STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

680 Drilling and Well Completions
organic filtration control agent should be added. One of the more important
requisites for proper mud misting is foaming agent. The exact amount to be
added depends on the particular foamer used, as most different brands have
different amounts of active materials. Since air is the lifting medium in mist
drilling fluid, the sufficient air velocity in the annulus should be from 2,000 to
3,000 ft/min. The approximate mud or water pumping rate is 10 bbl/hr.
Foam Drilling Fluids
Foam is gas-liquid dispersion in which the liquid is the continuous phase and
the gas is the discontinuous phase. The first use of foam in drilling was reported
in 1964.
Foam has been successfully used as a drilling fluid in several geological
conditions.
1. In air drilling areas, the use of air drilling technique can be prolonged
when formation water enters the hole by adding a small stream of liquid surfactant
to the air stream. The addition of surfactant forms foam at the contact
with formation water. The foam carries out cuttings and produced water.
Considerable volumes of formation water can be held using this technique.
2. In hard rock drilling areas with loss of circulation, the application of
preformed (mixed at the surface) stable foam shows four to ten times
higher penetration rate than clay-based muds.
3. In oil-producing formations with high fluid loss, drilling in with foam and
foam completion proves beneficial. Usually, these formations cannot stand
a column of water-so it is impossible to establish returns with conventional
mud. The use of foam for drilling in and completion results in substantial
increases in production.
Stable foam systems consists of a detergent, freshwater, and compressed air.
Gel-foam system includes bentonite added to the water-detergent mix. Additives
may be included in the mixture for special purposes. To be used effectively as
a circulating medium, foam must be preformed. That is, it must be generated
without contact with the solid and liquid contaminants naturally encountered
in the well. Once formed, foam systems have stabilizing characteristics that
make them resistant to well-bore contaminants. Foam should have a gas-toliquid
volume ratio from 3-50 ft3/gal depending on downhole requirements.
The water-detergent solution that is mixed with gas to form foam can be
prepared using a wide range of organic foaming agents (0.1-1.0 parts of
foaming agent per 100 parts of solution). Foams can be prepared with densities
as low as 0.26 lb/gal. Viscosity can be varied so high lifting capacities
result when circulating at 300 fpm annular velocity. BHP measurements
have indicated actual pressures of 15 psi at 1000 ft and 50 psi at 2,900 psi
while circulating.
Drilling Fluid Additives
The classification of drilling fluid additives is based on the definitions of the
International Association of Drilling Contractors [30].
a. Alkalinity or pH control additives are products designed to control the
degree of acidity or alkalinity of a drilling fluid. These additives include
lime, caustic soda, and bicarbonate of soda.