The Microlog - A New Electrical Logging Method for Detailed Determination of Permeable Beds

Abstract

Abstract A new electrical logging method is described which measures the resistivity ofsmall volumes of material near and behind the wall of the bore holes. The verysmall electrode systems used are supported in the face of the flexibleinsulator pad of appropriate dimension maintained in contact with the wall. Themeasurements are recorded simultaneously with two different electrodespacings. This method provides a very detailed record of the formations and particularlyof the permeable beds traversed by the bore holes. The application of MicroLog to the location of permeable zones within compactformations, such as limestone, and to the determination of the amount of sandexisting in sequences of thin shale and sand streaks is discussed. The possibility of quantitative determination, providing a step toward theevaluation of formation porosity and mud cake thickness, is also examined. Field examples illustrate the essential features of the method. Introduction In conventional electrical logging, the spontaneous potential (SP) log is usedto delineate the permeable beds, and the resistivity logs are used primarily toprovide indications concerning the fluid content of the beds. When the formations are much more resistive than the mud, as happens, forexample, in limestone fields, the SP currents are short-circuited by the moreconductive mud column, with the result that the SP log is quite rounded. Inthat case, the SP log generally gives the approximate location of the permeableformations but it cannot be used for an accurate determination of theboundaries of each permeable bed. Solutions for the problem of obtaining a better determination of the permeablebeds in limestone fields were developed from two angles. One approach consistedin improvements to the logging of the SP, as given by Selective SP logging andStatic SP logging. These new methods, which have been described in an earlierpaper, give good results when the mud is not too salty, but they are still in asomewhat experimental stage, mostly because the development efforts have latelybeen concentrated on another approach to the problem, i.e., the MicroLog. T.P. 2880