Characterization of Ultra-High Molecular Weight Oilfield Polyacrylamides Under Different pH Environments Using Asymmetrical Flow FFF and Multi-Angle Light Scattering Detector

Abstract

Abstract 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.