3D Survey:The acquisition of seismic data as closely spaced receiver and shot lines such that there typically are no significant gaps in the subsurface coverage. A 2D survey commonly contains numerous widely spaced lines acquired orthogonally to the strike of geological structures and a minimum of lines acquired parallel to geological structures to allow line-to-line correlation of the seismic data and interpretation and mapping of structures.

3D Seismic Data:A set of numerous closely-spaced seismic lines that provide a high spatially sampled measure of subsurface reflectivity. Typical receiver line spacing can range from 300 m [1000 ft] to over 600 m [2000 ft], and typical distances between shotpoints and receiver groups is 25 m [82 ft] (offshore and internationally) and 110 ft or 220 ft [34 to 67 m] (onshore USA, using values that are even factors of the 5280 feet in a mile). Bin sizes are commonly 25 m, 110 ft or 220 ft. The resultant data set can be "cut" in any direction but still display a well sampled seismic section. The original seismic lines are called in-lines. Lines displayed perpendicular to in-lines are called crosslines. In a properly migrated 3D seismic data set, events are placed in their proper vertical and horizontal positions, providing more accurate subsurface maps than can be constructed on the basis of more widely spaced 2D seismic lines, between which significant interpolation might be necessary. In particular, 3D seismic data provide detailed information about fault distribution and subsurface structures. Computer-based interpretation and display of 3D seismic data allow for more thorough analysis than 2D seismic data.

Acidize: The treatment of a reservoir formation with a stimulation fluid containing a reactive acid. In sandstone formations, the acid reacts with the soluble substances in the formation matrix to enlarge the pore spaces. In carbonate formations, the acid dissolves the entire formation matrix. In each case, the matrix acidizing treatment improves the formation permeability to enable enhanced production of reservoir fluids. Matrix acidizing operations are ideally performed at high rate, but at treatment pressures below the fracture pressure of the formation. This enables the acid to penetrate the formation and extend the depth of treatment while avoiding damage to the reservoir formation.

Anticline:An arch-shaped fold in rock in which rock layers are upwardly convex. The oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur. Anticlines form many excellent hydrocarbon traps, particularly in folds with reservoir-quality rocks in their core and impermeable seals in the outer layers of the fold. A syncline is the opposite type of fold, having downwardly convex layers with young rocks in the core.

Amplitude Anomaly:An abrupt increase in seismic amplitude that can indicate the presence of hydrocarbons, although such anomalies can also result from processing problems, geometric or velocity focusing or changes in lithology. Amplitude anomalies that indicate the presence of hydrocarbons can result from sudden changes in acoustic impedance, such as when a gas sand underlies a shale, and in that case, the term is used synonymously with hydrocarbon indicator.

Artificial Lift:Any system that adds energy to the fluid column in a wellbore with the objective of initiating and improving production from the well. Artificial-lift systems use a range of operating principles, including rod pumping, gas lift and electrical submersible pumps.

Barrels of Oil per Day:A common unit of measurement for the daily volume of crude oil produced by a well or from a field. The volume of a barrel is equivalent to 42 US gallons, abbreviated BOPD.

Base Log:The log used as the reference for depths in the well. Each log may record formation features at slightly different depths due to their different response and the difficulty of aligning depths. It is therefore important to select one log to which the other logs are depth matched, and which is used as the reference for well-to-well correlation and mechanical operations such as perforating. The gamma ray is most often used as the base log since it can be recorded in both open and cased holes.

BCF:Abbreviation for billion cubic feet, a unit of measurement for large volumes of natural gas.

Bottom Hole Pressure (BHP): The downhole pressure, measured or calculated at a point of interest, generally the top of the perforated interval.

Casing (Drilling): large diameter pipe lowered into an openhole and cemented in place. The well designer must design casing to withstand a variety of forces, such as collapse, burst, and tensile failure, as well as chemically aggressive brines. Most casing joints are fabricated with male threads on each end, and short-length casing couplings with female threads are used to join the individual joints of casing together, or joints of casing may be fabricated with male threads on one end and female threads on the other. Casing is run to protect fresh-water formations, isolate a zone of lost returns or isolate formations with significantly different pressure gradients. The operation during which the casing is put into the wellbore is commonly called "running pipe." Casing is usually manufactured from plain carbon steel that is heat-treated to varying strengths, but may be specially fabricated of stainless steel, aluminum, titanium, fiberglass and other materials.

Casing (Completion): steel pipe cemented in place during the construction process to stabilize the wellbore. The casing forms a major structural component of the wellbore and serves several important functions: preventing the formation wall from caving into the wellbore, isolating the different formations to prevent the flow or crossflow of formation fluids, and providing a means of maintaining control of formation fluids and pressure as the well is drilled. The casing string provides a means of securing surface pressure control equipment and downhole production equipment, such as the drilling blowout preventer (BOP) or production packer. Casing is available in a range of sizes and material grades.

