Qualifying stress direction from borehole shear sonic anisotropy
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Copyright 2015 ARMA, American Rock Mechanics Association.A method is presented to qualify the maximum horizontal stress direction on basis of dipole shear sonic anisotropy in near-vertical wellbores. The proposed scheme follows a similar qualification standard to that used for stress observations on the basis of image logs and four-arm caliper logs in the World Stress Map Project. Image log analysis and shear wave anisotropy analysis will often complement one another and add confidence when both are observed. The combination of geological setting and rock properties, together with drilling practices, does not always result in clear borehole failure, limiting the ability to quantify stress direction from images alone. Shear sonic anisotropy is often able to identify horizontal stress imbalance where borehole failure has yet not occurred. Herein, we review the methodology to determine stress direction on the basis of dipole borehole sonic data, including examination of the effect of hole ovality. The use of slowness frequency dispersion curves is particularly important, as dispersion curve analysis is essential for distinguishing shear sonic anisotropy due to horizontal differential stresses from that caused by lithological fabric and natural fractures.
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