Analysis of pore-scale nonaqueous phase liquid dissolution in etched silicon pore network

Pore-scale mechanisms that control NAPL dissolution were investigated in silicon-based micromodels that have a 1 × 1 cm2 pore network with highly controlled pore channels. NAPL dissolution was observed in three different micromodels: a homogeneous micromodel with a uniform and repeating pore geometry, a heterogeneous micromodel with a pore structure created from an image of a sand matrix, and a preferential flow micromodel that contained a high permeability zone bounded on either side by low permeability zones.; NAPL stained with a hydrophobic fluorescent dye was trapped in each pore network, which was then purged with water stained with a hydrophilic fluorescent dye. During dissolution a fluorescence microscope equipped with an automated stage and digital camera was used to capture 2-D images of trapped NAPL. The NAPL and water dyes provided a sharp contrast between NAPL and other phases in the images. As a result, a digital image analysis system was used to determine the surface area and perimeter length of each NAPL blob. Because pores contained vertical walls, the surface area and perimeter length of each NAPL blob was converted to 3-D volumes and interfacial areas, respectively, by multiplying by the micromodel depth.; NAPL volumes and surface areas were used as input parameters to calculate mass transfer coefficients for dissolution in all micromodel experiments. Mass transfer coefficients for the homogeneous and heterogeneous micromodels initially increased with water velocity but then leveled off to a relatively constant value. Also, these mass transfer coefficients varied with pore and NAPL blob geometry. Specifically, the mass transfer coefficient decreased with increasing blob size because larger blobs had more NAPL-water interfacial area not adjacent to flowing water, and with decreasing pore throat width to pore throat depth because NAPL interfaces in deeper more narrow pores were adjacent to more stagnant water. For each of these cases the mass transfer coefficient decreased because the average diffusion length adjacent to NAPL blobs increased.