1. adj. [Drilling]

Referring to a situation when the pressure (or force per unit area) exerted on a formation exposed in a wellbore is less than the internal fluid pressure of that formation. If sufficient porosity and permeability exist, formation fluids enter the wellbore. The drilling rate typically increases as an underbalanced condition is approached.

Antonyms: overbalance

See related terms: coiled tubing drilling,  hydrostatic pressure,  kill