Overview

Underbalanced drilling is defined as drilling operations where the drilling fluid pressure in the borehole is less than the pore pressure in the formation rock in the open-hole section. By using light drilling fluids, the driller intentionally keeps the borehole pressure from balancing the formation pore fluid pressure. The light fluids used in underbalanced drilling are usually air, gas, foam, and aerated water. However, unaerated oil, water, and even weighted mud can be used for underbalanced drilling in areas where formation pore pressure gradients are higher than the hydrostatic pressure gradient of water.

Underbalanced drilling technology is an integrated technology, resulting from merging traditional air and gas drilling techniques with aerated and foam drilling methods developed during the past 15 years. The compressor is a key equipment required for providing pressurized gas in underbalanced drilling. Portable compressors were first utilized in the late 1880s in the mining industry to drill in-mine pneumatic percussion boreholes.1 It is not known when the air drilling technique was first used to drill water wells, but we know that deep petroleum and natural wells were drilled utilizing portable air compressors in the 1920s.2 The more popular use of air as a circulating drilling fluid began in the early 1950s.3 By the late 1970s, it was estimated that air and gas technology was being used on about 10% of the deep wells drilled and completed.4 ^,5

Aerated drilling fluids have been used to drill boreholes since the 1930s.6 Stable foam systems were first used...