Cement Squeeze:A remedial cementing operation designed to force cement into leak paths in wellbore tubulars. The required squeeze pressure is achieved by carefully controlling pump pressure. Squeeze cementing operations may be performed to repair poor primary cement jobs, isolate perforations or repair damaged casing or liner.

Completion:A generic term used to describe the assembly of downhole tubulars and equipment required to enable safe and efficient production from an oil or gas well. The point at which the completion process begins may depend on the type and design of well. However, there are many options applied or actions performed during the construction phase of a well that have significant impact on the productivity of the well.

Choke:A device incorporating an orifice that is used to control fluid flow rate or downstream system pressure. Chokes are available in several configurations for both fixed and adjustable modes of operation. Adjustable chokes enable the fluid flow and pressure parameters to be changed to suit process or production requirements. Fixed chokes do not provide this flexibility, although they are more resistant to erosion under prolonged operation or production of abrasive fluids.

Christmas Tree:An assembly of valves, spools, pressure gauges and chokes fitted to the wellhead of a completed well to control production. Christmas trees are available in a wide range of sizes and configurations, such as low- or high-pressure capacity and single- or multiple-completion capacity.

Compressor:A device that raises the pressure of air or natural gas. A compressor normally uses positive displacement to compress the gas to higher pressures so that the gas can flow into pipelines and other facilities.

Condensate:A natural gas liquid with a low vapor pressure compared with natural gasoline and liquefied petroleum gas. Condensate is mainly composed of propane, butane, pentane and heavier hydrocarbon fractions. The condensate is not only generated into the reservoir, it is also formed when liquid drops out, or condenses, from a gas stream in pipelines or surface facilities.

Continuous Gas Lift:An artificial-lift method in which the gas-lift system is operated on a continuous basis to sustain liquid production at an efficient rate.

Cores:A sample of geologic formation, usually reservoir rock, taken during or after drilling a well. Cores can be full-diameter cores (that is, they are nearly as large in diameter as the drill bit) taken at the time of drilling the zone, or sidewall cores (generally less than 1 in. [2.5 cm] in diameter) taken after a hole has been drilled. Cores samples are used for many studies, some of which relate to drilling fluids and damage done by them.

Cross-Section Map:A diagram of a vertical section through a volume, as opposed to the surface, "bird's eye," or plan view of a map. Cross sections are useful for displaying the types and orientations of subsurface structures and formations.

Dehydrator: A device used to remove water and water vapors from gas. Gas dehydration can be accomplished through a glycol dehydrator or a dry-bed dehydrator, which use a liquid desiccant and a solid desiccant, respectively.

Derrick:The structure used to support the crown blocks and the drillstring of a drilling rig. Derricks are usually pyramidal in shape, and offer a good strength-to-weight ratio. If the derrick design does not allow it to be moved easily in one piece, special ironworkers must assemble them piece by piece, and in some cases disassemble them if they are to be moved.

Disposal Well:A well, often a depleted oil or gas well, into which waste fluids can be injected for safe disposal. Disposal wells typically are subject to regulatory requirements to avoid the contamination of freshwater aquifers.

Downdip: Located down the slope of a dipping plane or surface. In a dipping (not flat-lying) hydrocarbon reservoir that contains gas, oil and water, the gas is updip, the gas-oil contact is downdip from the gas, and the oil-water contact is still farther downdip.

Drill Bit:The tool used to crush or cut rock. Everything on a drilling rig directly or indirectly assists the bit in crushing or cutting the rock. The bit is on the bottom of the drillstring and must be changed when it becomes excessively dull or stops making progress. Most bits work by scraping or crushing the rock, or both, usually as part of a rotational motion. Some bits, known as hammer bits, pound the rock vertically in much the same fashion as a construction site air hammer

Dry Hole:A wellbore that has not encountered hydrocarbons in economically producible quantities. Most wells contain salt water in some zones. In addition, the wellbore usually encounters small amounts of crude oil and natural gas. Whether the well is a "duster" depends on many factors of the economic equation, including proximity to transport and processing infrastructures, local market conditions, expected completion costs, tax and investment recovery conditions of the jurisdiction and projected oil and gas prices during the productive life of the well.

Electric Log:A wireline log of formation resistivity produced by a simple, unfocused arrangement of current emitting and measure electrodes. Conventional electrical devices have four electrodes-current emitting (A), current return (B), measure (M) and measure reference (N)-which may be placed in different configurations. The two most common configurations are the normal and lateral. The currents used are low frequency, typically less than 500 Hz. H.G. Doll recorded the first electrical log on September 5, 1927, in the Pechelbronn field, Alsace, France.

Filtration:The process of separating components of a slurry by leaving the suspended solids as filter cake on a filter medium while the liquid passes through. The process can be either static or dynamic.

Fishing:The application of tools, equipment and techniques for the removal of junk, debris or fish from a wellbore. The key elements of a fishing operation include an understanding of the dimensions and nature of the fish to be removed, the wellbore conditions, the tools and techniques employed and the process by which the recovered fish will be handled at surface.

Flowline:A surface pipeline carrying oil, gas or water that connects the wellhead to a manifold or to production facilities, such as heater-treaters and separators.

