Curvature-Based Equation of State for Microemulsion-Phase Behavior

Abstract

An accurate description of microemulsion phase behavior is critical for many industrial applications, including surfactant flooding in enhanced oil recovery. Recent phase behavior models have assumed constant-shaped micelles, typically spherical, using net average curvature (NAC), which is not consistent with scattering and microscopy experiments that suggest changes in shapes of the continuous and discontinuous domains. Based on the strong evidence of varying micellar shape, principle micellar curves were recently used to model interfacial tensions (IFT). Huh's scaling equation was also coupled to this IFT model to generate phase behavior estimates, but without accounting for micellar shape. In this paper, we present a novel microemulsion phase behavior equation-of-state (EoS) that accounts for changing micellar curvatures under the assumption of a general prolate spheroidal geometry, instead of through Huh's equation. This new EoS improves phase behavior modeling capabilities and eliminates the use of NAC in favor of a more physical definition of characteristic length. Our new EoS can be used to fit and predict microemulsion phase behavior irrespective of IFT data availability. For the cases considered, the new EoS agrees well with experimental data for scans in both salinity and composition. The model also predicts phase behavior data for a wide range of temperature and pressure, and is validated against dynamic scattering experiments, to show the physical significance of the approach.

Description of Micellar Radii for Phase Behavior and Viscosity Modeling of Aqueous Surfactant Solutions and Microemulsions

“…where ν is the average micellar velocity and τ E is the excess correlation time. 37 Lattice Elliptical Model. Consider a two-dimensional lattice of thickness d. In this lattice, micelles are approximated using ellipses that can be defined by the radii r ai and r bi , noting that r bi ≥ r ai , as illustrated in Figure 1.…”

“…The solubilization ratio of component i is defined by where V im is the volume of component i in the microemulsion phase and s is the surfactant component. The correlation time from scattering (τ) can be expressed in terms of the correlation length or average diameter of a micelle (ξ) by where ν is the average micellar velocity and τ E is the excess correlation time …”

Review on Microemulsions for Conformance Improvement Technology: Fundamentals, Design Considerations, and Perspectives

“…Microemulsions are proposed as a potential solution for conformance improvement because of their thermodynamic stability and favorable physicochemical properties . The type of surfactant and other emulsifiers determine the phase behavior of microemulsion, which, in optimal composition, aids in improved conformance control.…”

“…A design consideration during conformance improvement is the selection of a proper slug concentration and type. ,, The emulsifier(s) must be chosen for a specified reservoir, such that it can be delivered at reasonable injectivity. The emulsification process must be evident at certain shear rate and pressure conditions within preferable adsorption limits and cost constraints. , The optimal slug concentration aids in the formation of a Winsor III microemulsion system in the laboratory under predetermined salinity, pH, and temperature conditions.…”

“…Microemulsion treatment may promote flow divergence of the injected phase to contact the unswept oil volume of the reservoir. This mechanism provides dual functions: it plugs high-permeability streaks and thief zones and diverts flow toward low-permeability regions. , At a molecular scale, enhanced viscosity of microemulsions is a result of decreasing the aggregate/micelle size and increasing the degree of intermolecular associations. , Figure shows viscosity plots of microemulsion as a function of the oil volume fraction and co-solvent concentration …”