Page 221 - Reservoir Geomechanics

202 Reservoir geomechanics

virtual excess mud pressure is introduced that would tend to stabilize the wellbore
whereas if A m > A p ,a virtual underbalance results which would be destablilizing
(Mody and Hale 1993).
In Figure 6.18a, the portion of the wellbore wall that goes into failure is shown as a
function of membrane efficiency and water activity of the mud. As discussed at some
length in Chapter 10,a convenient rule-of-thumb is that if less than half the wellbore
wall goes into failure, wellbore stability will not be particularly problematic. As can
be seen, for very high A m (low mud salinity), the wellbore is very unstable, regardless
of membrane efficiency. At intermediate values of A m , increasing membrane efficiency
can dramatically improve wellbore stability. Another way of saying this is that where
A m < A p , restricting ion transfer enhances wellbore stability. It should be pointed out
that a possible consequence of excessive mud salinity is desiccation and fracturing of
the shale in the borehole wall. This has the potential to mechanically weaken the shale
although the importance of this effect is controversial.
Figure 6.18b illustrates the fact that because the effect of chemical interactions
between drilling mud and shaley formations effectively weakens the rock, one can
sometimes use mud weight to offset the effect of weakening (although this effect may
diminish over time due to chemo-poroelastic processes). For example, for A m = 0.7, a
◦
mud weight of 11 ppg results in breakout widths of 100 .As this results in more than
half the wellbore circumference failing, it would result in a relatively unstable wellbore.
Raising the mud weight to 11.5 ppg reduces breakout widths to about 60 , thus resulting
◦
in a much more stable wellbore. A similar result could have been achieved with 11 ppg
mud by lowering A m to 0.67. Figure 6.18c shows what happens to the zone of wellbore
failure when A m = 0.5 and A w = 0.88 for a membrane efficiency of 0.1. When the mud
is much more saline than the formation fluid, the wellbore actually becomes more stable
with time as indicated by the strength of the rock required to avoid failure as a function
of distance from the wellbore wall and time. This is because the saline mud actually
causes pore pressure to decrease in the wellbore wall as a consequence of induced fluid
flow from the formation into the wellbore.
Comprehensive discussion of chemical effects on wellbore stability is beyond the
scope of this book. However, it needs to be remembered that the time dependence
of ion exchange is not considered in the calculations shown (i.e. ion exchange is only
consideredinthecontextofmembraneefficiency).Anotherissueaffectingionexchange
is the physical size of the ions involved such that the rate of ion exchange is slower for
large ions (such as K and Ca ++ ) than small ions (such as Na and Mg ++ )(Van Oort,
+
+
Hale et al. 1995).

Multiple modes of breakout formation
In the discussion of breakouts so far in this chapter, I have focused on the case when
σ 11 = σ θθ , σ zz = σ 22 and σ 33 = σ rr but this is not the only case of compressive