Page 32 - Reservoir Geomechanics

17 The tectonic stress field

Observations of stress-induced wellbore breakouts are a very effective technique for
determining stress orientation in wells and boreholes (Chapter 6). Breakouts are related
to a natural compressive failure process that occurs when the maximum hoop stress
around the hole is large enough to exceed the strength of the rock. This causes the
rock around a portion of the wellbore to fail in compression (Bell and Gough 1983;
Zoback, Moos et al. 1985; Bell 1989). For the simple case of a vertical well when S v
is a principal stress, this leads to the occurrence of stress-induced borehole breakouts
that form at the azimuth of the minimum horizontal compressive stress. Breakouts are
an important source of stress information because they are ubiquitous in oil and gas
wells drilled around the world and because they also permit stress orientations to be
obtained over a range of depths in an individual well. Detailed studies have shown that
these orientations are quite uniform with depth, and independent of lithology and age
(e.g. Plumb and Cox 1987; Castillo and Zoback 1994). Breakouts occurring in devi-
ated wells are somewhat more complicated to analyze (Peska and Zoback 1995), but
as discussed in Chapter 8,have the potential for providing information about stress
orientation and stress magnitude.
Drilling-induced tensile fractures are another type of wellbore failure yielding useful
information about stress orientations (Moos and Zoback 1990; Brudy and Zoback
1999). These fractures form in the wall of the borehole at the azimuth of the maximum
horizontal compressive stress when the circumferential stress acting around the well
locally goes into tension. As shown by Wiprut, Zoback et al.(2000), drilling-induced
tensile fractures can define stress orientations with great detail and precision. As with
breakouts, drilling-induced tensile fractures observed in deviated wells (Brudy and
Zoback 1993; Peska and Zoback 1995)have the potential for providing information
about stress orientation and stress magnitude (Chapter 8).

Earthquake focal mechanisms

Because they are so widespread, earthquake focal plane mechanisms would seem to
be a ubiquitous indicator of stress in the crust. While there is indeed important infor-
mation about stress magnitudes and relative orientations inherent in focal mechanism
observations, these data must be interpreted with caution. Focal mechanisms are dis-
cussed at greater length in Chapter 5. The pattern of seismic radiation from the focus
of an earthquake permits construction of earthquake focal mechanisms as illustrated
by the figures (beach ball diagrams) in the right column of Figure 1.2.At this point,
it is only necessary to recognize that there are two types of information about stress
that are obtainable from well-constrained focal mechanisms of crustal earthquakes.
(By well-constrained we mean that the earthquake is recorded at a sufficient number of
seismographs that the orientation of the focal planes can be reliably determined.) First,
the style of faulting that occurred in the earthquake can be determined (i.e. normal,
strike-slip, or reverse faulting) which, in turn defines the relative magnitudes of S Hmax ,