The Varying Radial Geometrical Factors of the Induction Log
Authors: Woodhouse Richard , Taylor Philip A. 
Published: December 2007

This paper was prepared for the SPE-European Spring Meeting 1974 of the Society of Petroleum Engineers of AIME, held in Amsterdam, the Netherlands, May 29–30, 1974. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
Discussion of this paper is invited. Three copies of any discussion should be sent to the Netherland Section of the Society of Petroleum Engineers, P. O. Box 228, The Hague, the Netherlands. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
Induction logging theory, including the skin and propagation effects has been previously presented in the literature. previously presented in the literature. This paper reviews the radial response of a typical modern deep-reading multi-coil sonde in both homogeneous and invaded formations. For the case of a homogeneous medium of 0.2 ohm.m half the measured signal is shown to be derived outside a diameter of 60". The standard geometrical factor (no skin effect) suggests that half the signal is derived from outside 130".
The function of the skin-effect correction circuit is described. It provides accurate corrected resistivities provides accurate corrected resistivities for homogeneous formations.
For invaded formations the pseudo-geometrical factor is shown to vary for all pseudo-geometrical factor is shown to vary for all values of Rxo and Rt. A set of pseudo-geometrical factor curves is presented. The use of these curves improves estimates of Rt when only an induction log is available.
If the induction tool is used with salty muds in high porosity formations it is shown that Rt can be seriously underestimated if the classical Geometrical Factor is used and that it is possible to miss hydrocarbons if invasion is not shallow. This reinforces the usual recommendation that the induction log alone should not be relied upon in very salty muds.
When both laterolog and induction logs are available use of a new series of charts for various Rxo will lead to improved estimates of Rt and di.

Introduction
The induction log is in common use in the oil industry as a means of measuring formation resistivity. The interpretation of resistivity logs is complicated by many factors, including invasion. Invasion profiles can only be evaluated if sufficient profiles can only be evaluated if sufficient different resistivity measurements are available and if the radial response of the tools is well defined.
The radial response of the induction log was therefore investigated. This required the use of complex mathematics and a knowledge of the induction tool construction.