Page 226 - Reservoir Geomechanics

207 Determination of S 3 from mini-fracs

When significant mud losses are noted during drilling, it can denote the accidental
hydraulic fracturing of a well, requiring that the mud weight be reduced to a value
less than the least principal stress, or frac-gradient. Finally, wellbore ballooning noted
during logging-while-drilling (LWD) operations indicate that the wellbore pressure is
very close to the least principal stress (see Chapter 10).
In many problems encountered in geomechanics, knowledge of the magnitude of
the maximum horizontal principal stress at depth, S Hmax ,is especially important. For
example, an accurate determination of S Hmax is usually very important in problems
related to wellbore stability such as the determination of optimal mud weights, well
trajectories, casing set points, etc. (Chapter 10). As explained in Chapter 6,in the area
of maximum stress concentration where breakouts form in vertical wells, the hoop
stress results from a value of S Hmax that is amplified by a factor of 3 at the wellbore
wall. Hence, an accurate estimate of S Hmax is often a critically important element of a
wellbore stability analysis. The same thing is true when trying to assess the likelihood
of shear failure on pre-existing faults (Chapter 11). As discussed in detail in Chapter
5, determination of shear and normal stress on an arbitrarily oriented fault requires
knowledge of all three principal stresses.
Despite the importance of the determination of S Hmax in geomechanics, it has long
been recognized that this is the most difficult component of the stress tensor to accu-
rately estimate, particularly as it cannot be measured directly. Because making stress
measurements at great depth offers a unique set of challenges, we review in this chapter
techniques developed that have proven to be especially efficacious for determination
of S Hmax in relatively deep wells. These techniques were reviewed by Zoback, Barton
et al. (2003). The type of integrated stress measurement strategy utilized here was
first employed to estimate the magnitude of the three principal stresses in the Cajon
Pass scientific research borehole (Zoback and Healy 1992) and KTB scientific drilling
project in Germany (Zoback, Apel et al. 1993; Brudy, Zoback et al. 1997). Hydraulic
fracturing was used to estimate the least principal stress, S hmin ,to6 km depth. Know-
ing this, observations of drilling-induced tensile fractures and/or the width of wellbore
breakouts (w BO ) were used to constrain the magnitude of S Hmax . While these stress tech-
niques support the concept that brittle crust is in a state of frictional failure equilibrium
(Chapters 1 and 9)in the context of laboratory friction measurements and Coulomb
faulting theory (Chapter 4), the viability of these techniques for application to a vari-
ety of practical problems encountered in geomechanics has been confirmed through
numerous case studies world wide. A sampling of these types of studies is presented in
subsequent chapters.
The widespread use of wellbore imaging devices has been an important develop-
ment that has made possible the application of the techniques for estimating S Hmax ,
described below. As illustrated in Chapter 6, ultrasonic borehole televiewers (Zemanek,
Glenn et al. 1970) and electrical imaging devices (Ekstrom, Dahan et al. 1987) yield
detailed information about wellbore failure that is critically important in assessing