Page 252 - Reservoir Geomechanics

233 Determination of S 3 from mini-fracs

If the different mechanical properties of the faults were the cause of the rotation,
the orientation of the breakouts would be perturbed over a much greater length of the
wellbores than that observed. Brudy, Zoback et al. (1997) studied breakout and tensile
fracture orientation with depth in the ultradeep KTB research borehole using wellbore
image data and the interactive analysis technique referred to above. They documented
the fact that while the average stress orientation to ∼8km depth was quite consistent,
numerous relatively minor perturbations of stress orientation (at various wavelengths)
are superimposed on the average orientation due to slip on faults at various scales.
Through interactive analysis of the shape of the wellbore at various depths, the
orientation of the breakouts can be accurately determined as a function of depth using
ultrasonic imaging data (Barton, Tessler et al. 1991). The orientation of the breakouts
on the left side of Figure 7.14 are shown by the + symbols in the image on the right.
Barton and Zoback (1994) used dislocation modeling to replicate the observed breakout
rotations in the KTB wellbore at 5.4 km depth (Figure 7.14) and showed how modeling
could be used to constrain the magnitude of S Hmax based on knowing (i) the magnitudes
of S hmin and S v , (ii) the unperturbed orientation of S Hmax and (iii) the strike and dip of
the causative fault. Note that the modeling results (triangles on right hand image) were
used to replicate the breakout rotation observed in televiewer data (left image) which
are shown by + symbols in the right image. The breakouts do not form right next to the
fault that slipped due to the stress drop on the fault (Shamir and Zoback 1992). There
is also a temperature gradient anomaly at the position of this fault due to fluid flow
into the borehole along this fault (see Barton, Zoback et al. (1995). The magnitude of
S Hmax determined from modeling the breakout rotation was consistent with the range
of values obtained from analysis of drilling-induced tensile fractures and breakouts by
Brudy, Zoback et al. (1997)in this well (Barton and Zoback 1994)asshown by the +
at 5.4 km depth in Figure 7.7.

Summary

In the sections above, we outlined two techniques for determination of the maximum
horizontal principal stress, S Hmax , which use observations of compressive and tensile
wellbore failure in vertical wells. This requires independent knowledge of the least
horizontal principal stress, S hmin , which is determinable from carefully conducted leak
off tests, or mini-fracs. These techniques are extended to deviated wells in Chapter 8.
In numerous field studies, the techniques described above have yielded consistent
values at various depths in a given well and multiple wells in a given field (e.g. Zoback,
Barton et al. (2003). In cases where the wells are drilled in areas of active faulting, the
values obtained for S Hmax are consistent with predictions of frictional faulting theory
(Zoback and Healy 1984; Zoback and Healy 1992; Brudy, Zoback et al. 1997;Townend