A Hand-Based Method for Calculating Microemulsion Phase Equilibrium in Chemical Enhanced Oil Recovery.

Microemulsions have been increasingly applied in oil reservoirs for enhanced oil recovery because they can significantly reduce water-oil interfacial tension and increase the capillary number. However, during subsurface flow, microemulsions undergo phase transitions among Winsor I, II, and III as the salinity, water-oil ratio, and surfactant concentration change. The effectiveness of microemulsions in enhancing oil recovery is strongly dependent on their phase type. Therefore, accurately characterizing the phase-transition behavior is crucial for reservoir development design. The Hand method, which is widely used in numerical simulations, faces challenges, such as low computational efficiency and limited applicability, when dealing with phase equilibrium calculations involving microemulsions and excess phases. To overcome these limitations, this study proposes a simplified two-dimensional Cartesian coordinate method for calculating the microemulsion phase equilibrium. This method reduces complexity by mapping pseudocomponent compositions and the nodal lines of pseudoternary phase diagrams at different salinities onto a planar coordinate system. Compared with the Hand and HLD-NAC methods, the proposed approach improves the computational efficiency while maintaining accuracy. Moreover, validation against experimental data demonstrates that the method effectively captures the salinity-, composition-, and surfactant-dependent phase behaviors of microemulsions. This work provides theoretical support and a practical computational tool for the phase identification and numerical simulation of microemulsions in enhanced oil recovery applications.