Bedding Anisotropy and Effective Stress Law for the Permeability and Deformation of Clayey Sandstones

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 kappa(perpendicular to) of permeability for flow perpendicular to bedding was consistently larger than the corresponding kappa(parallel to) for parallel flow. The effective stress coefficient beta(parallel to) for pore volume changes parallel to bedding and corresponding coefficient beta(perpendicular to) 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.

Automated summary

The study showed that bedding anisotropy significantly influences the effective stress coefficient for permeability, but has little effect on the coefficient for pore volume change. With the closure of preexisting microcracks, the effective stress coefficients for permeability in stage II were typically larger than in stage I.