Adding surfactant to the oil-brine system helps with the displacement of trapped oil in the reservoir by reducing the interfacial tension (IFT) of oil and brine. During this process, depending on the mass fraction of surfactant, a third liquid phase that is called microemulsion may form. Estimation of the microemulsion dynamic viscosity at reservoir condition is important as it is directly connected to the oil recovery predictions and optimization design. The dynamic viscosity of the microemulsion phase not only depends on the phase behavior, but also the microstructure of this phase. Although extensive research has been done on the phase behavior and thermodynamic analysis of the microemulsion phase for a range of temperatures, pressures, and compositions, limited studies has focused on the connection between the microemulsion microstructure and viscosity, and the above-mentioned parameters. Here we aim at fundamentally understanding, and quantifying the relevance of microemulsion viscosity to the surfactant concentration, brine salinity, pressure and temperature, by conducting numerical design of experiments using molecular dynamics (MD) simulation. We computationally measure the viscosity of the system using the Einstein relation which is the reformulated Green-Kubo formula, to calculate and track the change of viscosity with the above-mentioned conditions. After validating our model through comparing our results with the available experimental data, we change different parameters to find a correlation (or multiple correlations), which relates the viscosity, to the variable pressures, temperatures, and concentrations. Doing so, we introduce a physics-based model which can be used to estimate viscosity accurately at the dynamic reservoir condition.

Fundamental studies of the oil/brine/surfactant microemulsion phase viscosity using molecular dynamics simulation

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