KG²B, a collaborative benchmarking exercise for estimating the permeability of the Grimsel granodiorite—Part 2: modelling, microstructures and complementary data

C David 1, * J Wassermann 2 F Amann 3 J Klaver 3 C Davy 4 J Sarout 5 L Esteban 5 E Rutter 6 Q Hu 7 L Louis 8 Pierre Delage 9 D Lockner 10 Patrick Selvadurai 11 T Vanorio 12 A Amann hildenbrand 3 P Meredith 13 J Browning 13 T Mitchell 13 C Madonna 14 J Billiotte 15 T Reuschlé 16 D Lasseux 17 J Fortin 18 R Lenormand 19 Didier Loggia 20 F Nono 21 G Boitnott 8 E Jahns 22 M Fleury 23 G Berthe 23 P Braun 24 David Grégoire 25 Laurent Perrier 25 P Polito 26 Yves Jannot 27 A Sommier 17 B Krooss 3 A Clark 12
Abstract : Measuring and modelling the permeability of tight rocks remains a challenging task. In addition to the traditional sources of errors that affect more permeable formations (e.g. sample selection, non-representative specimens, disturbance introduced during sample acquisition and preparation), tight rocks can be particularly prone to solid–fluid interactions and thus more sensitive to the methods, procedures and techniques used to measure permeability. To address this problem, it is desirable to collect, for a single material, measurements obtained by different methods and pore fluids. For that purpose, a benchmarking exercise involving 24 laboratories was organized for measuring and modelling the permeability of a single low-permeability material, the Grimsel granodiorite. The objectives of the benchmark were: (i) to compare the results for a given method, (ii) to compare the results between different methods, (iii) to analyse the accuracy of each method, (iv) to study the influence of experimental conditions (especially the nature of pore fluid), (v) to discuss the relevance of indirect methods and models and finally (vi) to suggest good practice for low-permeability measurements. To complement the data set of permeability measurements presented in a companion paper, we focus here on (i) quantitative analysis of microstructures and pore size distribution, (ii) permeability modelling and (iii) complementary measurements of permeability anisotropy and poroelastic parameters. Broad ion beam—scanning electron microscopy, micro-computerized tomography, mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) methods were used to characterize the microstructures and provided the input parameters for permeability modelling. Several models were used: (i) basic statistical models, (ii) 3-D pore network and effective medium models, (iii) percolation model using MICP data and (iv) free-fluid model using NMR data. The models were generally successful in predicting the actual range of measured permeability. Statistical models overestimate the permeability because they do not adequately account for the heterogeneity of the crack network. Pore network and effective medium models provide additional constraints on crack parameters such as aspect ratio, aperture, density and connectivity. MICP and advanced microscopy techniques are very useful tools providing important input data for permeability estimation. Permeability measured—orthogonal to foliation is lower that—parallel to foliation. Combining the experimental and modelling results provide a unique and rich data set.
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C David, J Wassermann, F Amann, J Klaver, C Davy, et al.. KG²B, a collaborative benchmarking exercise for estimating the permeability of the Grimsel granodiorite—Part 2: modelling, microstructures and complementary data. Geophysical Journal International, Oxford University Press (OUP), 2018, 215 (2), pp.825 - 843. ⟨10.1093/gji/ggy305⟩. ⟨hal-01858321⟩

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