The recovery of viscosity of HPAM solution in presence of high concentration sulfide ions

Partially hydrolyzed polyacrylamide (HPAM) is widely used as a viscosity-enhancing agent in oil recovery. However, significant viscosity loss has been reported for HPAM solutions in the presence of S2-. Surprisingly, in this study, we found that this loss could be recovered by adding more S2-. To address this issue, the rheological behavior, hydrodynamic radius, and chemical structure of HPAM were investigated simultaneously using rheometry, dynamic light scattering (DLS), Fourier-transform infrared (FT-IR) spectroscopy, and 1H nuclear magnetic resonance (NMR) spectroscopy. The viscosity of the HPAM solution dropped sharply in the presence of trace S2- (<10 mg/L), then partially recovered as S2- concentration increased from 10 to 400 mg/L, and finally plateaued at higher concentrations. Also, the change in viscosity with increase of S2- concentration was found to be in consistent with that in hydrodynamic radius of HPAM in aqueous solutions. Furthermore, FT-IR and 1H NMR spectra suggest that the amide amino groups were chemically altered by S2-, changing the chemical environment of the methyne proton. Interaction between amide groups and the trace S2- reduced the hydrodynamic radius and destroyed the HPAM network structure, thus reducing viscosity. In contrast, with increase in S2- concentration, more and more S2- linked to amide groups via interactions between them, resulting in the enhanced electrostatic repulsion between these S2-. Consequently, the repulsion made the HPAM molecule more stretched and the network fluffier, helping to recover the viscosity loss. Herein, the whole picture and mechanism of S2--induced viscosity changes in an HPAM solution are evaluated for the first time.