Underbalanced drilling

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Underbalanced drilling

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.

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Encyclopedia

Blowout (well drilling)

A blowout is the uncontrolled release of formation fluid from a well drilling, typically for petroleum production. A blowout is caused when a combination of well control systems fail ? primarily drilling mud hydrostatics and blow-out preventers ? and formation pore pressure is greater than the wellbore pressure at depth....
, 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.

Advantages

Underbalanced wells have several advantages over conventional drilling including:

Eliminated formation damage. In a conventional well, drilling mud
Drilling mud

In geotechnical engineering, drilling mud, also known as spud mud , is a drilling fluid used to drill boreholes into the earth. Often used while drilling oil well and natural gas wells and on exploration drilling rigs but can also be used for much simpler boreholes, such as water wells....
is forced into the formation in a process called invasion
Drilling fluid invasion

Drilling fluid invasion is a process that occurs in a well being drilled with higher wellbore pressure than formation pressure. The liquid component of the drilling mud continues to "invade" the porous and permeable formation until the solids present in the mud, commonly bentonite, clog enough pores to form a mud cake capable of preventing...
, 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

Lost circulation is one of the more serious problems that can arise during drilling. Circulation is said to be lost, when the mud flows into surrounding formations instead of returning up the annulus....
. 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
Differential sticking

File:Differential sticking.svgDifferential sticking is a problem that occurs when drilling a Oil well with a greater well bore pressure than formation pressure, as is usually the case....
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.

Disadvantages

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

Geosteering

In the process of well drilling, geosteering is the act of adjusting the borehole position on the fly to reach one or more geological targets....
if it is a deviated well

Directional well

A directional well, as it is known in the oil industry, is an oil well with a borehole that deviates from a vertically straight line. This is normally an intentional decision and done purposefully, to hit several target sands, for instance....
). 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.

References

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.