Effect of borehole deviation on breakout orientations

Well bore breakouts are zones of spalling and fracture that form on opposite sides of a well bore and tend to change the cross‐sectional shape of the borehole from circular to roughly elliptical. In vertical holes drilled in areas where one principal stress ( S v ) is vertical, breakouts tend to form at opposite ends of the borehole diameter parallel to the least compressive horizontal principal stress direction ( S h ). This paper uses an analytical elastic solution for stress at the wall of a borehole to analyze the rotation of breakout orientations away from the direction of S h as the borehole deviates from the direction of the vertical principal stress. The calculated orientations of breakouts in deviated boreholes depend on the type of faulting regime in which the well was drilled (i.e., normal, strike‐slip, or thrust), on the deviation angle ø of the borehole axis from vertical, on the angle θ between the horizontal projection of the borehole axis and the direction of S h , and on the relative magnitudes of the three principal stresses ( S h ; S H , the greatest compressive horizontal stress; and S v ). In the strike‐slip faulting regime, regardless of the values of θ, S v , S H , and S h , a borehole must deviate at least 35° from vertical before the horizontal projection of the breakout orientation differs by more than 10° from S h . In the normal and thrust faulting regimes, however, the borehole deviation angle ø crit required to rotate projected breakout orientations by 10° from S h approaches zero as the value of S h approaches that of S H . In the thrust faulting regime, only about 3% of all combinations of θ, S v , S H , and S h will produce ø crit values less than 10°, while about 12% of all such combinations will yield ø crit < 10° in the normal faulting regime. About 12% and 33% of such combinations will yield ø crit < 20° in the thrust and normal faulting regimes, respectively. Careful study of changes in breakout orientation as a function of borehole deviation may improve the resolution of inferred stress directions when studying breakouts in deviated boreholes in these two faulting regimes.