Effects of flow rate and pore size variability on capillary barrier effects: a microfluidic investigation

Capillary barrier effects (CBEs) have been applied in capillary barrier systems as effective means of protecting underground regions from wetting. However, the microscale behavior of CBEs and related influencing factors are not well understood. This study utilized microfluidics to investigate effects of flow rate and pore size variability on CBEs at the microscale. Imbibition processes of water displacing air were imposed on three water-wet microfluidic chips with different degrees of pore size variability and injection rates. The obtained results demonstrated that the increasing injection rate changed water invasion pattern from finger growth to compact displacement. The CBEs could increase S wf (water saturation of fine sections at the onset of breakthrough) by up to 44%. The increase in S wf became smaller at a higher injection rate but was insensitive to the pore size variability within the investigation range. This study also elaborated the specific effect of inertia on pore body invasions under different pore characteristics. Among the materials with close average pore size, those with more uniform pore sizes are recommended for the construction of capillary barrier systems. For capillary barrier systems with robust CBEs, increasing the thicknesses of coarse layers has insignificant effect on reducing deep percolation.