Universal Viscosifying Behavior of Acrylamide-based Polymers Used in EOR - Application for QA/QC, Viscosity Predictions and Field Characterization

Abstract

Summary Conventional EOR polymers are acrylamide-based copolymers. 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… Moreover, solutions are non-Newtonian, they exhibit a Low shear Newtonian plateau viscosity at low shear rate followed by a shear thinning region at higher shear rate. In the absence of predictive model, for any new polymer grade or lot, any new or slightly varying field conditions, 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 polymers solutions in various physicochemical conditions (variation of the polymer microstructure, monomer composition, molecular weight, brine salinity and temperature). The flow curves in both dilute, semidilute non-entangled and semidilute entangled regimes were modelled by only two adjustable parameters: the intrinsic viscosity [η] and the relaxation time in the diluted regime λd. The low shear plateau viscosity η0 (more specifically, the specific viscosity ηspe) and the shear thinning index n obey mastercurves which are solely function of the overlap parameter C[η]. The relaxation time λ depends on C[η] and the relaxation time in the diluted regime λd. All these results are consistent with predictions for a neutral polymer in good solvent. By using these mastercurves, intrinsic viscosity of any polymer/brine system can be easily obtained at various temperatures from a single measurement in the semi-dilute regime in which viscosity is higher than water, and classic rheometers are very sensitive. The whole flow curve (η=f(γ)) can be predicted at any concentration, temperature and molecular weight. For any unknown polymer/brine system, the determination of λd enables to determine the viscosimetric average molecular weight M of the polymer. Finally, by using the additive property of the intrinsic viscosity of binary solutions, a method is proposed to evaluate molecular weight of field samples. Polymer physics is today considered as 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 molecular weight rather than a viscosity value, on-site and lab QA/QC will be greatly improved.