HR: 1340h
AN: B33C-1045    [Abstracts]
TI: Upscaling of reactive-transport processes in porous media: From pore to continuum scales
AU: * Kang, Q
EM: qkang@lanl.gov
AF: Hydrology, Geochemistry, and Geology Group, Los Alamos National Laboratory, Los Alamos, NM 87545 United States
AU: Lichtner, P C
EM: lichtner@lanl.gov
AF: Hydrology, Geochemistry, and Geology Group, Los Alamos National Laboratory, Los Alamos, NM 87545 United States
AB: Multi-component reactive transport in porous media is critical to a wide range of fields. It poses great challenges to theoretical, experimental, as well as numerical studies, because it usually involves multiple processes (advection, diffusion, reaction) and multiple scales (molecular, pore, laboratory, field). It is virtually impossible to solve the small-scale governing equations at the field scale because of the large number of nodes that would be required. Current modeling approaches commonly employ a continuum description and rely on volume averages. In porous media, averages are taken over scales larger than typical grain sizes. Hence, spatial heterogeneity below the scale of volume averaging is not resolved explicitly. Therefore, to better understand the multi-component reactive transport processes in porous media requires integrating fundamental behavior at multiple scales. For porous media, upscaling starts at the pore scale, where pore-scale geometry and other properties affect both the macroscopic transport equations and their parameters. Therefore, modeling pore-scale processes and subsequent upscaling to a macroscopic scale allow one to identify key parameters and physical-chemical processes that control macroscopic phenomena and to provide constitutive relations needed for continuum modeling. To date, the majority of pore-scale modeling studies have focused on the basic transport properties of porous media including effective diffusion, conductivity, permeability, and elasticity. Little has been done to upscale reactive transport in porous media. In this study, we apply a lattice Boltzmann pore-scale model for multi-component reactive transport in porous media developed in a previous study, to various simple, hypothetical chemical systems in a two-dimensional, artificially-constructed heterogeneous medium. The reactive-transport processes are simulated at the pore scale, with systematic consideration of the pore-scale flow field, diffusion, homogeneous reactions among multiple aqueous species, heterogeneous reactions between the aqueous solution and minerals, as well as the resulting changes in solid and pore geometry. The results are averaged over vertical slabs which are considered as REVs, and are compared with one-dimensional continuum-scale simulations. Our studies show that even for the simple geometry and chemical systems considered here, the results averaged from the pore-scale simulations are only in agreement with continuum-scale simulations in very narrow parameter ranges. Our studies also show that some phenomena observed in the pore-scale simulations, such as the incomplete replacement of primary minerals by the secondary minerals at small solubility, are missing in the continuum-scale simulations. This discrepancy is due to the neglect of spatial and temporal changes in tortuosity in the continuum approach.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1839 Hydrologic scaling
DE: 1849 Numerical approximations and analysis
DE: 4475 Scaling: spatial and temporal (1872, 3270, 4277)
SC: Biogeosciences [B]
MN: Fall Meeting 2005