Page 257 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 233

2.5.8
Drilling Fluids Program [ ]

Conventional wisdom says that adding small amounts of oil also
reduces density and provides some shale inhibition. Adding some oil
does initially decrease density but this reduces the efficiency of the
hydrocyclones and can lead to an increase in solids content, affecting
rheology and ultimately allowing density to increase again unless the
oil percentage is high. As far as shale stabilization is concerned, there
is no evidence to support the common assertion that an oil in water
emulsion will have any stabilizing effect whatsoever. In addition, as
water is the continuous phase the oil does not prevent clay dispersion.
Invert emulsion oil muds. In this case, water, or dominantly brine,
is emulsified in oil (or other hydrocarbon liquids). This is the most
common type of oil mud that is used in drilling. The brine is emulsified
in the oil as minute droplets by the presence of cationic emulsifiers.
Oil-base mud. This term, according to convention, has covered
fluids where there is no brine emulsified in the continuous phase of oil
or where it is present in quantities of less than 10%. Another term that
might be used is “all oil systems.” Today, the terms oil mud, oil-base
mud, etc., are interchangeable and really define fluids where the con-
tinuous phase is oil and if an emulsion is present, it is as an invert
emulsion. The presence of a water phase in an oil mud allows for ver-
satility in control of parameters such as rheology.

2.5.8. Components of Invert Oil Emulsion Muds

The oil (continuous) phase. This is the main component of the
system and often called “base oil.” It may be made up from (in histor-
ical order of usage):
Crude oil. This was the first continuous phase to be used but sel-
dom now in use because of environmental concerns, potential elas-
tomer solvency, high-inherent viscosities, and high flammability.
Diesel oil. This was most commonly used until the early 80s when
low aromatic oils began to appear. It is still used in some cases, partic-
ularly when it is regulated that oil mud cuttings have to be processed
at a disposal or treatment site. It is less expensive and often logistical-
ly more convenient than low-aromatic mineral oil. Because the kine-
matic viscosities of diesel (and crude oils) are higher than most low
aromatic mineral oils, they will demonstrate, in the oil mud, corre-
spondingly higher plastic viscosities.

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