Oil & Gas Glossary

A well stimulation technique where acid (typically hydrochloric or hydrofluoric acid) is pumped into the formation to dissolve rock and enlarge pore spaces, improving the flow of oil or gas into the wellbore.

The space between two concentric objects, such as between the drill string and the wellbore wall, or between two strings of casing. The annulus is monitored during drilling for signs of kicks and is used for circulating drilling fluids.

A scale developed by the American Petroleum Institute to measure the relative density of liquid petroleum products. Higher API gravity indicates lighter crude oil. Light crude is above 31.1° API, medium is 22.3°–31.1°, and heavy is below 22.3°.

Any method used to raise oil to the surface when reservoir pressure is insufficient for the well to flow naturally. Common methods include rod pumps (beam pumps or horsehead pumps), electric submersible pumps (ESPs), gas lift, and progressive cavity pumps.

The standard unit of measurement for crude oil. One barrel equals 42 U.S. gallons (approximately 159 liters). Production rates are typically reported in barrels per day (bpd or BOPD for barrels of oil per day).

The lower portion of the drill string, consisting of the drill bit, drill collars, stabilizers, reamers, MWD/LWD tools, and other components. The BHA design affects drilling direction, rate of penetration, and hole quality.

An uncontrolled release of oil, gas, or other well fluids from the wellbore. Blowouts occur when formation pressure exceeds the pressure exerted by the drilling fluid column and well control equipment fails. The Deepwater Horizon disaster in 2010 was caused by a blowout.

A large, specialized valve assembly installed on the wellhead to seal, control, and monitor the well during drilling. BOPs can close around the drill pipe (pipe rams), seal the open hole (blind rams), or shear through the drill pipe to seal the well completely (shear rams). The BOP is the primary well control safety equipment.

A unit of measurement for crude oil production rate. An individual well might produce anywhere from a few BOPD to over 10,000 BOPD for a high-rate well.

Steel pipe cemented into the wellbore to prevent the hole from collapsing, isolate different geological formations, prevent fluid migration between zones, and protect freshwater aquifers. Wells typically have multiple casing strings of decreasing diameter: conductor, surface, intermediate, and production casing.

The process of pumping cement slurry into the annulus between the casing and the wellbore wall. Cementing secures the casing in place, provides zonal isolation between formations, and prevents fluid migration behind the casing. A cement job failure can lead to sustained casing pressure or environmental contamination.

An assembly of valves, spools, and fittings installed on the wellhead of a completed well to control production flow. Surface trees sit above ground; subsea trees are installed on the seafloor for deepwater wells. Named for its branching appearance.

A continuous string of small-diameter steel tubing spooled on a large reel. Used for well interventions including cleanouts, acidizing, nitrogen kicks, fishing, and milling without pulling the production tubing. Coiled tubing units are faster to rig up than conventional workover rigs.

The process of making a drilled well ready for production. Includes running production casing or liner, cementing, perforating, installing production tubing and downhole equipment, and often hydraulic fracturing. The completion design significantly impacts well productivity and lifespan.

The operator's on-site representative who oversees drilling operations and makes decisions on behalf of the operating company. Also called company representative or wellsite leader. The company man has final authority on the rig and works closely with the toolpusher and service company personnel.

The first and largest-diameter casing string set in a well, typically driven or drilled to 50–150 feet. Provides structural support for the wellhead equipment and prevents the shallow hole from caving in during initial drilling operations.

The daytime 12-hour shift on a drilling rig, typically running from 6:00 AM to 6:00 PM (sometimes called the "day shift" or "daylight tower"). The opposite shift is the "night tour" or "graveyard tower." Pronounced "tower" in the oil field.

The daily fee charged by a drilling contractor for the use of a drilling rig and its crew. Day rates vary widely based on rig type, market conditions, and location — from $15,000–$30,000/day for a land rig to $200,000–$500,000+/day for a deepwater drillship.

The tall structure above the rig floor that supports the traveling block and provides the height needed to handle stands of drill pipe. Modern rigs use a mast (which can be raised and lowered) rather than a permanent derrick, but the term is still commonly used interchangeably.

A rig crew position responsible for working on the monkeyboard (a platform high on the derrick) to handle the upper ends of drill pipe stands during tripping operations. Also monitors and maintains the mud system, mixing chemicals and testing fluid properties. The derrickhand position is one step above roughneck in the rig career ladder.

The practice of drilling a wellbore along a planned trajectory to reach a target that is not directly below the rig location. Enables reaching multiple targets from a single pad, drilling under obstacles, and accessing reservoirs at optimal angles. Horizontal wells are a type of directional well drilled at or near 90° from vertical.

