1. n. [Drilling] The amount of pressure (or force per unit area) in the wellbore that exceeds the pressure of fluids in the formation. This excess pressure is needed to prevent reservoir fluids (oil, gas, water) from entering the wellbore.

What is the difference between underbalanced and overbalanced drilling?

If hydrostatic pressure in a well is higher than the reservoir pressure, the difference is called overbalance pressure, or simply overbalance. Conversely, if reservoir pressure is more than hydrostatic pressure, the difference is called underbalance.

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.