Page 256 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 232

[ ] Well Programming
2.5.7

These are some but not all first principles in avoiding formation
damage. Some or most might be applied in exploration wells. To real-
ly be sure of minimizing the damage to a reservoir, nothing can
replace knowing the reservoir in depth. The reservoir conditions, the
connate water conditions, the mineralogy, the permeability, and the
porosity. There is no standard answer to this and the solutions are
highly reservoir specific.

2.5.7. Oil Muds

As described above, certain clay formations contain clay minerals,
which will readily hydrate, expand, and disperse when supplied with a
polar liquid such as water. The hydration of a clay is the reverse of the
process of diagenesis.
Oils and many other (but not all) hydrocarbon liquids are nonpo-
lar or have very low polarity. They will not have present dissociated
hydroxyl or hydrogen ions which can interact with a clay mineral and
cause it to destabilize. This fact is an attribute of the use of oil (or cer-
tain other hydrocarbon liquids) in certain drilling fluids. It is possible
to make the definition that the term oil mud is directed to oils or other
hydrocarbon liquids that are not miscible in water and (these oils)
comprise the continuous phase. Glycols and glycerols when employed
as a component of a drilling fluid could not be described in such a way
since they are miscible in water.
The use of oil in drilling fluids can be divided into three categories:
emulsion muds, invert emulsion oil muds, and oil-base mud. Only the
latter two are properly described as oil muds.
Emulsion muds. In this case, oil is emulsified into water or a brine
and the water will be the continuous phase. These types of muds are
often called straight emulsions. They are really water-base muds with
between 5 and 45% of oil emulsified through the water phase. Oil is
sometimes added to increase lubricity or assist in lowering fluid loss.
Droplets of oil are dispersed through the continuous phase (water)
and held in dispersion by anionic emulsifiers. In some cases in the past,
lignosulfonates were effective in emulsifying oil in water if the amount
of oil was not substantial. These muds are not at all common except for
some amounts of oil emulsified in a water-base mud. Those with high
oil contents are more likely to be used for specific cases such as
reduced density (below 1.0 specific gravity).

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