Interpretation techniques for induction logs

SPE71701

CASE STUDIES USING ADVANCED INTERPRETATION TECHNIQUES FOR INDUCTION LOGS

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SPE 71707 Case Studies Using Advanced Interpretation Techniques for Induction Logs
Thomas D. Barber, SPE, and Frank Shray, SPE, Schlumberger Oilfield Services

Copyright 2001, Society of Petroleum Engineers Inc. This paper was prepared for presentation at the 2001 SPE Annual Technical Conference andExhibition held in New Orleans, Louisiana, 30 September–3 October 2001. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented,does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineersis prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

maximum-entropy processing methods. Introduction The largestproblem with resistivity and induction logging measurements is the very large volume of investigation needed to obtain adequate radial depth of investigation to overcome invasion. The volume that contributes to the logs can be many meters above and below the tool. To make useful logs, the effects of the volume above and below the layer of interest must be carefully removed. In the old dual-inductiontools, this was attempted by hardware focusing of the coils. Doll introduced the concept of deconvolution (or, in essence, software focusing) of induction logs in the 1950’s, but the technology to do this was not realized until the mid 1980’s with the introduction of the Phasor* induction tool. Phasor processing1, 2 allowed the shoulder effect to be removed from above and below a layer about 8 ftthick. Although higher resolutions were available, in practice, borehole rugosity complicated the enhancement of resolution. The Array Induction Imager (AIT*) family tools—AIT, PlatformExpress* AIT (AIT-H) , SlimAccess* AIT (SAIT), Xtreme* AIT (XAIT), and SlimXtreme* AIT (QAIT)3, 4— produce wellsite resistivity logs having simultaneously high resolution and high rejection of borehole rugosityeffects. These tools produce logs that are corrected for the most common environmental effects—borehole effect and shoulder effect— and that are processed radially to obtain Rxo, Rt, and invasion radii, all in real time. However, the real-time processing does not account for the very non-linear effect of dip. To remove dip effect the AIT data are processed through maximum-entropy resistivity loginversion (Merlin) processing at the computer center. Figure 1 shows a high-level flow diagram of these processing options. Wellsite Processing Borehole Correction The first link in the chain of AIT-family log processing is to correct the raw data from all eight arrays for borehole effect. Borehole corrections5 for the AIT tools are based on inversion
* Throughout this paper an asterisk (*) is used todenote a Mark of Schlumberger.

Abstract Because of their considerable range of investigation, inductionlogging instruments are susceptible to a variety of environmental effects. The problems arising from high relative dip or high shoulder-bed contrasts have proven difficult to solve. The industry trend of using oil-based muds to drill these formations has prevented the use of the preferred...