Definition of Underbalanced Drilling The drilling process in which the wellbore pressure is intentionally engineered (i.e., a low “weight” drilling fluid or a gasified drilling fluid) to be less than that of the target reservoir, therefore allowing for the continuous flow of formation fluids into the well. It is important to note that underbalanced drilling is essentially a drilling process whereby the flow of fluids to surface is carefully monitored and controlled, and that it is not a situation in which the well is out of control (i.e., gas kick or blowout).