Surfactant-influenced oil-water slip and flow in nanochannels studied by molecular dynamics and theoretical modeling

This work uses molecular dynamics (MD) methods to investigate oil-water two-phase flow in hydrophilic nanochannels with surfactants adsorbed at the oil-water interface. We reveal two new discontinuous effects of nanoscale fluid flow: (i) the velocity distribution in the water layer is not a univariate function of the pressure gradient, and (ii) the apparent viscosity in the oil-water interface region is a function of surfactant concentration and pressure gradient. Based on the MD results, a layered theoretical model describing the discontinuous effects at the nanoscale is developed. The theoretical model provides quantitative two-dimensional functions for oil-water interface slip and fluid mass flux with pressure gradient and surfactant concentration. Surfactants significantly inhibit oil-water slip and reduce fluid mass flux. This study thus provides important insights into the theoretical and numerical models for shale or tight oil production predictions.