SPWLA Glossary

(1) That space between a drill pipe and the 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 beds when both hydrocarbon and interstitial water phases have mobility and their mobilities are different.

When R_xo > R_o the annulus will be more conductive than the flushed zone (R_xo) or the uninvaded oil-bearing zone (R_t). When R_xo < R_o, the resistivity of the annulus will be intermediate between that of the flushed zone (R_xo) and the uninvaded oil-bearing zone (R_t).