Nanoparticles for Enhanced Oil Recovery: Influence of pH on Aluminum-Cross-linked Partially Hydrolyzed Polyacrylamide-Investigation by Rheology and NMR

New methods are continuously being evaluated for Enhanced Oil Recovery (EOR). Nanoparticle flooding is an intriguing new approach in which one of the main applications is microscopic diversion. Linked polymer solution (LPS) is a nanoparticle system that consists of partially hydrolyzed polyacrylamide (HPAM) cross-linked by aluminum­(III). The source of aluminum is an aluminum citrate (AlCit) complex, where citrate serves as a carrier ligand. The large size and flexibility of HPAM as well as the multicomponent species of AlCit present at most relevant reservoir conditions make the system highly complex and challenging to characterize. In the literature, there is a lack of systematic and consistent data describing the various chemical species involved in LPS. This work used nuclear magnetic resonance (NMR) spectroscopy and dynamic rheology to investigate the reaction between HPAM and Al³⁺ as a function of solution pH. The results of NMR spectra indicate that AlCit is more reactive at near neutral pH and cross-linking reactions should preferably be performed at this condition. Structural conformation and viscoelastic properties of the HPAM-Al complex appear to be dominated by the protonation–deprotonation state of the carboxylic acid groups as result of pH changes