Underbalance Perforation
Underbalance perforation is the process of perforating the well with hydrostatic pressure inside the casing or liner less than the reservoir pressure. This has a distinct advantage that soon after the perforation, reservoir fluid tends to flow into the wellbore. In the opposite scenario of pressure inside casing or line being more than reservoir pressure, the wellbore fluid tends to flow into the perforation tunnel and could damage the reservoir.
Perforating underbalance without appropriate pressure control equipment has a risk of developing into a well control situation. When using Tubing Conveyed Perforation (TCP), it is possible to reduce the hydrostatic pressure inside tubing by displacing part of the tubing to lighter fluid or Nitrogen before perforation. Pressure can be safely contained and observed on surface soon after perforation.