BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Mist Drilling
To overcome some of the problems associated with water encroachment while air drilling, the
technique of injecting a small amount of water containing a foaming agent into the air stream was
devised. The foaming agent aids in trapping water as a mist in the circulated air stream, thereby
removing it from the hole. Hence, the name mist drilling. Air is still the external phase of this
dispersion and principal lift medium. Mist drilling does not overcome the problem of waterwetting
the shale formation and, in sensitive formations, this condition may result in hole
instability. Formation water may also enter the hole at such a rate that mist drilling becomes
impractical.
Aerated Fluid
Where air or mist drilling cannot be used effectively, air is sometimes injected into the drilling
fluid, creating an aerated condition. This technique reduces the hydrostatic head sufficiently to
allow fluid circulation. Otherwise, it would be lost. An aerated fluid system requires high
volumes of both air and fluid. It is often difficult to maintain satisfactory air-fluid dispersion on
connections or during any extended period of shutdown.
Stiff Foam
To overcome some of the difficulties with aerated fluids, special fluid formulations have been
developed to maintain more stable air-fluid dispersions. This has enabled the industry to prepare
lighter fluids with excellent hole cleaning abilities. Though some operators consider the addition
of a foaming agent to water alone to be sufficient, the normal practice is to add a viscosifier and
film strengthening agent. This latter practice produces foam with shaving cream consistency and
is referred to as stiff foam drilling. A typical fluid slurry composition for a stiff foam preparation
in fresh water is as follows.
Table 3-15
Typical Formulation for Air-Fluid Dispersions
SUPER-COL ®
XAN-PLEX ® D
caustic soda
6 lbm/bbl
¼ to ½ lbm/bbl
½ lbm/bbl
The amount of foaming agent depends on the volume of water invasion from the hole. The more
water coming in, the more foaming agent required. The intrusion of oil has a dampening effect
on foam. When oil is encountered, the addition of a high-molecular polyacrylate such as
CYPAN ® may be used.
In some cases, it may be desirable to use brackish or saline water for slurry make-up. AMPLI-
FOAM TM should be used as the foaming agent with this type of water because it will be more
effective in this environment.
As mentioned earlier, air-fluid dispersions require special rig equipment. Requirements for stiff
foam drilling are listed below:
• Air compressor capability of 800 to 1000 ft 3 /min.
• Rotating head
• Blooie line – length recommended of 100 to 150 ft., size 6 to 10 in diameter.
• Two injection pumps – triplex capable of injecting fluid rates up to 55 to 60 bbl/hr.
• Air charts for constant recording of ft 3 /min.
• Meter for determining gallons required.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-47