(42e) Shear-Induced Structural Transitions and Gelation in Ultra-Low Interfacial Tension Microemulsions

A microemulsion system originally studied for use in enhanced oil recovery undergoes a shear-induced gelation phenomenon above a critical shear rate. The resulting fluid has a greatly increased viscosity, a considerable amount of viscoelasticity, and is highly thixotropic, with the shear-induced structures requiring several days to fully relax. In enhanced oil recovery, tuned surfactant formulations are injected into the reservoir, and has the potential to greatly increase oil recovery during primary, secondary, and tertiary phases of oil production. The enhanced recovery is the result of surfactant adsorption at the oil/brine interface, producing an ultra-low (<0.01 mN/m) interfacial tension between the aqueous and non-aqueous phases, reducing the capillary number and clearing oil from smaller pores than possible in brine-only floods. The viscoelastic properties of the system studied here are highly relevant in this application, where the shear-induced viscosity increase could be beneficial (by improving sweep efficiency) or harmful (by clogging pores, reducing injection and production flow rates). The fluid consists of an industrially available extended surfactant (ethoxylated-propoxylated alkyl sulfate), brine, and a small amount of a refined alkane oil. RheoSANS (Small Angle Neutron Scattering) is used to characterize the rheological and microstructural transitions of this fluid, and demonstrate our ability to tune and optimize its behavior. Additionally, we use a model system to investigate the mechanism underlying the observed rheological behavior by replacing the refined alkane oil with a variety of n-alkanes. This swap allows us to blend deuterated and hydrogenated alkanes to contrast match the oil and water phases, isolating the scattering, and thus structure, of the surfactant layer. We can also vary the alkane carbon number to deconvolute the effect of salt concentration from that of interfacial tension.