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.

What is overbalance pressure?

Overbalance pressure is the amount by which the hydrostatic pressure exceeds the pore pressure of a formation.

Why are drills underbalanced?

Increased reservoir knowledge

During an underbalanced drilling operation, reservoir productivity and the produced fluids can be measured and analyzed while drilling. This allows a well to be drilled longer or shorter, depending on production requirements.