Investigation Of Napl Dissolution And Instability Using A Pore Scale Network Model

Author : Mir Ahmad Seyedabbasi
Publisher :
ISBN 13 : 9781267215161
Total Pages : pages
Book Rating : 4.2/5 (151 download)

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Download or read book Upscaling Mass Transfer for NAPL Dissolution Fingering written by Mir Ahmad Seyedabbasi and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Laboratory experiments and numerical simulations in homogeneous porous media were used to investigate the influence of porous medium wettability on the formation and growth of preferential dissolution pathways, dissolution fingers, during nonaqueous-phase liquid (NAPL) dissolution. As the porous medium became increasingly NAPL-wet, dissolution fingers grew wider and slower. This result was observed in physical experiments with 0 and 100% NAPL-wet conditions and confirmed with numerical simulations at these and intermediate wettabilities. A previously derived expression for an upscaled mass transfer rate coefficient that accounts for the growth of dissolution fingers was used to quantify the effect of fingering on overall NAPL removal rates. For the test cases evaluated, NAPL dissolution fingering controlled the overall rate of NAPL dissolution after the dissolution front moved 9 cm in 0% NAPL-wet conditions and 34 cm in 100% NAPL-wet conditions. Thus, even in completely NAPL-wet media dissolution fingering may control the overall rate of NAPL dissolution after relatively short travel distances. The influence of aqueous-phase boundary conditions on dissolution fingering was also investigated. For the range of conditions studied here, constant head and constant flux boundary conditions resulted in similar finger growth and morphology of the interface between clean and NAPL-contaminated regions: the interface roughness and the growth of this roughness with pore volumes flushed were independent of boundary condition type. Laboratory studies were conducted to evaluate the influence of heterogeneity on NAPL dissolution fingering. During a spill event heterogeneous porous media favor the formation of nonuniform NAPL saturation fields, including NAPL residual and NAPL pools, that may affect the fingering mechanism. A light transmission technique was used to measure trichloroethylene saturation fields at a 0.05-cm resolution that resulted from spills in two heterogeneous packings of a laboratory test cell. The correlation length of the permeability field transverse to the mean water flow direction was selected to be similar to (1.0 cm) or significantly greater than (6.0 cm) the expected wavelength of dissolution fingers. As the entrapped NAPL dissolved into water, preferential NAPL dissolution patterns occurred in both experiments, with patterns strongly affected by the heterogeneities. Experimental results were used to validate the utility of a numerical simulator for capturing the growth of centimeter-scale preferential NAPL dissolution patterns. Using data from these experiments, four different methods for upscaling the mass transfer rate coefficient for NAPL dissolution were examined [Basu et al., 2008; Christ et al., 2006; Imhoff et al., 2003a; Saenton and Illangasekare, 2007]. These models were developed to account for the influence of dissolution fingering or NAPL architecture on the long-term flux of contaminants from NAPL source zones. In the packing where the correlation scale of permeability perpendicular to the mean water flow direction was 6.0 cm, greater than the scale of the dissolution fingers, all upscaling approaches predicted effluent concentrations reasonably well. When the correlation scale of the heterogeneities was smaller (1.0 cm), the models performed much more poorly. These results and their implications on the applicability of the upscaling models will be discussed. Among the four different upscaling approaches tested in Chapter 4 the stream-tube approach developed by Basu et al. [2008] seemed very promising in predicting the upscale mass transfer and effluent concentrations in the two physical experiments we conducted with different degrees of heterogeneities in permeability and NAPL saturation distribution. A simple stream-tube approach was developed to predict the dissolution front dynamics and effluent concentration profiles. This simple model worked poorly when used for the large correlation length system where the permeability heterogeneity was close to homogeneous, where the dissolution rate is controlled by dissolution fingering. However, when the correlation scale of heterogeneity decreased for the small correlation length system, the dissolution rate was controlled solely by the heterogeneities in permeability and NAPL saturation fields and not by the dissolution fingering mechanism. In this case the stream-tube model was able to predict the effluent concentration profile and capture the tailing in that very accurately. This model needs to be combined with the dissolution fingering model and be tested for wider range of heterogeneous systems. This is outlined as a recommendation for the future work.