What is Underbalanced Drilling (UBD)?
A rigorous definition of UBD is the situation in which the exerted circulating pressure of the drilling fluid in contact with the formation is less than the effective pore pressure in the adjacent section of the matrix. The desirable course of action is to have this occur along the entire exposed section of the net productive pay of the reservoir under consideration, resulting in a net inflow of oil, water or gas (which may be contained in the matrix) into the wellbore. These produced fluids are then returned to the surface along with the circulating drilling fluid. A number of nomenclatures for a description of UBD exist in the literature. They can be defined as follows: Overbalanced Drilling. A situation in which the equivalent circulating density of the drilling mud is sufficient that, at bottom hole conditions, the drilling fluid pressure is greater than the formation pressure, resulting in an effectively killed state of the well (where no inflow of formation of fluids occurs) duri
