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Common oilfield terminology


Abandon To permanently plug a well, usually with cement plugs, and recover all possible downhole equipment. A well is abandoned if it is dry or has ceased to produce.

Abnormal pressure Well pore pressure that is either higher or lower than normal formation pressure.

Acidize Acid is pumped into the producing zone to increase the flow of oil or gas. The acid dissolves formation limestone improving the flow into the well.

Air drilling Compressed air is used in place of drilling fluids to carry cuttings from the drill bit to the surface.

Annular velocity The velocity or speed of a fluid, normally drilling fluid or cement, circulating in the annulus.

Annulus The space between either the drill string and well bore/casing or between the production tubing and casing.

Anticline A geological structure in which the strata are curved or domes.

API gravity An American Petroleum Institute (API) standard measurement of the density of liquid hydrocarbons. The lighter the oil, the higher the API gravity.

Appraisal well A well drilled to determine the size, reserves and possible production rates of a newly discovered field.

Aquifer The production of groundwater from a formation below the water table.

Associated gas Natural gas found with oil, either in the dissolved form or as a cap of free gas above the oil.


Barite Barium sulphate, a mined mineral used to increase the density of drilling fluids.

Barrel of oil One barrel is equal to 42 US gallons, approx 159 litres or 35 imperial gallons.

bbl One barrel of oil.

bcf Billion cubic feet. A unit often used to measure the size of a gas field.

bcm Billion cubic metres.

Bend restrictors Usually fitted around a flowline in static load applications to prevent damage where excessive bending may occur.

Bend stiffeners Designed to provide support for cables or flowlines as they emerge from permanent structures.

Bentonite Colloidal clay consisting predominantly of sodium montmorillonite. It is used as a viscosifier and fluid loss reducer in drilling fluids.

Bit Cutting tool fixed to the bottom of the drill string.

Blooie line Flowline used when drilling with air or gas.

Blowout Uncontrolled escape of drilling fluid, gas, oil or water from the well when formation pressure is greater than the hydrostatic pressure of the fluid in the well.

Blowout preventers (BOP) Stack of high pressure valves at the top of the casing to control or shut off the flow of gas or fluids to the surface.

bopd Barrels of oil per day.

Bottom-hole pressure (BHP) Pressure of the formation or reservoir at the bottom of the well.

Bottom-hole assembly (BHA) Lower section of the drill string typically consisting of bit, bit sub, stabilisers and drill collars.

Bottoms-up Stage at which drilling fluid and cuttings arrive at the surface having been circulated up from the bottom of the well.

bpd Barrels of oil per day.

Bridging agent Materials added to drilling fluid to control or prevent loss of fluid into the formation.

Bromide brine Sodium, calcium or zinc bromide added to water to increase its density. The brines are used in well work-overs and completions.

BTU British Thermal Unit, measurement of heat needed to raise the temperature of one pound of water by 1º Fahrenheit.

Buckle mitigation buoyancy Mitigates the effects of longitudinal buckling along a steel pipeline.

Buoyancy The buoyancy (upward) force on an object is equal to the weight of fluid that it displaces. This applies to the atmosphere as well as the hydrosphere and works for airships, balloons, submarines, boats and buoys.


Calliper log Tool that measures the diameter of a well over its depth.

Cap rock Rock with very low permeability that forms an effective cap or seal above a reservoir. Shales, salts and anhydrite are typical types of cap rock.

Casing Steel pipe that is used to line and protect the well after it’s been drilled. The casing string is formed from sections of pipe screwed together.

Casing point Depth at which the drilling of a particular section of hole is stopped and casing is run.

Cementing Procedure where cement is used to fill the space between casing and well bore. The cement holds the casing in place and isolates one section of the well from another.

Centrifuge Device designed to mechanically separate high specific gravity solids from a drilling fluid.

Choke Valve-like device used to control flow when testing a well.

Choke line Pipe that runs from the blowout preventer (BOP) to the choke manifold. The choke line and manifold are used during well control procedures.

Christmas tree Series of valves mounted on top of a well to control production.

Circulation Movement of drilling fluid from the mud pits, through the pump, drill pipe, bit and annulus before returning to the mud pits.

Coiled tubing Continuous length of pipe used in place of drill pipe.

Completion Range of steps taken to bring a well into production.

