Mechanistic Model for Nanoparticle Retention in Porous Media

With sizes larger than molecules but smaller than colloidal particles, nanoparticles exhibit unique transport properties in porous media. They can easily pass through typical pore throats in reservoir formations with micron diameters, but may get retained by physicochemical interaction with the pore walls. Based on detailed analysis of nanoparticle retention data from an extensive series of transport experiments, we examine the limitations of classical models of transport and interaction with a stationary phase. Some features of nanoparticle transport and retention are similar to those of adsorbing/desorbing solutes, while others are similar to those of depositing colloids. But neither solute sorption nor colloid filtration alone can explain all nanoparticle retention features, and of particular importance for subsurface applications, neither model can predict the effect of changing flow conditions on nanoparticle retention. The model that accounts for most observations is an independent two-site model which postulates physically independent sites of fixed capacity: one for reversible attachment and the other for irreversible attachment. We validate the model against five distinctly different groups of experimental data from the literature, through a rigorous approach of obtaining the model parameters from one experiment and blind testing against data from other experiments when experimental conditions vary.