Binary surfactant systems have demonstrated superior oil recovery capabilities in enhanced oil recovery (EOR) applications compared to single surfactant systems. This is due to their ability to form mixed micelles, which exhibit lower interfacial tension (IFT) and greater solubilization capacity than single surfactant systems. Thus, understanding their interactions and properties is crucial for maximizing their beneficial effects and determining their synergism. Therefore, in this study, we conducted a systematic experimental study involving eight surfactants and six binary surfactant mixtures at various ratios to determine their critical micelle concentrations (CMCs). Additionally, we applied Rubingh's Regular Solution Theory to characterize the behavior of these binary surfactant mixtures and to assess potential interactions among the surfactants. Our findings reveal a consistent synergistic phenomena in all binary surfactant systems, with the concentration of non-ionic surfactants playing a crucial role. Increasing the non-ionic surfactant concentration improved synergistic interactions, resulting in low CMC when combined with anionic, cationic, and zwitterionic surfactants. However, an excess concentration of the cationic surfactant exhibited "weak" synergistic effects, which can be attributed to its relatively smaller hydrophobic tail.
Carbonate reservoirs, constituting more than 60% of the world's hydrocarbon reserves, are of significant importance for efficient oil production (Adibhatla & Mohanty, 2006). These reservoirs often exhibit a high degree of heterogeneity, complex pore structures, and substantial presence of impurities. Some carbonate formations are further complicated by high reservoir temperatures and high salinity conditions (Lu et al., 2014). These pose significant challenges in reservoir characterization, production, and management.
Consequently, oil recovery in these reservoirs frequently falls below 40% (Høgnesen et al., 2005). Historically, surfactants have been utilized in enhanced oil recovery (EOR) applications, displaying promising outcomes (Ahmadi & Shadizadeh, 2013; Ivanova et al., 2020). Surfactants, being amphiphilic molecules, effectively reduce the interfacial tension (IFT) between oil and water that in turn enables the mobilization of trapped oil within the reservoir and its displacement towards production wells (Bello et al., 2022). However, a significant limitation in the application of single surfactants in carbonate formations arises from the presence of impurities like clay minerals and the physiochemical conditions of the aqueous medium, such as salinity and pH, which can influence the surface charge of the rock, and might lead to unfavorable results (Pal et al., 2018). This calls for the exploration of alternative strategies, such as binary surfactant solutions. Binary surfactant systems involve the combination of two distinct surfactants, each with its unique properties and behaviors.
