Some perspective on electrical borehole image logs for the end user

Abstract

Electrical borehole image logs have been available as a commercial wireline service since circa 1986. They have now become a staple for many logging programs, not just in exploratory wells, but frequently for development wells. While the physical technology of the logging tools has progressed and is more robust, the processing and interpretation of the data sets can be still be very perplexing, requiring some art and science from several disciplines. For log QA/QC and data processing, knowledge of the tool design and components, and well logging practices is necessary. For interpretation of data sets, knowledge of geology is essential. Potential pitfalls collecting and interpreting the image data sets include several issues. QA/QC of orientation calibrations, proper application of speed corrections, and depth matching are vital. Image tool pads have a set radius and ideally would match borehole diameter, however, borehole condition, i.e. washouts, key seats, and constrictions can alter borehole radius and thus resulting in pad to borehole wall mismatch, causing some degradation of image quality. Image tools are designed to operate within certain mud system limitations, and changes in mud programs can degrade image quality. Logging speed and pad pressure can affect tool travel and the limits of data transmission. Once a data set is collected, properly QA/QC'd and processed, a number of interpretation techniques are available to the experienced analyst. First auto-pick is generally run on the data set. An experienced analyst can discard invalid picks and further discriminate various picks as to bedding, fractures and faults, various stratigraphic or sedimentological dips, and often recognize facies. Various techniques include use of: SCAT and dip-azimuth cross plots; classic dip-meter interpretation based on pattern recognition; walkout and vector diagrams; use of various image filters which can enhance certain dips; and known or inferred structural dip subtraction. Quality interpretations are time consuming and the most meaningful interpretations will come from interaction with the end user, who can assist with the bigger picture and resolution of ambiguous findings.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016