Underbalanced Drilling
Underbalanced drilling, or UBD, is the process of drilling a well by maintaining a hydrostatic head in the wellbore intentionally lower than the fluid pressure in the formation. The difference between the fluid pressure in the formation and the hydrostatic pressure in the well is called 'Underbalance'. The hydrostatic head can either be naturally under balance or by injecting natural gas, nitrogen, or air. If there is enough porosity and permeability in the rock, the formation fluid flows into the wellbore and up to the surface while drilling.
Underbalanced conditions do not allow filter cake build-up and also avoid formation damage due to the invasion of drilling mud and solids into the formation. Drilling rates in Underbalanced Drilling are typically high. In UBD, the influx of formation fluids must be controlled to avoid well-control problems. With the well flowing, the Blow Out Preventor (BOP) system is kept closed while drilling by using a rotating head to ensure maintain control of the well.