Abstract
A pressure overbalance during conventional drilling can cause significant fluid filtrate invasion and lost circulation. Fluid invasion into the formation can lead to formation damage, high mud costs, a need for expensive completions, and well productivity impairment. Because underbalanced drilling creates a natural tendency for fluid and gas to flow from the formation to the borehole, successful underbalanced drilling depends upon the appropriate selection of circulating fluid. The use of a compressible fluid in the circulating system, referred to as air drilling, lowers the downhole pressure, allowing drilling into and beyond these sensitive formations. The paper discusses the equipment needed; well control; downhole air requirements; air drilling techniques using dry air, air-mist, stable foam, stiff foam, and aerated-fluid; downhole fires; directional air drilling; and well completions.
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