Completion fluid Non-damaging, low solids fluid designed to complete a well. The fluid is pumped into the hole before production operations start.

Concession Specific area over which a company has a licence to carry out exploration and development activities.

Condensate Hydrocarbons which are gaseous under reservoir conditions but liquid when pressures and temperatures are reduced.

Conductor pipe First string of casing used on a well to isolate the shallow unconsolidated formations.

Connate water Water that was trapped containing sedimentary deposits during rock formation.

Coring Sampling of an in-situ column of rock using a core barrel.

Crown Series of pulleys at the top of a derrick.

Cuttings Particles of rock cut by the drill bit and circulated to the surface by the drilling fluid (mud).


Darcy Unit of permeability. A porous medium has a permeability of 1 Darcy when a pressure of 1atm on a sample 1cm long and 1sq cm in section will force a liquid of 1cp viscosity through the sample at the rate of 1cc per sec.

Day rate Cost of renting a drilling rig for a day.

Depletion Reduction in hydrocarbons following production.

Density The density of an object is a measure of the mass per
unit volume = M/V

Derrick Tapered towered mast used in a crane-like function to support the drill string.

Derrickman Member of the crew that works in the derrick, racking and unracking drill pipe when manoeuvring it into and out of the hole.

Desander Cyclone type device used to separate solid particles, typically sand, from a drilling fluid.

Development well Well drilled into a proven hydrocarbon reservoir before going into full production.

Deviated well Non-vertical well.

Differential pressure sticking This occurs when part of the drill string, normally the collars, sticks to the wall cake resulting in the non-uniform distribution of pressure around the drill string.

Directional drilling Controlled drilling of a well in a pre-determined direction or path.

Distributed buoyancy System used to provide flexible pipe and subsea umbilicals

Dogleg Bend or elbow caused by a change of direction in the well.

Doghouse Drilling crew office and coffee room on the rig floor.

Drill collar Thick walled section of the drill string fitted between the drill bit and the drill pipe providing the necessary weight on the bit for efficient drilling.

Drill stem test Test to determine whether oil and/or gas have been found in commercial quantities.

Drill-in fluid Low solids, non-damaging fluid formulated specifically for drilling through reservoirs.

Drilling fluid Viscous water or oil-based fluid circulated down through the drill pipe and up the annulus to carry the cuttings to the surface while maintaining good hole conditions.

Drilling fluid engineer Person responsible for the formulation, testing and treatment of a drilling fluid.

Dry gas Natural gas of methane and limited amounts of ethane, propane and butane.

Dry hole Well that has failed to find oil or gas in commercial quantities.

DST Drill stem test.

Dual completion Completion of a well with two producing reservoirs.

Durafloat™ RIS Marine drilling riser buoyancy featuring residual integrity system.

Duraguard™ Cable protection system used on flowlines, umbilicals, hoses and cables.

Duster Dry hole.

Dynamic positioning Method used to maintain a semi-submersible or drillship on its exact location. Thrusters are used to counter adverse weather and sea conditions.


Electric log Electrical survey of an uncased well to obtain information concerning the porosity, permeability and fluid content of the formations drilled.

Elevators Hinged clamp-like device which closes around the top of the drill string to pull it out of or lower it into the hole.

Enhanced oil recovery (EOR) Number of processes used to produce oil from a reservoir that has lost its natural pressure.

Evaporite Sedimentary rock formed by evaporation. Limestone, gypsum and anhydrite are examples.

Exploration well Well drilled to find new deposits of hydrocarbons


Farm in/Farm out Agreement between oil companies where the owner of a block assigns an interest in the block to another willing to fund part or all of the exploration costs.

Fault Geological term denoting a break in the formation, upward or downward, in the subsurface strata.

Field Geographical area covering an oil or gas reservoir.

Filter cake Suspended solids from drilling fluid that are deposited on a porous medium (the formation), normally under pressure. The filter or wall cake formed on the sides of the hole should be tough, thin and slick with low permeability.

Fish Name applied to any equipment or tooling left in the well. It could be hand tools, frill pipe or other specialist equipment.

Fishing Variety of procedures used to retrieve tools, pipe or other objects lost in the well.

Flaring Burning of unwanted produced gas.

