WELLOG WALL RESISTIVITY
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Techniques of Water-Resources Investigations of the
Application of Borehole Geophysics to Water Resources Investigations (chapter E1), 1971,1976
By: W. Scott Keys and L. M. MacCary
WALL RESISTIVITY DEVICES:
The unfocused wall-resistivity devices measure mainly the resistivity of the mud cake. Wall resistivity devices a very short spaced electrode devices that are mounted on motorized caliper pads so that they can be compressed against the borehole wall. They provide fine lithologic detail and are also used to delineate porous and non-porous formations on the basis of the presence or absence of mud cake. Micro-normal and micro-lateral electrode configurations are used.
The electrodes are dime sized contacts spaced one inch apart on an insulated pad. The electrodes are designated as A, M, M2 from bottom to top. The electrode group AM2 provides a micro-normal configuration with AM spacing of 2 inches. The electrode group AMM2 provides a micro-lateral (three active electrodes) with an AO of 1 ½ inches.
The micro-lateral sees only 1 ½ inches into the wall of the hole. The micro-normal measures resistivities just beyond the mud cake and gives a resistivity that is mostly influenced by formation resistivities. When the 2 inch normal continuously records a higher resistivity than the micro-lateral, it indicates a uniformly thick mud cake. In general, a uniformly thick mud cake occurs on more porous formations. In shale, there is no mud cake, and both micro-curves read the same resistivity.
Determination of permeability using Sxo:
Sxo = (Rmf/Rxo)1/2
Note: Rmf measurement is routinely obtained from mud analysis using a mud cup.
Determining porosity from micro-resistivity:
Use this chart for determining porosity from micro-resistivity.
It is possible that a resurgence of these very useful resistivity methods will occur due to their relative low cost and high amount of geophysical borehole information provided.
For more information on wall resistivity devices and methods contact firstname.lastname@example.org.