Underbalanced drilling is defined as drilling with the hydrostatic pressure of the fluid intentionally less than that of the formation pore pressure. The IADC classifies UBD wells according to the drilling fluid used to achieve underbalanced conditions.
IADC UBD Well Classification by Fluid Design
Gas Drilling - using only gas as the drilling medium. No intentional fluid added.
Mist Drilling - drilling with liquid entrained in a continuous gaseous phase. Typical, < 2.5% liquid content.
Foam Drilling - drilling with a two phase fluid with a continuous liquid phase generated from the addition of liquid, surfactant, and gas. Typical foams range from 55% to 97.5% gas.
Gasified Liquid Drilling - drilling with a gas entrained in a liquid phase.
Liquid Drilling - drilling with a single liquid phase.
For well classifications 1 - 4, gas is used to reduce the hydrostatic head of the fluid column to the desired value. Gas accumulations due to separation from the 2-phase state can at times accumulate at the surface of the well and may require removal via a lubrication technique. The reason may be to kill the well prior to tripping or, that the gas cannot be circulated from the well in a conventional sense.
Gas accumulated beneath the BOPs can represent a potential safety risk and may need to be "lubricated" from the well. The "volume" method of lubrication is most often used for this purpose but a simple and little known procedure ("pressure" method) should be used in many instances. Either method has application and limitations but the pressure method is the only accurate technique for wells experiencing downhole fluid losses.