Underbalanced drilling is a procedure used to drill oil and gas wells where the pressure in the wellbore is kept lower than the fluid pressure in the formation being drilled. As the well is being drilled, formation fluid flows into the wellbore and up to the surface. This is the opposite of the usual situation, where the wellbore is kept at a pressure above the formation to prevent formation fluid entering the well.
If the formation pressure is relatively high, using a lower density mud will reduce the well bore pressure below the pore pressure of the formation. Sometimes nitrogen gas is injected into the drilling mud to reduce its equivalent density and hence its hydrostatic pressure throughout the well depth.
Compressors and high pressure boosters are used to inject high pressure air in the wellbore for air underbalanced drilling
NCA products used in oil & gas production drilling: