Impact of fractures on diffusively dominated reactive transport: application to radioactive waste storage studies

Benjamin Delfino 1 Jean-Raynald De Dreuzy 2 Jocelyne Erhel 1 Benoit Cochepin 3 Yves Méheust 2
1 FLUMINANCE - Fluid Flow Analysis, Description and Control from Image Sequences
IRMAR - Institut de Recherche Mathématique de Rennes, IRSTEA - Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture, Inria Rennes – Bretagne Atlantique
Abstract : Even in small numbers, fractures must be carefully considered for the geological disposal of radioactive wastes. They critically enhance di usivity, 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 identi ed 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 simpli ed modeling frameworks on the basis of some key properties : (i) transport is mostly di usive 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 di usive 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 di ering di usion 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 di usion in the fracture than in the matrix, and validate them against elemen- tary fracture structures and simpli ed reactivity. We intend to use the developed methods on di erent fracture structures to simulation reactivity over long periods of time. We determine the possible relevance of the most classical simpli ed frameworks for fracture- matrix including : (i) fully homogenized models with adapted porosity, permeability and surface to volume ratio to reco- ver localization e ects, (ii) models with isolated fractures within "in nite 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 [1] D. Roubinet, J. R. Dreuzy, and D. M. Tartakovsky. Semi-analytical solutions for solute transport and ex- change in fractured porous media. Water Resources Research, 48(1), 2012. [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.
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Soumis le : mercredi 30 novembre 2016 - 12:17:40
Dernière modification le : jeudi 11 janvier 2018 - 06:28:13


  • HAL Id : hal-01405716, version 1


Benjamin Delfino, Jean-Raynald De Dreuzy, Jocelyne Erhel, Benoit Cochepin, Yves Méheust. Impact of fractures on diffusively dominated reactive transport: application to radioactive waste storage studies. Computational Methods for Water Resources (CMWR), 2016, Toronto, Canada. 〈hal-01405716〉



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