In situ Viscosity Measurements of Microemulsions in Cores with Different Permeabilities

In enhanced oil recovery (EOR) surfactants are used to reduce the oil/brine interfacial tension (IFT) by several orders of magnitude to mobilize and produce residual, capillary-trapped, oil. The reduction in IFT is usually accompanied by the formation of a separate microemulsion phase. Microemulsions are thermodynamically stable mixtures of oil, brine and surfactants, and have ultra-low IFT with both excess oil and brine phases which they are in contact with. Such ultra-low IFT facilitates oil mobilization even under low pressure gradients. However, the existence of a third-phase microemulsion introduces further unknowns to flow behavior in the porous media. For example, it is commonly observed that the microemulsions have considerably higher viscosities than their single components. Consequently, such higher viscosities will affect surfactant transport in the porous medium, and can lead to high surfactant retention values. Understanding the microemulsion flow properties is essential not only in the design of optimized surfactant systems, but also in modelling the relevant EOR processes. In this paper the microemulsion rheological behavior is analyzed in the rheometer and under flowing conditions in the cores. Large volumes of microemulsion were injected into outcrop core plugs with different permeabilities in the range of 72 mDarcy to 2 Darcy. The cores were mounted in a core holder and the pre-mixed microemulsion was injected at several fixed rates. Pressure along the core was continuously recorded during the entire experiment. An in situ, or apparent, viscosity was calculated using the Darcy flow equation. All measurements were performed at 30 °C. Comparisons between the rheometer-measured microemulsion viscosity and the in situ viscosity in the core revealed that the permeability had a significant impact on the microemulsion rheology. The highest permeability core showed in situ viscosity values similar to the rheometer viscosity, while in the lowest permeability core the in situ viscosity was a factor of two higher than the rheometer-measured one. Here, several possible mechanisms for this permeability-dependent microemulsion viscosity will be discussed, such as pore plugging, effect of shear, separation into oil and brine, and microemulsion structure size.