Pore-scale visualization of colloid straining and filtration in saturated porous media using micromodels

Colloid transport was studied at the pore scale in order to gain insight into the microscale processes governing particle removal. Monodisperse suspensions of colloids and water-saturated micromodels were employed. Experiments were carried out for different particle sizes, grain surface roughness, solution ionic strength, and flow rates. Straining and attachment were observed and measured by tracking the trajectory and fate of individual colloids using optical microscopy. Classical filtration theory proved appropriate for throat to colloid ratios (T/C) larger than 2.5 but did not take into account the possibility of straining that becomes an important capture mechanism for smaller T/C ratios. Spatially within the porous medium, straining occurred within the first 1-2 pore throats, while interception and attachment was seen from the inlet to the first 6-10 pore spaces, depending on particle size. Once a particle passed the initial region, the probability of attachment was very small. Colloid attachment increased with increasing solution ionic strength or decreasing flow rate, whereas straining was mainly independent of flow rate. Surface roughness of the grains also played a significant role in colloid capture, increasing collision efficiency by a factor of 2-3. The mechanisms of removal and the spatial distribution of colloid retention differed noticeably as a function of the T/C ratio. Micromodel visualizations clearly showed that physical straining and the effect of surface roughness should be taken into account when predicting the transport of colloids in saturated porous media.