Page 271 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 247

2.5.12
Drilling Fluids Program [ ]

Mist drilling. Similar to air drilling, mist drilling is an option when
small amounts of water are produced by the well. Increased air vol-
umes are needed and a mist pump is used to inject small quantities of
a water foaming agent solution. The benefits of mist drilling are simi-
lar to air drilling, but at a higher cost.
Stable foam drilling. This is the most versatile of the air-assisted
drilling techniques. The liquid is a continuous phase and contains
encapsulated air bubbles within it. The percentage of liquid will vary
between 2% and 15% by volume. An annular back pressure valve is
used which, with a varying liquid fraction, allows a variable bottom
hole gradient of between 0.026 psi/ft (0.5 ppg) and 0.312 psi/ft (6.0
ppg). The lifting capacity of stable foam is superior to that of drilling
muds and it is possible to displace fluids from the hole using foam.
Advantages of foam are similar to those for air and mist drilling.
Oil and salt water influxes are likely to destroy the foam stability, pre-
cluding the use of foam in those conditions.
Stiff foam drilling. Stiff foam is a very stable emulsion of air in
mud. It is formed by injecting air into a thin mud slurry, with air-mud
ratios of between 100:1 to 300:1. There are several formulations for the
mud slurry, depending on what fluids will be encountered in the well-
bore. The foam can be tailored to resist salt water or oil in small quan-
tities. High volumes of water or oil produced will cause problems.
This type of system may be used where the air-assisted tech-
niques mentioned above are not suitable. Again, specialist advice
should be sought.

Particular applications that may be suitable are:

1. Large diameter holes where air requirements for dust or mist
drilling would be excessive. Air requirements for a 26 in diameter
hole using stiff foam are only 400 to 600 cubic feet per minute
(CFM).
2. Drilling unconsolidated formations. Stiff foam can stabilize this
kind of formation.
3. Preventing downhole fires, which sometimes happens when
drilling oil zones with air.

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