Abstract

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.

1. INTRODUCTION

Borehole dipole (flexural) sonic waves provide the opportunity to identify and quantify shear anisotropy. The dipole flexural waves polarize into a fast and slow shear in the presence of elastic anisotropy in the planes containing the borehole axis. Anisotropy may be the result of mechanical anisotropy, fractures, or stress. Slowness-dispersion analysis provides the ability to identify stress-induced azimuthal anisotropy. If anisotropy is confirmed to be a result of a differential stress, one can then determine the direction of maximum horizontal stress from the direction of the fast shear wave in a near-vertical wellbore [1, 2, 3, 4]. This technique is commonly used in the petroleum industry for complementing other methods, such as borehole failure measured from calipers and images, to deduce the direction and magnitude of the present-day horizontal stresses. Stress characterization methods that rely on the presence of borehole failure using images are limited to boreholes that exhibit failure. Similarly there are situations where high differential stresses are observed from borehole failure and yet shear wave anisotropy is absent. Theory and laboratory testing illustrate that all rocks have some degree of acoustic sensitivity to changes in stress, which is related to the compliance of the grain-to-grain contacts. In the borehole, this phenomenon is also limited by the accuracy of acoustic logging technology [30]. These data are then used for future geomechanics-related studies for predicting sand production, planning stimulation treatments, reservoir engineering, and wellbore stability [6, 7, and 8].

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