Page 319 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 295

2.9.1
Drilling Problems--Avoidance Planning [ ]

If filtrate invasion is allowed to occur, then a tiny influx into the
pore spaces will cause an increase in pore pressure. This pressure can-
not dissipate far into the shale since permeability is extremely small,
therefore, it will build up in the near wellbore region until it equals the
total driving force pushing filtrate in. The extra pressure from the ECD
will further increase the local pore pressure as drilling continues.
Note that the API fluid loss value has no effect on filtrate invasion
of shales. The pore size is much smaller than the passages through fil-
ter cake (as previously noted) and in any case, filter cake will not build
up on a shale surface because the amount of filtrate moving into the
shale is too small.
The only ways to affect the level of filtrate invasion is either by
making the filtrate more viscous and/or by blocking the pore throats
with sufficiently small materials. See the material on inhibitive glycol
and silicate water-based muds in Section 2.5.5, “Nondispersed or
Polymer Water-Based Muds.”
Oil-based muds are so inhibitive because capillary forces prevent oil
filtrate from entering the pores with no local pore pressure change, and
of course if there is no water, there is no hydration. If mud can both pre-
vent hydration and give pore-pressure isolation, together with provid-
ing sufficient hydrostatic support against the in-situ rock stresses, then
the wellbore will be stable. However, even with oil-based muds, prob-
lems can occur. Many shales are fractured and, in this case, oil mud will
enter those fractures and cavings due to pressure fluctuations will
result. If shale cavings are seen on bottoms up after a trip with correct-
ly maintained oil mud, it is most likely due to fractured shales. With
fractured shales, the trick is to minimize trips, minimize mud density
(higher mud density will drive mud into the fractures), use good con-
nection practices, and use sized additives to block off those fractures as
they are exposed.
There are several chemical and physical forces tending to push fil-
trate into or away from the pores. The net resulting force will deter-
mine whether filtrate invasion may or may not occur; indeed it may be
possible to tailor these forces to give a net dehydrating effect by mov-
ing pore fluids out of the near wellbore region.
The forces to consider are:

Hydraulic—overbalance
Chemical—if the mud is able to form a semi-permeable membrane

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