Enhanced mass transfer of residual NAPL by convection in stagnant zone

Cosolvent flushing is a promising method to improve NAPLs (non-aqueous phase liquids) removal. Nevertheless, the removal performance is undermined by the tailing phenomenon, i.e., the contaminant concentration tends to remain unchanged in later period as NAPL is residually trapped in the stagnant zone. High proportion of stagnant zone leads to more severe tailing phenomenon and how to accelerate dissolution of NAPL in the stagnant zone is important for NAPL remediation. However, few work has studied the remediation process at pore-scale due to limited observation techniques. As a result, the fundamental process of convection enhancement remains unclear. In this work, dynamics of NAPL removal in the stagnant zone is quantitatively investigated in transparent micromodels via the use of Raman spectroscopy. Impacts of injection condition and properties of the porous media are systematically investigated. Results indicate that convection dominants the mass transfer in aqueous phase and enhances the NAPL dissolution. Increasing the flushing velocity can enhance convection, but when the flushing velocity exceeds 3 cm/min, convection in the region away from the dominant channel is little affected by the flushing velocity. Differently, increasing the pore throat size will enhance convection in the entire stagnant zone and accelerate the whole dissolution process.