Re-entrant transition of aluminum-crosslinked partially hydrolyzed polyacrylamide in a high salinity solvent by rheology and NMR (original) (raw)

Linked polymer solution (LPS) is a nanoparticle polymer and designed by crosslinking a high molecular weight partially hydrolyzed polyacrylamide (HPAM) with aluminum (III). It has been applied in the oil industry to enhance oil recovery by improving sweep efficiency and by microscopic diversion in porous media. To achieve good propagation properties, aggregates formed by intermolecular crosslinking and gel formation should be avoided. To our knowledge, there is no established method to distinguish between intra-and intermolecular crosslinking for high molecular weight (>10 3 10 6 Da), low concentration (<1000 ppm), polydisperse solutions of partially hydrolyzed polyacrylamides in high salinity solvents (5 wt % NaCl). The high salinity solvent is relevant to represent for formation water in many oil reservoirs. The main objective of the present study is to establish an experimental method for determining phase transition of LPS from monomeric coiled state to aggregated state in a high salinity solvent. No single experimental methods are conclusive and we have therefore applied a combinatorics approach including two-dimensional NMR, dynamic rheology, and UV spectroscopy. The different techniques show similar trends, which allow overall interpretations of phase transitions to be made. The experimental results indicated that the LPS solution at high salinity solvent underwent a phase transition by chain re-expansion, called reentrant transition. The transition point was observed at addition of 100 ppm of Al 31. Higher concentrations of Al 31 suppressed the rate of reentrant transition, most likely because of intramolecular crosslinking of HPAM chains by Al 31. Intermolecular crosslinking reaction was not observed at these conditions.