Page 185 - Reservoir Geomechanics

168 Reservoir geomechanics

stress concentration. In Chapter 7,I discuss the importance of drilling-induced tensile
fractures as means of obtaining important information about stress orientation and
magnitude as well as the manner in which hydraulic fractures yield extremely important
information about the magnitude of the least principal stress. Of course, if hydraulic
fracturing occurs unintentionally during drilling (due to excessively high mud weights),
lost circulation can occur. This is another serious problem during drilling, especially
in areas of severe overpressure. Options for avoiding lost circulation during drilling in
overpressured areas are discussed in Chapter 8.
In the sections of this chapter that follow, I first introduce the concept of stress con-
centrations around a vertical well, how this stress concentration can lead to compressive
and tensile wall failures and how such failures are used to determine the orientation
of the horizontal principal stresses that exist in situ. The majority of stress orientation
data shown in the maps presented in Chapter 1 and throughout this book (and utilized
by the World Stress Map project, see Zoback, 1992) come from wellbore failures and
earthquake focal mechanisms. Hence, after introducing breakouts and drilling-induced
tensile fractures in the first part of this chapter, we discuss the quality ranking criterion
developed by Zoback and Zoback (1989, 1991) for mapping the intraplate stress field.
Next, I extend the discussion of tensile failures to discuss hydraulic fracturing and the
determination of the least principal stress, S 3 , from hydrofracs in reservoirs or extended
leak-off tests at casing set points. As S 3 ≡ S hmin in normal and strike-slip faulting areas

(the most common stress states around the world), establishing the magnitude of S hmin
is a critical component of determining the full stress tensor. When one principal stress
is vertical, S v is obtained by integration of density logs as discussed in Chapter 1.
Pore pressure can be either measured directly or estimated using the techniques
described in Chapter 2.With knowledge of the orientation of the horizontal princi-
pal stresses obtained from wellbore failures and the magnitude S hmin , determination of
the complete stress tensor requires only the magnitude of S Hmax to be determined. In
Chapters 7 and 8 we discuss determination of S Hmax utilizing observations of wellbore
failures with independently determined values of S v , P p and S hmin .To set the stage for
these discussions, we consider in this chapter both drilling-induced tensile wall frac-
tures as well as compressive failures (breakouts) to include the influence of thermal
stresses and excess mud weight on the formation of such fractures.
As a full understanding of compressive failure is also critically important for evalu-
ation of wellbore stability (Chapter 10), we briefly consider at the end of this chapter
a number of other processes that affect compressive failure around wellbores. These
include the way in which the presence of weak bedding planes can induce anisotropic
rock strength (previously introduced in Chapter 4) and briefly consider, theoretically,
compressive failure when elastic–plastic constitutive laws (introduced in Chapter 3)
are more appropriate for a given formation than the strength of materials approach
used throughout most of this book. This is most applicable for the case of drilling
through poorly cemented sands. Finally, we briefly broaden discussion of wellbore