What does "underbalance drilling" refer to in drilling operations?

Underbalance drilling refers to a technique in drilling operations where the pressure exerted by the drilling fluid is less than the pressure in the formation being drilled. This approach is employed to enhance the efficiency of the drilling process, to reduce the risk of formation damage, and to improve the flow of hydrocarbons from the rock into the wellbore.

When the fluid density is less than the formation pressure, it promotes better penetration rates and can aid in wellbore stability. It also helps in controlling the influx of formation fluids, leading to improved well control and can prevent issues like loss of circulation. This method can also optimize the reservoir's production potential during the drilling phase, allowing for better recovery rates once the well is put into production.

In contrast, drilling with a fluid density greater than the formation pressure can lead to formation damage, and drilling with a fluid density equal to formation pressure does not provide the benefits associated with underbalance techniques. Drilling without any fluid would not be practical or safe, as drilling mud serves critical functions in cooling the drill bit and removing cuttings from the wellbore.