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ABSTRACT: Determination of in-situ stress and other structural elements using sonic cross-dipole waveforms and image logs

Manescu, Adrian and Peter Bright
Baker Atlas, Perth, Australia

Potential effects of stress and other structural elements on the wellbore are key questions when it comes to considering borehole stability in vertical or horizontal wells. Sonic cross-dipole measurements and image logs help to answer these questions.

The cross-dipole technique utilizes two orthogonal dipole sources and two orthogonal sets of receivers to measure azimuthal anisotropy around the wellbore. A reservoir with near vertical, aligned fractures will show azimuthal anisotropy with the shear wave velocities split into a fast mode (particle displacement parallel to the fracture plane) and a slow mode (particle displacement perpendicular to the fracture plane). Cross-dipole measurements determine both mode velocities and the direction of their axes.

Causes of shear wave velocities split can be structural elements as aligned fractures and thin bedding planes inclined to the borehole or from unequal stresses within the formation, or borehole ellipticity. The magnitude of the anisotropy between the fast and slow shear modes can be related to the fracture density or stress variation. Not only can large fractures be detected but also zones of aligned micro-cracks that will change the elastic properties of the rock. These may be undetectable on borehole image data. Fractures parallel to the direction of maximum horizontal stress may open up; those perpendicular to it may be closed. Examined together with borehole imaging measurements many ambiguities are eliminated.

The analysis of the features observed on borehole image data and of the cross-dipole shear anisotropy help plan field development, avoids sanding problems and ensures drilling safety.

©Copyright 2000. The American Association of Petroleum Geologists. All rights reserved.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia