Principles of Log Calibration and Their Application to Log Accuracy
Authors: Cochrane J.E. 
Published: December 2007

Abstract
Accurate measurement and recording of log data is necessary for effective well evaluation. Calibration data auxiliary to the log, when reviewed with a knowledge of the calibration procedure, will demonstrate the degree of log accuracy. The general method for calibrating logging systems is described. Variations of the general method are described and examples cited. Use of calibration data to confirm log accuracy is discussed and a field example of profitable accuracy control is given. The Appendix presents the necessary detail information that is needed to use calibration data for accuracy control of Induction-Electric/ gamma ray logs.

Introduction
Improved logging techniques have made use of log data for formation evaluation possible on a broad basis. Because of these advances, the accuracy of log measurements has become of prime importance.inaccurate well logs lead to a confused evaluation picture and the advantages of the new techniques are lost. Fig. I represents a case history which illustrates this point. R., analysis with the first run Induction log and Sonic log indicated high water saturation (Table 1). The Induction log had not been calibrated recently and no calibration records were available. On test the well flowed clean oil at 500 B/D. The remainder of the zone was drilled and logs recorded with complete and accurate sets of calibration records. Differences in shale conductivities tended to confirm first run Induction log miscalibration rather than invasion perturbations such as an annulus effect. R, analysis with the second run Induction log gave the expected saturation values. In this case a productive zone was not passed by because of a faulty log, but an expensive drill-stem test was necessary to confirm the presence of oil. If log accuracy can be demonstrated by the calibration records which usually accompany the log then calibration data may be a valuable tool to the oil company in maintaining accuracy control. This paper discusses the theory and methods of log calibration and their application to log accuracy. Hopefully, this information will help oil company personnel to understand and make profitable use of calibration data.

Calibration Theory and Methods
Fig. 2 describes a basic logging system. To calibrate such a system, it is necessary to relate the magnitude of a voltage (recorder signal) to the magnitude of the measured parameter; i.e., resistivity, conductivity, acoustic travel time, etc. The general method used by logging service companies consists of standardizing the recorder's voltage response when the sensing component (sonde) is in one or more environments for which the magnitude of the desired log parameter is accurately known. Such environments are termed "standard media". As an illustration, suppose a hypothetical resistivity system is to be calibrated over a moderate range of resistivities (Fig. 3). One might proceed by first placing the sonde in a standard medium of low resistivity such as salt water, the magnitude of which is accurately calculated from the salinity and temperature. The recording device is adjusted to read this resistivity. The sonde is next placed in a standard medium of high resistivity such as fresh water and the recorder is again adjusted to read the known high value. By alternating between the standard media and making appropriate adjustments, the sensitivity and scale of the recorder is set.
JPT
P. 817ˆ