A bend or deviation in the wellbore. Measured in degrees per 100 feet (dogleg severity or DLS). Excessive doglegs can cause drill string fatigue, casing wear, and problems running equipment into the well.

Refers to anything that occurs or is located within the wellbore, below the surface. Opposite of "surface" or "topside." Example: "downhole tools," "downhole pressure," "downhole conditions."

The primary hoisting mechanism on a drilling rig. A large winch that spools the drilling line, providing the power to raise and lower the drill string and casing. The driller controls the drawworks from the driller's console on the rig floor.

Thick-walled, heavy steel tubulars placed in the lower part of the drill string (in the BHA) to provide weight on the bit (WOB). The weight of the drill collars, not the drill pipe, creates the downward force needed to make the bit cut into the rock.

Hollow steel pipe that makes up the majority of the drill string. Transmits rotary motion and drilling fluid from the surface to the bit. Standard drill pipe lengths (joints) are approximately 30 feet, and three joints connected together form a stand (approximately 90 feet).

The entire assembly from the kelly or top drive down to the drill bit, including drill pipe, drill collars, and bottomhole assembly components. Transmits rotation to the bit, provides a conduit for drilling fluid, and allows weight to be applied to the bit.

The crew member who operates the drilling rig from the driller's console on the rig floor. Controls the drawworks, rotary table or top drive, mud pumps, and well control equipment. The driller is the senior rig floor position and is responsible for the safe execution of all drilling operations during their shift.

A specially formulated fluid circulated through the drill string and up the annulus during drilling. Serves multiple critical functions: cools and lubricates the bit, carries rock cuttings to the surface, maintains hydrostatic pressure to prevent blowouts, and stabilizes the wellbore. Can be water-based, oil-based, or synthetic-based.

Techniques used to increase oil extraction beyond primary (natural flow) and secondary (waterflooding) recovery. EOR methods include thermal injection (steam), chemical injection (polymers, surfactants), and gas injection (CO2, nitrogen). EOR can recover 30–60% of a reservoir's original oil, compared to 20–40% with conventional methods.

A multistage centrifugal pump powered by an electric motor, installed downhole in the wellbore to lift fluids to the surface. ESPs are commonly used in high-volume wells and are one of the most efficient artificial lift methods. Require electrical power supplied via a cable run alongside the production tubing.

An entry-level rig crew position responsible for handling pipe and equipment on the rig floor during drilling operations. Duties include making and breaking connections, handling tongs, operating slip elevators, and general rig floor maintenance. Also known as a roughneck on many rigs.

A vessel used in offshore oil and gas production that processes hydrocarbons on board and stores the processed oil until it can be offloaded to tankers. FPSOs are used in deepwater fields far from pipeline infrastructure, particularly in West Africa, Brazil, and Southeast Asia.

The pressure at which the formation rock will fracture. Expressed as a pressure per unit of depth (psi/ft or equivalent mud weight). Drilling fluid weight must be kept below the fracture gradient to avoid lost circulation (fracturing the formation and losing fluid into it).

The complete set of equipment and crew needed to perform a hydraulic fracturing operation. Includes pump trucks (typically 15–30+ units), blender, sand chief (proppant handling), hydration units, chemical additive equipment, data vans, wireline units, and support equipment.

An artificial lift method where compressed gas is injected into the production tubing through gas lift valves to reduce the hydrostatic pressure of the fluid column, allowing reservoir pressure to push oil to the surface. Common in Gulf of Mexico and Middle East operations.

The process of adjusting the wellbore trajectory in real time based on geological data to keep the bit within the target formation or sweet spot. Uses MWD/LWD data, gamma ray readings, and formation evaluation to make directional decisions while drilling.

A well drilled vertically to a target depth (kickoff point), then gradually turned to drill horizontally through the producing formation. Horizontal laterals in shale plays can extend 1–3 miles. Horizontal drilling dramatically increases the wellbore's contact with the reservoir compared to vertical wells.

Health, Safety, and Environment — the department and discipline responsible for managing occupational health, workplace safety, and environmental compliance on oil and gas operations. HSE professionals develop safety programs, conduct risk assessments, investigate incidents, and ensure regulatory compliance.

A well stimulation technique where fluid (water, sand, and chemical additives) is pumped at high pressure into the formation to create fractures in the rock, allowing oil and gas to flow more freely into the wellbore. Combined with horizontal drilling, hydraulic fracturing enabled the U.S. shale revolution. A single stage may use 300,000–600,000 gallons of water and 200,000–400,000 pounds of proppant.

