1. n. 
Alternate Form: annuli
2. n. 
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.