reservoir geomecanics

311 Wellbore stability
60 80 100 120 140 160 180
Breakout width (w BO )
Figure 10.7. Illustration of the importance of drilling direction on success (after Zoback, Barton
et al. 2003). The figure shows modeled breakout width in the shales in an oil field in South America
at a depth of 2195 m TVD as a function of drilling direction assuming a mud weight of 10 ppg and
C 0 = 17.2 MPa. The total circumference of the wellbore that fails is twice the breakout width. The
symbols illustrate the number of days it took to drill the respective well, which is a measure of
drilling problems associated with wellbore stability. The asterisks indicate wells that were not
problematic (30 days). The color scale ranges from
an acceptable breakout width (70 ◦ , blue) in which less than half the wellbore circumference fails to
an excessive amount failure corresponding to over half the circumference failing (breakout widths
over 100 ◦ , dark red). Reprinted with permission of Elsevier.
Another illustration of the importance of well trajectory on wellbore stability is
shown in Figure 10.7 where we compare wellbore stability predictions based on the
derived stress state to actual drilling experience in a field in a sub-Andean foreland
basin in northwestern South America (Zoback, Barton et al. 2003). We divided the
wells in the field into three categories depending on the time needed to drill each well:
wells that were drilled in less than 20 days are considered not to be problematic; wells
that took more than 20 days are considered to be problematic and those that required
more than 30 days are considered to be quite problematic. In Figure 10.7 we compare
the predicted failure width and the drilling experience (drilling time) as a function of
drilling direction for all wells in the field.