Underbalanced drilling

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Underbalanced drilling, sometimes abreviated UBD, is the procedure used at times to drill wells in the oil and gas industry where the pressure in the wellbore is kept lower than the fluid pressure in the formation being drilled. This means 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 in order 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 and is considered very dangerous. In underbalanced drilling, however, there is a "rotating head" at the surface - essentially a seal that diverts produced fluids to a seperator while allowing the drill string to continue rotating.

If the formation pressure is relatively high, simply using a lower density mud will suffice to reduce the well bore pressure below the pore pressure of the formation. More commonly, however, inert gas is injected into the drilling mud to reduce its equivalent density and hence its hydrostatic force throughout the well depth. This inert 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.

[edit] Advantages

Wells are drilled underbalanced because it provides certain advantages. These advantages include:

Eliminated formation damage. In a conventional well, drilling mud is forced into the formation in a process called invasion, which is a frequent cause 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 starts to flow 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, the mud will not enter the formation in the first place and the problem can be entirely 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 essentially pushed away from the walls, eliminating the differential sticking problem.

[edit] Disadvantages

Underbalanced drilling is usually more expensive than conventional drilling, and has safety issues of its own. This is particularly true when combustible and corrosion inducing gasses like processed flue gas and oxygen are injected into the drilling mud to lower its equivalent 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 (knowing the bottom hole pressure is very important in underbalanced drilling, as is other information for geosteering if it is a deviated well). Unfortunately, when compressible gas is injected into the drilling mud, the usual mud pulse telemetry of Measurement While Drilling 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.