A Consistent Approach for Stress Field Orientation Determination from Multiple Measurements: Calipers, Images and Sonic

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 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. A new machine-learning-based analysis of the dispersion data is used to uniquely identify the crossover characteristics of stress-sensitive zones. 1 INTRODUCTION Stress field characterization is an important input for all subsurface geomechanics applications. Improved understanding of the stress direction and the stress regime will help to reduce the risk of drilling operations and enhance the success of hydraulic fracturing (Fjaer et al., 1992; Zoback, 2010). Over the last 30 years four-arm caliper (Plumb and Hickman, 1985) and borehole image data (Zoback et al., 1985) have been established as the industry standard measurements to interpret the in-situ horizontal stress direction. The interpretation of the stress direction is closely linked to the near-wellbore stresses that are a function of the far-field stresses and the borehole pressure. Breakouts (BO) can be observed when compressive stresses on the bore-hole wall exceed compressive shear strength of the formation (usually the case when borehole pressure is too low).