Existing methods and a newly proposed method are used to estimate the magnitudes of the horizontal principal stresses from stress-induced borehole breakouts and leak-off tests. Under the condition that the depth interval between leak-off and breakout is sufficiently small, so that the stress ratios sigma H /sigma v and sigma h /sigma v can be reasonably regarded as constants within the depth range, both horizontal principal stresses sigma H and sigma h can be resolved with some knowledge of breakout width. A comparison of the methods has been conducted with well data from the North West Shelf of Australia. The estimated stress magnitudes are generally consistent with the measured trend from neighbouring modified leak-off (or hydraulic fracturing) tests, and also indicate that the tectonic stress regime in the depth range from 2.6 to 2.9 km, at the location of the borehole, is likely to be on the boundary between extension and strike-slip. Knowledge of the in situ stress field provides a crucial development factor in petroleum engineering, such as the optimum design of inclined or horizontal drilling for maximizing wellbore mechanical stability and recovery from fractured reservoirs.

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