Development and Evaluation of EOR Polymers Suitable for Hostile Environments: II—Copolymers of Acrylamide and Sodium AMPS

Abstract

Abstract Copolymers of acrylamide (Am) and sodium-2-acrylamido-2-methylpropane sulfonate (NaAMPS) have been evaluated for flooding applications at elevated temperatures. The brines employed in this study include synthetic seawater (447 ppm Ca++, 1256 ppm Mg++ and 33,560 ppm TDS) and others containing a It 10 weight ratio of CaCl2 to NaCl and having TDS ranging from 0.55 to 16.5%. AMPS is a registered trade mark of the Lubrizol Corporation. While NaAMPS homopolymer solutions are thermally stable at 121°C (250°F), and hydrolyze very slowly and to about 4% after extended aging, copolymerization of NaAMPS with acrylamide does not appear to protect the later against thermal hydrolysis and consequent precipitation in hard brines. Only Am-NaAMPS copolymers containing less than 9 wt.% Am might tolerate a brine such as seawater at 121°C (250°F). Comparison of these results with those reported for vinylpyrrolidone (Vp) and acrylamide copolymers which showed stability for a copolymer containing 50 wt.% Am in seawater at 121°C (250°F) is evidence that Vp protects acrylamide against thermal hydrolysis, but NaAMPS does not. Evaluation of various weight ratios of Am-NaAMPS copolymers for flooding applications at 93°C (200°F) resulted in an optimum composition near 40/60 wt. ratio of Am/NaAMPS. Given the fact that polyacrylamides are suitable for flooding applications up to 75°C (167°F), inclusion of about 60 wt.% of a more expensive NaAMPS monomer into the copolymer composition to increase its usability limit by less than 20°C (36°F), does not appear economically viable. It is believed that many of the so called "high temperature" polymers commercially available, are NaAMPS based materials containing high levels of acrylamide and are not suitable for polymerflooding at temperatures significantly higher than those of polyacrylamide.