Page 196 - Reservoir Geomechanics

178 Reservoir geomechanics

In Figure 6.5b, the area around the wellbore wall in which tensional stresses exist
is at the azimuth of the maximum horizontal stress and is shown in white. As noted
above, under normal circumstances, drilling induced tensile fractures are not expected
to propagate more than a cm from the wellbore wall. Thus, the formation of drilling-
induced tensile fractures will not lead to a hydraulic fracture propagating away from
the wellbore (which could cause lost circulation) unless the mud weight exceeds the
least principal stress. In the case of deviated wells, this is somewhat more complicated
and is discussed in more detail in Chapter 8.
Because drilling-induced tensile fractures do not propagate any significant distance
away from the wellbore wall (and thus have no appreciable effect on drilling), wellbore
image logs are essentially the only way to know if drilling-induced tensile fractures
are present in a well. This can be seen quite clearly in the two examples of electrical
image logs in Figure 6.6 (from Zoback, Barton et al. 2003). As predicted by the simple
theory discussed above, the fractures form on opposite sides of the wellbore wall (at the
azimuth of S Hmax ,90 from the position of breakouts) and propagate along the axis of
◦
the wellbore. As discussed in Chapter 8, the occurrence of axial drilling-induced tensile
fractures is evidence that one principal stress is parallel to the axis of the wellbore. Note
that in the televiewer image shown in Figure 6.4, there are tensile fractures on opposite
◦
sides of the wellbore wall that are 90 from the midpoints of the wellbore breakouts. In
other words, this well was failing simultaneously in compression and tension as it was
being drilled. However, because the tensile fractures do not affect the drilling process,
and because the breakouts were not excessively large (see Chapter 10) there were no
problems with wellbore stability. In Figure 6.6c. the orientations of S Hmax determined
from breakouts and drilling-induced tensile fractures in a section of a well are shown
(the orientation of breakouts were rotated 90 as they form at the azimuth of S hmin ).
◦
Note that the breakouts and tensile fractures form 180 apart, on opposite sides of the
◦
◦
well and the breakouts and tensile fractures form 90 apart, exactly as predicted on the
basis of the simple theory described above.
To illustrate how robust drilling-induced tensile fractures are as stress indicators, a
stress map of the Visund field in the northern North Sea is shown in Figure 6.7 (after
Wiprut and Zoback 2000). In the Visund field, an extremely uniform stress field is
observed as a function of both depth and position in the oil field. Drilling-induced
tensile fractures were observed in five vertical wells (A−E). The depth intervals logged
are shown in white in the lower right corner of the figure and the intervals over which
the tensile fractures were observed is shown by the black lines. The rose diagrams show
the orientation and standard deviation of the drilling-induced tensile fractures observed
in wells A–E as well as a compilation of the 1261 observations made in all of the wells.
Note that numerous observations in each well indicate very uniform stress with depth
(standard deviations of only ∼10 ). As these observations come from depths ranging
◦
between 2500 m and 5200 m and from wells separated by up to 20 km, a spatially
uniform stress field is observed.