FlexLink™ Umbilical buoyancy used to prevent ROV/AUV vehicles from damaging their control lines.

Flowline Large diameter pipe carrying the circulating drilling fluid from the top of the bell nipple to the shale shakers.

Fluid loss Measurement of the relative amount of fluid lost from the drilling fluid to a permeable formation when under pressure.

Formation Group of rocks of the same age.

Formation damage Damage done to the productivity of a well as a result of invasion into the formation by drilling fluid solids or filtrate.

Formation pressure Bottom hole pressure of a well when it is shut in.

Formation water Salty water underlying oil or gas in a formation.

FPSO Floating Production Storage and Offloading facility.

Fracture Crack within a rock.

Fracture gradient Pressure needed to create fractures in a formation at a specific depth.

Fracturing Applying hydraulic pressure to a reservoir to create or enlarge fractures through which hydrocarbons can be produced.


Galena Lead sulphide (Pbs) with a specific gravity of approximately 7.0 is used to formulate very high density drilling fluids, normally for well control purposes.

Gas cut Oil or drilling fluid, which has been invaded by gas.

Gas hydrate Result of natural gas molecules being trapped in molecules of ice and formed in conditions of extreme cold.

Gas injection Re-injection of gas into a well either for the maintenance of reservoir pressure or for conservation purposes.

Gas/oil ratio Volume of gas produced per barrel of oil.

Geopressure Pressure within a formation.

Geothermal gradient Measurement of the increase in temperature with increases in depth. Average geothermal gradients range from 25-30ºC per kilometre.

Geronimo line Steel cable running from the derrick to the ground at a safe distance from the rig floor. The cable is used by the derrickman to slide down as a quick route of escape should there be a requirement to descend rapidly.

Gauge hole Drilled hole that is close to the same diameter as the drill bit.

Gumbo Hydrophilic formation (shale) that swells and becomes soft and sticky when wet, often plugging the annulus, flowline and shaker screens.

Gunk plug Slurry in crude or diesel oil, normally containing bentonite, cement or guar gum used to control losses of drilling fluid into a formation.


Hook Piece of equipment attached to the bottom of the travelling block from which the swivel, kelly and drill string are hung.

Horizontal well Section of a well drilled at more than 80º from vertical. Horizontal well sections are normally drilled through a reservoir to maximise production.

HTHP High temperature, high pressure. An HTHP well is one where both pressure and temperature are higher than average. Generally this means a bottomhole temperature in excess of 149ºC and a pressure gradient greater than 0.8psi/ft.

Hydrocarbon Compounds of hydrogen and carbon, which may exist as a solid, gas or liquid and generally used to describe oil, gas or condensate.

Hydrocyclone Range of devices to reduce the solids content of drilling fluids. The hydrocyclones with larger cones are used as desanders while those with smaller cones are used as desilters.

Hydrogen sulphide (H2S) Poisonous gas which is lethal even at low concentrations. The gas smells like rotten eggs at low concentrations but is odourless at high concentrations. It is also highly corrosive.

Hydrostatic head Pressure exerted by a column of fluid usually expressed in pounds per square inch.


Inhibitive mud Drilling fluid that has been formulated to minimise the hydration and swelling of clays and shales.

Injection well Well used for the pressured injection of water or gas into a reservoir.

Intermediate casing String or strings of casing run into the hole to isolate the hole sections below the surface casing but above the production string.

Invert emulsion-drilling fluid Water in oil emulsion drilling fluid where fresh or salt water is the dispersed or internal phase and oil is the external phase.


Jacket Lower section of an offshore platform.

Jackup rig Offshore rig with support legs that are lowered onto the seafloor to raise the actual rig above the water and out of the way of any wave action.


Kelly Hexagonal shaped section of pipe mounted above the drill pipe to transfer rotary drive from the rotary table to the drill pipe and the bit. The Kelly has a hole through its centre through which the drilling fluid is pumped down the drill string to the bit.

Keyseat Groove worn into the side of a well that has a smaller diameter than the hole. The groove is normally similar in diameter to the drill pipe meaning the larger sections of tooling, such as the bit, are unable to go through the grove and become stuck.

Kick Influx of oil, water or gas while drilling. This occurs because the hydrostatic head of the drilling fluid in the well is less than the pressure in the formation.

