Dedicated to Charles Fairhurst for his pioneering contributions to stress measurement in rock by hydraulic fracturing.
ABSTRACT: Elongated breakout cross sections for different rock stresses and strengths have been produced using a numerical simulation based on boundary element methods and a micromechanics model of extensile splitting. Although these breakout cross sections are completely stable for an elastic-brittle rock they are not related uniquely to the stresses and strengths, and depend upon the sequence in which holes are drilled and the stresses applied. The depth of any breakout depends mainly on the initial breakout angle. The same initial breakout angle can result from many different combinations of stress. We conclude that breakout shapes cannot be used to infer the magnitudes of the stresses orthogonal to a borehole. The interpretation of hydraulic fracturing measurements based on the stress around a circular hole does not result in large errors if the hole has moderate breakouts. Large breakouts result in tensile fractures that preclude measurements of breakdown pressures.
The phenomenon of rock fracturing by spalling from the walls of boreholes, is referred to as "borehole breakout". The breakout problem has been the subject of intensive field studies and observations (Leeman 1960; Cox 1970; Bell and Gough 1979, 1982; Hickman et al. 1985; Plumb and Hickman 1985; Kim et al. 1986), laboratory experiments (Gay 1976; Mastin 1984; Haimson and Herrick 1985) and theoretical analysis (Gough and Bell 1982; Zoback et al. 1985; Zheng and Cook 1985, Ewy et al. 1987, Guenot, 1987 and Maury, 1987). All the studies agree that breakouts take place in regions of high stress concentration and that the elongation of the diameter of a borehole is parallel to the direction of the minimum principal stress orthogonal to the borehole axis. In this paper, we wish to report recent theoretical results concerning the effect of borehole breakouts on in situ stress measurements. We investigate the possibility of finding the magnitudes of the in situ stresses from borehole breakouts and the effect of breakouts on the results of stress measurement by hydraulic fracturing. Results from the numerical simulations show that borehole breakouts are stress history dependent, that is, the shape and size of a breakout depends on the sequence in which the hole is drilled and the stresses are applied. Results also show that the initial breakout angle is the main factor that controls the breakout depth and the same initial breakout angle can be obtained from different stress- strength combinations so that there is a non-unique relationship between the in situ stresses and the breakout shape and size. The effect of borehole breakout on the hydraulic fracture technique, however, is not important for moderate breakouts but for larger breakouts, in which tensile fractures have already been generated, only the "shut in" pressure can be obtained.
2 NUMERICALLY GENERATED BREAKOUTS AND THEIR NONUNIQUE RELATIONSHIP TO IN situ STRESSES
When a borehole is drilled, the original stress field changes and results in concentration of tangential stress around the borehole boundary. The stress distribution around a circular borehole can be obtained from the Kirsch's solution (Jaeger and Cook, 1979).