Investigation of NAPL dissolution and instability using a pore-scale network model: sensitivity analysis of porous medium characteristics

Nonaqueous phase liquids (NAPLs) introduced into an aquifer can cause long-term contamination. While some of the NAPL may be recoverable, most will dissolve into the surrounding water over a period of years or centuries. A three-dimensional pore-scale network model was developed to investigate the NAPL dissolution process in porous media. The model explicitly investigated two dissolution mechanisms: stagnant layer diffusion and channel flow dissolution. Numerical experiments were conducted to determine the effects of ganglion size, aqueous phase flow rate, and porous medium characteristics on the NAPL dissolution rate, ganglion configuration, and ganglion instability, both initial and dissolution-induced. It was found that flow rate has a strong effect on the dissolution rate, while porous medium characteristics are more important for ganglion instability. Single ganglion results were translated into a column study using population analysis, and the resulting relationships between modified Sherwood number, Reynolds number, and Peclet number agreed well with published results. A six-factor full factorial numerical experiment was conducted to systematically investigate the effects of pore mean size, pore size standard deviation, aspect ratio, correlation length, ganglion size and Capillary number. Aspect ratio was found to be the single most important factor in determining the ganglion instability types, with snap-off dominating at high aspect ratios. Further investigation indicated that the effect of aspect ratio is conditioned on the values of other porous medium characteristics, such as the pore size standard deviation and correlation length