Kill Procedures used to stop a well from flowing out of control. Higher density drilling fluids are used to increase the hydrostatic head.

Kill line Pipe connected to the annulus below the blowout preventers allowing drilling fluids to be pumped directly into the annulus when the blowout preventers are closed.


Lag time Time taken for cuttings to be carried from the bit to the surface.

Leakoff test Test carried out to find the pressure required to break down a formation allowing fluid to leak off into it. The test is generally carried out on the formation immediately below the casing shoe to establish what the maximum drilling fluid weight can be in the particular hole section.

Limestone Sedimentary rock predominantly of calcium carbonate.

Liner Section of pipe set through the reservoir that does not go to the top of the well but is suspended from the bottom of the previous casing string.

Lipophilic Affinity for oil.

Liquefied natural gas (LNG) Naturally occurring gas, predominantly methane, that has been liquefied for transportation purposes.

Liquefied petroleum gas (LPG) Hydrocarbon that is gaseous at atmospheric pressure and temperature but is kept in the liquid state by pressure to allow for effective storage and transportation.

Logging while drilling (LWD) Evaluation of formations as they are being drilled. The measuring tools are fitted into the bottomhole assembly.

Lost circulation Loss of drilling fluid into the formation creating partial or total failure to return to the surface.


Making a connection Approx 9m length of drill pipe connected to the drill string to increase its length as the depth of the well grows.

Marine drilling riser buoyancy (MDRB) Serves many functions including the reduction of the submerged riser weight and the provision of ‘lift’.

Mass The mass of an object is a measure of the amount of matter that it contains.

mboe Million of barrels of oil.

Millidarcy 1/1000 darcy, a measurement of the permeability of rocks. Please refer to “Darcy”.

mcfd Millions of cubic feet per day. Gas measurement.

Modular buoyancy Used predominantly as suspended moorings or subsea markers.

Moonpool Large opening located in the centre of a drillship through which the drilling process is carried out.

Mousehole Hole in the rig floor used to hold a length of drillpipe before adding it to the drill string when making a connection.

mtoe Millions of tons of oil equivalent.

Mud Generic term for water or oil based drilling fluids.

Mud logging Continuous analysis of the cuttings and drilling mud circulating from the bottom of a well.

Multilateral Central wellbore from which a number of wells branch out in different directions.


Natural gas Naturally occurring mixture of hydrocarbons with methane as the predominant part.

Net pay Part of a formation that has oil and/or gas in commercial quantities.


Offset well Well that is close to an existing well from which data is taken to assist in the planning of the new well.

Oceanus™ floats A range of standard buoyancy units suitable for all ocean depths

Oceanus Oceanus was a waterway which the Greeks and Romans believed to be an enormous river encircling the world. In Greek mythology, this world-ocean was personified as a Titan, a son of Uranus and Gaia.
In Hellenistic and Roman mosaics, this Titan was often depicted as having the upper body of a muscular man with a long beard and horns and the lower torso of a serpent.

Oil base mud Drilling fluid in which oil is the continuous or external phase.

Oil show Small amount of oil found in the well or in a sample of rock.

OPEC Organisation of Petroleum Exporting Countries.

Open hole Section of the well bore that has not been cased.

Operator Company that holds the rights to drill wells and produce hydrocarbons from a specific block or field. The operating company may be a member of a consortium that acts on its behalf.


Packer Piece of equipment used to pack off or seal the wellbore. The device is run into the hole to the depth required and then expanded using a variety of mechanisms.

Pay zone Section of rock from which oil and/or gas is produced.

PDC bit Type of drill bit which uses polycrystalline diamond compact cutters to cut the rock formations.

Penetration rate Measurement of the number of feet or metres drilled per hour.

Perforations Holes made in the casing, cement and formation to allow formation fluids to enter the well bore.

Permeability Measurement of a rock’s capacity in relation to the movement of fluids through its pores.

Pipe rams Valve-like part of the blowout preventer comprising semi circular holes that, when closed, form a seal around the drill pipe.

Platform Fixed offshore structure attached to the seabed.

Plug and abandon Permanently close a well down either because it has ceased being productive or because it has failed to find commercial quantities of hydrocarbons.

Pore pressure The pressure of fluids in a reservoir.

Porosity Measurement of pore space as a percentage of total rock mass.

