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A–Z reference

Oilfield Glossary

Plain-English definitions of 52+ oilfield terms — from annulus to workover. Search or jump by letter.

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A

Annulus
The space between two concentric cylinders downhole — for example, between the drill pipe (or casing) and the wellbore wall. Cement and fluids are circulated through the annulus. Learn more →
API Gravity
A measure of how light or heavy a crude oil is relative to water, in degrees (°API). Calculated as (141.5 ÷ SG) − 131.5. Higher API means lighter oil. Learn more →
Artificial Lift
Any method — pumpjack, ESP, gas lift — that adds energy to a well to lift fluids to surface when natural reservoir pressure is insufficient. Learn more →

B

Bakken
A prolific tight-oil formation in the Williston Basin of North Dakota and Montana that helped launch the US shale boom. Learn more →
Barrel (bbl)
The standard unit of oil volume, equal to exactly 42 US gallons (about 159 liters). Learn more →
Blowout
An uncontrolled release of formation fluids from a well. Prevented by drilling mud and the blowout preventer (BOP). Learn more →
Blowout Preventer (BOP)
A stack of high-pressure valves at the wellhead that can seal a well in seconds to control a kick and prevent a blowout. Learn more →
BOE
Barrel of oil equivalent — a unit normalizing gas and oil to a common energy basis, where ~6 Mcf of gas ≈ 1 BOE ≈ 5.8 MMBtu. Learn more →

C

Casing
Steel pipe run into a wellbore and cemented in place to line and stabilize the hole and isolate zones. Learn more →
Cementing
Pumping cement slurry into the annulus between casing and rock to seal and support the casing and isolate zones. Learn more →
Christmas Tree
The assembly of valves, spools and chokes on a producing wellhead that controls and regulates flow from the well. Learn more →
Completion
The operations that make a drilled well ready to produce: casing, cementing, perforating, stimulating and installing production equipment. Learn more →

D

Decline Curve
A plot of a well's production rate over time used to forecast future output and estimate reserves. Learn more →
Derrick
The tall structural tower over a well that supports the drill string and hoisting equipment. Learn more →
Directional Drilling
Deliberately steering a wellbore away from vertical to reach a target or run horizontally through a reservoir. Learn more →
Downstream
The refining, marketing and distribution segment of the industry that turns crude into fuels and products. Learn more →

E

Enhanced Oil Recovery (EOR)
Tertiary recovery methods — CO2, steam or chemical injection — that recover oil left behind by primary and secondary production. Learn more →
ESP
Electric submersible pump — a multistage centrifugal pump set downhole and powered from surface, used for high-volume artificial lift. Learn more →

F

Frac (Hydraulic Fracturing)
Pumping fluid and proppant into rock at high pressure to create fractures that let oil and gas flow to the well. Learn more →

G

Gas Lift
An artificial-lift method that injects gas into the wellbore to lighten the fluid column so reservoir pressure can lift it. Learn more →
GOR
Gas-oil ratio — the volume of gas produced per barrel of oil, usually in standard cubic feet per stock-tank barrel.

K

Kerogen
Solid organic matter in source rock that matures into oil and gas under heat and pressure. Learn more →
Kick
An influx of formation fluid into the wellbore that, if uncontrolled, can lead to a blowout. Controlled with mud weight and the BOP. Learn more →

L

LACT Unit
Lease Automatic Custody Transfer unit — an automated skid that meters, samples and transfers ownership of crude oil; the 'cash register' of the lease. Learn more →

M

Mcf
One thousand cubic feet of natural gas (M = Roman numeral for 1,000). Learn more →
Midstream
The transport, storage and processing segment linking production to refining — pipelines, gathering and gas plants. Learn more →
Mud (Drilling Fluid)
The fluid circulated while drilling to clean the hole, cool the bit, and control pressure with its weight. Learn more →
Mud Weight
The density of drilling fluid, usually in pounds per gallon (ppg), which sets the hydrostatic pressure controlling the well. Learn more →

P

PDC Bit
A fixed-cutter drill bit using polycrystalline diamond compact cutters that shear rock; common in stable formations. Learn more →
Perforation
Punching holes through casing and cement into the reservoir, usually with a perforating gun, to let fluids flow in. Learn more →
Permeability
A measure of how easily fluid flows through the connected pores of a rock. Learn more →
Permian Basin
The largest US producing basin, in West Texas and SE New Mexico, containing the Midland and Delaware sub-basins. Learn more →
Pig
A device sent through a pipeline to clean it or inspect it ('pigging'). Short for Pipeline Inspection Gauge.
Porosity
The percentage of void (pore) space in a rock that can hold fluids such as oil, gas or water. Learn more →
Proppant
Granular material — frac sand, resin-coated sand or ceramic — that holds hydraulic fractures open after pumping stops. Learn more →
Pumpjack
The iconic surface beam unit that drives a sucker-rod string to operate a downhole pump. Learn more →

R

Reservoir Rock
Porous, permeable rock — often sandstone or carbonate — that stores and transmits hydrocarbons. Learn more →
Roustabout
An entry-level general laborer on an oilfield site who handles cleaning, maintenance and moving equipment.

S

Separator
A surface vessel that splits produced fluid into gas, oil and water phases. Learn more →
Shale Shaker
A vibrating screen that is the first solids-control device, separating cuttings from returning drilling mud. Learn more →
Source Rock
Organic-rich rock, usually shale, in which oil and gas are generated from kerogen. Learn more →
Spud
To begin drilling a well — the moment the bit first penetrates the ground.
Sweet / Sour Crude
Sweet crude has low sulfur and is easier to refine; sour crude has high sulfur and needs more processing. Learn more →

T

Tight Oil
Oil held in very low-permeability rock such as shale, producible only with horizontal drilling and fracturing. Learn more →
Top Drive
A motor suspended in the derrick that rotates the drill string from above, replacing the rotary table. Learn more →
Tubing
The smaller pipe run inside casing through which oil or gas flows to surface; can be pulled during workovers. Learn more →

U

Upstream
The exploration and production (E&P) segment — finding and extracting oil and gas. Learn more →

W

Water Cut
The percentage of produced water in the total produced liquid stream; it rises as a well ages.
Wellhead
The pressure-sealing equipment at surface where the casing strings terminate and the tree mounts. Learn more →
Wildcat
An exploratory well drilled in an unproven area with no nearby production.
Wireline
A cable used to lower tools downhole for logging, perforating, setting plugs and retrieving equipment.
Workover
An intervention to repair, restore or enhance a producing well — e.g. replacing a downhole pump or recompleting a zone.

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