Nanoparticles for Enhanced Oil Recovery: Phase Transition of Aluminum-Cross-Linked Partially Hydrolyzed Polyacrylamide under Low-Salinity Conditions by Rheology and Nuclear Magnetic Resonance

There is booming interest in the application of nanoparticles for enhanced oil recovery. In this work, a polymer nanoparticle that is generated by cross-linking a high-molecular weight partially hydrolyzed polyacrylamide with aluminum(III) and known as linked polymer solution (LPS) has been studied. The size and conformational state of LPS particles are influenced by the concentration of Al(III). To the best our knowledge, there is no current established method for determining the conformational state, i.e., single coiled particle, coil aggregates, or gel, for a polymer solution with a high molecular weight (>10 X 10(6) Da) and a low concentration (<1000 ppm). In this work, therefore, the phase transition of LPS is investigated by employing two-dimensional H-1-H-1 nuclear magnetic resonance (nuclear Overhauser effect spectroscopy and diffusion ordered spectroscopy), UV-visible spectroscopy, and oscillatory rheological methods. Each method is limited to determining the conformational state; however, the combined methods provided a consistent tool for mapping various interactions regarding the conformational changes of LPS as a function of Al(III) concentration. The results of our study revealed that the phase transition is a stepwise process; the transition from a random polymer coil to structured coils (intramolecular cross-linking) was observed by reduction of the hydrodynamic radius and an increase in the rate of diffusion, followed by coil aggregates as a function of Al3+ concentration. Ultimately, networked weak gels are formed by coil aggregates (intermolecular cross-linking) above the threshold concentration.