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This study investigates the effects of yield stress (τ0) and shear banding on the fluidic behaviors of cetyltrimethylammonium bromide/sodium salicylate wormlike micellar solutions flowing through a microfluidic planar contraction (8:1) geometry. Test solutions with different surfactant concentrations (Cd = 75, 87.5, and 100 mM) at a fixed molar ratio of salt to surfactant (R = 0.32) were characterized by shear and extensional rheometry. While the lower concentrated test solution (Cd = 75 mM) with low τ0 (≈ 0.02 Pa) and no shear banding showed a Newtonian-like flow behavior for Mach number, Ma < 1, the flow with corner vortices was formed when Ma exceeds unity. For higher Cd (87.5 and 100 mM), new fluidic phenomena are documented: (i) even at a low volumetric flow rate (Q), the fluid velocity at upstream corners was slower than that of Newtonian-like flows and (ii) at higher Q, the secondary flow with a quasi-static condition was formed at Ma well lower than unity. Micro-particle image velocimetry showed the lower shear rates at upstream corners, which can be understood by the effects of contraction entry, shear thinning, and high yield stress. The quasi-static secondary flow region was not induced by generation of elastic shock waves; instead the shear banding was found to be the underlying mechanism for the separation of the region from the main flow. In addition, the length of secondary flow regions showed a close correlation with the Deborah number, which was calculated using the extensional relaxation time.

While traditional oil recovery methods are limited in terms of meeting the overall oil demands, enhanced oil recovery (EOR) techniques are being continually developed to provide a principal portion of our energy demands. Chemical EOR (cEOR) is one of the EOR techniques that shows an efficient oil recovery factor in a number of oilfields with low salinity and temperature ranges. However, the application of cEOR under the harsh conditions of reservoirs where most of today’s crude oils come from remains a challenge. High temperatures, the presence of ions, divalent ions, and heterogeneous rock structures in such reservoirs restrict the application of cEOR. Polymer solutions, surfactants, alkaline-based solutions, and complex multi-components of them are common chemical displacing fluids that failed to show successful recovery results in hostile conditions for various reasons. Wormlike micellar solutions (WMS) are viscoelastic surfactants that possess advantageous characteristics for overcoming current cEOR challenges. In this study, we first review the major approaches and challenges of commonly used chemical agents for cEOR applications. Subsequently, we review special characteristics of WMS that make them promising materials for the future of cEOR.