A magnetic resonance imaging study of dense nonaqueous phase liquid dissolution from angular porous media

Magnetic resonance imaging (MRI) was used to determine the effects of pore-scale heterogeneity on the dissolution of a nonaqueous phase liquid (NAPL) in water-saturated flow-through columns (1.2 cm in diameter) packed with either approximately 500 or approximately 1,000 micron diameter angular silica gel (referred to as SG500 and SG1000, respectively). Columns were contaminated with 1,3,5-trifluorobenzene at residual saturation and then purged with water at a constant Darcy velocity of 1.83 m/day. Three-dimensional 19F images were acquired every 2-5 h at an imaging resolution of 59 x 234 x 234 microm3. Imaging results show that the specific NAPL surface area (a(t)) is linearly related to the NAPL volumetric fraction (theta(n)) and that the constant of proportionality between these parameters is determined by the blob size and geometry distribution. Overall (expressed as the modified Sherwood number, Sh') and intrinsic (expressed as the apparent Sherwood number, Sh(apt)) mass transfer rate coefficients were calculated. Values of Sh' and Sh(apt) for SG500 were approximately three times less than those for SG1000. For both solids, Sh' first increased or stayed the same and then decreased with decreasing theta(n), while Shapt generally increased with decreasing theta(n). These results suggest that during dissolution new flow paths were created (i.e., bypass zones were eliminated) as NAPL dissolved, decreasing the fraction of NAPL-water interfaces adjacent to pores filled with stagnant water and the average diffusion length scale. Since at for SG500 was dominated by less spherical multipore blobs (as opposed to more spherical singlets for SG1000), these results also suggest that the extent of flow bypassing (and the average diffusion length scale) increases in systems with more irregular blobs. These results are important because Sh' correlations and a "sphere" dissolution model do not account for transient changes in the fraction of NAPL surface area that contributes to dissolution or for the effect of initial blob size and geometry distribution on this fraction.