Abstract
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 [η] and the relaxation time in the dilute regime λd. The zero-shear viscosity η0 (more specifically, the specific viscosity ηsp) and the power law index n obey master curves that are solely functions of the overlap parameter C[η]. The relaxation time λ depends on C[η] and the relaxation time in the dilute regime λd. 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 η(γ˙) can be predicted at any concentration, temperature, and molecular weight. For any unknown polymer/brine system, the determination of λd enables us 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 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.
Highlights
Intrinsic viscosity is a key property that traduces the volume occupied by 1 g of polymer in solution and results from the influence of all physicochemical parameters such as molecular weight (Mw), polydispersity, temperature, salinity, etc
Its relevance to determine the average molecular weight and the size of polymer chains is evident and complementary of size exclusion chromatography (SEC) that is pushed to its limits [7,8,9] when dealing with very high Mw polymers used in enhanced oil recovery (EOR) for which Mw is ranging between one and several tens of million g/mol
Its classical determination requires to measure the viscosity of a series of polymer solutions at different concentrations in the dilute regime corresponding to C[η], 1
Summary
Intrinsic viscosity is a key property that traduces the volume occupied by 1 g of polymer in solution (units are in dl/g) and results from the influence of all physicochemical parameters such as Mw, polydispersity, temperature, salinity, etc The measurement of this quantity allows us to get information on polymer molecular weight (Mw), its interaction with the solvent, the architecture and the rigidity of the chain (linear, ramified flexible coils, and semiflexible rods), the size of the chains in solution (radius of gyration Rg), and the critical concentration at which chains overlap (C*). Results obtained on polymers with different microstructures (Flopaam series from SNF vs ZLPAM from ZL Chemicals, blocky/statistic structure), monomer compositions (by varying the proportion of acrylamide/ sodium acrylate/acrylamido tertiobutylsulfonate), molecular weights (Flopaam series 3130–3630), salinities (6–250 g/l), and temperatures (25–90 °C) in a very wide range of concentration (30–17 000 ppm) are modeled based on the scaling laws of polymer physics. We review the possible applications of these findings for quality control in laboratory or during operations in oilfield
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