Underbalanced drilling vs Conventional drilling

Underbalanced drilling, or UBD, 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.

In conventional, or “Overbalanced”, drilling fluid is pumped into the well shaft at pressure higher than in the reservoir. This keeps the oil and gas in the reservoir during drilling, but the high pressure can damage the rock around the wellbore.