The pressure exerted by a column of fluid at rest due to its weight. In drilling, the hydrostatic pressure of the drilling fluid column must exceed the formation (pore) pressure to prevent formation fluids from entering the wellbore (a kick). Calculated as: pressure = 0.052 × mud weight (ppg) × true vertical depth (ft).

A mobile offshore drilling unit with retractable legs that is towed to a location, then jacks its hull above the water surface on its legs, which rest on the seafloor. Used for drilling in shallow water (up to approximately 400 feet). Common in the Gulf of Mexico shelf area, North Sea, Middle East, and Southeast Asia.

A single length of drill pipe, casing, or tubing, typically about 30 feet (9.1 meters) long. Three joints screwed together form a stand (approximately 90 feet).

A heavy square or hexagonal steel pipe that fits through a matching kelly bushing in the rotary table. The rotary table turns the kelly, which turns the drill string. Largely replaced on modern rigs by top drive systems, but still used on some older rigs.

An unplanned influx of formation fluids (oil, gas, or water) into the wellbore, occurring when formation pressure exceeds the hydrostatic pressure of the drilling fluid. A kick is the precursor to a blowout if not properly controlled. Warning signs include increased flow rate, pit gain, drilling break, and gas cut mud.

A high-pressure line connected to the BOP stack used to pump heavy drilling fluid (kill mud) into the well during well control operations. The kill line allows fluid to be pumped into the well even when the BOP is closed.

The horizontal section of a directional or horizontal well that runs through the producing formation. In U.S. shale plays, laterals typically range from 5,000 to 15,000 feet (approximately 1–3 miles), with longer laterals becoming increasingly common to improve economics.

Natural gas cooled to approximately -260°F (-162°C), at which point it becomes a liquid and occupies about 1/600th of its gaseous volume. LNG allows natural gas to be shipped by tanker to markets not connected by pipeline. Major U.S. LNG export facilities are on the Gulf Coast.

The loss of drilling fluid into the formation through natural fractures, vugs, or induced fractures. Results in a drop in the mud pit level and loss of hydrostatic pressure, which can lead to a kick. Treated with lost circulation materials (LCM) such as nut shells, mica, or cellophane.

The practice of recording formation evaluation measurements (resistivity, density, porosity, gamma ray) during the drilling process using sensors integrated into the bottomhole assembly. Provides real-time geological data for geosteering and formation evaluation without the need to stop drilling.

A unit of measurement for natural gas volume. M is the Roman numeral for thousand. MMCF = million cubic feet. BCF = billion cubic feet. TCF = trillion cubic feet. Gas production is reported as MCF/day or MMCF/day.

A small platform near the top of the derrick where the derrickhand works during tripping operations, handling the upper end of drill pipe stands as they are pulled from or run into the hole. One of the highest and most physically demanding positions on the rig.

A service company specialist responsible for designing, mixing, monitoring, and maintaining the drilling fluid (mud) system. Tests mud properties (weight, viscosity, pH, solids content) regularly and adjusts the formulation to address wellbore conditions. Also called a drilling fluids engineer.

The process of monitoring and recording drilling parameters (rate of penetration, weight on bit, torque, pump pressure) and analyzing rock cuttings brought to the surface by the drilling fluid. Mud loggers identify the formations being drilled and detect hydrocarbon shows. Often the first technical role for new graduates.

The density of drilling fluid, measured in pounds per gallon (ppg) or equivalent specific gravity. Mud weight determines the hydrostatic pressure exerted on the formation. Must be maintained between the pore pressure (to prevent kicks) and the fracture gradient (to prevent lost circulation).

Technology that transmits real-time wellbore survey data (inclination, azimuth, toolface) from the bottomhole assembly to the surface while drilling. Enables directional drilling by providing the directional driller with the information needed to steer the well along the planned trajectory.

Hydrocarbons found in natural gas that are liquid at surface conditions. Includes ethane, propane, butane, isobutane, and natural gasoline (pentanes plus). NGLs are separated from the gas stream at processing plants and are valuable petrochemical feedstocks and fuel products.

To assemble (nipple up) or disassemble (nipple down) equipment, particularly the BOP stack. "Nippling up the BOPs" means assembling and installing the blowout preventer stack on the wellhead. A critical operation that must be done correctly for well control.

