High molecular weight polymers used for heavy oil recovery exhibit viscoelasticity that can influence the oil recovery during chemical enhanced oil recovery. Different polymers having similar molecular weight and shear rheology may have different elongation flow behavior depending on their extensional properties. Displacing slugs are more likely to stretch than shear in tortuous porous media. Therefore, it is critical to seek an analytical tool that can characterize extensional parameters to improve polymer selection criteria. This article focuses on the extensional characterization of two polymers (hydrolyzed polyacrylamide and associative polymer) having identical shear behavior using capillary breakup extensional rheometer to explain their different porous media behavior. Maximum extensional viscosity at the critical Deborah number and Deborah number in porous media classified the associative polymer as the one having high elastic-limit. Extensional characterization results were complemented by significantly higher pressure drop, marginally increased oil recovery of associative polymer in porous media.

Associative polymer (AP) and hydrolyzed polyacrylamide (HPAM), the two commonly used polymers for heavy oil recovery, are reported to exhibit very different flow behaviour in porous media despite having similar shear viscosity. The kind of hydrophobic association (intramolecular or intermolecular) that AP exhibit is concentration dependent and will influence both shear and elongational flow in the porous media. To understand flow behaviour of these two polymers in porous media, the role of hydrophobic association on shear and extensional rheology and its effect on the resistance factor (RF) and residual resistance factor in porous media are investigated over a non‐associating HPAM polymer. The results suggest that shear rheology and the Deborah number cannot explain the porous media flow behaviour. However, direct measurements of extensional properties using a capillary breakup extensional rheometer have shown the marginal difference between two polymers at a lower concentration (1000 ppm) and the considerable difference at a higher concentration (2000 ppm), indicating an intramolecular and intermolecular association, respectively. These results are in accordance with porous media observations where both polymers have shown a similar RF at 1000 ppm. Whereas, at 2000 ppm AP showed a much higher RF at low/intermediate fluxes, and a similar RF at high flux, suggesting the transformation from intermolecular association to intramolecular association. The significant drop in the extensional viscosity at high strain rates after exhibiting the maxima explains this behaviour.

Summary Various types of ultrahigh-molar-mass polyacrylamides (HPAMs) and their copolymers and terpolymers used not only in enhanced oil recovery (EOR) but also in drilling, fracturing, water treatment, and tailing applications require an accurate description of polymer molar mass (Mw) and hydrodynamic size for their optimal design. The range of Mw for various types of available HPAMs is between 4 and 30 million g/mol and is typically determined by use of intrinsic-viscosity measurement. Molecular-weight distribution (MWD) cannot be determined because neither standard with low polydispersity index (PDI) nor gel-permeation-chromatography (GPC) or size-exclusion-chromatography (SEC) techniques exist today for such ultrahigh-molar-mass polymers. Moreover, the solution environment in underground reservoirs, characterized by high temperatures, pH values, and the presence of monovalent and divalent ions, may often lead to changes in polymer-macromolecular conformation. Current techniques, SEC, ultraviolet-visible measurements, and liquid chromatography, are not capable of accurately investigating these complex macromolecular structures for various reasons. In this paper, the asymmetrical-flow field-flow fractionation (AF4) system was used to fractionate four different ultrahigh-molecular-weight HPAM samples, varying in molar mass and commercially used for oilfield applications, in various carrier pH values ranging from 12 to 3 (pH values of 12, 7.4, and 3). The system uses field-flow fractionation (FFF), a family of analytical techniques developed specifically for separating and characterizing macromolecules, colloids, and particles. The theoretical separation range for AF4 is between 103 to 1012 g/mol. Other advantages over conventional GPC/SEC include minimum shear degradation, mild operating conditions, and no sample loss caused by adsorption. The flow system was equipped with a multiangle-light-scattering (MALS) and refractive-index (RI) detectors to measure molar mass and radius of gyration (Rg). The results show that the observed molecular weight of the polymer aggregate increased substantially as the pH value of the carrier solution decreased from 12 to 3, especially for higher-molar-mass polymers. The sample Rg showed the opposite trend, decreasing as the pH of the carrier solution changed from basic to acidic. For ultrahigh molecular HPAM at high pH, a narrower molar mass and radius distribution was observed with disaggregated molar mass and increased branching or swelling (therefore larger hydrodynamic radius). Use of this direct separation and measurement technique can improve understanding of polymer-macromolecular structure and corresponding changes in the reservoir brines.

Various types of ultrahigh molar mass polyacrylamides (PAMs) or HPAMs and their co- and ter-polymers used not only in enhanced oil recovery, but also in drilling, fracturing, water treatment and tailing applications require an accurate description of polymer molar mass (Mw) and hydrodynamic size for their optimal design. The range of Mw for various types of available HPAMs is between 4 and 30 million g/mol and is typically determined using intrinsic viscosity measurement. Molecular weight distribution (MWD or PDI) cannot be determined since neither standards with low PDI nor GPC/SEC techniques exist today for such ultrahigh molar mass polymers. Moreover, the solution environment in underground reservoirs, characterized by high temperatures, pH and the presence of monovalent and divalent ions, may often lead to changes in polymer macromolecular conformation. Current techniques, such as light scattering or microscopy, SEC, ultraviolet visible measurements and liquid chromatography, are not capable of accurately investigating these macromolecular complex structures for various reasons. In this paper the Asymmetrical Flow Field Flow Fractionation system was utilized to fractionate four different ultrahigh molecular weight HPAM samples, varying in molar mass and commercially used for oilfield applications, in different carrier pH values ranging from 12 to 3 (pH 12, pH 7.4 and pH 3). The system uses field flow fractionation a family of analytical techniques developed specifically for separating and characterizing macromolecules, colloids and particles. Other advantages over conventional GPC/SEC include minimum shear degradation, mild operating conditions and no sample loss due to adsorption. The flow system was equipped with a multiangle light scattering and refractive index detectors to measure molar mass and radius of gyration. The results show that the samples molecular weights increased substantially as the pH (or the ionic strength) of the carrier solution decreased from 12 to 3, especially for higher molar mass polymers. The samples radius of gyrations showed the opposite trend decreasing as the pH of the carrier solution changed from basic to acidic. For ultrahigh molecular HPAM at high pH, a narrower molar mass and radius distribution was observed with disaggregated molar mass and increased branching or swelling (therefore higher hydrodynamic radius). Use of this direct separation and measurement technique can improve understanding of polymer macromolecular structure and respective changes in the reservoir environments to enable optimal chemical dosage in oilfield applications.

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