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abandon |
To cease producing hydrocarbons from a well when it becomes unprofitable. Sometimes, before a well is abandoned, some of the casing is removed and salvaged. Usually, one or more cement plugs are placed in the borehole to prevent migration of fluids between the various formations. |
abnormal pressure |
Any pore pressure which differs from normal pore pressure. |
absolute permeability |
The permeability of a rock measured when only one fluid phase is present in the rock. Usually measured in millidarcies or darcies. |
acid stimulation |
Acidizing of a well. Process of introducing acid into a well to dissolve acid soluble materials, in the wellbore or in reservoir rock near the wellbore, which might be restricting fluid flow. Acid-soluble materials in the reservoir rock might occur naturally or might have been introduced during the drilling or completion process of the well. Commonly used acids are hydrochloric and mixtures of hydrochloric and hydrofluoric; although other mineral acids and organic acids have been used, notably citric acid and acetic acid. |
acoustic log |
In acoustic logging, a transmitter located in the borehole emits a pulse of mechanical energy which is recorded by one or more receivers located in the borehole some distance away from the transmitter. In full waveform acoustic logging, the complete acoustic wave at each receiver is recorded digitally. The character of the acoustic signal that is detected by the receivers is affected by, among other things, the mechanical properties of the rock around the borehole. |
acoustic travel time |
The total time required for a specific acoustic wave to travel from one point to another. |
acoustic wave |
An elastic wave, seismic wave. A sound wave in which the disturbance propagated through a medium is an elastic deformation of the medium. The wave train which results from the emission of an acoustic pulse by a well logging device is detected by one or more detectors mounted on the sonde and various aspects of it are measured (see acoustic log). The wave train is a composite of various modes of energy transfer. The first arrival usually results from compressional waves traveling in the formation; the inverse of its velocity is measured by sonic tools. A second arrival is sometimes identified as shear waves traveling in the formation. The velocity of the compressional waves traveling through the mud is frequency dependent and has relatively high frequency content. On the other hand, surface waves of relatively low frequency travel along the borehole wall at lower velocities and are usually called tube waves or pseudo-Stoneley waves. See also wave train. |
air drilling |
A drilling technique whereby gases (typically compressed air or nitrogen) are used to cool the drill bit and lift cuttings out of the wellbore, instead of the more conventional use of liquids. The advantages of air drilling are that it is usually much faster than drilling with liquids and it may eliminate lost circulation problems. The disadvantages are the inability to control the influx of formation fluids into the wellbore and the destabilization of the borehole wall in the absence of the wellbore pressure typically provided by liquids. |
anhydrite |
The common name for the naturally occurring calcium sulphate. Anhydrite is calcium sulphate (CaSO4 ) existing in the orthorhombic crystalline form. The anhydrite crystal cannot become a gypsum crystal (monoclinic form) in a single hydration step. Anhydrite must first become dissociated in solution before it can crystallize as gypsum (or recrystallize as anhydrite). Compare gypsum. |
anisotropy |
The property of a rock which allows it to show different responses or measurements when measured along different axes. Microscopic anisotropy relates to the variation of a property measured perpendicular to the bedding from that measured parallel to the bedding because plate-like mineral grains and interstices tend to orient themselves parallel to the bedding. Macroscopic anisotropy relates to the variation resulting from thin beds and fracture planes whose properties differ appreciably from those of the remaining volume over which a measurement is made. |
annulus |
The space between two concentric objects, such as between the wellbore and casing or between casing and tubing, where fluid can flow |
anticline |
A fold or fold system in the form of an arch |
apparent resistivity |
Resistivity recorded on a resistivity well log which may differ from true resistivity because of the influence on the measured response caused by the presence of the mud column, invaded zone, adjacent beds, borehole cavities, etc. These values may need correction prior to use in any computation. |
aquifer |
A water-bearing bed of strata, either by virtue of its porosity or because it is pervious |
arenaceous |
A group of detrital sedimentary rocks, typically sandstones, in which the particles range in size from 1/16mm to 2mm. |
argillaceous |
Rocks or substances composed of clay minerals, or having notable proportion of clay in their composition such as shale and slate. |
arkose |
arenaceous rocks |
artificial lift |
Any method used to raise oil to the surface through a well, after reservoir pressure has declined to the point at which the well no longer produces by means of natural energy. |
asphalt |
A dark coloured bitumen found as a natural deposit. Crude oil of high asphaltic content will leave asphalt as a residue when subjected to distillation to remove the lighter fractions. Asphalt is dark brown or black in colour and at room temperatures usually is solid. |
attenuation |
When a form of energy is propagated through a medium, its amplitude is decreased. This decrease is termed attenuation. |
authigenic |
A term implying a development in place during or after deposition. |
azimuth |
(1) In well logging. In a horizontal plane, it is the clockwise angle of departure from magnetic north. |