Time during drilling operations when no progress is being made toward the well objective. Includes equipment failures, weather delays, waiting on equipment or services, fishing operations, and well control events. Minimizing NPT is a key performance metric for drilling operations.

The senior authority on an offshore platform or drilling rig. Responsible for all personnel safety, operations, and emergency response. Equivalent to the captain of a ship — the OIM has ultimate authority and responsibility for the installation. The most senior operational position in offshore operations.

The company that holds the rights to explore for and produce oil and gas from a lease or concession. The operator manages and directs drilling and production operations, often hiring service companies and drilling contractors. Examples: ExxonMobil, Chevron, EOG Resources, Pioneer Natural Resources.

The practice of drilling multiple wells from a single prepared location (pad). Reduces the environmental footprint, allows the rig to quickly skid or walk to the next well location, and improves operational efficiency. Standard practice in modern unconventional development — a single pad may support 8–24+ wells.

The process of creating holes in the production casing and cement to allow formation fluids to flow into the wellbore. Performed using shaped explosive charges (perforating guns) lowered into the well on wireline or tubing. Perforation design (shot density, phasing, penetration depth) affects well productivity.

The process of permanently sealing a well that is no longer productive or needed. Involves placing cement plugs at designated intervals, cutting and pulling casing, and restoring the surface location. Regulated by state agencies to prevent environmental contamination and ensure long-term wellbore integrity.

The pressure of the fluids within the pore spaces of the reservoir rock. Also called formation pressure. Normal pore pressure follows a hydrostatic gradient of about 0.433 psi/ft (for freshwater). Formations with higher-than-normal pore pressure are called overpressured or geopressured.

Granular material (typically sand, resin-coated sand, or ceramic beads) pumped into hydraulic fractures to hold them open after pumping pressure is released. Without proppant, the fractures would close under the weight of the overlying rock, eliminating the flow pathway.

The pipe installed inside the production casing through which oil and gas flow from the reservoir to the surface. Smaller in diameter than casing, production tubing protects the casing from corrosive well fluids and allows for easy intervention and workover operations.

An older term for a production operator or lease operator — the person responsible for monitoring and maintaining producing wells and associated production equipment on a daily basis. Still widely used in parts of Oklahoma, Kansas, and other mid-continent producing regions.

The study of how reservoir fluids (oil, gas, water) behave under varying conditions of pressure, volume, and temperature. PVT analysis is essential for reservoir engineering, production forecasting, and facilities design.

The speed at which the drill bit advances through the rock, measured in feet per hour (ft/hr). ROP depends on rock hardness, bit type, weight on bit, rotary speed, hydraulics, and drilling fluid properties. Higher ROP reduces drilling time and cost.

A subsurface rock formation with sufficient porosity and permeability to store and transmit hydrocarbons. Reservoir rock is typically sandstone, limestone, or dolomite. A reservoir must be overlaid by an impermeable seal (cap rock) to trap the hydrocarbons.

The process of disassembling, transporting, and reassembling a drilling rig from one location to another. Land rig moves involve dozens of truckloads and can take 3–7 days. Offshore rig moves involve towing or self-propulsion and may take days to weeks depending on distance.

A member of the drilling rig crew who works on the rig floor, handling drill pipe, operating tongs, and performing manual labor during drilling operations. The term is used both as a specific position title and as a general term for rig workers. Historically one of the toughest jobs in the oil field.

The most entry-level position on a drilling rig or oilfield operation. Responsible for general labor including cleaning, painting, grounds maintenance, loading and unloading supplies, and assisting other crew members. The starting point for most rig careers.

A rotating platform on the rig floor that transfers rotational energy to the drill string via the kelly bushing and kelly. On modern rigs, the rotary table is often supplemented or replaced by a top drive, but it is still used for pipe handling and as a backup rotation method.

A company that provides specialized equipment, personnel, and services to operators and drilling contractors. Major service companies include SLB (formerly Schlumberger), Halliburton, and Baker Hughes. Services include drilling fluids, cementing, wireline, directional drilling, hydraulic fracturing, and well testing.

A fine-grained sedimentary rock formed from compacted clay and silt. Shale formations can be both source rocks (where hydrocarbons are generated) and reservoir rocks (when produced using horizontal drilling and hydraulic fracturing). Major U.S. shale plays include the Permian Basin, Eagle Ford, Bakken, and Marcellus.

To close the valves and stop production from a well, or to close the BOP and stop flow during a well control event. A well may be shut in for economic reasons (low prices), maintenance, or safety. During drilling, the well is shut in when a kick is detected to prevent a blowout.

