Dictionary:Annulus
(an’ y∂ l∂s)
1. The space between two concentric cylindrical objects.
2. The space between a drill-pipe and the borehole wall through which the returning drilling fluid (mud) returns to the surface.
3. The space between tubing and casing or between casing and formation.
4. A low-resistance ring about a borehole sometimes produced by invasion of mud filtrate into hydrocarbon-bearing beds. Because of their greater mobility, hydrocarbons may be displaced farther beyond the invaded zone than conductive formation water. See Figure I-6.
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