Page 259 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 235

2.5.8
Drilling Fluids Program [ ]

Some very brief generalizations for this group are:

1. It is easier for anaerobic bacteria to break down stabilized esters
than ethers because the chemical bonds are different.
2. Ether systems have higher temperature stability than ester systems.
3. Nonmineral oil and synthetic oil systems are substantially more
expensive in varying degrees than oil mud systems based upon
mineral oils. The economic viability of using such systems depends
on the following factors, at least:
a) An operation with a high total daily operational cost
b) Depth/time curve will be greatly reduced by using an oil mud
c) The use of any other form of oil mud would be banned
4. Although these systems are biodegradable in seawater, the oil mud
system can be reused on multiwell programs.
5. In some systems, care must be taken to avoid hydrolysis caused by
too alkaline or acid an environment.

The brine (or water) phase. This is the other liquid phase in an
invert emulsion oil mud and will be discontinuous. Although it is usu-
ally present as a brine, it is described as the water phase as it is quan-
tified by volume on the basis of the percentage of water present in the
total volume of the oil mud. It consists of a salt, normally calcium chlo-
ride but in some cases sodium chloride, dissolved in water. This brine
will be present in the form of droplets, sub-micron to a few microns in
size and surrounded by emulsifier.
The brine phase of an invert oil emulsion mud has two functions.
It allows for flexible control of certain fluid parameters. It also gener-
ates an osmotic force to initiate water transport from shales to the fluid
or at the least prevent water penetration of the bore hole wall.
Properties of the brine phase—osmosis. The emulsified brine will
not be completely isolated from the formation because the layer of
emulsifiers around each droplet of brine can act as a semi-permeable
osmotic membrane. If an osmotic gradient is set up between the brine
emulsified in the oil and formation water, it is possible for water trans-
port from the formation to occur. The osmotic gradient will depend on
the concentration of the salt(s) in the brine. This concentration is
known as the water phase salinity (WPS). It can be quantified by chem-

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