Retention and remobilization of colloids during steady-state and transient two-phase flow

In this work, we study colloid transport through a porous medium under steady-state and transient two-phase flow conditions. The porous medium was a PDMS micromodel and the immiscible fluids were water and fluorinert-FC43. Given that the micromodel was hydrophobic, fluorinert was the wetting phase, and water was the nonwetting phase. We used hydrophilic fluorescent microspheres (dispersed in water) with mean diameter of 300 nm. We directly observed colloid movement and fluids distribution within pores of the micromodel using a confocal laser scanning microscope. We also obtained concentration breakthrough curves by measuring the fluorescence intensity in the outlet of the micromodel. The breakthrough curves showed that, under steady-state flow at different water saturations, more colloids were retained at lower saturations. Our visualization results suggested that the enhanced attachment was due to the retention of colloids onto fluorinert-water interfaces (FWIs) and fluorinert-water-solid contact lines (FWSCs). At the end of a steady-state two-phase flow experiment, we changed the micromodel saturation by injecting either water (drainage) or fluorinert (imbibition). We found remobilization of colloids during imbibition events, but no remobilization was observed during drainage. Visualization showed that colloids deposited on solid-water interfaces (SWIs) were dislodged by moving FWSCs during imbibition. We simulated breakthrough curves by modeling colloids interactions with SWIs and FWIs separately. Remobilization of colloids attached to SWI was modeled as a first-order kinetic process and the rate coefficient was assumed proportional to temporal rate of change of saturation. Colloids attachment to and detachment from FWIs were modeled as an equilibrium process. Generally, good agreements between experimental results and simulation were obtained. This is the first study of colloid transport in two-phase flow, where pore-scale visualization, breakthrough concentration measurement, and modeling of results are combined.