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Geomechanical characterisation of organic‑rich calcareous shale using AFM and nanoindentation

Lookup NU author(s): Dr Mohamed Rouainia, Thomas Fender

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This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).


Abstract

The geomechanical integrity of shale overburden is a highly significant geological risk factor for a range of engineering and energy-related applications including CO2 storage and unconventional hydrocarbon production. This paper aims to provide a comprehensive set of high-quality nano- and micro-mechanical data on shale samples to better constrain the macroscopic mechanical properties that result from the microstructural constituents of shale. We present the frst study of the mechanical responses of a calcareous shale over length scales of 10 nm to 100 μm, combining approaches involving atomic force microscopy (AFM), and both low-load and high-load nanoindentation. PeakForce quantitative nanomechanical mapping AFM (PF-QNM) and quantitative imaging (QI-AFM) give similar results for Young’s modulus up to 25 GPa, with both techniques generating values for organic matter of 5–10 GPa. Of the two AFM techniques, only PF-QNM generates robust results at higher moduli, giving similar results to low-load nanoindentation up to 60 GPa. Measured moduli for clay, calcite, and quartz-feldspar are 22 ± 2 GPa, 42 ± 8 GPa, and 55 ± 10 GPa respectively. For calcite and quartz-feldspar, these values are signifcantly lower than measurements made on highly crystalline phases. High-load nanoindentation generates an unimodal mechanical response in the range of 40–50 GPa for both samples studied here, consistent with calcite being the dominant mineral phase. Voigt and Reuss bounds calculated from low-load nanoindentation results for individual phases generate the expected composite value measured by high-load nanoindentation at length scales of 100–600 μm. In contrast, moduli measured on more highly crystalline mineral phases using data from literature do not match the composite value. More emphasis should, therefore, be placed on the use of nano- and micro-scale data as the inputs to efective medium models and homogenisation schemes to predict the bulk shale mechanical response.


Publication metadata

Author(s): Graham SP, Rouainia M, Aplin AC, Cubillas P, Fender T, Armitage P

Publication type: Article

Publication status: Published

Journal: Rock Mechanics and Rock Engineering

Year: 2020

Pages: ePub ahead of print

Online publication date: 19/10/2020

Acceptance date: 23/09/2020

Date deposited: 03/12/2020

ISSN (print): 0723-2632

ISSN (electronic): 1434-453X

Publisher: Springer Wien

URL: https://doi.org/10.1007/s00603-020-02261-6

DOI: 10.1007/s00603-020-02261-6


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