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{{#category_index:A|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.

FIG. I-6. Invaded zone. (Courtesy Schlumberger.)

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