Page 269 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 245

2.5.11
Drilling Fluids Program [ ]

has been justified where real cases of formation damage have been
experienced. In general, formation damage caused by an oil mud is not
at all common. Possible causes of formation damage specific to oil
muds are described below.
Oil wetting. In theory, the emulsifiers and oil-wetting agents that
will oil wet droplets of brine to form an invert oil emulsion will also oil
wet a sandstone reservoir. Oil wetting will destroy the layer of water on
the sand grains of the reservoir over which the crude oil flows. It will
result in decreased relative permeability to oil. Consequently, mobility
of crude oil flow is reduced. (See Section 2.5.6.)
This type of damage is usually very localized because of the low fil-
tration rate of oil muds into the formation. Usually penetration of fil-
trate is a few inches or at least within the distance that can be perfo-
rated out. Obviously, running a low, tightly controlled filtration rate
and not having high differential pressures will minimize filtrate inva-
sion of the reservoir. The use of a bridging agent that can be back
flushed will also be beneficial.
As well as the action of the emulsifier, wettability might also be
changed by an increase in oil saturation if there is a substantial inva-
sion of oil filtrate. Low-permeability reservoirs would be more likely to
be affected. This type of damage is not so likely if the filtrate of the oil
mud is kept very low (which is usually the case). However, in the past,
before the advent of PDC bits, oil muds usually drilled slower than
water-based muds. Engineering the oil mud to have a high filtration
could enhance penetration rate with an oil mud. Oil wetting of the
reservoir might have been more likely with this type of mud.
Oil muds with high filtration rates are no longer used. Advances
in bit technology now provide all the penetration that is desired.
A “relaxed” filtration rate will have the only effect of increasing
dilution costs.
Emulsion blockage. The filtrate of an oil mud is normally oil with
some dissolved emulsifiers. This filtrate if mixed with formation water
might form an emulsion in the formation. Such an emulsion would
have an increased viscosity and thus impair the mobility of crude oil.
Unreacted or partially soluble emulsifier. If emulsifier present in
oil mud filtrate were to become insoluble, it could cause damage by
blocking pore throats. This was more likely to occur with chemical
change of soap-type emulsifiers. With advances in emulsifier chem-
istry, this type of damage is not likely in most cases.

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