HR: 10:35h
AN: H42A-02    [Abstracts]
TI: The role of small-scale aperture variability on the formation of large-scale dissolution channels in rough-walled fractures exposed to reactive fluid flow
AU: * Detwiler, R L
EM: detwiler@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550 United States
AU: Ezzedine, S M
EM: ezzedine1@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550 United States
AU: Rajaram, H
EM: hari@colorado.edu
AF: University of Colorado, 428 UCB, Boulder, CO 80309 United States
AU: Morris, J P
EM: morris50@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550 United States
AB: Flow of reactive fluids through variable-aperture fractures can lead to geochemical alteration of the fracture surfaces resulting in localized changes in fracture permeability. For a pore fluid that is undersaturated with respect to the minerals forming the fracture surfaces, chemical reactions lead to dissolution and increases in local fracture apertures. Local dissolution rates tend to be positively correlated with fracture aperture, (i.e. larger flow rates lead to locally higher flow rates and faster dissolution) resulting in a positive feedback mechanism that can lead to instabilities in the reaction front. Under certain conditions, reaction front instabilities lead to the formation of distinct dissolution channels that significantly alter the large-scale transport properties of fractured media. The formation of dissolution channels and the rate of their propagation are sensitive to parameters including the dimensionless Peclet and Damkohler numbers (Pe=advective/diffusive transport Da=surface reaction rate/advective transport), and the statistics (mean and variance), spatial correlation structure, and size of the fracture aperture field. We have developed an enhanced computational model of reactive fluid flow through variable aperture fractures in which the small-scale variability of the fracture aperture is represented explicitly. Fluid flow, transport of dissolved components and surface reactions are all calculated using depth-averaged formulations of the governing equations, which allows us to explore the influence of small-scale aperture variability on the formation of dissolution at scales much larger than the aperture correlation length. Enhancements to this code include implementation of a higher order advection scheme to minimize the influence of numerical dispersion and parallelization of the algorithm to allow large-scale simulations (>>10 million nodes). Direct comparison to dissolution experiments has demonstrated that this model effectively simulates the observed behavior over a range of Pe and Da. We present results from a series of fracture dissolution simulations in which we systematically varied Pe, Da, and fracture size in synthetic fractures with identical aperture statistics (mean and variance) and correlation structure. Results demonstrate that the length scale required for development of distinct dissolution channels increases for larger Pe and smaller Da. Furthermore, the spacing of distinct dissolution channels scales with the length of the longest channels. These results suggest the potential for the development of generalized scaling relationships to quantify the channeling induced by reactive fluid flow in variable aperture fractures. This work was performed under the auspices of the DOE by the University of California, Lawrence Livermore National Laboratory under contract No. 7405-Eng-48.
DE: 1832 Groundwater transport
DE: 1847 Modeling
DE: 5104 Fracture and flow
SC: Hydrology [H]
MN: Fall Meeting 2005