We are introducing a new 1-11/16-inch multi-function pulsed neutron instrument. The Reservoir Performance Monitor (RPM) has operating modes allowing pulsed neutron decay, pulsed neutron spectrometry, pulsed neutron holdup, neutron activation water flow, and radioisotope measurements. The tool is combinable with fluid density, spinner flowmeter, or more advanced production logging instruments. The new instrument includes three gamma-ray detectors arrayed above a new neutron generator that can be pulsed at different frequencies and modes for different types of measurements. The system employs short tool sections for ease in shipping and handling. In the pulsed neutron capture mode, the tool pulses at 1 kHz, and records a complete time spectrum for each detector. An energy spectrum is also recorded for maintaining energy discrimination levels. Time spectra from short-spaced and long-spaced detectors can be processed individually to provide traditional thermal neutron capture cross section information, or the two spectra can be used together to automatically correct for borehole and diffusion effects and produce results that are very close to intrinsic formation values. In the pulsed neutron spectrometry mode, the instrument pulses at 10 kHz, and records full inelastic and capture gamma ray energy spectra from each detector. These data are processed to determine critical elemental ratios including carbon/oxygen and calcium/silicon from the inelastic spectra and silicon/calcium from the capture spectra. The pulsed neutron holdup imager mode yields both energy spectra and time decay spectra from each detector simultaneously. Measurements can be used to determine holdups of gas, oil, and water, and when combined with other production logs can provide a comprehensive production profile picture, even in deviated or horizontal wells. The neutron activation mode provides water-flow measurements using one of several data acquisition methods. Stationary measurements are made in either of two modes, and measurements at different logging speeds can be used to segregate different flow rates in either an annulus or in an adjacent tubing string. With the neutron generator turned off, the RPM can also be used to detect the distribution of materials, tagged with radioactive tracers, that are injected into the well during well treatments. In this manner, the effectiveness of operations such as hydraulic fracturing or gravel pack placement can be evaluated.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper presents new hardware and analytical methods for determining formation gas saturation behind casing using pulsed neutron instruments. Instrumentation, characterization, algorithms, and limitations are discussed, and log examples are presented that illustrate the application of the methods in limestone and sand/shale formations.
Summary Compensated neutron logging (CNL(SM)) uses a two-detector system that was developed to reduce borehole effects. The ratio of counting rates from the detectors provides the basic tool response from which a porosity index is obtained. Each detector in this system has a different vertical resolution because of its spacing. A new method of processing the counting rates has been developed to enhance the vertical resolution capabilities of the neutron porosity index by exploiting the better vertical resolution of the near detector. Because no additional or new measurements are required, data from older wells can easily be re-evaluated. Results from the new method have been compared with microspherically focused logs (MicroSFL(SM)) and electromagnetic propagation logs (EPT(SM)). They show repeatable thin-bed resolution on the order of 1 ft [0.3 m] for data sampled at 6-in. [15-cm] intervals; the typical vertical resolution from ratio processing is approximately 2 ft [0.6 m]. The statistical precision of the high-resolution processing is superior to that of the standard ratio method. An additional parameter, obtained with the new processing method, provides information about borehole effects. This parameter can be used for qualitative indications of gas when invasion is not deep and environmental effects are not large. The new method has been applied successfully in carbonate and laminated sand formations. Studies show that thin beds can be detected in high-porosity formations where normal processing has significant statistical variations resulting from reduced counting rates. Introduction The basic principle of CNL was developed to overcome the sensitivity of neutron logs to borehole factors such as borehole size, salinity, and sonde standoff. Allen et al. showed that the slope of thermal neutron flux curves approached a constant value at large distances from the source. Furthermore, they demonstrated that, at large distances from the source, the magnitude of the slope was independent of borehole size or tool position. Thus, the ratio of counting rates from two detectors placed sufficiently far from the source tended to be less subject to borehole perturbations. Because the slope of the flux curves was dependent on porosity, the ratio could be transformed into a porosity index. The main advantage of ratio processing is that the resultant porosity is less affected by environmental factors than a porosity obtained from a single detector. It does not, however, capitalize on two important features of the CNL hardware design, the difference in the vertical resolution of the near and far detectors and the better statistical behavior of the near detector. Because of its distance from the source, the near detector is nearly twice as sensitive as the far detector to contrasts in porosity at bed boundaries and it produces a higher counting rate. In the early days of CNL, surface instrumentation was based largely on analog circuitry because digital electronics had not reached today's level of maturity. Consequently, digital filtering techniques to match the vertical resolution of the two detectors could not be performed by the surface panel. The introduction of Cyber Service Unit (CSU(SM)) wellsite instrumentation offered a means to implement new methods for processing CNL data. However, the choice was made to emulate the surface panel's ratio processing, which had become widely accepted. A new method of processing the CNL counting rates has been developed to enhance the vertical resolution of the porosity index. The method consists of four steps. The vertical resolution of the near and far detectors is matched to minimize bed boundary anomalies. An environmentally corrected porosity index is then computed from the ratio of the matched counting rates. A continuous calibration factor for the near-detector porosity response is computed using the environmentally corrected porosity index. The calibration factor is used to correct the raw near-detector counting rate to produce an accurate porosity index that retains the high statistical precision and improved vertical resolution of the near detector. Basic Approach Counting rates for each detector in the CNL tool have been measured in the laboratory and may be expressed as functions of porosity: (1) and (2) where N near and N far are the average counting rates for the near and far detectors, respectively. Eqs. 1 and 2 represent the responses of the two detectors for ideal conditions because laboratory formations are precisely drilled, homogeneous, nonbedded, experimental constructions containing a single fluid. In a real borehole the counting rates generally do not match those from the laboratory; however, they may be expressed as (3) and (4) where the xj's represent such uncontrolled environmental factors as mudcake and salinity. With respect to laboratory conditions, the alpha functions quantify the counting-rate attenuations caused by the environmental parameters affecting the measurement. In this context the functional behavior of the alpha functions is expected to be such that their rate of change, with depth, will usually be less rapid than the rate of change of porosity. Normal processing of CNL data is based on the premise that, to a first approximation, alpha near = alpha far so that (5) and (6) which leads to a ratio FN, (7) from which porosity can be computed by means of a functional relationship: (8) Eqs. 5 and 6 can be rewritten for counting rates that have been resolution-matched to give (9) and (10) where phi R is obtained by substituting the ratio of the resolution-matched counting rates, FRN, into Eq. 8: (11)
