Because underbalanced drilling creates a natural tendency for fluid to flow from the formation into the borehole, successful underbalanced drilling depends on appropriate selection of circulating fluid. Under these conditions, use of conventional mud systems often results in lost circulation, formation damage, high mud costs and a need for expensive completions. Use of compressible fluids, on the other hand, can inhibit or eliminate many of the problems associated with drilling in environments in which formation damage is likely.

Use of a compressible fluid in the circulating system, referred to as air drilling, lowers the downhole fluid pressure, allowing drilling into formations where loss of circulation and damage to productive formations are problems of major concern.

Other advantages to air drilling include increased penetration rates, improved drill bit performance, and contamination-free drill solids for ready detection of hydrocarbons.

Reduced pressure air drilling techniques include not only gas continuous phase methods and use of dry gas and gas mist, but also gas internal systems with stable foams and aerated fluids.

This discussion of problems related to underbalanced drilling addresses types of drilling equipment used, and provides an overview of experience gained from both successful and unsuccessful wells.


The concept of using compressible fluids (i. e., gases) as a drilling media to remove cuttings from a drilled hole was first recorded in a United States patent issued in 1866. Since then science and technology have transformed this idea into a sophisticated industry of specialized drilling techniques.

Reduced pressure drilling techniques involve using compressed gas, (most commonly, atmospheric air), as the circulating fluid. Depending on specific drilling conditions, this gas may be used alone, or in conjunction with water and other additives.

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