Abstract
AbstractThis study investigates the nano to core‐scale geomechanical properties of a maturity series of organic‐rich, calcareous shales buried to 100°C–180°C, with a focus on: (a) the mechanical properties of organic matter; (b) the elastic response and anisotropy of the shale composite at micro and core scale; and (c) the creep response. Atomic force microscopy was used to target kerogen at nanoscale resolution, and it was found that the elastic stiffness increased with thermal maturity from 5.8 GPa in an immature sample to 11.3 GPa in a mature sample. Nanoindentation testing of the shale matrix showed that diagenesis is a key factor in determining the bulk elasticity, with increasingly intense carbonate cementation at higher thermal maturities contributing to a stiffer response. A multiscale model was formulated to upscale the elastic properties from nanoscale solid clay minerals to a microcracked composite at core scale, with good predictions of the micro and core‐scale stiffness in comparison to indentation and triaxial results. A negative correlation was found between the creep modulus and clay/kerogen content, with greater creep displacement observed in nanoindentation tests in the immature clay‐ and kerogen‐rich sample compared to samples of higher thermal maturity.
Highlights
The robust geomechanical characterization of shale underpins the understanding and prediction of many processes of importance to both geology and engineering
This study investigates the nano to core-scale geomechanical properties of a maturity series of organic-rich, calcareous shales buried to 100°C–180°C, with a focus on: (a) the mechanical properties of organic matter; (b) the elastic response and anisotropy of the shale composite at micro and core scale; and (c) the creep response
A negative correlation was found between the creep modulus and clay/kerogen content, with greater creep displacement observed in nanoindentation tests in the immature clay- and kerogen-rich sample compared to samples of higher thermal maturity
Summary
The robust geomechanical characterization of shale underpins the understanding and prediction of many processes of importance to both geology and engineering These include the development and nature of folds, faults and fracture systems as a result of tectonic deformation (Albertz & Lingrey, 2012; Hesse et al, 2010; Mitra, 2003; Obradors-Prats et al, 2017) and the flow and leakage of fluids in the context of CO2 sequestration, the storage of both hydrogen and nuclear waste and unconventional hydrocarbon production (Allen et al, 2020; Dewhurst & Hennig, 2003; Gale et al, 2014, 2007; Holt et al, 2020; Nygård et al, 2006; Imber et al, 2014). Correlations between mineralogy and the macroscopic mechanical response have been observed in some experimental studies, the relationship is complex and dependent on additional factors such as microstructure and the grain-scale diagenetic processes which cement, lithify and strengthen the granular mix without necessarily changing the bulk mineralogy (Herrmann et al, 2018, 2020; Sone & Zoback, 2013a, 2013b)
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