To begin drilling a new well. The spud date is the date the drill bit first penetrates the ground surface. "We spudded the well on Monday" means drilling started on Monday.

Two or three joints of drill pipe connected together and stored vertically in the derrick during tripping operations. A stand of three joints is approximately 90 feet long. Handling pipe in stands rather than individual joints speeds up tripping operations.

A condition where the drill string cannot be rotated, raised, or lowered. Causes include differential sticking (the pipe is held against the wellbore wall by differential pressure), mechanical sticking (key seats, undergauge hole), and pack-off (cuttings accumulation around the pipe). Stuck pipe is one of the most costly drilling problems.

Equipment or operations located on the seafloor, below the water surface. Subsea systems include wellheads, Christmas trees, manifolds, and flowlines installed on the ocean floor in deepwater operations where surface facilities are not practical.

The second casing string set in a well (after conductor), typically run to 1,000–3,000 feet. Its primary purpose is to protect freshwater aquifers from contamination by drilling fluids and wellbore fluids. Required by regulation in all U.S. states, with minimum depth requirements set by state agencies.

The area within a reservoir with the best production potential, characterized by optimal thickness, porosity, permeability, pressure, and hydrocarbon saturation. Identifying and targeting sweet spots is a primary goal of exploration and geosteering.

The maximum depth reached by a well, measured along the wellbore (measured depth) or vertically (true vertical depth). "The well reached TD at 22,000 feet measured depth."

The senior drilling supervisor on a rig, responsible for overall rig operations, crew management, and coordination with the operator's company man. The toolpusher manages two crews (day and night shifts) and handles logistics, maintenance, and safety. Also called a rig manager or senior toolpusher.

A motor suspended from the traveling block that rotates the drill string from above. Replaced the kelly and rotary table on most modern rigs. Advantages include the ability to drill with stands (90 feet) instead of singles (30 feet), faster connections, and the ability to rotate and circulate while tripping.

A work shift on a drilling rig, pronounced "tower" in the oil field. The daylight tour (day shift) and night tour (night shift) are each 12 hours. "He's on the night tour" means he works the night shift.

The process of pulling the drill string out of the hole (trip out or POOH — Pull Out Of Hole) or running it back in (trip in or RIH — Run In Hole). Tripping is done to change the bit, run casing, or perform other downhole operations. A full trip on a deep well can take 8–16 hours.

See Production Tubing. Also used generically to refer to any small-diameter pipe used in wellbore operations.

A planned, periodic shutdown of a refinery, processing plant, or production facility for maintenance, inspection, repair, and equipment replacement. Turnarounds can last 2–8 weeks and require hundreds of additional workers. A major source of temporary employment in the oil and gas industry.

The hole drilled into the earth. Includes both the open hole (uncased portion) and the cased portion. The path of the wellbore from surface to total depth.

The surface termination of a wellbore, providing the structural and pressure-containing interface between the casing strings, the tubing, and the surface control equipment (BOP during drilling, Christmas tree during production).

Any operation performed on a well after initial completion to maintain, restore, or improve production. Includes wireline operations, coiled tubing work, acidizing, workovers, and fishing. May be performed without a full workover rig.

A continuous record of formation properties measured as a function of depth. Logs measure properties like resistivity, porosity, density, natural radioactivity (gamma ray), and acoustic velocity. Used for formation evaluation, correlation, and completion design. Can be run on wireline or measured while drilling (LWD).

The amount of downward force applied to the drill bit, measured in thousands of pounds (klbs). WOB is provided by the weight of the drill collars in the BHA, not by pushing down with the drill pipe. Optimal WOB depends on bit type, formation, and drilling objectives.

Major maintenance or remedial operation on a producing well that requires pulling the production tubing and/or other downhole equipment. Common workover operations include replacing ESPs, squeezing perforations, sidetracking, and deepening the well. Requires a workover rig or well servicing unit.

A slender metal cable used to lower and retrieve tools, instruments, and measurement devices into and out of a well. Braided wireline (slickline) is a solid wire used for mechanical operations. Electric wireline (e-line) has conductors for transmitting data from downhole logging tools to the surface.

A specific geological interval or formation in the subsurface. Wells may produce from multiple zones, each requiring separate perforation and potentially different completion techniques. "The Wolfcamp A zone" or "the upper zone" identifies a particular producing interval.

A completion technique where hydraulic fracturing alternates between two adjacent horizontal wells. While one well is being perforated, the other is being pumped, minimizing equipment downtime. Creates stress interactions between the wells that can improve fracture complexity and production.