underbalanced

Underbalanced drilling (UBD) is a drilling technique in which the wellbore pressure during drilling is intentionally kept below the formation pore pressure. The resulting influx of formation fluids into the wellbore, or controlled gas influx, is allowed and managed rather than being prevented by the hydrostatic pressure of the drilling mud. The approach contrasts with conventional overbalanced drilling, where wellbore pressure exceeds formation pressure to stabilize the well.

UBD employs low-density drilling fluids, aerated or foamed mud, or the use of inert gas such as nitrogen to reduce hydrostatic pressure. Techniques include aerated mud, foamed drilling, and gas-assisted systems to maintain underbalanced conditions, along with surface or downhole pressure-control devices such as rotating heads and diverters to handle kicks.

UBD aims to minimize formation damage caused by mud filtrate invasion, preserve reservoir properties, and improve penetration rates and early production by avoiding mud invasion.

Disadvantages include higher risk of kicks and blowouts if not properly controlled, potential wellbore instability and lost circulation, and safety hazards from gas influx; it requires specialized equipment, trained personnel, and careful pressure management. The approach can be more costly and complex than conventional drilling and requires robust kick detection and containment capabilities.

UBD is typically used in reservoirs with high pore pressure relative to fracture gradient or in offshore or depleted formations where minimizing damage and preserving permeability is important. Not all wells are suitable for underbalanced conditions; success depends on accurate formation pressure data, appropriate surface containment, and reliable downhole pressure control.