Ababou, Rachid and Cañamón Valera, Israel and Poutrel, Adrien Macropermeability distribution and anisotropy in a 3D ﬁssured and fractured clay rock: ‘Excavation Damaged Zone’ around a cylindrical drift in CallovoOxfordian Argilite (Bure). (2011) Physics and Chemistry of the Earth, 36 (1718). 19321948. ISSN 14747065

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Official URL: http://dx.doi.org/10.1016/j.pce.2011.07.032
Abstract
The Underground Research Laboratory at Bure (CMHM), operated by ANDRA, the French National Radioactive Waste Management Agency, was developed for studying the disposal of radioactive waste in a deep clayey geologic repository. It comprises a network of underground galleries in a 130 m thick layer of Callovo Oxfordian clay rock (depths 400–600 m). This work focuses on hydraulic homogenization (permeability upscaling) of the Excavation Damaged Zone (EDZ) around a cylindrical drift, taking into account: (1) the permeability of the intact porous rock matrix; (2) the geometric structure of microﬁssures and small fractures synthesized as a statistical set of planar discs; (3) the curved shapes of large ‘chevron’ fractures induced by excavation (periodically distributed). The method used for hydraulic homogenization (upscaling) of the 3D porous and fractured rock is based on a ‘frozen gradient’ superposition of individual ﬂuxes pertaining to each fracture/matrix block, or ‘unit block’. Each unit block comprises a prismatic block of permeable matrix (intact rock) obeying Darcy’s law, crossed by a single piece of planar fracture obeying either Darcy or Poiseuille law. Polygonal as well as disc shaped fractures are accommodated. The result of upscaling is a tensorial Darcy law, with macropermeability Kij(x) distributed over a grid of upscaling subdomains, or ‘voxels’. Alternatively, Kij(x) can be calculated pointwise using a moving window, e.g., for obtaining permeability proﬁles along ‘numerical’ boreholes. Because the permeable matrix is taken into account, the upscaling procedure can be implemented sequentially, as we do here: ﬁrst, we embed the statistical ﬁssures in the matrix, and secondly, we embed the large curved chevron fractures. The results of hydraulic upscaling are expressed ﬁrst in terms of ‘equivalent’ macropermeability tensors, Kij(x,y,z) distributed around the drift. The statistically isotropic ﬁssures are considered, ﬁrst, without chevron fractures. There are 10,000 randomly isotropic ﬁssures distributed over a 20 m stretch of drift. The resulting spatially distributed K ij tensor is nearly isotropic (as expected). At the scale of the whole EDZ, the global K FISSURES is roughly 5000 times larger than permeability matrix KM. The detailed distribution of the equivalent K FISSURES (x, y, z) deﬁned on a grid of voxels is radially inhomogeneous, like the statistics of the disc ﬁssures. In addition, a moving window procedure is used to compute detailed radial proﬁles of K FISSURES versus distance (r) to drift wall, and the results compare favorably with in situ permeability proﬁles (numerical vs. experimental boreholes at Bure’s GMR drift). Finally, including the large curved chevron fractures in addition to the random ﬁssures, the resulting K ij (x, y, z) appears strongly anisotropic locally. Its principal directions are spatially variable, and they tend to be aligned with the tangent planes of the chevron fracture surfaces. The global equivalent Kij of the whole EDZ is also obtained: it is only weakly anisotropic, much less so than the local Kij’s. However, because of the radially divergent structure of the ‘chevrons’ (although not quite cylindrical in geometry), it is recognized that the global Kij due to chevrons lacks physical meaning as a tensor. Considering only the magnitude, it is found that the permeability due to ‘chevrons’ (K CHEVRONS ) is about 4 orders of magnitude larger than that due to statistical ﬁssures (K FISSURES ), assuming a hydraulic aperture a CHEVRON = 100 microns. By a simple argument, K CHEVRONS would be only one order of magnitude larger than K FISSURES with the choice a CHEVRON = 10 microns instead of 100 microns. This signiﬁcant sensitivity is due to several factors: the large extent of chevron fractures, the assumption of constant hydraulic aperture, and the cubic law behavior based on the assumption of Poiseuille ﬂow. The equivalent macropermeabilities obtained in this work can be used for large scale ﬂow modeling using any simulation code that accommodates Darcy’s law with a full, spatially variable permeability tensor Kij(x).
Item Type:  Article 

Additional Information:  Thanks to Elsevier editor. The definitive version is available athttp://www.sciencedirect.com/science/article/pii/S1474706511001483 
Audience (journal):  International peerreviewed journal 
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Institution:  French research institutions > Agence Nationale pour la Gestion des Déchets Radioactifs  ANDRA (FRANCE) French research institutions > Centre National de la Recherche Scientifique  CNRS (FRANCE) Université de Toulouse > Institut National Polytechnique de Toulouse  Toulouse INP (FRANCE) Université de Toulouse > Université Toulouse III  Paul Sabatier  UT3 (FRANCE) Other partners > Universidad Politécnica de Madrid (SPAIN) 
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Deposited On:  10 Jan 2014 10:21 
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