Statistical and rheological evaluation of the effect of salts in partially hydrolyzed polyacrylamide solutions

Partially hydrolyzed polyacrylamide (PHPA) is the most common anionic copolymer used in Enhanced oil recovery (EOR), but the use of this polymer presents some limitations in the presence of divalent cations. The objective of this work was to evaluate the thermal stability of PHPA in brines containing Na+, Mg2+, and Ca2+ cations (isolated or combined). In this study, the PHPA used at a concentration of 2500 mgL-1 has a MW of 20 x 106 Da. The stability of the polymeric solutions was monitored through rheological analyses for 180 days at 70 degrees C, in the absence of oxygen, using a central composite rotational design. In the absence of dissolved oxygen and cations, the PHPA solution was basically the same as that observed over a period of 30 days. It was observed that Ca+2 ion had the greatest influence on reducing the viscosity of PHPA in all cases. Polymeric solutions with Total dissolved solids (TDS) greater than 1000 mg L-1 showed complete degradation of the polymer in 180 days. The statistical data corroborate the rheological results, showing that only the main effect of Na+ was not statistically significant and that the concentrations of Mg2+ and Ca2+ presented statistically significant effects in their linear components and quadratic components. Representative scheme of the PHPA polymer chain in the presence of different concentrations of Na+, Mg2+, and Ca2+ salts. image