Impact of fractures on diffusively dominated reactive transport: application to radioactive waste storage studies
Résumé
Even in small numbers, fractures must be carefully
considered for the geological disposal of radioactive
wastes. They critically enhance diusivity, speed up
solute transport, extend mixing fronts and, in turn,
modify the physicochemical conditions of reactivity
around possible storage sites. Fractures occur at se-
veral places in the cement surrounding the containers
and in the Excavation Damaged Zones (EDZ) of the
galleries. They even occur in clays like in the French
Callovo-Oxfordian formation mostly because of the de-
saturation conditions induced in the operational time
of the galleries.
Numerous studies in various elds (e.g. radioactive
waste storage, CO2 sequestration, geothermal storage,
hydrothermal alteration) have shown that fractures
cannot be simply integrated within an equivalent po-
rous medium with a simple enhancement of its petro-
physical properties (porosity and permeability). Frac-
tures cannot either be accurately identied and fully
deterministic modeling approaches are precluded.
We propose a combined numerical and experimental
approach to determine the in
fluence on reactivity of
typical fracture patterns classically found in radioac-
tive waste applications. We investigate the possibility
to apply simplied modeling frameworks on the basis
of some key properties :
(i) transport is mostly diusive and much faster in
the fractures than in the porous matrix [1],
(ii) reactivity occurs predominantly in the matrix be-
cause of the large surface to volume ratio favorable
to dissolution/precipitation processes,
(iii) reactivity within the surrounding matrix is at
equilibrium, or equivalently much faster than
the diusive transport. Reactivity is assumed
transport-limited rather than rate-limited.
Based on the separation of the fracture and ma-
trix domains, we develop a reactive transport mo-
del with diering diusion conditions in the fracture
and in the matrix, appropriate
flow-rock interactions
at equilibrium in the matrix and fracture-matrix ex-
change conditions at their interface. Using preferen-
tially existing software, we propose simulation methods
that comply with much faster diusion in the fracture
than in the matrix, and validate them against elemen-
tary fracture structures and simplied reactivity.
We intend to use the developed methods on dierent
fracture structures to simulation reactivity over long
periods of time. We determine the possible relevance
of the most classical simplied frameworks for fracture-
matrix including :
(i) fully homogenized models with adapted porosity,
permeability and surface to volume ratio to reco-
ver localization eects,
(ii) models with isolated fractures within "innite ma-
trix" assuming implicitly the localization of reac-
tivity in the immediate vicinity of the fracture [2],
(iii) double porosity models characterized by single or
multiple exchange coecients[3].
Following the outcome of the numerical simulations,
we will investigate experimentally the most critical li-
mitation of reactivity. It might a priori be either the
fracture to matrix exchange law especially if fracture
is desaturated and matrix saturated.
Within the radioactive waste framework, we aim at
including fractures in the safety assessment work
flow.
We intend to determine to which extent fractures faci-
litate the access to reactive surfaces, the increase of the
bulk reactivity, the corrosion potential and the pertur-
bation of the chemical conditions.We frame as much as
possible the reference simulations in realistic physical
and chemical conditions including the main operatio-
nal phases of the radioactive waste repository. Results
will be reported as comprehensive evolution scenarios.
References
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Semi-analytical solutions for solute transport and ex-
change in fractured porous media. Water Resources
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[2] C. I. Steefel and P. C. Lichtner. Multicomponent reac-
tive transport in discrete fractures : I. controls on reac-
tion front geometry. Journal of Hydrology, 209(1) :186{
199, 1998.
[3] T. Xu and K. Pruess. Modeling multiphase non-
isothermal
fluid
flow and reactive geochemical transport
in variably saturated fractured rocks : 1. methodology.
American Journal of Science, 301(1) :16{33, 2001.