Underbalanced Drilling (also called Managed Pressure Drilling) is the term most commonly used when drilling through hydrocarbon-bearing payzones with a circulating system that exerts a hydrostatic pressure less than or equal to the reservoir pressure. Depending on the application, the circulating system can be identical to that used for gas- (air, nitrogen, natural gas), mist-, foam-, or an aerated or nitrified fluid drilling application. However considerable precautions need to be taken since the underbalanced downhole conditions will result in flows of oil and/or gas into the wellbore.
Underbalanced Drilling is gaining increased popularity as operators realize how wells drilled underbalanced result in a marked reduction or even elimination of formation damage. This increases the production from these wells, resulting in faster payback and earlier positive cash flow. The main advantages of Underbalanced Drilling are as follows: