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 $Hmax (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 SHmax is ~N42OE, perpendicular to a major growth fault penetrated by the well, the magnitude of SHnmx is relatively close to the vertical stress and the effective in situ compressire rock strength is about 22-32 MPa. We also illustrate how drillinginduced 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 $v from vertical (~10-12o).
Improved knowledge of the complete stress tensor and effective rock strength is important in many problems involving underground openings. In hydrocarbon reservoirs, these factors are important in a number of problems ranging from borehole stability and sand production to hydrocarbon migration and hydraulic fracturing. We have conducted a comprehensive series of calculations of the occurrence of compressive and tensile failures in arbitrarily-inclined boreholes (Peska and Zoback 1995) that demonstrates the general relationship between failure of inclined wellbores, tectonic stress and rock strength. In this paper, we present two specific applications of this theory borehole breakouts in an inclined hole in an area of active normal faulting in the Gulf of Mexico and inclined tensile wall-fractures in a strike-slip faulting environments in the KTB pilot hole, Germany. We demonstrate through these case studies how one can use wellbore failure and leak-off/hydrofrac data to estimate the complete stress tensor and to utilize this_ information to constrain effective rock strength.
Stress-induced compressive failures around wellbores are commonly known as stress-induced breakouts (e.g., Bell and Gough 1979, Zoback et al. 1985) and can be observed with either 4-arm caliper dipmeters or borehole televiewers (e.g., Plumb and Hickman 1985). Numerous compilations from nearvertical holes show that wellbore breakouts are reliable indicators of principal stress directions (see, for example, Bell and Gough 1979, Gough and Bell 1981, Zoback and Zoback 1980, 1989, 1991, Plumb and Cox 1987, Zoback et al. 1989, Adams and Bell 1991, Milllet et al. 1992, Zoback 1992). In contrast to drilling-induced hydraulic fractures which propagate away from the borehole and are associated with lost circulation (e.g., Stock et al. 1985, Moos and Morin 1991), tensile wall-fractures are very fme fractures in the wall of the hole that occur due to the stress concentration immediately adjacent to the wellbore wall (e.g., Aadnoy 1990b, Brudy and Zoback 1993, Zoback et al. 1993) and can be detected only through car
