Underbalance

Drilling

Underbalance refers to a state when the pressure exerted by the drilling mud is less than the pressure in the formation. In general, this should be avoided as it would allow the well to 'kick' and have formation fluid enter the wellbore.

The primary rule of well control is to always ensure the drilling fluid exerts more pressure than the formation, this known as overbalance. There are some special situations where underbalance is desirable, such as when perforating (to avoid damage and to clean the perforation tunnel).