Page 217 - Reservoir Geomechanics

198 Reservoir geomechanics

with time (Figure 6.14c). Thus, wellbore cooling does tend to stabilize breakout growth
with time, but will not affect breakout width observations used for estimation of S Hmax
in Chapter 7 nor will it be a viable technique for minimizing problems of wellbore
stability (Chapter 10).
There are other complexities affecting the formation of breakouts that will arise when
we consider wellbore stability in Chapter 10:
the effect of rock strength anisotropy that results from the presence of weak bedding

planes in shale;
the possibility that the strength of materials approach utilized here oversimplifies the

breakout formation process;
the relation between mud chemistry, rock strength and wellbore stability;

other modes of breakout formation; and

penetration of mud into fractured rock surrounding a wellbore.

Rock strength anisotropy

The theory of compressive failure of rocks with weak bedding planes was discussed
in Chapter 4.Anexample of how slip on weak bedding planes affects breakout for-
mation is illustrated in Figure 6.16. The breakouts seen in the unwrapped ultrasonic
televiewer data in Figure 6.16a are unusual in that there are four dark zones around
the circumference of the well (indicating low reflectivity of the acoustic pulse from the
borehole wall) rather than two as seen in Figure 6.4. The cross-sectional view (Figure
6.16b) indicates that the breakouts on each side of the well are each double-lobed,as
originally hypothesized by Vernik and Zoback (1990) when considering the formation
of breakouts in the KTB hole when there were steeply dipping foliation planes cutting
across the hole.
Modeling the formation of breakouts when weak bedding or foliation planes cut
across a wellbore at a high angle is shown in Figure 6.16c. In such cases, breakouts
formduetotwoprocesses:whenthestressconcentrationexceedstheintactrockstrength
and when the stress concentration activates slip on the weak planes thus enlarging the
failure zone. The reason that the double lobes appear is related to the fact that the
stress trajectories bend around the wellbore as shown in Figure 6.1 such that there are
zones where the orientation of S 1 becomes optimal for inducing slip on the bedding
plane. As shown in Figure 6.16c, this occurs at the edges of places around the wellbore
where normal breakouts would form (i.e. those forming as a result of the concentrated
stresses exceeding the intact rock strength). As is the case with breakout formation in
homogeneous and isotropic rock, increases in mud weight tend to stabilize wellbores
and reduce the size of breakouts (Figure 6.16d). This problem was also investigated
by Germanovich, Galybin et al.(1996). The issue of breakout formation in such cases
will be important in Chapter 10 in two contexts: in areas where there are steep bedding
planes due to the tilting of overburden units and when highly deviated wells are drilled