Drilling Glossary

An uncontrolled, accidental release of well pressure, either to the surface or to another formation (in this case, called an underground blow-out).

Blow-out preventer (BOP)
A series of valves, which offshore can be as high as 50 feet, to control the well and prevent a blow-out.

Steelpipe used to protect the well-bore from caving in. When casing is cemented in place, it forms a hydraulic seal with the rock formations, preventing well fluid from migrating up or down the outside of the casing. It also keeps the well fluids separate from groundwater sources adjacent to the wellbore.

Analogousto the human circulatory system, mud is pumped into the well from a reservoir called a mud pit. It circulates down the inside of the drill pipe, flows out through ports in the bit and circulates back to the surface up the outside of the drill pipe, where it is filtered, de-gassed and returned to the pit.

Slang for retrieving pipe, tools, cable or objects that have been dropped into the well.

Fracturing (frac job)
If the target reservoir lacks sufficient porosity or permeability to produce on its own in commercial quantities, this stimulation technique can improve the flow. Enormous pressure is applied to the reservoir by pumping in massive amounts of fluid (water or polymer) to enlarge the channels between pores in the rock.

An unplanned influx of formation fluid into the wellbore, caused by the unexpected presence of hydrocarbons.

Lost circulation
Leak-off of mud into a subsurface formation or through a hole in the well casing. Can be stopped by circulating plugging material similar to radiator stop-leak used in automobiles.

Mixture of water and chemicals that occupies the borehole during drilling or completion of a well. Its main task is to exert hydrostatic pressure on the reservoir to balance the natural formation pressure and prevent an accidental influx of formation fluid into the borehole. It prevents the sides of the well from caving in as the hole is cut. And, it transports the rock cuttings from the bottom of the hole to the surface, where the geologist can examine them for clues as to the type of rock being penetrated.

A mechanical seal between tubing and casing, usually set just above the producing formation.

A downhole perforating gun, lowered by wireline, that fires shaped charges through the casing into the desired rock formation, resulting in perforations through which reservoir fluids may flow into the wellbore and up to the surface.

Plug & Abandon (P & A)
When a well is depleted of economically recoverable oil and/or gas, a permanent plug is set to seal the bottom of the well, as much casing as possible is recovered, and the surface location is restored to its original condition. The well is abandoned and a report is filed with the governing authority.

Seismic exploration
Sound waves are pulsed into the earth. They reflect off subsurface layers and the reflected waves are processed to create a subsurface image of the earth from which promising rock formations can be identified for potential drilling.

The act of beginning to drill a borehole, usually starting with driving a piece of large diameter pipe (casing) into the ground (or seabed) to guide the bit and protect the surface immediately surrounding the borehole.

The act of removing the drill pipe from the borehole (to put on a fresh drillbit, for example) and/or reinserting the pipe into the hole. Each phase has a name: trippingout, tripping-in or round-tripping. This function is usually done manually by roughnecks, or it can be done by automatic pipe-handling equipment.

High-pressure pipe runs inside the casing through which the oil or gas is produced.

An exploratory borehole drilled in virgin territory, usually at least a mile or two away from the nearest production of oil or gas. A rank wildcat is a well drilled many miles away from the nearest production and is inherently more exploratory in nature, and thus more risky. On the other hand, the rank wildcat may tap into a virgin subsurface zone that has high pressure and offers much greater daily oil or gas production.

An oilfield term meaning “overhaul.” Periodically, wells must be worked over to address cleaning, wear and corrosion of the downhole equipment, or pressure issues. Special light-duty rigs called workover rigs perform this task at a fraction of the cost of a drilling rig.