Pore-scale investigation of wettability impact on residual nonaqueous phase liquid dissolution in natural porous media

Abstract

Understanding the dissolution behavior of residual nonaqueous phase liquids (NAPLs) in soil and aquifer is essential for remediation activities and related prediction models. In this study, the impact of wettability of pore surfaces on the residual NAPL dissolution was investigated in natural porous media using X-ray microtomography. Three-dimensional pore-scale characteristics of NAPL blobs were determined in water-wet and neutral-wet media. Time-resolved images enable to estimate the temporal changes in the residual saturation and interfacial area, then to calculate the mass transfer coefficient. The results showed that the wettability has a significant impact on the morphology of residual NAPL; thus, affecting the dissolution behavior in natural porous media. In the neutral-wet system, residual NAPL blobs are trapped in small and large pores, resulting in a uniform distribution and a higher specific total interfacial area. The NAPL dissolution rate in the neutral-wet media is faster due to the higher specific interfacial area and more uniform spatial distribution. Considering the total interfacial area as the active mass transfer place, the wettability has a slight effect on the mass transfer coefficient. The mass transfer coefficient is independent of the residual saturation, whereas the lumped mass transfer coefficient decreases significantly during the dissolution process. Under different flow rates, the experimental data of Sherwood number Sh and modified Sherwood number Sh' could be well fitted with Sh ~ Ren and Sh′ ~ Renθn. These results are essential because the effect of wettability on dissolution was first studied from the perspective of pore scale, and the prediction models will help optimize the remediation strategies.