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224 Part III Underbalanced Drilling Systems

agent is employed. A typical formula for 1 barrel of a stable foam slurry
should include the following:
• 1 bbl of freshwater
• ½–2 gallons of foaming agent
• ¼ gallon of corrosion inhibitor
• 4 lbs of lime

A typical formula for 1 barrel of a stiff foam slurry should include the
following:

• 12–14 lbs of bentonite
• 1–1.5 lbs of soda ash
• ½–¾ lb of CMC
• 0.4–0.8 gallons (0.5 to 2% by volume) foaming agent

10.3.2 Foam Stability Control
It has been observed in laboratories that foams are stable when their gas
contents are between 55% and 97.5% (Sanghani, 1982). The gas content
is also called the foam quality index or simply the foam quality in foam
drilling. It is defined as
Γ = Gas Volume (10.2)
Total Foam Volume

or
4:07T
Q go
Γ = P (10.3)
4:07T Q go + 1 Q l + 5:615
P 7:48 60 Q fx
2
where T = temperature in °R; P = pressure in lb/ft ; Q go = gas injection
rate in scfm; Q l = liquid injection rate in gpm; and Q fx = formation fluid
influx rate in bbl/hour. Apparently, foam quality drops as the pressure
increases with depth.
Foam drilling operations are usually designed with the maximum foam
quality at the top hole being equal to 0.95 and the minimum foam quality
at the bottomhole being equal to 0.60. However, these conditions are not
maintained when the depth is beyond 5,000 ft. For deep drilling
operations with foams, it is vitally important to use a high enough liquid
injection rate for hole cleaning purposes because foam stability is not guar-
anteed in the lower section of the annulus. Guo and colleagues (2003)