Probable reserves Reserves of oil or gas which have yet to be proven but which are thought to have a reasonable chance of being produced.

Proven reserves Reserves of oil or gas which are near certain to be recoverable under current operating and commercial conditions.

PSA Production sharing agreement.


Rathole Hole in the rig floor in which the kelly is stored when not in use.

Reaming Technique used to return an under gauge hole to its correct gauge size.

Recoverable reserves Amount of oil and/or gas that is expected to be produced from a well.

Reservoir rock Permeable and porous rock (limestone, sandstone, dolomite) containing commercial quantities of petroleum.

Reverse circulation Circulation of drilling fluid down through the annulus and back up through the drill pipe. This is the opposite of that normally used.

Riser Large diameter pipe connecting a rig to the subsea blowout preventer and the hole itself.

Rotary table Circular section of the drill floor that is rotated clockwise transmitting its power through the Kelly to the drill string.

Roughneck Member of the drilling crew working on the rig floor.

Round trip Removal of the drill pipe and bottom hole assembly from the well, typically to change the bit, before running the complete drill string back into the hole.

Roustabout General rig worker.


Salt-water flow Uncontrolled invasion of salty water into the wellbore.

Secondary recovery Recovery of petroleum by injecting gas or water into a reservoir to increase reservoir pressure.

Sedimentary rock Rocks formed by the laying down of matter in the sea, river or lake. Limestone, sandstone and shale are sedimentary rocks.

Seismic survey Seismographic investigation of subsurface rock formations using low frequency soundwaves, recording and analysis.

Shale Sedimentary rock consisting of clay and sand.

Shale shaker Series of vibrating sieves over which the drilling fluid is passed to screen out the drilled cuttings carried up from the bottom of the well by the fluid.

Shut in pressure Pressure of a well after it has been shut in for a defined period.

Sidetrack Well that kicks off from the original well normally to bypass a section of the original hole because it has collapsed or contains junk.

Sour gas Gas which contains acid such as hydrogen sulphide and carbon dioxide.

Source rock Sedimentary rock capable of producing hydrocarbons.

Specific gravity Weight of a specific volume of a material compared to the weight of an equal volume of water.

Spud First stage of the actual drilling of a well.

Step-out well Well drilled to find the outer limits of a field.

Stimulation Variety of processes used to increase the productivity of a well, normally acidising or fracturing.

Supatherm Bundle hybrid offset riser buoyancy system

Suspended well Well that has been temporarily capped.

Swab Wireline technique using special tooling designed to initiate production from a well.

Sweet crude Oil with very low sulphur and hydrogen sulphide content.

Syntactic foam A composite material consisting of manufactured hollow spheres embedded in a resin matrix.

Synthetic drilling field Invert emulsion drilling fluid that uses a synthetic fluid as the external continuous phase in place of oil.


tcf Trillion cubic feet.

Tight hole A well drilled under strict terms of secrecy.

Tongs Large wrenches used to tighten joints on the drill string.

Tool joint Box and thread at the end of drillpipe sections used to join individual sections together.

Toolpusher Drilling contractor’s rig supervisor.

tpes Total primary energy supply.

Topdrive Piece of equipment used to rotate the drill string. Mounted below the hook, the topdrive can be used in place of a rotary table and kelly.

Tubing Small diameter length of pipe run inside the casing through which oil or gas are produced.

Turnkey contract Contract in which one of the parties agrees to drill a well to a certain depth or stage for an agreed specific sum.

tvd True vertical depth, ie, the vertical distance from the bottom of the well to the rig.


Underbalance When the formation pressure is greater than the hydrostatic pressure.

Underground blowout Unrestricted influx of fluid from one formation to another.

Underream Enlarging a well’s diameter from its original drilled size.


Viscosity Internal resistance of a fluid to flow.

Volume The volume of an object is a measure of the space that it occupies.


Water flooding Secondary recovery method of increasing production of oil by injecting water into a reservoir.

Well control Variety of techniques used to control the flow of formation fluids into the wellbore.

Whipstock Device used to deflect or change the direction of a well.

Wildcat well Well drilled in an unproven area.

Wiper trip Withdrawal of the drill string to clean up problematic sections of open hole.

Workover Remedial work carried out on an existing well to increase production.