The use of air or gas as a circulating medium was introduced in the early 1950's. Even though initial attempts were crude, significant increases in penetration rate and bit life were obtained. Since these initial attempts, development of air and gas drilling techniques have expanded and are widely accepted today as a method to reduce drilling times and cut cost of many wells. Along with the time and resultant dollar savings, other advantages such as immediate and continuous hydrocarbon detection, minimum damage to liquid sensitive pay zones, better control of lost circulation, and cleaner cores are obtained.

Today's air drilling technology is attributed to many drilling people whose initiative and accumulative experience have refined the method and determined situations where the technique is most applicable. The lack of understanding, rather than experience, is often the reason for not accepting air drilling. Drilling with air does involve special consideration in the use of equipment and drilling techniques that are not commonly encountered with other drilling media. For example, air, unlike fluids, compresses readily and requires a somewhat more sophisticated engineering approach to achieve the desired results.

This paper discusses the mechanics of air drilling, modifications such as mist or foam drilling, unique equipment requirements, and downhole problems that have been encountered. Special attention is given to presenting techniques developed to prevent or control downhole problems.

Mechanics of Air Drilling

Air is the ultimate low density drilling media. In order to achieve optimum results and greatest economy from air drilling, there are several factors which should be considered. Hard formations which are dry or produce relatively few formation liquids provide the best results while air drilling. When the formation is completely dry, or the influx of liquids is slight enough to be absorbed in the air stream, the drill cuttings return to the surface in the form of dust. Also, this allows for immediate and continuous evaluation of hydrocarbons.

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