Coupled two-phase flow and surfactant/PFAS transport in porous media with angular pores: From pore-scale physics to Darcy-scale modeling

Abstract

Two-phase surfactant-laden fluid flow and surfactant transport in porous media are critical to many natural and engineering applications. Surfactants modify two-phase flow by altering interfacial tension and wettability, while two-phase flow controls surfactant transport pathways and adsorption sites. These coupled processes are commonly modeled by combining Darcy-type two-phase flow equations with advection–dispersion–adsorption transport equations, with capillary pressure–saturation relationships scaled using the Leverett $J$-function. However, the Leverett $J$-function simplifies the porous medium as bundles of cylindrical tubes and decouples interfacial tension and wettability, limiting representation of angular pore geometries and coupled interfacial tension and wettability effects. We present a modeling framework that incorporates pore angularity and interfacial tension–wettability coupling effect into Darcy-scale surfactant-laden fluid flow and surfactant transport models. Within this framework, we derive two-phase flow properties for angular pores, upscale them across pore size distributions, and obtain explicit and closed-form expressions for the upscaled properties. These expressions are incorporated into a coupled flow–transport model for simulating transient two-phase flow and surfactant transport processes. Modeling results suggest a nonmonotonic and nonlinear dependence of two-phase flow properties on pore structure (angularity and size distribution) and interfacial tension (controlled by surfactant type and concentration). Example simulations of water flow and PFAS (surfactant-like contaminants) migration in unsaturated soils indicate that surfactant-induced flow effects on PFAS leaching are generally minor under typical site conditions, whereas pore angularity exerts dominant control on water flow, interfacial area, and consequently PFAS retention. Overall, the upscaling framework offers a more physically grounded approach for modeling two-phase surfactant-laden fluid flow and surfactant transport in porous media.