Air Drilling

Aerated-Fluid Drilling


The aeration of fluid with compressed air or nitrogen
is used to lower the weight of the column of fluid on
the formation and to reduce the potential for lost circulation, without changing the properties of the
drilling fluid. A smooth transition can be made from a foam system to an aerated system by minimizing or eliminating the surfactant and increasing the Liquid Volume Fraction (LVF). Combining the advantages of
a conventional drilling fluid with air drilling techniques, aerated-fluid drilling is best suited for use in highly unstable formations where fluid loss is a concern.
This method has the slowest penetration rate of the underbalanced techniques.

Aerated-fluid systems can be the most corrosive of all reduced-pressure drilling methods. With the proper selection of supply water, pH control, and use of Total Separation Solutions corrosion inhibitors, aerated fluid
systems have been successfully used worldwide.

Air-Gas Drilling Techniques

There are four general types of air drilling techniques, each with a specific purpose for drilling underbalanced reservoirs:

• Dry air-gas (dusting)
• Mist
• Foam or stiff foam
• Aerated fluid

By choosing the right technique, the operator can successfully drill into and beyond low-pressure formations. Reduced pressure systems provide fluid densities ranging from near 0 to 7 ppg. These low-density fluids are ideally suited for a variety of specialized drilling operations.

Dry Air-Gas Drilling


This technique is commonly called "dusting” and is
used for drilling dry formations or where any water
influx is slight enough to be absorbed by the air-gas stream. Drilling through unstable formations with straight air is not advised, because air cannot carry wellbore stability additives. When water-saturated formations are encountered, problems occur:

• Wet cuttings form slugs that can plug pipe
• Intermittent passing of slugs effects surface 
   equipment
• Fluid build-up in the annulus can wet water-
   sensitive shales
• Borehole walls can become unstable

As the water influx increases, a number of warning
signs are observable at the surface:

• Loss of returns
• Pressure buildup
• Slugs of fluid at the blooie line

If the hole cannot be dried, mist drilling should be used.