Surfactant–Polymer (SP) binary flooding has emerged as a promising enhanced oil recovery (EOR) technique that addresses several limitations associated with Alkaline–Surfactant–Polymer (ASP) flooding. By eliminating alkaline agents, SP systems avoid issues such as scaling, emulsion stability challenges, and formation damage, while maintaining—and in some cases improving—oil recovery efficiency through the synergistic action of polymers and surfactants.
1. Research Status of SP Flooding Systems
Recent studies have demonstrated that SP systems can achieve ultra-low interfacial tension (IFT) between the displacing fluid and crude oil, a key factor in mobilizing residual oil. For instance, formulations combining sulfonate mixtures with hydrolyzed polyacrylamide have shown IFT values in the ultra-low range (10⁻³ mN/m) and retained over 80% of their viscosity after 90 days of aging. Core flooding experiments have reported incremental oil recoveries exceeding 32%, with the SP contribution accounting for more than 20% of the total oil produced.
Compared to polymer-only or surfactant-only flooding, SP systems offer improved injectivity, lower resistance to flow, and greater volumetric sweep efficiency. These advantages make SP flooding particularly attractive for mature reservoirs with high water cut.
In recent years, attention has turned to applying SP flooding in high-temperature, high-salinity carbonate reservoirs—traditionally considered challenging for chemical EOR. A notable development includes a seawater-based SP formulation designed for reservoirs up to 115 °C, which achieved ultra-low IFT and reduced residual oil saturation to below 5% in core flood tests.
Looking forward, research continues to focus on optimizing polymer–surfactant combinations, understanding the molecular interactions governing performance, and developing environmentally sustainable separation technologies for produced fluids. These efforts aim to enhance the technical and economic feasibility of SP flooding across a broader range of reservoir conditions.
2. Mechanisms of Oil Displacement in SP Flooding
The efficiency of SP flooding lies in the complementary action of its two components. The polymer increases the viscosity of the aqueous phase, improving the mobility ratio between the injected fluid and the oil. This reduces viscous fingering and enhances both vertical and areal sweep efficiency, particularly in heterogeneous formations.
At the pore scale, the displacement process involves complex multiphase interactions. As the SP solution propagates through porous media, it mobilizes oil ganglia and promotes the formation of continuous oil threads along pore walls. These threads are subjected to shear forces that generate wave-like instabilities at the oil–water interface. The viscoelastic nature of the polymer plays a critical role here: it exerts normal stresses that vary along the interface—higher at convex regions and lower at concave ones—thereby stabilizing the oil thread and preventing its breakup.
Simultaneously, the surfactant reduces IFT, weakening the cohesive forces within the oil and facilitating the formation of smaller droplets under shear. These droplets contribute to the development of an oil-in-water emulsion, which enhances microscopic displacement efficiency. The polymer further supports this process by increasing the viscosity of the continuous phase, which slows droplet coalescence and improves emulsion stability.
Additionally, the polymer chains act as a protective colloid, shielding surfactant molecules from interactions with divalent cations such as Ca²⁺ and Mg²⁺ in formation water. This preservation of surfactant activity ensures sustained IFT reduction throughout the displacement process.
In summary, SP flooding integrates the strengths of both chemical agents: the polymer improves sweep efficiency through viscosity control and flow stabilization, while the surfactant enhances oil mobilization through IFT reduction and emulsification. Their combined effect results in a robust EOR method capable of addressing the limitations of conventional waterflooding and single-agent chemical flooding.