Underbalanced drilling is a procedure used to drill oil and gas wells where the pressure in the wellbore is kept lower than the fluid pressure in the formation being drilled. As the well is being drilled, formation fluid flows into the wellbore and up to the surface. This is the opposite of the usual situation, where the wellbore is kept at a pressure above the formation to prevent formation fluid entering the well. In such a conventional "overbalanced" well, the invasion of fluid is considered a kick, and if the well is not shut-in it can lead to a blowout, a dangerous situation. In underbalanced drilling, however, there is a "rotating head" at the surface - essentially a seal that diverts produced fluids to a separator while allowing the drill string to continue rotating.
Advantages of running the Underbalanced Drilling System:
Eliminated formation damage from invasion
Increased Rate of Penetration (ROP). With less pressure at the bottom of the wellbore, it is easier for the drill bit to cut and remove rock
Reduction of loss circulation
Differential sticking is eliminated
Increases reservoir knowledge
Disadvantages of running the Underbalanced Drilling System:
Increased drilling costs
Possible wellbore stability problems
Compatibility with conventional MWD systems
Generally higher risk with more inherent problems
Possible excessive borehole
There is a way to mitigate the risk when it comes to openhole logging in underbalanced drilling scenarios. Cordax's Logging While Tripping (LWT) is a patented formation evaluation technique in which open hole logs are acquired safely in horizontal and hostile hole conditions that you would experience in underbalanced drilling scenarios.