Galford, J.E., SPE, Schlumberger Well Services Flaum, C., SPE, Schlumberger Well Services Gilchrist Jr., W.A.,* SPE, Schlumberger Well Services Soran, P.D.,** SPE, Schlumberger Well Services Gardner, J.S.,+ SPE, Schlumberger Well Services Summary. The basic openhole responses and environmental correction algorithms for compensated neutron logging (CNL(TM)) tools have been updated. The improved processing is based on an extensive set of laboratory formation measurements to which mathematical modeling calculations have been added. In all, the new algorithms include basic responses for the three principal formation matrix types and corrections for seven environmental effects and formation-fluid salinity. A total of 467 laboratory formation measurements have been augmented with 245 data points generated through mathematical modeling. This data base has been used to define more accurately the effects on the tool response of variations in logging conditions from those considered standard in the laboratory. More accurate corrections for the effects of formation pressure, temperature, mudcake, natural or barite mud, and borehole salinity have been defined. Certain other effects depend on more than one parameter. For example, the effect of formation salinity is somewhat matrix-dependent; therefore, the corrections are handled differently for sandstone, limestone, and dolomite. The effect of tool standoff depends on the borehole size; consequently, the standoff correction is larger for larger boreholes. The porosity crossplots and environmental correction charts based on the new algorithms represent a significant evolutionary improvement over previous techniques. They should be an important aid to the use and interpretation of neutron logs. Introduction The openhole porosity response functions and environmental correction algorithms for the CNL log have been updated. The new algorithms have been field tested and will become available on a routine basis both at computing centers and on field service units. This paper presents a discussion of the new procedures. Environmental correction charts and interpretation crossplots similar to those published by Schlumberger are also included. Finally, log examples are used to illustrate some of the differences between the new algorithms and those they are replacing. Neutron logging has been an important measurement for porosity and lithology determination for more than 20 years. The dual-spacing CNL log was first described in 1971 by Alger et al. and has become a mainstay measurement for formation evaluation. In combination with other services, the CNL log provides information on porosity, lithology, and the presence of gas. The dual-detector design with ratio processing was chosen to reduce environmental effects on the measurement. The downhole device has remained virtually unchanged through the evolution from analog surface instrumentation to the sophisticated computer-based units of today. Even with the reduced environmental effects of the dual-detector design, it is still necessary to apply corrections for certain borehole conditions and for variations in formation-fluid salinity. Correction plots have been published in Ref. 1. Environmental corrections have been routinely applied to field logs before processing at field log interpretation centers. Corrections for a subset of the effects have been used at the wellsite on the Cyber Service Unit (CSU(TM)). Over the years, our understanding of the neutron measurement has continued to improve through the acquisition of additional laboratory measurements and, more recently, through mathematical modeling. This has been reflected from time to time in improvements to response functions and environmental correction algorithms and in periodic upgrades to the log interpretation charts. In addition, several papers have been published that deal with specific areas of CNL response or interpretation. Laboratory and Modeling Data The new response functions and environmental correction algorithms discussed in this paper are based on an extensive collection of more than 467 laboratory measurements. Measurements were made in limestone, quartz sandstone, and dolomite at porosities ranging from zero to the water point. Two types of laboratory formations were used, homogeneous quarried rock samples and test-tank formations made in the laboratory for specific measurements. The formations used were produced and characterized in the same way as those described by Tittman et al. Each measurement was made for a period of time sufficient to reduce statistical error well below the uncertainty in our knowledge of the porosity of the formations. The estimated uncertainty in the formation porosities was determined as discussed by Tittman et al. Although many of the measurements were made to improve the characterization of the CNL porosity response, most of the data were taken to aid in understanding the environmental effects. Data for variations in hole size, tool standoff, mudcake, mud weight, and borehole salinity were included. Measurements for various formation-fluid salinities were also made. In addition, measurement for certain combinations of effects, such as hole size and standoff, were part of the data base. In addition to the laboratory data, 245 data points were generated by mathematical modeling. Both Monte Carlo and deterministic techniques were used as described in earlier publications. Monte Carlo calculations were used for cases where a three-dimensional treatment was necessary. In some cases, however, it was possible to use a two-dimensional (2D) deterministic model described by Ellis and Case. All modeling data were calculated from the reaction rate of He(n, p) with 100% efficient detectors and no dead time. Consequently, it was necessary to normalize the calculated results so that they could be used with the experimental data. The calculations used for this study were normalized to measurements in the 13.2-p.u. freshwater-filled limestone formation with an 8-in. [20-cm] -diameter freshwater-filled borehole. The uncertainties for the calculated data were determined from the statistical precision of the results. Modeling points were used to extend the laboratory data into areas that were difficult to measure experimentally. Several calculations were used in the critical range between 0 and 15 p.u. SPEFE P. 371^
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