Oil and Gas Terms Beginning with “A” — Page 5

An amplitude anomaly is a localized deviation in seismic reflection amplitude from the regionally expected background trend, manifesting as an unusually high amplitude (bright spot), unusually low amplitude (dim spot),… Read more →

Amplitude variation with offset (AVO) is the change in seismic reflection amplitude as a function of source-receiver distance (offset) or, equivalently, as a function of the angle of incidence of the seismic ray at the… Read more →

Anaerobic describes any process, organism, or environment occurring in the complete or near-complete absence of molecular oxygen (O₂), in contrast to aerobic conditions where dissolved oxygen is present and governs… Read more →

In petroleum geology and reservoir engineering, an analog is a well-characterised geological setting, producing field, or surface outcrop that is used as a reference model to estimate the properties, performance, and… Read more →

The angle of approach is the acute angle formed between an incoming seismic ray and the reflecting or refracting interface at the precise point of intersection, measured from the interface plane itself rather than from… Read more →

The angle of incidence is the acute angle formed between an incoming seismic ray and the normal (perpendicular) to a reflecting or refracting interface at the point of intersection, measured in degrees from the normal… Read more →

Angular dispersion is the variation of seismic wave velocity as a function of propagation direction, encompassing both azimuth (compass bearing within the horizontal plane) and dip (angle from vertical or from… Read more →

An angular unconformity is a buried erosional surface separating a younger sequence of sedimentary strata from an older sequence whose bedding planes dip at a measurably different angle, so that the two packages appear… Read more →

Anhydrite is the anhydrous (water-free) calcium sulfate mineral with the chemical formula CaSO 4 , belonging to the orthorhombic crystal system. It forms transparent to white, gray, or pale blue crystals with a vitreous… Read more →

The aniline point test is a standardized laboratory procedure defined by ASTM D611 and ISO 2977 that determines the minimum temperature at which equal volumes of aniline (C 6 H 5 NH 2 , a nitrogen-bearing aromatic… Read more →

An anion is an ion that carries a net negative electric charge, formed when a neutral atom or molecule acquires one or more additional electrons beyond its neutral electron configuration. The charge of an anion is… Read more →

An anisotropic formation is a subsurface rock body whose physical properties vary measurably with the direction of observation or measurement. In an isotropic material, a seismic wave, an electrical current, a fluid… Read more →

Anisotropy is the property of a material or medium in which one or more physical properties have different values when measured along different directions. The word derives from the Greek for "not equal measure," and it… Read more →

An annubar is a multi-port averaging Pitot tube flow meter inserted diametrically across a pipe to measure the volumetric or mass flow rate of gas, liquid, or steam by sensing the differential pressure between the… Read more →

An annular blowout preventer (annular BOP) is a hydraulically actuated wellbore barrier that seals the annular space around any drill pipe, casing, tubing, or wireline that may be in the wellbore, or around an open… Read more →

Annular flow is a multiphase flow regime in which the lighter fluid (typically gas) occupies the central core of a pipe or wellbore at high velocity while the heavier fluid (typically liquid) exists simultaneously as a… Read more →

Annular gas flow (AGF) is the migration of formation gas upward through the annular space between a casing string and the borehole wall, traveling through or around the cement sheath that was placed during primary… Read more →

Annular pressure is the fluid pressure acting within the annular space between two concentric tubular strings or between a tubular string and the open borehole wall, measured at any point along the wellbore axis from… Read more →

Annular production is the production of hydrocarbons or other reservoir fluids through the annular space between the production tubing string and the innermost casing string, rather than exclusively through the interior… Read more →

The annular space is the ring-shaped (toroidal) geometric void that exists between the outer surface of one cylindrical object and the inner surface of a larger concentric cylinder surrounding it. In oil and gas well… Read more →

Annular velocity (AV) is the average linear speed at which a fluid travels upward through the annular space between the drill string and the borehole wall or casing inner diameter during drilling operations, expressed… Read more →

Annuli is the plural form of annulus , referring to two or more separate ring-shaped spaces that exist concurrently within a single wellbore between successive concentric tubular strings. A modern oil and gas well is a… Read more →

The annulus is the ring-shaped void formed between any two concentric cylinders inside a wellbore, and it is one of the most consequential geometric features in well engineering. Every cased-and-cemented well contains… Read more →

An anode is the electrode in an electrochemical cell from which conventional current flows into the electrolyte, or equivalently, the site where oxidation (electron loss) occurs. In practical petroleum industry terms,… Read more →

An anomaly in petroleum geoscience is any observed value of a physical, chemical, or geological property that deviates measurably from the expected background level, signalling a subsurface condition that may represent… Read more →

Anoxic describes any environment in which dissolved free oxygen is absent or reduced to negligible concentrations, generally below 0.2 millilitres of O₂ per litre of water. The prefix derives from the Greek an-… Read more →

An anticlinal trap is a structural hydrocarbon trap whose geometry is controlled by an anticline , an upwardly arching fold in rock where the strata curve to a high point and then dip away on all sides. Hydrocarbons… Read more →

An anticline is an arch-shaped fold in layered rock in which the strata are upwardly convex: the beds curve upward to a crest and then dip away on both flanks, with the oldest rocks exposed or encountered in the core of… Read more →

An antifoam agent (also called a defoamer) is a surface-active chemical additive that prevents or rapidly collapses foam generated during mixing, pumping, or processing operations in oil and gas applications. Foam is a… Read more →

The antisqueeze effect is a thin-bed measurement artifact that occurs in laterolog resistivity tools when a relatively high-resistivity bed is sandwiched between beds of lower resistivity. Under this geometrical… Read more →