Inversion of borehole breakout orientation data : J Geophys Res V96, NB12, Nov 1991, P20093–20107

Abstract

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 is vertical, breakout orientations tend to parallel the direction of the regional minimum principal stress (compressive). Such breakout orientations are often valuable indicators of the direction of the principal stresses. We will show in this paper that the variation of the breakout direction as the borehole deviates from vertical gives further information about the relative magnitudes of all the three principal stresses. Based upon the variation of breakout directions as a function of deviations and azimuths of the borehole along its depth, an inverse problem is formulated to estimate both the direction of the minimum horizontal stress and the relative magnitudes of the horizontal stresses to the vertical stress. Tests with synthetic data sets show that the power of the method is dependent on the stress regime (strike-slip, normal and thrust). Generally the inverse problem in the case of thrust faulting is less constrained, whereas, in the case of normal faulting, it allows for a determination of all three parameters (SH/SV, SK/SV and ν) with a reasonable certainty. Interpretation of the data from the Siljan Deep Drilling Project in Sweden shows that the azimuth of the minimum principal horizontal stress is very well constrained to N(18.4°±0.3°)E, while the ratio of the maximum principal horizontal stress to the vertical stress is constrained to 1.1 ±0.1. The ratio between minimum principal horizontal stress and principal vertical stress is poorly constrained to the interval (0.0–0.95). Information of drilling mud leakage and borehole stability analysis provides a value of 0.61 [Stephansson et al., 1990]. These results are in fair agreement with the results from earthquake fault plane solutions in South Central Fennoscandia with dominant strike-slip faulting and an inferred minimum horizontal stress direction of N45°E [Slunga, 1981].