Page 231 - Applied Petroleum Geomechanics

226 Applied Petroleum Geomechanics

For a vertical or near vertical borehole, the maximum horizontal stress
orientation is aligned with the direction of drilling-induced tensile fractures
and is perpendicular to the direction of borehole breakouts. Oriented caliper
and image logs can be used to identify the directions of wellbore breakouts and
drilling-induced tensile failures. In hydraulic fracturing operations, the
direction of the hydraulic fracture propagation (except natural fractures) is
parallel to the maximum horizontal stress direction, and microseismic
measurements can identify it (Dohmen et al., 2017).

6.5.1 From borehole breakouts
Borehole breakouts are stress-induced enlargements of a wellbore. When a
borehole is drilled, the formations removed from the borehole are no
longer supporting the surrounding rocks. As a result, the stresses become
concentrated around the wellbore. Borehole breakout occurs when the
critical stress (obtained from a failure criterion; e.g., the Mohr-Coulomb
criterion) around the borehole exceed rock strength. Breakouts are
mostly caused by shear failures, including maybe compressive or tensile
spalling failures of the wellbore (Zhang, 2002; Zhang et al., 2003). Shear
failures in a wellbore are primarily caused by the development of inter-
secting conjugate shear planes, causing pieces of the borehole wall to spall
off, as shown in Fig. 6.23. Around a vertical borehole, stress concentration
(e.g., the tangential stress) is the greatest in the direction of the minimum
horizontal stress (S h in Fig. 6.23). Hence, the long axes of borehole
breakouts are oriented approximately parallel to the minimum horizontal
stress orientation (S h ), as shown in Fig. 6.23.

Figure 6.23 Results of a hollow cylinder lab test simulating borehole breakouts
(performed by the CSIRO Division of Geomechanics). Intersection of conjugate shear
failure planes results in enlargement of the cross-sectional shape of the wellbore
(Reinecker et al., 2003).