(1) That space between a drill pipe and the [[formation|formations]] through which the drilling fluid ([[mud]]) returns to the surface.

(2) The space between tubing and casing or between casing and formation.

(3) A ring of [[interstitial water]] produced by invasion processes in hydrocarbon-bearing [[bed|beds]] when both hydrocarbon and [[interstitial water]] phases have mobility and their mobilities are different.

When R<sub>xo</sub> > R<sub>o</sub> the annulus will be more conductive than the flushed zone (R<sub>xo</sub>) or the uninvaded oil-bearing zone (R<sub>t</sub>). When R<sub>xo</sub> < R<sub>o</sub>, the resistivity of the annulus will be intermediate between that of the flushed zone (R<sub>xo</sub>) and the uninvaded oil-bearing zone (R<sub>t</sub>).

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