HR: 1340h
AN: H33F-0537    [Abstracts]
TI: Network Modeling of Anorthite and Kaolinite Reaction Rates in Porous Media
AU: * Li, L
EM: lili@princeton.edu
AF: Environmental Engineering and Water Resources Program, Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544 United States
AU: Peters, C A
EM: cap@princeton.edu
AF: Environmental Engineering and Water Resources Program, Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544 United States
AU: Celia, M A
EM: celia@princeton.edu
AF: Environmental Engineering and Water Resources Program, Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544 United States
AB: Although subsurface systems consist of porous media with a wide range of physical and chemical properties, reactive transport modeling commonly employs a continuum approach, where each grid block is characterized by spatially-averaged properties and reaction rates are calculated using uniform concentrations. Such spatial averaging can introduce significant error in the representation of geochemical reaction rates. In this work, we use network models to examine the effects of pore-scale heterogeneities on continuum-scale rates of anorthite and kaolinite reactions, and we identify conditions under which the effects of pore-scale heterogeneities are significant in reaction rate up-scaling. The network is constructed to represent consolidated sandstone, with about 10% reactive minerals (anorthite and kaolinite) distributed in reactive pore clusters according to prescribed configurations. The minerals react with acidic brine saturated with high-pressure CO2, representing conditions relevant for geological CO2 sequestration. The reaction rates computed from the network model are compared with those from a continuum model, which simulates the porous medium using spatially-averaged concentrations. Simulation results show that aqueous concentrations at the pore scale vary by orders of magnitude, and their distributions are highly skewed and, in some cases, bimodal. These concentration heterogeneities lead to significant spatial variations in pore-scale reaction rates. As a result, the continuum-scale rates from the network model are significantly different from those from the continuum model. In general, the continuum model overestimates the anorthite dissolution rates; for kaolinite, it either underestimates its precipitation rates, or predicts a different reaction direction from that of the network model. The effects of pore-scale heterogeneities are influenced by hydrodynamic conditions, as well as spatial distributions of reactive minerals. For anorthite dissolution, the continuum model overestimates its rates under all hydrodynamic conditions, with the degree of overestimation reaching a maximum in medium flow conditions. For kaolinite, the continuum model and the network model predict the same reaction direction at slow and fast flow conditions, but opposite reaction directions in medium flow conditions. With regard to the impact of reactive cluster size, larger cluster size leads to larger differences between the rates. These results provide guidelines on the conditions under which the effects of pore-scale heterogeneities are important in reactive transport modeling.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1869 Stochastic processes
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting