Synergistic Effects between Different Types of Surfactants and an Associating Polymer on Surfactant–Polymer Flooding under High-Temperature and High-Salinity Conditions

The synergistic effects of surfactant–polymer (SP) flooding under high-temperature and high-salinity (HTHS) conditions were investigated for a hydrophobically associating polyacrylamide (HAPAM) mixed with different types of surfactants. The hydrophobic group of the polymer is positively charged. All surfactants contain 12 carbons at the hydrophobic chain. The list of surfactants included a cationic surfactant (DTAB: dodecyl trimethylammonium bromide), an anionic surfactant (SDS: sodium dodecyl sulfate) and a nonionic surfactant (DDG: dodecyl glucopyranoside). The physical properties of various SP solutions, prepared in high-salinity water, were evaluated using formulations composed of 0.2 g/L polymer and different concentrations of surfactants. Upon the addition of surfactants, the zero-shear viscosity (η0) of the polymer solution was found to peak at ∼0.15 g/L (0.4 mM) of surfactant for the HAPAM/SDS and HAPAM/DDG solutions. However, the HAPAM/DTAB solution did not show a viscosity maximum with increases in surfactant concentration due to the electrostatic repulsion between the hydrophobic units of the polymer and DTAB. At a representative reservoir temperature of 90 °C, the η0 value of the HAPAM/SDS solution is significantly higher than that of other SP solutions when η0 peaks, indicating that more HAPAM polymer chains are connected at a high temperature via the electrostatic attraction between hydrophobic pendants of the polymer and SDS. The interfacial tension (IFT) results demonstrate an SP synergistic effect on the IFT reduction with the HAPAM/SDS and HAPAM/DTAB solutions at 90 °C, while no such effect was observed for the HAPAM/DDG solution, suggesting that the arrangement of DDG molecules at the oil–water interface is not altered by the polymer. At 90 °C and a surfactant concentration of 0.4 g/L, the HAPAM/SDS solution exhibits the highest viscosity along a sufficiently low IFT. Subsequent micromodel experiments indicated that the volumetric sweep efficiency was significantly enhanced by HAPAM/SDS flooding. The efficiency of oil production with the HAPAM/SDS formulation in the secondary recovery mode was higher than with other SP systems. This study probes the synergistic interactions in SP slugs at a molecular level under HTHS reservoir conditions to reveal their potential effects and correlation to the effectiveness of oil production enhancement.