Though not as common as overbalanced drilling, underbalanced drilling is achieved when the pressure exerted on the well is less than or equal to that of the reservoir. ... In contrast to conventional drilling, flow from the reservoir is driven into the wellbore during underbalanced drilling, rather than away from it.

Why is drilling underbalanced?

Underbalanced drilling prevents formation damage that can occur during conventional, or overbalanced drilling processes. The negative differential pressure obtained during underbalanced drilling between the reservoir and the wellbore encourages production of formation fluids and gases.

What will happen if we drill while in underbalanced mud hydrostatic?

Lost circulation is when drilling mud flows into the formation uncontrollably. Large amounts of mud can be lost before a proper mud cake forms, or the loss can continue indefinitely. If the well is drilled under-balanced, mud will not enter the formation and the problem can be avoided.