Cased-Hole KUTh Logging Using A PNS Tool

Abstract

Traditionally, all major service companies have had an openhole spectral gamma tool in their arsenal that will measure the naturally occurring potassium (K), uranium (U), and thorium (Th) radioactivity surrounding a borehole. KUTh data are used in various ways, such as subtracting out high API uranium indications from total API Gamma Ray readings in radioactive formations and using the relative percentages of these elements as input to proprietary mineral calculation programs. Recently, a standard C/O logging tool with the neutron generator turned-off was used for cased-hole KUTh logging with good results. Although this pulsed neutron spectrometry (PNS) tool was a slim tool (2 1/8-in. diameter), the log quality was comparable to that achieved by earlier, large diameter gamma spectrometry tools because this particular PNS tool utilized a high density scintillator. This experience clearly demonstrated that reliable spectral gamma measurements can be made in cased hole. Calibration with a standard thorium calibration sleeve allows the operator to set the spectral gain of the system (using the Th peak); and this value of gain is used during the logging operation. After logging, the data is reprocessed with software that tracks and corrects any gain drift due to temperature producing the final corrected log. Concentrations of K, U, and Th are extracted using a weighted-least-squares (WLS) fit of standard elemental (basis) spectra to the log data during the re-log phase. Measurements made in this way can readily precede the standard formation evaluation run in a well and use essentially the same instrumentation and thus replace traditional openhole KUTh measurements. The enhanced design and the software associated with it provide high quality formation evaluation information; and this information can be made available with no associated drilling rig cost if the tool is run going in the hole. This paper will show field examples of KUTh data. The paper will also discuss procedures necessary to obtain quality measurements, will point out various savings that can be realized by an operating oil company using the new design and the information it gathers, and will present the interpretation procedures necessary to obtain a final product. Introduction Openhole spectral gamma-ray (SGR) tools have been available for several decades and measure the naturally occurring potassium (K), uranium (U), and thorium (Th) radioactivity surrounding a borehole. KUTh measurements made with a pulsed neutron spectrometry (PNS) tool can readily precede the standard PNS formation evaluation run in a well while using essentially the same instrumentation. The PNS tool can thus replace traditional openhole KUTh measurements. Knowledge of these elements allows the analyst a more detailed stratigraphic analysis, and depth correlation, than can be obtained with conventional through-tubing gamma ray tools. By using the KUTh analysis, the analyst can also better differentiate lithology, as well as identify the type and volume of clay contained in the logged interval.[W3] Technical innovation has always been critical to the optimization of mature, sometimes declining fields. Recent high oil prices, coupled with possible supply shortages, are forcing operators to continually look to these existing fields as a potential source of increased production. As production water flow increases in the permeable zones of these wells, it tends to deposit uranium salts around the perforations, on the casing, inside of the production tubing, and also in the formation itself. Such deposition can make formation evaluation difficult for even the most experienced petrophysicist. Uranium can also indicate a fractured interval when combined with a low potassium and thorium content. Recognizing clay types to optimize infield drilling procedures -and drilling mud types - is also very helpful. All of these interpretation problems are now solved much easier by using the new through-tubing pulsed neutron KUTh log and its interpretation procedures. KUTh information can also be combined with a photoelectric measurement, as an additional indicator, to aid in identifying lithology and clay typing.