reservoir geomecanics

222 Reservoir geomechanics
the hydrofrac) would be proportional to the change in system volume. However, when
a tensile fracture opens at the wellbore wall, the change of V s is negligible because V s
is so large. Thus, P b represents unstable fracture propagation into the far-field (fluid
is flowing into the fracture faster than the pump is supplying it) but fracture initiation
could have occurred at any pressure. Because equation (7.4) assumes that hydrofracs
initiate at the breakdown pressure, if the actual initiation pressure cannot be observed
due to the large system volume, it is obvious that hydraulic fracturing pressure data
cannot be used to determine S Hmax in most circumstances.
Wellbore failure and the determination of S Hmax
As mentioned above the type of integrated stress measurement strategy utilized here
and summarized by Zoback, Barton et al. (2003) was first employed to estimate the
magnitude of the three principal stresses in the Cajon Pass (Zoback and Healy 1992) and
KTB scientific drilling projects (Zoback, Apel et al. 1993; Brudy, Zoback et al. 1997).
Figure 7.7 presents a summary of the stress results for the KTB Project. Hydraulic
fracturing was used to estimate the least principal stress, S hmin ,to6kmdepth, as well
as the magnitude of S Hmax to a depth of ∼3kmusing a modification of the conventional
hydraulic fracturing method described above (Baumgärtner, Rummel et al. 1990). The
magnitude of S hmin determined from hydraulic fracturing and estimates of rock strength
from laboratory measurements along with observations of wellbore breakouts were
used to constrain the magnitude of S Hmax between depths of 1.7 and 4 km (the open
and filled triangles indicate lower and upper bound estimates). Observations of drillinginduced
tensile fractures between 3 and 4 km allowed us to independently estimate the
lower and upper bound of S Hmax (+’s and ×’s, respectively), again using the magnitude
of S hmin determined from hydraulic fracturing. Note how well the estimates of S Hmax
from the three techniques compare between ∼1.7 and 4 km. At greater depth, it was
necessary to combine the observations of tensile fractures and breakouts (the wellbore
wasfailing simultaneously in compression and tension in the manner illustrated in the
left panel of Figure 6.4)toconstrain the magnitude of S Hmax . Because of the large uncertainty
in temperature at which the tensile fractures formed, there is a correspondingly
large uncertainty in the magnitude of S Hmax at great depth (see Brudy, Zoback et al.
1997). Modeling of a breakout rotation at 5.4 km depth using the technique described
at the end of this chapter provided an independent estimate of the magnitude of S Hmax
consistent with the combined analysis (Barton and Zoback 1994).
As discussed in Chapter 6, breakouts form in the area around a wellbore where
the stress concentration exceeds the rock strength and once a breakout forms, the
stress concentration around the wellbore is such that breakouts will tend to deepen.
Because breakout width is expected to remain stable even as breakout growth occurs
after initiation, Barton, Zoback et al. (1988) proposed a methodology for determination