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Understanding fluid transport through claystones from their 3D nanoscopic pore network

type de publication      article dans une revue internationale avec comité de lecture
date de publication 2016
auteur(s) Song Yang; Davy Catherine A.; Bertier P.; Troadec David
journal (abréviation) Microporous and Mesoporous Materials (Microporous and Mesoporous Materials)
volume (numéro) 228
pages 64 – 85
résumé This paper investigates the complex and nanoscopic pore network of claystones, after their retrieval from the geological layer, and their further conditioning and drying, as it is usually done to assess their fluid permeability (i.e. their transport properties).
Following the leading research of Keller et al. [1] on the Swiss Opalinus clay, and because no percolating pore network is obtained at bigger scales, we provide micrometric pore volumes for a French Toarcian claystone and for a Callovo-Oxfordian claystone [2], by Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) imaging. The voxel size ranges from 5.94 × 7.54 × 10 nm3 to 8.49 × 10.78 × 50 nm3, and the investigated volumes range between 28 and 553 μm3. Comparison with nitrogen adsorption data is proposed.
One originality of our research is to prepare the samples as for the assessment of macroscopic fluid transport, by moderate drying at centimetric size. It is observed that, at the scale imaged by FIB/SEM, fluid transport occurs through very limited percolating parts of the pore network (0.7–2.1%). For both claystones, pore volumes generally percolate by sub-micrometric cracks, attributed to drying, and more seldomly by tortuous parts (not of a crack nature). Fluid transport is predicted by Katz–Thompson equation from the 3D geometry of the shortest percolating path. This provides permeability values on the order of 10−21–10−20 m2 (1–10 nD), in good agreement with experimental data. This study hints at a mechanism of fluid transport by fingering through pores as small as 20 nm diameter, rather than homogeneously through the whole claystone volume.
mots clés Focused ion beam/scanning electron microscopy (FIB/SEM); Claystone; Pore structure; Cracking; Permeability
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