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. (References and illustrations at end ofpaper.) 135Reduced 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.Underbalanced Drilling... SPE 27351Essentially, there are three ways to affect the drag in the well: