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| Titel |
Solute transport in a plane horizontal fracture: influence of density contrasts and fracture-matrix exchange |
| VerfasserIn |
Yves Meheust, Jérémy Bouquain, Laure Michel, Jean de Bremond d'Ars, Philippe Davy |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056136
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| Zusammenfassung |
| Fractures are preferential paths for fluids and solutes inside hard rock formations in the
Earth’s upper crust. We address the advective and dispersive transport of a buoyant solute in a
horizontal fracture with no wall roughness, under laminar flow conditions. Our reference
configuration is that with impervious fracture walls an no density-driven coupling between
flow and transport; it gives rise to the classic one-dimensional longitudinal Tayor-Aris
dispersion process. In reality the solute usually has a negative buoyancy, so that the
fluid density is spatially distributed according to the solute concentration field,
which induces significant perturbations to the Poiseuille flow inside the fracture.
We study this impact of density constrasts on the longitudinal dispersion, using a
two-dimensional finite elements numerical simulation [1]. The asymptotic Taylor–Aris
effective dispersion coefficient is observed to be reached eventually, but buoyancy
effects at short and moderate times are responsible for a systematic retardation of the
asymptotic mean solute position with respect to the frame moving at the mean fluid
velocity, as well as for a time shift in the establishment of the asymptotic dispersion
regime. We characterize these time delays as a function of the Péclet number and of
another non-dimensional number that quantifies the ratio of buoyancy to viscous
forces. Depending on the Péclet number, the asymptotic dispersion is measured to be
either larger or smaller than what it would be in the absence of buoyancy effects.
Breakthrough curves (an important measurement in hydrogeological applications)
measured at distances larger than the typical distance needed to reach the asymptotic
dispersion regime are impacted accordingly. We also discuss conditions under which
density effects related to fracture wall roughness are likely to be significant, or
not.
Another effect that can strongly influence the transport process is the small but finite
porosity of the rock matrix, which allows part of the solute present in the vicinity of the
fracture wall do diffuse into the matrix. We carry an experimental study of this effect [2]. The
analog fracture model consists of a 1000 Ã 50 Ã 5 mm3 plexiglass box with a porous lower
wall made of 1mm-large glass beads. A permanent laminar water flow is forced through the
fracture at controlled mean velocity (-ă 1 mm/s). A dye (patent blue) injection system
simulates a point source of contaminant along the center plane of the experimental fracture.
The two-dimensional equivalent longitudinal concentration field is measured as a function of
time using a visualization system based on 4 cameras positioned side by side. Mass
transfer between the fracture and the bounding porous matrix is measured at different
volumetric flows and for various concentrations of the injected dye, and this in different
geometries (roughness) of the fracture-matrix interface. Here also, buoyancy effects
play a significant role in the trapping of the solute in the vicinity of the porous
wall.
[1] J. Bouquain, Y. Meheust and P. Davy, J. Contam. Hydrol., in press (2010)
[2] L. Michel, PhD thesis, Université Rennes 1 (2009) |
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