KG²B, a collaborative benchmarking exercise for estimating the permeability of the Grimsel granodiorite – Part 1: measurements, pressure dependence and pore-fluid effects

C. David 1 C Wassermann 2 Marc Fleury 3 Guillaume Berthe 3 Alexandra Amann-Hildenbrand 4 Bernhard Krooss 4 Joel Billiotte 5 Catherine Davy 6 Pierre Delage 7 Philipp Braun 7 Jerome Fortin 8 David Grégoire 9 Laurent Perrier 9 Qinhong-Max Hu 10 Eberhard Jahns 11 Jop Klaver 4 Didier Lasseux 12 Yves Jannot 12 Alain Sommier 12 Roland Lenormand 13 David Lockner 14 Laurent Louis 15 Gregory Boitnott 15 Claudio Madonna 16 Florian Amann 16 Philip Meredith 17 John Browning 17 Tom Mitchell 17 Franck Laurel Nono Nguendjio 18 Didier Loggia 18 Peter Polito 19 Thierry Reuschle 20 Ernie Rutter 21 Joël Sarout 22 Lionel Esteban 22 Patrick Selvadurai 23 Tiziana Vanorio 24 Anthony Clarke 24
Abstract : Measuring 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 collaborative benchmarking exercise involving 24 laboratories was organized for measuring the permeability of a single low permeability material, the Grimsel granodiorite, at a common effective confining pressure (5 MPa). 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 analyze 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. In total 39 measurements were collected that allowed us to discuss the influence of (i) pore-fluid, (ii) measurement method, (iii) sample size and (iv) pressure sensitivity. Discarding some outliers from the bulk data set (4 out of 39) an average permeability of 1.11 × 10−18 m² with a standard deviation of 0.57 × 10−18 m² was obtained. The most striking result was the large difference in permeability for gas measurements compared to liquid measurements. Regardless of the method used, gas permeability was higher than liquid permeability by a factor approximately 2 (kgas = 1.28 × 10−18 m² compared to kliquid = 0.65 × 10−18 m²). Possible explanations are that (i) liquid permeability was underestimated due to fluid-rock interactions (ii) gas permeability was overestimated due to insufficient correction for gas slippage and/or (iii) gases and liquids do not probe exactly the same porous networks. The analysis of Knudsen numbers shows that the gas permeability measurements were performed in conditions for which the Klinkenberg correction is sufficient. Smaller samples had a larger scatter of permeability values, suggesting that their volume were below the Representative Elementary Volume. The pressure dependence of permeability was studied by some of the participating teams in the range 1–30 MPa and could be fitted to an exponential law k = ko.exp(–γPeff) with γ = 0.093 MPa−1. Good practice rules for measuring permeability in tight materials are also provided.
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C. David, C Wassermann, Marc Fleury, Guillaume Berthe, Alexandra Amann-Hildenbrand, et al.. KG²B, a collaborative benchmarking exercise for estimating the permeability of the Grimsel granodiorite – Part 1: measurements, pressure dependence and pore-fluid effects. Geophysical Journal International, Oxford University Press (OUP), 2018, 215 (2), pp.799-824. ⟨10.1093/gji/ggy304⟩. ⟨hal-01992587⟩

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