Stability of boreholes in a geologic medium including the effects of anisotropy |
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Authors: | Dinesh Gupta Musharraf Zaman |
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Institution: | School of Civil Engineering and Environmental Science, University of Oklahoma, Norman, OK 73019, U S A |
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Abstract: | An analytical formulation is developed to investigate the stability of
a deep, inclined borehole drilled in a geologic medium and subjected to an internal pressure
and a non_hydrostatic stress field. The formulation consists of a three_dimensional (3_D)
analysis of stresses around a borehole, combined with internal pressurization of the borehole
to obtain an approximate solution of the overall stress distribution. The orientation of the
borehole, the in_situ stresses and bedding plane can all be arbitrarily related to each other to
represent the actual field situations. Both tensile failure and shear failure potentials of a
borehole are investigated. The failure criteria applied assume that when the least principal
stress exceeds the strength of the formation in tension, a tensile failure occurs. Shear failure is
represented using the modified Drucker_Prager failure criterion for anisotropic materials. A
parametric study is carried out to assess the effect of material anisotropy, bedding plane
inclination and in_situ stress conditions on borehole stability. Results of the parametric study
indicate that wellbore stability is significantly influenced by a high borehole inclination, high
degree of material anisotropy, in_situ stress conditions and high formation bedding plane
inclination. The stability of a borehole in an elasto_plastic medium is also investigated. In
order to evaluate the extent of the plastic zone around a borehole and the effect of anisotropy of
the material on this plastic zone, a mathematical formulation is developed using theories of
elasticity and plasticity. The borehole is assumed to be vertical, subjected to hydrostatic
stresses, and drilled in a transversely isotropic geologic medium. A parametric study is carried
out to investigate the effect of material anisotropy on the plastic behavior of the geologic
medium. Results indicate that the stress distribution around a borehole, the extent of the plastic
zone, and the failure pressure are influenced by the degree of material anisotropy and value of
in_situ overburden stresses. It was observed that the borehole becomes less stable as the
degree of anisotropy of the geologic medium increases. |
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Keywords: | stability of borehole tensile failure shear failure failure criteria stress distribution |
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