Formation:The fundamental unit of lithostratigraphy. A body of rock that is sufficiently distinctive and continuous that it can be mapped. In stratigraphy, a formation is a body of strata of predominantly one type or combination of types; multiple formations form groups, and subdivisions of formations are members

Frac:The process of pumping into a closed wellbore with powerful hydraulic pumps to create enough downhole pressure to crack or fracture the formation. This allows injection of proppant into the formation, thereby creating a plane of high-permeability sand through which fluids can flow. The proppant remains in place once the hydraulic pressure is removed and therefore props open the fracture and enhances flow into the wellbore.

Gas Lift:Reduce the hydrostatic pressure of the fluid column. The resulting reduction in bottomhole pressure allows the reservoir liquids to enter the wellbore at a higher flow rate. The injection gas is typically conveyed down the tubing-casing annulus and enters the production train through a series of gas-lift valves. The gas-lift valve position, operating pressures and gas injection rate are determined by specific well conditions.

Geologic Map:A map showing the type and spatial distribution of rocks at the surface of the Earth. Rock formations are color-coded and symbols for geological structures are annotated, so age relationships are evident. Topographic contours and cultural features can also appear on geologic maps.

Gun Barrel:   A settling tank used for treating oil. Oil and brine are separated only by gravity segregation forces. The clean oil floats to the top and brine is removed from the bottom of the tank. Gun barrels are found predominantly in older or marginal fields. A gun barrel is also called a wash tank.

Natural Gas:A naturally occurring mixture of hydrocarbon gases that is highly compressible and expansible. Methane [CH₄] is the chief constituent of most natural gas (constituting as much as 85% of some natural gases), with lesser amounts of ethane [C₂H₆], propane [C₃H₈], butane [C₄H₁₀] and pentane [C₅H₁₂]. Impurities can also be present in large proportions, including carbon dioxide, helium, nitrogen and hydrogen sulfide.

Oil/Water Ratio:Ratio of the volume percent oil to the volume percent water in an oil mud, where each is a percent of the total liquid in the mud. OWR is calculated directly from the retort analysis of an oil mud. For example, if a mud contains 60 vol.% oil and 18 vol.% water, the oil percentage is [60/(60 + 18)]100 = 77% and the water percent is [18/(60 +18)] = 23%. That OWR is written as 77/23.

Packer:A downhole device used in almost every completion to isolate the annulus from the production conduit, enabling controlled production, injection or treatment

Perforate:To create holes in the casing or liner to achieve efficient communication between the reservoir and the wellbore. The characteristics and placement of the communication paths (perforations) can have significant influence on the productivity of the well

Plug & Abandon:To prepare a well to be closed permanently, usually after either logs determine there is insufficient hydrocarbon potential to complete the well, or after production operations have drained the reservoir.

Prospect:An area of exploration in which hydrocarbons have been predicted to exist in economic quantity. A prospect is commonly an anomaly, such as a geologic structure or a seismic amplitude anomaly, that is recommended by explorationists for drilling a well

Separator:A cylindrical or spherical vessel used to separate oil, gas and water from the total fluid stream produced by a well. Separators can be either horizontal or vertical.

Shale:Shale can include relatively large amounts of organic material compared with other rock types and thus has potential to become a rich hydrocarbon source rock, even though a typical shale contains just 1% organic matter.

Shut-in Pressure:The surface force per unit area exerted at the top of a wellbore when it is closed at either the Christmas tree or the BOP stack. The pressure may be from the formation or an external and intentional source. The SIP may be zero, indicating that any open formations are effectively balanced by the hydrostatic column of fluid in the well. If the pressure is zero, the well is considered to be dead, and can normally be opened safely to the atmosphere.

Spud:To start the well drilling process by removing rock, dirt and other sedimentary material with the drill bit.

Squeeze:The careful application of pump pressure to force a treatment fluid or slurry into a planned treatment zone.

Structure Map:A type of subsurface map whose contours represent the elevation of a particular formation, reservoir or geologic marker in space, such that folds, faults and other geologic structures are clearly displayed

Swabbing:To reduce pressure in a wellbore by moving pipe, wireline tools or rubber-cupped seals up the wellbore. If the pressure is reduced sufficiently, reservoir fluids may flow into the wellbore and towards the surface

Tank Battery:In the tank battery, the oil volume is measured and tested before pumping the oil into the pipeline system.

Total Depth:The planned end of the well, measured by the length of pipe required to reach the bottom.

Waterdrive:A reservoir-drive mechanism whereby the oil is driven through the reservoir by an active aquifer

Well Control:  The technology focused on maintaining pressure on open formations (that is, exposed to the wellbore) to prevent or direct the flow of formation fluids into the wellbore

Wireline: A general term used to describe well-intervention operations conducted using single-strand or multistrand wire or cable for intervention in oil or gas wells.

Workover:   The process of performing major maintenance or remedial treatments on an oil or gas well. In many cases, workover implies the removal and replacement of the production tubing string after the well has been killed and a workover rig has been placed on location.