Effect of potential determining ions on sulfonated polyacrylamide behavior during smart water-polymer injection into carbonate reservoirs

In low salinity polymer flooding (LSPF), an advanced hybrid method for enhanced oil recovery (EOR), less attention has been given to the impacts of potential determining ions on polymer behavior in carbonate reservoirs. Therefore, seawaters spiked with divalent ions were used with sulfonated polyacrylamide (SPAM) polymer to investigate the effects of potential determining ions on SPAM performance in wettability alteration, polymer adsorption, carbonate surface charge, viscosity enhancement, emulsion type, and oil recovery. Among divalent anions and cations, only excess amounts of Mg²⁺ in a smart water-polymer solution could alter the wettability from oil-wet to neutral-wet and make the rock/brine zeta potential positive. Additionally, higher SPAM adsorption onto carbonate surfaces was observed as Mg²⁺ concentration was doubled, driven by interactions between sulfonate groups (–SO₃⁻) and the positively charged rock surface. Conversely, excess SO₄^2- impeded interactions between –SO₃^– and positively charged carbonate rock species, reducing SPAM adsorption. At 5000 ppm SPAM concentration, excess divalent ions increased solution viscosity due to the shielding effect, with the highest viscosity achieved by doubling Mg²⁺ concentration. However, at 10,000 ppm SPAM concentration, only SO₄^2- improved viscosity, while Ca²⁺ and Mg²⁺ reduced the viscosity of smart water-polymer solutions. As for emulsions produced by smart water-polymer solutions, the presence of SPAM in smart water led to the production of water-in-oil (W/O) emulsions and increased the mean droplet size of water droplets due to the salt-out effect. According to the results obtained from calcite-coated micromodel flooding experiments, the ultimate oil recovery for SW + SPAM (5000 ppm) was 34.2%. Also, a two-fold increase in the Mg²⁺ concentration rose the oil recovery by 6.5%.