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Oil and Gas Glossary

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Excessive borehole enlargement caused by solvent and erosional action by the drilling fluid.

water-base mud

A drilling mud in which the continuous phase is water.

water cut

The volume fraction of water in the total volume of liquid produced from a well.


Of a well, having gone to water.

water encroachment

The movement of water into a producing formation as the formation is depleted of oil and gas by production

water flooding

A method of secondary recovery in which water is injected into a reservoir in order to move additional quantities of oil toward producing wells.

water wet

Describing the preference of a solid to be in contact with a water phase rather than an oil or gas phase. Water-wet rocks preferentially imbibe water. Generally, sandstones and carbonates are water-wet before contact with crude oil, but may be altered by components of the crude oil to become oil-wet. Certain minerals, as well as different crystallographic faces of the same mineral, may be variably prone to being oil- or water-wet.

well bore

A borehole. The bore of a well, whether cased or uncased. Often modernized to one word, wellbore, particularly when the term is used as an adjective.

wellbore storage effect

Distortions in the reservoir response due to wellbore storage. The characteristic trends are an early unit slope trend with pressure change and the derivative overlain on the log-log plot, followed by a "hump" in the pressure derivative that gradually disappears as reservoir trends become recognizable. Complex behaviour in the wellbore, such as wellbore phase distribution, can result in a more complex transient trend. A crucial part of the transient analysis is to distinguish the effects of wellbore storage from the interpretable reservoir response.

well completion

The activities and methods necessary to prepare a well for the production of oil and gas; the method by which a flow line for hydrocarbons is established between the reservoir and the surface.


The surface termination of a wellbore that incorporates facilities for installing casing hangers during the well construction phase. The wellhead also incorporates a means of hanging the production tubing and installing the Christmas tree and surface flow-control facilities in preparation for the production phase of the well.

well log

wireline log, borehole log. The product of a survey operation, also called a survey, consisting of one or more curves. Provides a permanent record of one or more physical measurements as a function of depth in a well bore.


The preference of a solid to contact one liquid or gas, known as the wetting phase, rather than another. The wetting phase will tend to spread on the solid surface and a porous solid will tend to imbibe the wetting phase, in both cases displacing the nonwetting phase. Rocks can be water-wet, oil-wet or intermediate-wet. The intermediate state between water-wet and oil-wet can be caused by a mixed-wet system, in which some surfaces or grains are water-wet and others are oil-wet, or a neutral-wet system, in which the surfaces are not strongly wet by either water or oil. Both water and oil wet most materials in preference to gas, but gas can wet sulphur, graphite and coal.


A long, steel casing that uses an inclined plane to cause the bit to deflect from the original borehole at a slight angle. Whipstocks are commonly used in controlled directional drilling, to straighten crooked boreholes, or to sidetrack to avoid unretrieved fish.

wireline log

Well log run on logging cable.

work over

To perform one or more of a variety of remedial operations on a producing oil well to try to increase production.

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