Effects of Pore-Scale Heterogeneity on Macroscopic NAPL Dissolution Efficiency: A Two-Scale Numerical Simulation Study

Interphase mass transfer or dissolution coefficient of nonaqueous phase liquids (NAPL) is an important parameter in predicting the transport of contaminant species in porous media. While the literature offers valuable insights into the dependence of this coefficient on different parameters at the continuum scale (e.g., contaminant saturation and Darcy velocity), effects of pore-scale heterogeneity on macroscopic dissolution coefficient have received little attention. In this work a three-dimensional pore-scale model is developed to simulate interphase mass transfer over different synthetic pore network structures with various pore radii correlation lengths. The pore network modeling simulates dissolution of immobile NAPL into water (single phase) through diffusive throats for the water-NAPL interface. The impacts of pore network spatially correlated heterogeneities, NAPL saturation/distribution, and aqueous phase velocity on NAPL mass transfer coefficient and water-NAPL interfacial surface area are studied. These macroscopic properties are then employed in two-dimensional continuum-scale domains formed by concatenating 20 by 20 pore networks in x and y directions. The results highlight the impact of pore-scale heterogeneity on the distribution of NAPL and subsequently on the dissolution rate (i.e., dissolution coefficient). An uncorrelated distribution of pore radii consistently leads to higher NAPL dissolution coefficient than spatially correlated heterogeneity. The results of continuum modeling show that NAPL dissolution rates are only different between domains formed by correlated and uncorrelated pore networks at very high flow rates and Darcy velocities. However, for typical values of Darcy velocity in groundwater systems, variation in mass transfer coefficient due to pore-scale heterogeneity is minimal for efficient mass removal.