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The nanogranular nature of shale

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Abstract

Despite their ubiquitous presence as sealing formations in hydrocarbon bearing reservoirs affecting many fields of exploitation, the source of anisotropy of this earth material is still an enigma that has deceived many decoding attempts from experimental and theoretical sides. Sedimentary rocks, such as shales, are made of highly compacted clay particles of sub-micrometer size, nanometric porosity and different mineralogy. In this paper, we present, for the first time, results from a new experimental technique that allows one to rationally assess the elasticity content of the highly heterogeneous clay fabric of shales from nano- and microindentation. Based on the statistical analysis of massive nanoindentation tests, we find (1) that the in-situ elasticity content of the clayfabric at a scale of a few hundred to thousands nanometers is almost an order of magnitude smaller than reported clay stiffness values of clay minerals, and (2) that the elasticity and the anisotropy scale linearly with the clay packing density beyond a percolation threshold of roughly 50%. Furthermore, we show that the elasticity content sensed by nano- and microindentation tests is equal to the one that is sensed by (small strain) velocity measurements. From those observations, we conclude that shales are nanogranular composite materials, whose mechanical properties are governed by particle-to-particle contact and by characteristic packing densities, and that the much stiffer mineral properties play a secondary role.

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Notes

  1. For detailed chemical and mineralogy information on EPK Kaolin, see http://www.feldspar.com/minerals/epk.html.

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Acknowledgements

The research reported in this article was supported by funds provided by the MIT-OU GeoGenome Industry Consortium. The tested shales stem from shale cuttings of Chevron-Texaco, for which the mineralogy and dynamic measurements were provided by the team of Dr. Russ Ewy of ChevronTexaco. The nanoindentation tests and AFM imaging were carried out by Dr. Georgios Constantinides in MIT’s Nanolab facilities of the Department of Materials Science and Engineering.

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Authors and Affiliations

  1. Massachusetts Institute of Technology, Cambridge, MA, USA

    Franz-Josef Ulm

  2. PoroMechanics Institute, University of Oklahoma at Norman, Norman, OK, USA

    Younane Abousleiman

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  1. Franz-Josef Ulm
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  2. Younane Abousleiman
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Correspondence to Franz-Josef Ulm.

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Ulm, FJ., Abousleiman, Y. The nanogranular nature of shale. Acta Geotech. 1, 77–88 (2006). https://doi.org/10.1007/s11440-006-0009-5

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