Page 199 - Reservoir Geomechanics

181 Compressive and tensile failures in vertical wells

Numerous studies have shown that if carefully analyzed, breakout orientations are
also remarkably consistent in a given well and in a given oil-field (Bell and Bab-
cock 1986; Klein and Barr 1986; Mount and Suppe 1987; Plumb and Cox 1987), and
yield reliable measures of stress orientation in many parts of the world. The basis and
criteria associated with creating integrated maps of contemporary tectonic stress are
those developed by (Zoback and Zoback 1980, 1989, 1991) and subsequently used in
the World Stress Map Project (Zoback 1992). From the perspective of regional stress
studies, the depth range of breakouts, typically 1–4 km, provide an important bridge
between the depths at which most crustal earthquakes occur (3–15 km) and the depths
of most in situ stress measurements (<2 km) and geologic observations at the surface.
Such data are an integral part of the World Stress Map data base.
Figure 6.8aisa stress map of the Timor Sea (Castillo, Bishop et al. 2000), con-
structed by compiling abundant breakout and tensile fracture orientations at depth in
the numerous wells shown. In each well the variation of the maximum horizontal stress
direction determined from the breakouts and tensile fractures is less than 10 . Note
◦
that in each subregion, the stress field is remarkably uniform. Although the average
stress orientation in the area seems clearly to correspond to the convergence direction
between Australia and Indonesia, the origin of the variations among the subregions in
this tectonically active area is not known.

Determination of breakout orientation from caliper logs
Traditionally, most of the data used to determine the orientation of breakouts in wells
come from magnetically oriented four-arm caliper data which are part of the dipmeter
logging tool that is commonly used in the petroleum industry. Despite the relatively low
sensitivity of this technique, with sufficient care it is possible to use four-arm caliper
data to reliably determine breakout orientations. This has been clearly demonstrated
in western California (Figure 6.8b, from Townend and Zoback 2004) where S Hmax
orientations obtained from analysis of breakouts with four-arm caliper data (inward
pointed arrows) yield consistent stress orientations that correlate well with earthquake
focal plane mechanisms (data points with circle in center) and young geologic indicators
of deformation (the trends of fold axes and active reverse faults).
While the analysis of wellbore breakouts with four-arm caliper data appears to be
quite straightforward, it is important not to misinterpret key seats (grooves in the side of
the well caused by the rubbing of pipe) or washouts (enlargements of the entire wellbore
circumference) as stress-induced breakouts. In fact, relatively little information comes
from a standard dipmeter log (Figure 6.9a) – the diameter of the well as measured by
the orthogonal pairs of caliper arms (termed the C1–3 and C2–4 pairs as the arms are
numbered sequentially “looking down the hole”), the pad 1 azimuth, the deviation of
the well and the hole azimuth. The actual resistivity data measured on each pad are not
used in the breakout analysis.