This article reports on a novel simple method for transforming the high-salinity-incompatible petroleum sulfonates into a persistently stable oil-swollen micelles, referred to here as nanosurfactant. We present and discuss the effect of three different nanosurfactant formulations on the interfacial tension (IFT) between high-salinity water and crude oil, their phase behavior, and the effect of their dilution on IFT to assess their ability to reduce mobilize oil after injection into high-salinity and temperature reservoirs.
The three nanosurfactant formulations were prepared in high-salinity water following a direct-mixing procedure in which solutions in fresh water of 5 wt% petroleum sulfonate in mineral oil and three 4 wt% zwitterionic co-surfactants were mixed with high-salinity water at room temperature to give a combined concentration of all active ingredients of 0.2 wt%. The IFT between crude oil and different nanosurfactant formulations was measured using a spinning drop interfacial tensiometer at 90°C. IFT was measured every 5 minutes while the oil drop was spinning at ~4000 rpm. The phase behavior was investigated by monitoring the turbidity and UV absorbance changes in a system of crude oil atop of the nanosurfactant formulation over time at 100°C without any mechanical mixing.
The particle size of the three nanosurfactant formulations is in the range of 40 to 80 nm, depending on the co-surfactant used. All formulations were persistently stable, colloidally, and chemically under high-salinity (~56,000 ppm) and temperature (100°C) for more than four months. All formulations showed substantial reduction in IFT with crude oil compared to high-salinity water alone. Dilution with high-salinity water up to five times further reduced the IFT, suggesting improved performance after injection into the reservoir. This behavior was consistent with the observed gradual decrease in surface tension of the nanosurfactant formulation as its concentration decreases toward the CMC value. Phase behavior experiments showed enhanced formation of homogeneous micelles at 100°C without the aid of any mixing. Our results demonstrate the ability of nanosurfactants to solubilize oil under typical carbonate reservoir conditions. Their colloidal nature allows them to migrate deeper in the reservoir compared to conventional surfactants due to size exclusion and chromatographic effects.
Nanosurfactants are novel oil-swollen micelles of the inexpensive and abundant petroleum sulfonate salts that are efficient in reducing IFT under typical carbonate reservoir conditions. The formulation method can be extended to other surfactants and chemical treatments that are incompatible with high-salinity water at high temperatures. Their nanoparticle character and colloidal behavior suggest their ability to migrate and penetrate deep in the reservoir. Nanosurfactants can therefore help overcome some of the most critical drawbacks in conventional chemical EOR technologies.