Surfactant-Polymer-Oil-Brine Interactions: Coupled Phase Behavior, Interfacial Tension, and Rheological Characterization

Abstract

Surfactant–polymer flooding requires optimized chemical formulations to maintain stability and performance under reservoir conditions. While surfactants reduce interfacial tension (IFT) and polymers improve mobility control, their coupled behavior in multiphase systems remains insufficiently understood. This study investigates the interactions of phases to better characterize phase behavior, interfacial properties, and rheological responses relevant to chemical EOR applications. A systematic experimental investigation was conducted on surfactant–polymer–oil–brine systems under selected temperature, pressure and salinity conditions. n-Decane was used as the model oil, hydrolyzed polyacrylamide (HPAM) as the polymer, sodium dodecyl sulfate (SDS) as the surfactant, and isobutyl alcohol (IBA) as the co-surfactant, with sodium chloride (NaCl) brine as aqueous phase. Phase behavior and microemulsion transitions were evaluated through salinity scans ranging up to16 wt.% NaCl at 149°F. The optimal formulations within the 7.01–10.55 wt.% NaCl range were selected for further characterization. IFT measurements were conducted using a spinning-drop tensiometer at 15 and 800 psi and temperatures of 73 and 122°F, while viscosity of the aqueous phase was measured using a rotational rheometer at 73 and 122°F. The experimental results demonstrate that salinity, temperature, and pressure influence the interfacial and rheological behavior of the investigated system. Ultra-low IFT values were achieved near the optimal salinity corresponding to Winsor Type III microemulsion behavior. Salinity was found to be a key parameter determining microemulsion phase behavior, with increasing salinity shifting the system type. Temperature moderately shifted the optimal salinity and viscosity of aqueous phase, whereas pressure variations from 15 to 800 psi had a minor influence on IFT. Polymer addition into system significantly increased aqueous-phase viscosity and introduced shear-thinning behavior, while increasing salinity reduced the viscosity of the polymer-containing brine. Overall, this study provides an integrated characterization of complex bulk phase interactions by systematically combining phase behavior, IFT and aqueous phase viscosity measurement using consistent fluid formulations. The results provide practical guidance for designing robust surfactant–polymer formulations for chemical EOR applications.