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Acoustic sonic well logging	Recording the time required for sound to travel a specific distance through rock, using a wireline (see wireline log) of LWD instruments. The rate of travel varies with rock composition, porosity and fluid content (mud).

Annulus	The space between the drill pipe and the borehole, between tubing and casing, or between casing and formation.

API gravity	The weight/unit volume of crude oil or other liquid hydrocarbon, where a specific gravity (SG) of 1.0 is equivalent to 10° API. This figure was developed by the American Petroleum Institute (API) and is measured in degrees. Crude oil is a hydrocarbon liquid with an API of less than 45°. Condensates are hydrocarbon liquids with an API greater than 45°. Produced hydrocarbon liquids can generally be classified as Light oils with an API gravity greater than 31.1°. Intermediate oils with an API gravity from 31.1° to 22.3°. Heavy oils with an API gravity less than 22.3°. Crude oil with API gravity less than 10 °API is referred to as extra heavy oil or bitumen. The formula used to obtain this effect is: API gravity = (141.5/SG) - 131.5

Appraisal well	This is the phase of petroleum operations that immediately follow successful exploratory drilling (see exploration) with the intention to assess the characteristics of a proven hydrocarbon reservoir. During appraisal, delineation wells might be drilled to determine the size of the oil or gas field and how to develop it most efficiently.

Artificial lift	Artificial lift is a system that adds energy to the fluid column in a wellbore with the objective of initiating and improving the production (see oil production) from the well. Basically artificial-lift systems use a range of operating principles, including gas lift, electric submersible pump and rod pumping.

Associated gas	This is a form of natural gas, which usually is found associated with petroleum (see crude oil). Since it was considered as a waste product while searching for oil it was very common to flare or vent the associated gas. Due to environmental aspects (reducing CO2 footprint) and the economical usage (on site electricity generation, reinjection for enhanced oil recovery, etc) less and less associated gas is released into the atmosphere. See also non-associated gas.

Azimuth	Azimuth Direction, as in a compass direction. The clockwise angle of departure from a reference direction (typically geographic) north, measured in a horizontal plane. In diameter and directional surveys, it is the clockwise angle from magnetic north to the tool reference point or electrode. This measurement must be corrected for magnetic declination to compute true azimuth. The azimuth is generally expressed in degrees. (See Azimuthal)

Azimuthal	The characteristic of a logging (See MWD) tool to perform separate measurements in different directions (azimuths) around the axis of the tool. Currently, MWD sensors making azimuthal measurements are limited to density and tend to give measurements in quadrants around the borehole. Some MWD sensors are shielded on one side so that measurements are taken from only (primarily) the unshielded side. These are oriented measurements rather than true azimuthal measurements.

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