Multiscale modelling of miscible interfaces: Application on surfactant-enhanced aquifer remediation

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In situ chemically enhanced solubilisation through the application of surfactants in injection solutions is a promising method to recover these contaminants. The process, nonetheless, involves dynamic evolution of phase boundaries, and multiple driving forces interacting with each other including capillary and viscous forces. On one hand, quantifying the contribution of the main mechanisms of NAPL recovery (mobilisation of NAPL ganglia through reduced interfacial tension and enhanced solubilisation), requires integration of fundamental physics of interfaces with continuum scale predictive tools. On the hand, the textural heterogeneities that span across multiple length scales of the porous medium complicates predicting the migration paths of the contaminant species, impacts the displacement mechanisms and adds uncertainty to the success of remediation operation in terms of recovery efficiency. Despite accumulated interest in modelling of the processes that involve dynamically evolving interfaces and underpinned science of miscible displacement through recent advances in Computational Fluid Dynamics (CFD), microfluidic studies, micromodel experiments and thermodynamics of interfaces, the impact of multiscale heterogeneity on the mechanisms of NAPL recovery has not been systematically quantified. Our overall aim is therefore to address incorporation of micro- and macro-heterogeneities of the porous system, by devising a novel multiscale computational apparatus that integrates dynamics of miscible displacement in the context of surfactant-enhanced aquifer remediation. In WP1 we numerically undertake a dimensionless analysis of interacting driving forces in Darcy scale. In particular we will utilise dimensionless numbers including Damköhler number (to represent mass transfer to advection ratio), Péclet number, viscosity ratio, and geostatistical parameters of the absolute permeability distribution such as spatial correlation lengths. We construct a flow-regime diagram and delineate extent of interacting viscous and chemical dissolution fronts instigated by, respectively, viscosity difference between fluids and permeability-feedback mechanism. We demonstrate the interplay of permeability heterogeneities (in various forms such as channelised fluvial systems, long spatially correlated distributions, Gaussian permeability realisations, etc.) on the interaction of viscous and chemical dissolution fingering, and overall NAPL recovery.In WP2 we seek innovative pore network modelling to underpin the physical processes (ganglia snap-off and mobilisation vs. interphase diffusion and mass transfer) that crucially shape the displacement mechanisms at microscale. We use a CFD theoretical model of interface evolution and rigorous transport model of viscous and chemical displacement. We upscale the results of flow and transport solutions from pore-scale to obtain Representative-Elementary-Volume-averaged multiphase flow and transport macroscopic properties. Through novel pore network generation techniques, we delineate the effect of pore-level statistics, morphology and structure on upscaled properties, and reduce the reliance over from commonly used empirical correlations.In WP3 we integrate the two-scales of modelling through a novel spatio-temporal adaptive computational apparatus that will provide unique insights into underlying physical phenomena that determine the efficiency of surfactant-enhanced aquifer remediation processes. Beyond the specific application of the novel multiscale tool for aquifer remediation, the computational apparatus will serve the purpose of various disciplines of engineering, such as waste treatment, geological carbon sequestration, enhanced oil recovery, drug delivery, etc. where interphase mass transfer across dynamic interfaces is a ubiquitous feature.