Density-Coupled Transport in Heterogeneous Porous Media: Ob servations and Comparison to Stochastic Theory

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9817823WeltyDuring her 1997-98 sabbatical leave, the PI has worked at University of Kassel in Germany to build a large-scale experimental vessel containing structured, correlated, random porous media that can be used for observations of any fluid flow or mass transport problem of interest under controlled laboratory conditions. This effort has been carried out in collaboration with Dr. Tissa Illangasekare of the Colorado School of NEnes, who has nearly twenty years of experience in building such vessels and in carrying out large-scale laboratory experimental research. Dr. Manfred Koch, Director of the Department of Geohydraulics and Engineering Hydrology at University of Kassel, has provided equipment, supplies, lab technician time, and student support for the work.The principal objective of the research described in this proposal is to carry out, as a first set of experiments in the new apparatus, a series of runs to quantify the interaction between permeability heterogeneity and high solute concentrations and the effect of this interaction on macrodispersion in saturated porous media. In doing so, the apparatus developed during 1997-98 will be tested and will continue to undergo refinement, as a further step in developing a dedicated porous media observational facility available to researchers from around the world at the University of Kassel.The set of proposed experiments will test the hypothesis that macrodispersion in densitydependent transport can be predicted by existing stochastic theory. Although it is well known that the density of a solute plume can affect its mean motion, the effect of fluid density on fluid mixing or effective dispersion in statistically-characterized heterogeneous porous media has not been systematically documented from an observational standpoint. Qualitative experimental evidence and theoretical predictions indicate that interaction of the density gradients with degree of heterogeneity affects dispersion: the permeability heterogeneity can damp out or enhance the effects of the density gradients on solute spreading depending on the solute/solvent flow configuration.The experiment is to be conducted in the laboratory to ensure isothermal conditions and to avoid unwanted geochemical reactions. Fluids of contrasting density (laboratory-grade water and NaCl solutions) will be displaced through the constructed tank in (1) a step-input horizontal displacement, and (2) a density-stratified, horizontal flow configuration. The horizontal displacements will be run for both stable and unstable flow configurations; the stratified flow configuration will be run for a stable configuration. Each fluid displacement scenario will be run for two mean pore velocities and three salt concentrations. Measurements of solute concentration made using solution samplers over time and space will be compared to predictions of macrodispersion of dense, miscible fluids based on existing stochastic theory previously developed by the PI. The results will either serve to confirm previous theoretical work, or will provide impetus for further theory development in this area. Such an approach, together with ongoing numerical simulations, will contribute to knowledge needed for a unified understanding of this coupled process to aid in predicting large-scale transport of dense, miscible fluids in aquifers. This knowledge base can then serve as the foundation for evaluating the effect on dispersion of additional, realistic non-ideal processes typically associated with density-coupled miscible transport, such as then-nal processes and heterogeneous geochemical reactions.The work is proposed to be carried out at University of Kassel due to the unusually generous resource base that will continue to be available there for student and lab technician support. (Similar facilities at the Colorado School of Mines would not be available during the period of interest to carry out proposed work.) The budget in this proposal is to cover salary and related expenses of the PI to continue her collaborative effort at University of Kassel for an additional two years to further develop the porous media observational facility and to carry out a first set of experiments in this facility.