HR: 15:10h
AN: H13J-06    [Abstracts]
TI: Experimental observation of coupled geochemical alteration and geomechanical deformation of discrete variable-aperture fractures
AU: * Detwiler, R L
EM: detwiler@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550, United States
AU: Fisher, S
EM: fisher43@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550, United States
AB: Flow through fractures is controlled by the magnitude and variability of apertures within the fracture. Geochemical reactions and applied stresses can both alter fracture apertures leading to changes in transmissivity that are difficult to predict with existing models. Previous experimental studies in fixed displacement fractures have shown that flow of reactive fluids can lead to different dissolution patterns that are controlled by the Peclet (Pe ~ advection/diffusion) and Damkohler (Da ~ reaction/advection) numbers and range from relatively uniform dissolution throughout the fracture (high Pe, low Da) to development of distinct channels (low Pe, high Da). These results demonstrate that during uniform dissolution the smallest aperture regions dissolve more quickly than the larger apertures, leading to a smoothing of the fracture aperture field. This suggests that the addition of normal stresses, which can be significant in many subsurface environments, may lead to closure of the fracture surfaces under conditions of net dissolution from the fracture surfaces. We have developed an experimental system that allows application of a uniform confining stress to transparent analog, variable-aperture fractures during reactive fluid flow experiments. The fractures are fabricated by mating a rough, nonreactive surface (glass) with a smooth reactive surface (KH₂PO₄). Using this apparatus, we have conducted a series of dissolution experiments under steady confining pressure. Because the fractures are transparent, we can apply light transmission techniques to accurately (± 3 μm) measure fracture apertures at high spatial resolution (80 x 80 μm) over the entire flow field during dissolution experiments. Results show that fracture transmissivity initially increases as the fracture surfaces dissolve. However, gradual erosion of contacts leads to an accumulation of stress in the contacts until the remaining contacts deform resulting in sudden decreases in fracture transmissivity. This process repeats leading to periodic transmissivity oscillations over the duration of the experiments. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
DE: 1832 Groundwater transport
DE: 3653 Fluid flow
DE: 5104 Fracture and flow
DE: 5139 Transport properties
SC: Hydrology [H]
MN: 2007 Fall Meeting