Abs tra c t Several schemes have been described for removal of the capture gamma-ray contribution during a neutron burst in order to obtain a clean inelastic gamma measurement. However, even a clean inelastic count-rate ( CR) measurement -while substantially dependent on the gamma transport properties of the formation -still possesses a sizeable residual neutron transport effect. M onte-Carlo modeling has been used in the proj ect reported here to better understand the physics of the neutron-gamma transport problem. M onte Carlo modeling has also been used to develop a correction technique that leads to a compensated inelastic ratio dependent almost solely on the gamma transport ( density) properties of the formation surrounding the tool. This paper will discuss how a modeling database was constructed and how it has been used to develop the correction technique. An extensive database of modeling results validates the proposed technique.I n addition, since this technique does not employ any new measurements from the PNC tool ( it uses existing near and far detector inelastic and capture CR' s) , any existing field PNC log can be used to test the process where a suitable openhole density-neutron log is available. Several log examples demonstrate a reasonable correlation between this new PNC density technique and an OH density log. Introduc tionVarious physical arguments point to the inelastic gross countrates in the detectors of PNC/ PNS tools as having significant sensitivity to formation density. Thus, some measure of formation density in cased-holes ( CH) may be obtained by proper analysis of these inelastic count rates. Several schemes have been described for removal of the capture gamma-ray contribution during the neutron burst in order to obtain a clean inelastic gamma measurement. 1-5 However, as will be shown here, even a clean inelastic count-rate measurement -while substantially dependent on the gamma transport properties of the formation -still possesses a sizeable residual neutron transport effect. M onte-Carlo modeling has been used to better understand the physics of the neutron-gamma transport problem and to develop a correction technique that leads to a compensated inelastic ratio dependent almost solely on the gamma transport ( density) properties of the formation surrounding the tool. An extensive database of modeling results validates this correction technique.This correction technique employs measurements that have been on logs for quite some time, thus allowing evaluation of the technique on previously run field logs. A couple of examples will be examined where openhole ( OH) neutrondensity logs were available for comparison. Generally, a good correlation is observed; but it must be recognized that any CH density measurement will necessarily be somewhat inferior to an OH measurement because of the complexities and uncertainties of the CH environment. Precise information on tool standoff, BH size, cement quality, and casing corrosion may not be available when the CH log is run. Variation in thes...

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.

Modern Pulsed-Neutron Spectrometry (PNS) well logging tools producemeasurements (e.g., Carbon/Oxygen (C/O) ratio, lithology (Ca/Si) ratio, elemental yields, etc) that are impacted by internal as well as externalfactors. The external ones are well known: porosity, lithology, borehole size, casing size, cement thickness, borehole fluid type, centering, tubing size, toname a few. These factors have been discussed extensively in the literature andwill not be addressed here. Internal effects have not, however, been extensively discussed in theliterature. They arise from unit-to-unit variations in componentcharacteristics. Some examples are scintillator and photomultiplierperformance, dead-time and pulse pileup effects, electronic, andanalog-to-digital converter linearity. All of these will be discussed here. These internal effects generally lead to differences, from unit-to-unit, in theabsolute magnitude of output quantities measured (C/O ratio, lithology ratio, for example). Fortunately, the log character (i.e. the variations with depth)is usually preserved. Unit-to-unit variability is of concern in well monitoringprojects because of the likelihood that logs will be obtained with differenttools (perhaps even different vendors) over the lifetime of a well. These internal effects are examined using a database of measurements made inthe same zone of a single test well. The database was generated from severaldozen tools; each calibrated in the same fixture. Using this database, corrective schemes were devised that minimize internal effects, therebyproducing measurements that are more consistent from unit to unit. Thecorrective schemes are parameterized in the calibration process for each tooland applied to all log data run with that particular tool. Introduction Halliburton has produced 43 of its Reservoir Monitoring Tools1(RMT) over the last several years. This device is a 5.4cm (2–1/8-in.) dia. pulsed-neutron spectrometry system providing inelastic Carbon-Oxygen ratio, neutron decay, and several capture elemental yield measurements in cased wellbores. This system uses a sealed-tube D-T neutron source and is pulsed at arate 10 kHz. Two Bismuth Germanate (BGO) scintillators (3.6 cm dia. by 2.5 cmand 15.2 cm long respectively) are space 28 and 52 cm axially from the source. The neutron burst duration is 30 ?s but fast neutrons are only produced for thelast 20 ?s of the pulse because of the ion-source strike time of the tube. Aninelastic spectrum is recorded during this 20 ?s period. After a brief pausefor 5 ?s, a second spectrum measuring the capture gamma-rays spans theremaining time until the occurrence of the next neutron burst. At intervals of25 ms, a long 5 ms pause is inserted to measure natural and induced backgroundactivity. All three spectra for each detector are accumulated downhole in thetool and telemetered to the surface once a second where further accumulationtakes place. A PC based analysis system at the surface provides data processingand recording capability. At the conclusion of the manufacturing phase, each of these tools was run ina horizontal water-filled calibrator and later in a test well located at themanufacturing site. These measurements provided an extensive database forexamining intra- and inter-tool measurement consistency. Problems and Solutions Dead-time and Pileup Pileup is a common problem with nuclear counting circuits especially wherehigh counting rates are desired and expected. Pileup is not the same asdead-time although both are usually related. Dead-time (DT) is the totalelapsed time from the arrival of one pulse to the point in time when thecircuit is ready to receive and process another. Pileup is the arrival of asecond pulse so closely after the first that the analog-to-digital converter(ADC) can not distinguish between them and treats them as a single event. Inspectrometric circuits, pileup is a problem when 2 pulses arrive within theaperture time of the peak detector. However, this time is usually much lessthan the total pulse-height conversion time which determines the dead-time.