World Stress Map Project Guidelines: Four-arm Caliper Logs 1 Borehole breakout analysis from four-arm caliper logs J. Reinecker, M. Tingay and B. Müller Introduction Borehole breakouts are an important indicator of horizontal stress orientation, particularly in aseismic regions and at intermediate depths (<5 km). Approximately 19% of the stress orientation indicators in the World Stress Map (WSM) database have been determined from borehole breakouts. Here we present the procedures for interpreting borehole breakouts from four-arm caliper log data and for WSM quality ranking of stress orientations deduced from borehole breakouts. Borehole Breakouts Borehole breakouts are stress-induced enlargements of the wellbore cross-section (Bell and Gough, 1979). When a borehole is drilled the material removed from the subsurface is no longer supporting the surrounding rock. As a result, the stresses become concentrated in the surrounding rock (i.e. the wellbore wall). Borehole breakout occurs when the stresses around the borehole exceed that required to cause compressive failure of the borehole wall (Zoback et al., 1985; Bell, 1990). The enlargement of the wellbore is caused by the development of intersecting conjugate shear planes, that cause pieces of the borehole wall to spall off (Figure 1). Figure 1: Results of a hollow cylinder lab test simulating borehole breakout (performed by the CSIRO Division of Geomechanics). Intersection of conjugate shear failure planes results in enlargement of the cross-sectional shape of the wellbore. S H and S h refer to the orientations of maximum and minimum horizontal stress respectively. Around a vertical borehole stress concentration is greatest in the direction of the minimum horizontal stress S h. Hence, the long axes of borehole breakouts are oriented approximately perpendicular to the maximum horizontal stress orientation S H (Plumb and Hickman, 1985). Hole ovalisation caused by pieces of wellbore wall spalling off Original borehole shape Zones of failure that have not spalled off Conjugate shear failure planes Schematic of Photograph S H S h S h S H