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

Abstract

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 × 106 Da) and a low concentration (<1000 ppm). In this work, therefore, the phase transition of LPS is investigated by employing two-dimensional 1H–1H 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.

Re‐entrant transition of aluminum‐crosslinked partially hydrolyzed polyacrylamide in a high salinity solvent by rheology and NMR

“…In our previous study, HPAM and LPS were prepared in a low salinity (i.e., 0.5 wt % NaCl), the distance d CH2‐NHa varied between 3.0 and 4.1 Å, whereas d CH2‐NHb varied between 2.4 and 2.6 Å. However, at higher salinity (5 wt % NaCl), these distances are longer than 5 Å for all concentrations of Al 3+ (see Figure S1 in Supporting Information) and thereby, no cross peak signals.…”

“…The diffusion coefficient of HPAM solution prepared in 5 wt % NaCl diffuse faster than HPAM solution made in 0.5 wt % NaCl (log D = −12.0 m 2 /s) . This indicates that the effective hydrodynamic radius of 600 ppm HPAM is larger in 0.5 wt % NaCl, despite the re‐expansion occurring at 5 wt % NaCl.…”

“…The experiments were conducted to understand the phase transition of LPS as a function of Al 3+ concentration at high brine salinity (5 wt % NaCl). The influence of salinity can be evaluated by comparing with our previous results of LPS prepared in 0.5 wt % NaCl . The presence of monovalent counterions influences the HPAM conformation as the counterions condense on the polymer chain and screen the electrostatic interactions between charged monomer units.…”

“…Characterization of high molecular weight polymer with wide distribution is very challenging work. In fact, a methodology was suggested for characterization of phase transitions of LPS as a function of Al(III) concentration in low salinity solvents in our previous work . In the present work, we extended the scope of LPS phase transition study and attempt to validate the methodology for high saline conditions.…”

Influence of salinity on the properties of the different HPAM/Al3+systems

“…In addition, for the influence on the cross-linked polymer, that of Ca 2+ is the biggest, then Mg 2+ , the least being Na + . In 2014, Kedir et al [25] investigated that phase transition of aluminum cross-linked HPAM under a low salinity condition and revealed that the phase transition is a stepwise process. The first step is the transition from a random polymer coil to structured coils, the second step is that the networked weak gel is formed by coil aggregates above the threshold concentration.…”