Wellbore stability analysis in transverse isotropic shales with anisotropic failure criteria
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© 2019 Elsevier B.V. The stress concentrations in the wall of a horizontal well in transversely isotropic shale are quantified and the stable mud weight window is established using a modified Hoek-Brown failure criterion. The upper margin of the safe drilling window is assumed due to the tensile breakdown pressure, which is computed with an anisotropic failure criterion considering the tensile strength and degree of anisotropy of the shale rock. Shear failure occurs at the lower boundary of the safe drilling window and is computed observing the rock's compressive strength anisotropy. The results show that when the degree of tectonic in-situ stress anisotropy increases, the safe pressure window between the tensile breakdown and shear collapse pressures decreases. We suggest that the narrowing of the safe drilling window as compared to isotropic cases may explain the increased occurrence of wellbore instability when drilling in shale. When the anisotropy of the elastic moduli increases further, both the breakdown pressure and collapse pressure decrease. Consequently, the range of recommended mud weights in the window for stable wellbores quickly narrows. Sensitivity analyses of the Poisson's ratio anisotropy effect on the breakdown pressure and collapse pressure indicate that the influence of Poisson's ratio anisotropy on borehole stability is very limited. The opposite holds for the Young's moduli, which are key factors contributing to narrowing of the safe drilling window when these moduli become more anisotropic.
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