The dynamic behavior of microemulsion-forming water-oil-amphiphiles mixtures is investigated in a 2.5D micromodel. The equilibrium phase behavior of such mixtures is well-understood in terms of macroscopic phase transitions. However, what is less understood and where experimental data are lacking is the coupling between the phase change and the bulk flow. Herein, we study the flow of an aqueous surfactant solution-oil mixture in porous media and analyze the dependence of phase formation and spatial phase configurations on the bulk flow rate. We find that a microemulsion forms instantaneously as a boundary layer at the initial surface of contact between the surfactant solution and oil. The boundary layer is temporally continuous because of the imposed convection. In addition to the imposed flow, we observe spontaneous pulsed Marangoni flows that drag the microemulsion and surfactant solution into the oil stream, forming large (macro)emulsion droplets. The formation of the microemulsion phase at the interface distinguishes the situation from that of the more common Marangoni flow with only two phases present. Additionally, an emulsion forms via liquid-liquid nucleation or the Ouzo effect (i.e., spontaneous emulsification) at low flow rates and via mechanical mixing at high flow rates. With regard to multiphase flow, contrary to the common belief that the microemulsion is the wetting liquid, we observe that the minor oil phase wets the solid surface. We show that a layered flow pattern is formed because of the out-of-equilibrium phase behavior at high volumetric flow rates (order of 2 m/day) where advection is much faster than the diffusive interfacial mass transfer and transverse mixing, which promote equilibrium behavior. At lower flow rates (order of 30 cm/day), however, the dynamic and equilibrium phase behaviors are well-correlated. These results clearly show that the phase change influences the macroscale flow behavior.

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Summary The rheological behavior of microemulsion systems was systematically investigated with mixtures of oil, brine, surfactant, cosolvent, and in some cases polymer to determine their effects. A microemulsion-rheology model was developed and used to interpret the experimental results. The optimal microemulsion/oil-viscosity ratio without cosolvent was roughly 5:6, but it can be reduced to a more favorable ratio of approximately 2 by adding cosolvent. Even though the amount of cosolvent needed is case dependent, a clear trend of microemulsion-viscosity reduction with increasing cosolvent concentration was observed. Limited evidence suggests that large hydrolyzed polyacrylamide (HPAM) molecules with a narrow molecular-weight (MW) distribution have negligible partitioning to Type II and Type III microemulsions.

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Summary We recently used sodium hydroxide (NaOH) in Indiana limestone coreflood experiments to lower anionic-surfactant adsorption. This study presents analysis of the limestone geochemistry and the surfactant adsorption under static and dynamic conditions. Analysis of the effluent ionic composition using ion chromatography and inductively coupled plasma showed the presence of sulfate (SO42−) aluminum (Al), and iron (Fe), as well as calcium (Ca) and magnesium (Mg). To determine the likely source of each geochemical species and to characterize how the dissolution kinetics changes the slug chemistry, PHREEQC was used to inverse-model Indiana limestone rock using the bulk X-ray-diffraction (XRD) mineralogical composition and the influent and effluent water chemistry. Results showed that all Indiana limestone cores contained anhydrite, which was not detected by XRD. The effluent concentration of Al increased with pH to approximately 15 mg/L, whereas Fe concentration remained fairly independent of pH at 0.04 ± 0.02 mg/L. These trends suggest the likely source of Al and Fe to be either clay dissolution or the release of natural clay colloids with NaOH. Simulations suggested that in Fe-bearing carbonates, alkali consumption is fast but limited with NaOH, which is observed as pH-front delay, whereas alkali consumption is slow but severe with sodium carbonate (Na2CO3) resulting in minimal pH-front delay but lower effluent pH compared with influent pH for prolonged injection times. Using the PHREEQC calculations, the ionic composition of the chemical slug in subsequent alkali/surfactant/polymer (ASP) experiments was adjusted. In addition, the coupled adsorption/transport of anionic surfactants in carbonate rocks was also investigated using surface-complexation-model adsorption under static and dynamic conditions. Model predictions agree with the single-phase-adsorption coreflood results and suggest that the adsorption on the metal oxides or clay could be comparable with that on calcite. This arises from the higher surface area and the point of zero charge of pH (pHpzc) of metal oxides.