Observations of borehole breakouts and tensile wall-fractures in deviated boreholes: A technique to constrain in situ stress and rock strength

In this study we demonstrate the relationship between compressive and tensile failures of inclined wellbores and the tectonic stress field. We present illustrative polar diagrams from which one can determine the orientation and tendency for borehole failure as a function of the borehole orientation and the stress field. In the case of borehole breakouts, we propose a technique to constrain the magnitude and orientation of SHmax (as well as to estimate the effective in situ rock strength) from the orientation of breakouts in deviated boreholes. The method is based on the assumption that an estimate of the least principal stress is available from leak-off tests or hydrofracs, the overburden stress is obtained from integration of density logs and pore pressure is either known or can be estimated. Application of this technique in the GBRN/DOE ’’Pathfinder” well, Gulf of Mexico, shows that the azimuth of Shmin is ~N42°E, perpendicular to a major growth fault penetrated by the well, the magnitude of SHmax is relatively close to the vertical stress and the effective in situ compressive rock strength is about 22-32 MPa. We also illustrate how drilling induced tensile wall-fractures (detected from FMS/FMI logs) in deviated holes (or vertical holes in which one principal stress is not vertical) can be used to constrain the in situ stress orientation and magnitude. In the case of the KTB borehole, Germany, this relationship is used at a depth of 3213 m to estimate the magnitude of SHmax (correlates closely with a value determined from hydraulic fracturing) as well as the deviation of S ν from vertical (~10-12°).