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216 Applied Petroleum Geomechanics

drilling-induced tensile fracture and wellbore breakout data (Aadnoy, 1990;
Peska and Zoback, 1995; Okabe et al., 1998; Huang et al., 2010; Thorsen,
2011) can be applied to obtain in situ stresses in inclined wellbores.
In the normal stress regime for a horizontal well drilled in one of the
horizontal stress directions, the tensile stress (absolute value) is the largest at
the top and bottom in the cross section of the borehole; hence, the tensile
fractures (vertical ones) initiate at the top and bottom of the horizontal well.

6.4.2.2 In the reverse faulting stress regime
6.4.2.2.1 For a vertical well
In the reverse faulting stress regime, the vertical stress is the minimum
in situ stress; therefore, horizontal hydraulic fracture stress will be generated
(Hubbert and Willis, 1957). The drilling-induced tensile fractures can also
be horizontal in a vertical well, as shown in Fig. 6.15. In the reverse faulting
stress regime, hydraulic fractures are opening or closing against the vertical
stress because the axial stress in the borehole axial direction induces these
tensile fractures. This suggests that the closure pressure in the vertical well in

Orienta on North
0 120 240 360
Resis ve FMI Image Conduc ve

Depth (m)

2686

Pairs of
transverse,
electrically
2687 conductive
fractures
occurring at
the tensile
region of the
wellbore
wall.
138°°N 318°N
°
Figure 6.15 Drilling-induced horizontal tensile fractures interpreted on an image log
from the West Tuna 39 well of Australia. The fractures are electrically conductive,
transverse (the azimuth of s H : 138 N and 318 N), restricted to the tensile stress region

of the vertical wellbore (Nelson et al., 2005).