1. n. [Drilling]

The space between two concentric objects, such as between the wellbore and casing or between casing and tubing, where fluid can flow. Pipe may consist of drill collars, drillpipe, casing, or tubing.

Alternate Form: annuli

See related terms: annular velocity,  bridge,  casing centralizer,  cement,  cementing,  crossflow,  displacement,  drill collar,  eccentricity,  flapper valve,  pack off

2. n. [Formation Evaluation]

With reference to invasion, a region between the flushed zone and the undisturbed zone containing a buildup of formation water. The annulus forms during invasion and is caused by the different mobilities of oil and water. It only occurs in the presence of both, but is unstable and will dissipate vertically or horizontally with time. The annulus has approximately the same water saturation as the flushed zone but contains formation water. When the formation water is much more saline than the mud filtrate, the annulus forms a conductive ring around the borehole.

This conductivity will cause an induction log to read too low a resistivity, by an amount that depends on its depth of investigation and the radius of the annulus. (Laterologs are little affected since they respond to resistivity, not conductivity.) Often a medium log will be more affected than a deep log so that an annulus can be detected by out-of-order curves (medium curves reading less than either shallow or deep). Array induction logs contain enough information to solve and correct for the effect of the annulus.

See related terms: depth of invasion,  transition zone,  undisturbed zone

3. n. [Well Testing]

The space between two concentric pipe strings, such as between the production tubing and casing in a well. The term may also refer to the space between a pipe string and the borehole wall in an openhole completion or openhole drillstem test (DST).