or lot, any new or slightly varying field condition, it is necessary to perform a whole set of viscosity measurements at varying concentrations, which is tedious, time-consuming, and not valuable. Flow curves (viscosity vs shear rate) were measured on a great number of polymer solutions in various physicochemical conditions (variation of the polymer microstructure, monomer composition, molecular weight, brine salinity, and temperature). The flow curves in dilute nonentangled, semidilute nonentangled, and semidilute entangled regimes were modeled by only two adjustable parameters: the intrinsic viscosity [eta] and the relaxation time in the dilute regime lambda(d). The zero-shear viscosity eta(0)(more specifically, the specific viscosity eta sp) and the power law indexnobey master curves that are solely functions of the overlap parameter C[eta]. The relaxation time lambda depends on C[eta] and the relaxation time in the dilute regime lambda(d). All these results are consistent with predictions for a neutral polymer in a good solvent. By using these master curves, intrinsic viscosity of any polymer/brine system can be easily obtained at various temperatures from a single measurement in the semidilute regime in which viscosity is higher than water, and classic rheometers are very sensitive. The whole flow curve eta(gamma) can be predicted at any concentration, temperature, and molecular weight. For any unknown polymer/brine system, the determination of lambda denables us to determine the viscosimetric average molecular weightMof the polymer. Finally, by using the additive property of the intrinsic viscosity of binary solutions, a method is proposed to evaluate the molecular weight of field samples. Polymer physics is today considered well described and well known. However, the beauty and the usefulness of this physics have been partly ignored by the EOR community up to now. This study gives a methodology to predict the viscosifying behavior and the molecular weight of any acrylamide-based copolymer/brine system. By attributing the molecular weight rather than a viscosity value, on-site and lab quality control will be greatly improved.