Page 274 - Reservoir Geomechanics

255 Wellbore failure and stress determination in deviated wells

sinusoid. Hence, these are segments of pre-existing fractures that cannot be seen around
the entire wellbore, similar to those in Figure 8.12b. One interesting aspect of the partial
sinusoids seen in Figures 8.12a,b is that they tend to occur at the azimuth of minimum
compression around the wellbore. Note there are partial sinusoids in Figure 8.12c
that are at the same azimuth as the axial drilling-induced tensile fractures. We refer to
these as drilling-enhanced fractures (Barton and Zoback 2002). We interpret them to
be small-aperture natural fractures that would normally be too fine to be seen on image
logs if it were not for the preferential spalling of the fracture as the well is being drilled
in the portions of the wellbore circumference where σ tmin is minimum. While there
has been no specific modeling to confirm this interpretation, we have noted a strong
correlation between drilling-enhanced natural fractures and the orientation of S Hmax in
vertical wells.

Determination of S Hmax orientation from shear velocity anisotropy
in deviated wells

There are numerous observations of shear wave anisotropy (the polarization of shear
wavesinan anisotropic medium into fast and slow components) in the upper crust.
The mechanisms proposed to explain them fall into two general categories. First
is stress-induced anisotropy in response to the difference among the three principal
stresses. In this case, vertically propagating seismic waves will be polarized with
afast direction parallel to the open microcracks (Crampin 1985), or perpendicular
to closed macroscopic fractures (Boness and Zoback 2004). In both cases, the fast
shear direction is polarized parallel to S Hmax . Second is structural anisotropy due to
the alignment of sub-parallel planar features such as aligned macroscopic fractures
or sedimentary bedding planes. In this case the propagating shear waves exhibit a fast
polarization direction parallel to the strike of the structural fabric or texture. In geophys-
ical exploration, shear velocity anisotropy is commonly modeled with a transversely
isotropic (Maxwell, Urbancic et al. 2002) symmetry where the shear waves are polar-
ized parallel and perpendicular to the planes normal to the formation symmetry axis
(Thomsen 1986).
Utilization of cross-dipole sonic logs in vertical wells has been used to determine
stress orientation from the fast shear polarization direction when bedding planes are
sub-horizontal or aligned fractures are not likely to influence the polarization of the
shear waves. We limit our discussion to cross-dipole sonic shear wave logs (Kimball
and Marzetta 1984; Chen 1988; Harrison, Randall et al. 1990)in order to compare and
contrast the results from utilization of this technique with the other techniques discussed
in this book. The sondes used to obtain these data have linear arrays of transmitters
and receiver stations commonly spaced at 6 inch intervals and the transmitter on these
dipole sonic tools is a low-frequency dipole source operating in the frequency range of