Custom-Engineered Solutions
for Underbalanced Drilling

Mist Drilling


Similar to dry air drilling, this system relies on the annular velocity of the air-gas to transport cuttings
out of the hole. Air-mist drilling is used when the amount of water is high enough to preclude air-dust drilling, but not so high as to cause hole cleaning problems. A pretreated drilling fluid is injected with the air-gas, and the combination returns to the surface
as a mist. A small quantity of water containing a foaming agent (surfactant) is injected into the air-gas stream at the surface. The water mist is carried in a continuous air-gas system. A practical definition of
mist drilling is “the quantity of water being added is
just enough to saturate the air-gas so that the water influx encountered is carried out as droplets.”

The application of foaming agent (surfactant) reduces the interfacial tension of the water and cuttings in the hole and allows small water/cuttings droplets to be
dispersed as a fine mist in the returning air-gas stream. The cuttings and water are then removed from the hole without formation of mud rings and bit balling.

The proper soap/water ratio is very important in maintaining a continuous flow out of the well. Soap selection is influenced by many downhole conditions, such as: crude oil or salt water influx, extreme downhole temperature, influx of natural gas liquids (condensate) and overall system compatibility (anionic, cationic, amphoteric). Poor surfactant performance is often over-looked and problems are associated with other conditions.

Guidelines for mist drilling:

• Water injection volumes: 400-700 Gal/Hr
• Soap injection volumes: 1-5 Gal/Hr (0.1-0.5% by weight)
• Air volumes for mist tend to be less by 20-25% than for dry air-gas drilling
• Pressures generally run at 200-400 psi for mist, compared to 100-300 psi for dry
   air-gas drilling
• Insufficient air/soap/foam leads to cuttings carrying inefficiencies and slugging,
   with attendant pressure increases


Underbalanced Drilling

Air-Gas Drilling Techniques

Dry Air-Gas Drilling

Mist Drilling

Foam Drilling

Aerated-Fluid Drilling