What is an underbalanced reservoir?

• A. Pressure in the well is greater than the pressure in the reservoir.
• B. Pressure in the well is less than the pressure in the reservoir.
• C. Pressure in the well equals the reservoir pressure.
• D. Pressure in the reservoir is negligible.

Answer: (B)

An underbalanced condition means the pressure in the wellbore is lower than the pressure in the surrounding formation. When the reservoir pressure exceeds the wellbore pressure, fluids from the rock want to flow into the borehole, which is characteristic of an underbalanced situation. This setup can reduce formation damage from drilling fluids and improve drilling rate, but it also raises the risk of kicks or blowouts if not carefully controlled.