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Chapter 7 – DRILLING FLUIDS 171

problems. This can be addressed by using either salt-saturated water mud
or an oil-based mud.
In complex salt sequences containing the most soluble potassium and
magnesium salts, a mixed salt system is required to address the particular
mix of salts present in the formation.

Reservoir damage. Mud filtrate can be extremely damaging to

formation fluids. There are two areas of particular concern: pollution of

water sources and reduced productivity of the pay zone. There are two

approaches in these cases, and they may be used together. The first is to
prevent filtrate invasion by using additives to plug off pore throats where

the formation is exposed. The second approach is to use a mud that has a

nondamaging filtrate. In the pay zone, productivity damage from filtrate

may occur in several ways:

1. The filtrate may contain fines (small solid particles) that bridge
off the zone of invasion. If this zone is deep, perforations may
not be able to penetrate completely through, and so the well will
be less productive. If the fines were acid soluble, acid treatments

may remedy the situation partially or completely. However,
weighting agents and drilled solids are usually not acid soluble, so

fluid loss control becomes very important in the production hole
section. Acid-soluble materials (such as calcium carbonate) may
be used.

2. Chemical reaction between filtrate and formation fluids may

produce solid precipitates or blocking emulsions. As noted
above, if these are acid insoluble, the resulting damage may
be permanent.

3. The filtrate may react with the clays within the formation. Oil-
based muds should give only low amounts of oil filtrate; no water

should be present.
Corrosion of downhole steel components. Tools and tubulars used
in drilling, casing, and completing the well can be subject to corrosion
by the mud. For most casing strings, mud is left in the annulus after the
cement job, which will remain for the life of the well. Mud properties may
change over time due to bacteriological action. This can produce H S
2
(especially when the mud contains organic additives) or low pH levels. Oil
muds produce oil wetting of metal surfaces and will protect against CO ,
H S, and H O corrosion. 2
2
2

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