Surfactant Stabilized Oil-in-Water Nanoemulsion: Stability, Interfacial Tension, and Rheology Study for Enhanced Oil Recovery Application

Nanoemulsions are kinetically stable biphasic dispersion of two immiscible liquids typically stabilized by an emulsifier with droplet sizes in the range of 10-200 nm. Present work deals with the formulation and characterization of stable oil-in-water nanoemulsions using nonionic surfactant (Tween 40) and light mineral oil for their application in enhanced oil recovery. The stability study of the nanoemulsions formed by high energy and low energy method was accomplished by bottle testing method. The emulsions were characterized in terms of droplet size, morphology and inner structure, surface charge, interfacial tension, and rheology. Droplet sizes of 18-31 rim obtained by dynamic light scattering analysis and surface charge values above -35 mV obtained by xi potential measurement prove the higher kinetic stability of the formed emulsions. Cryo-TEM micrographs reveal the surface morphology and inner structure of nanoemulsions. A miscibility test was performed to determine the dissolving ability of the nanoemulsions with crude oil. Measurement of interfacial tension (IFT) by pendant drop method shows a considerable reduction in IFT values with the increase of surfactant concentration and temperature, which is highly desirable for recovering trapped oil from the fine pores of the reservoirs. The viscosity of the nanoemulsions remains stable at a wide temperature (30-70 degrees C) range, denoting its thermal stability. The viscoelastic property of prepared nanoemulsions shows the increase of storage modulus (G') and loss modulus (G") with the increase in surfactant concentration and angular frequency (rad/s). Specific frequency (SF), the crossover point of G' and G", indicates the transition between elastic and viscous phases of nanoemulsions. A stable value of loss modulus after SF denotes better flowability of the emulsion. To test the efficiency of nanoemulsion in enhanced oil recovery, flooding experiment was performed by injection of a small pore volume of emulsion slug in a sand pack system, and an additional recovery of 28.94% was obtained after conventional water flooding.