Page 224 - Reservoir Geomechanics

205 Compressive and tensile failures in vertical wells

of wells world-wide with independent stress observations (as illustrated in Chapters
1 and 9) demonstrate that conventional breakout formation (mode B) is the dominant
mode of compressive wellbore failure. When wellbore stability is addressed in Chapter
10, the Z mode of failure is included in the failure analysis.

Penetration of mud into fractured rock surrounding a wellbore
As a final note on the factors affecting the formation of wellbore breakouts, it is impor-
tant to emphasize the fact that the ability for mud weight to stabilize a wellbore is based
on the precondition that the wellbore wall is impermeable. Hence, σ rr is equal to the
difference between the pressure in the well and that in the formation (equation 6.5) and
mud weight in excess of pore pressure tend to decrease the zone of failure around a
wellbore. This was discussed conceptually in Figure 4.5 and illustrated quantitatively
by the difference of the width of the failure zones in Figures 6.3 and 6.5. When drilling
through fractured rock, penetration of the mud pressure into rock over time could poten-
tially result in time-dependent wellbore instability due to two processes. First, to the
degree that pressure tends to equalize around the wellbore and
P decreases, σ rr will
decrease and σ θθ will increase leading to increased instability of the wellbore wall.
Paul and Zoback (2006) discuss this process in the context of time-dependent wellbore
failure of the SAFOD scientific research borehole that was drilled through the San
Andreas fault in 2005. This is discussed at further length in Chapter 10.
A second process that can lead to enhanced wellbore failure if there is an increase in
pressure surrounding a wellbore due to penetration of the drilling mud, results from a
temporary underbalance of wellbore pressure. This can occur when the pressure within
the wellbore drops suddenly, either when the mud pumps are shut off or when the
drillpipe and bottom-hole assembly are tripped out of the well. As noted above, under-
balanced conditions tend to exacerbate wellbore failure. To avoid problems associated
with fluid penetration into the formation surrounding a wellbore one needs to consider
utilizing various additives in the drilling mud (in effect, lost circulation materials) to
stop penetration of the drilling mud into the region surrounding the wellbore in fractured
formations.