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Underbalanced drilling, or UBD, 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. In such a conventional "overbalanced" well, the invasion of fluid is considered a kick, and if the well is not shut-in it can lead to a blowout, a dangerous situation. In underbalanced drilling, however, there is a "rotating head" at the surface - essentially a seal that diverts produced fluids to a separator while allowing the drill string to continue rotating.

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. More commonly, inert gas is injected into the drilling mud to reduce its equivalent density and hence its hydrostatic force throughout the well depth. This gas is commonly nitrogen, as it is non-combustible and readily available, but air, reduced oxygen air, processed flue gas and natural gas have all been used in this fashion.


Underbalanced wells have several advantages over conventional drilling including:

  • Eliminated formation damage. In a conventional well, drilling mud is forced into the formation in a process called invasion, which frequently causes formation damage - a decrease in the ability of the formation to transmit oil into the wellbore at a given pressure and flow rate. It may or may not be repairable. In underbalanced drilling, if the underbalanced state is maintained until the well becomes productive, invasion does not occur and formation damage can be completely avoided.
  • Increased Rate of Penetration (ROP). With less pressure at the bottom of the wellbore, it is easier for the drill bit to cut and remove rock.
  • Reduction of lost circulation. 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 underbalanced, mud will not enter the formation and the problem can be avoided.
  • Differential sticking is eliminated. Differential sticking is when the drill pipe is pressed against the wellbore wall so that part of its circumference will see only reservoir pressure, while the rest will continue to be pushed by wellbore pressure. As a result the pipe becomes stuck to the wall, and can require thousands of pounds of force to remove, which may prove impossible. Because the reservoir pressure is greater than the wellbore pressure in UBD, the pipe is pushed away from the walls, eliminating differential sticking.


Underbalanced drilling is usually more expensive than conventional drilling, and has safety issues of its own. This is true when combustible and corrosive gasses like processed flue gas and oxygen are injected into the drilling mud to lower its density. Drilling underbalanced may be pointless from a formation damage standpoint if the underbalanced condition can not be maintained - which can be difficult when the drillstring needs to be removed to change a bit, or if the flow must stop in order to allow mud pulse telemetry to be sent. Information is frequently needed from the bottom of the well (knowledge of bottom hole pressure is very important in underbalanced drilling, as is information for geosteering if it is a deviated well). When gas is injected into drilling mud, standard mud pulse telemetry becomes impossible. "Killing" the well (making it overbalanced) may be necessary to send information, inducing formation damage. Underbalanced drilling also increases the chances of the wellbore collapsing in on itself.


Nas, Steve, Chapter 12 Underbalanced Drilling, from Petroleum Engineering Handbook, Volume II, Editor Robert Mitchell, 2007, pages II-519 to 569. Handbook available from Society of Petroleum Engineers. [1]



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