Bedding Anisotropy and Effective Stress Law for the Permeability and Deformation of Clayey Sandstones
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ORIGINAL PAPER
Bedding Anisotropy and Effective Stress Law for the Permeability and Deformation of Clayey Sandstones Fanbao Meng1 · Xingfu Li1 · Patrick Baud2 · Teng‑Fong Wong1 Received: 27 March 2020 / Accepted: 4 November 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract We performed a systematic investigation of the effective stress behaviors for permeability and deformation in relation to bedding anisotropy of two clayey sandstones. Permeability and deformation were measured in samples cored parallel and perpendicular to bedding over a broad range of hydrostatic pressures, covering ‘stage I’ for microcrack closure and ‘stage II’ for pore deformation. Our data show that bedding anisotropy has a significant influence on the effective stress coefficient for permeability, but little effect on the effective stress coefficient for pore volume change. The effective stress coefficient 𝜅 ⟂ of permeability for flow perpendicular to bedding was consistently larger than the corresponding 𝜅 || for parallel flow. The effective stress coefficient 𝛽 || for pore volume changes parallel to bedding and corresponding coefficient 𝛽 ⟂ values perpendicular to bedding coincided, because the scalar change of pore volume was not sensitive to the orientation of the samples. Furthermore, we confirmed that with the closure of preexisting microcracks, the effective stress coefficients for permeability in stages II were typically larger than the corresponding coefficients in stage I, and that the effective stress coefficients for axial strain and pore volume change decreased for samples both perpendicular and parallel to bedding. Our new results quantified the effect of bedding anisotropy and crack closure on the effective stress behavior of clayey sandstones. Keywords Pore pressure · Confining pressure · Mechanical anisotropy · Clay · Fluid flow · Pore volume
1 Introduction It is of critical importance in reservoir mechanics and geotechnical engineering to understand the effects of the applied stresses and pore pressure on mechanical failure, deformation, and fluid transport (Mavko et al. 2009; Ingebritsen et al. 2006; Zoback 2007). Laboratory studies have demonstrated that the rock physics properties of a saturated rock can often be characterized as a function of the effective stress 𝜎ij − 𝜉PP 𝛿ij , where 𝜎ij is the stress tensor (with compression taken as positive), PP is the pore pressure, and 𝛿ij is the Kronecker delta. In this formulation, the coupling of the stress field and pore pressure is encapsulated in the effective stress coefficient ξ.
* Fanbao Meng [email protected] 1
Earth System Science Programme, Faculty of Science, The Chinese University of Hong Kong, Hong Kong, China
Institut de Physique du Globe de Strasbourg (UMR 7516 CNRS, Université de Strasbourg/EOST), Strasbourg, France
2
Laboratory observations on rock have shown that, whereas the effective stress coefficient ξ for mechanical strength is close to unity (Jaeger et al. 2007; Paterson and Wong 2005; Baud et al. 2015), for ot
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