Page 254 - Reservoir Geomechanics

8 Wellbore failure and stress determination

in deviated wells

Many wells being drilled for oil and gas production are either horizontal, highly deviated
from vertical or have complex trajectories. Because of this, it is necessary to understand
the factors that control the occurrence of compressive and tensile failures in wells with
arbitrary orientation. In this section, I generalize the material presented in Chapters 6
and 7 for wells of any orientation. I begin the chapter by considering the ways in
which compressive failures (breakouts) and drilling-induced tensile fractures occur in
arbitrarily oriented wells in normal, strike-slip and reverse faulting environments. The
basic principles of failure of wells with arbitrary orientation will be utilized extensively
when applied to wellbore stability in Chapter 10.
Most analyses of breakouts and tensile fracture compilations do not include observa-
tions of failures in inclined boreholes. There are three reasons for this. First, four-arm
caliper logs (typically used to study breakouts) frequently track key seats as discussed in
Chapter 6. This makes it difficult to detect breakouts with caliper data. Second, break-
out directions in deviated holes vary significantly from what would occur in vertical
holes (e.g. Mastin 1988; Peska and Zoback 1995). The same is true of drilling-induced
tensile fractures. Thus, it is not straightforward to relate breakout and tensile fracture
observations to stress directions. Third, drilling-induced tensile fractures in deviated
wells occur in an en echelon pattern at an angle to the wellbore axis and can be difficult
to distinguish from natural fractures in the formation. I demonstrate theoretically (and
illustrate through several examples) how to distinguish the two.
Because of the complexities associated with breakout and tensile fracture occurrence
in deviated wells, it has been typical to ignore data from deviated wells when assessing
in situ stress orientations. In this chapter we take the opposite approach. I show that
the details of failure of deviated wells are sensitive to the exact stress conditions in
situ,so the study of such failures can provide important insight into stress orientations
and magnitudes. In this chapter we demonstrate that the analysis of wellbore failures
in deviated wells makes possible several new techniques for stress determination.
I will present the material in this chapter in the context of deviated wells and principal
stresses acting in horizontal and vertical directions. As alluded to in Chapter 1, this is
the usual case world-wide. However, complex stress fields, such as near salt bodies (as
illustrated in Figure 1.10) require knowing the orientation of the stress tensor when
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