An instrument system has been developed for field measurement of the relative carbon content of reservoir rocks. Gamma rays produced by inelastic scattering of 14-MeV neutrons are detected and analyzed in a sophisticated scintillation spectrometer system adapted for routine commercial well logging. A pulsed-neutron source similar to those in commercial use for other nuclear logs is employed. A van de Graaff generator provides the accelerating voltage to initiate the deuterium-tritium reaction, which produces the neutrons in a specially designed ion acceleration tube. Pulses from the radiation detector are transmitted to the surface oiler a conventional seven-conductor logging cable without degrading the spectrum. The log is available on a limited commercial basis and is recommended for identifying oil-saturated zones in cased wells where formation water is too fresh for reliable application of the Neutron Lifetime Log. Introduction A practical method of identifying the element carbon and assaying its abundance in cased-off formations has long been sought as a means for finding oil in situ. Since oil is largely composed of carbon, a measurement of carbon concentration is a direct method of evaluating the oil content of a formation. A nuclear logging technique whereby such a measurement can be made has been known since about 1950, and much effort has been expended in many laboratories to develop a system that could be used commercially to evaluate the hydrocarbon saturation of potentially productive reservoir rocks. Such a system is now available and has been field tested with gratifying results. This paper concerns the general features of the system, which is presently in commercial use on a limited basis. FIELD INSTRUMENTATION The subsurface instrument is 3-5/8 in. in diameter by 14 ft long. It is rated for a maximum temperature of 350 degrees F and pressure of 15,000 psi. It is equipped with a collar locator to enhance the accuracy of depth correlation and it can log to 7 ft above bottom. One or more neutron-type curves can also be run to assist in correlation with available open- or cased-hole logs. The instrument operates on a standard seven-conductor logging line, and curves are recorded on a conventional strip-chart recorder. Measurement of carbon-to-oxygen ratio can be recorded as a continuous curve vs borehole depth when desired. In usual practice, a continuous curve is of little value because statistical fluctuations in the measurement tend to obscure the true character of me log. For best results, stationary measurements are made at preselected points of interest. These are made for a time interval of 1 to 10 minutes each, depending on the accuracy desired and the number of zones to be surveyed in the time available for the logging operations. Typically, a logging operation comprises a continuous "correlation curve" over all zones of interest plus a station measurement at each of 10 to 50 specific points. PRINCIPLES OF OPERATION PRINCIPLES OF OPERATION The logging system comprises a subsurface instrument equipped to initiate and detect radiation from carbon, and surface gear to analyze and record the amount of such radiation in a format that facilitates formation evaluation. To initiate the emission of radiation, a source of 14-MeV neutrons is required. To detect the radiation, a scintillation spectrometer is used. And to assay the abundance of formation carbon by analyzing the detected radiation, a multichannel pulse-height analyzer system is employed. SPEJ P. 463

A new log has been developed for quantitative formation evaluation which is based on a measurement of the length of time slow neutrons survive before they are captured in the rocks and fluids. The logging instrument employs a cyclically pulsed neutron generator and a gated scintillation counter which is synchronized with the source. The source emits short, intense bursts of 14 mev neutrons once every 1,000 microsec and is quiescent between bursts. During the period the source is quiescent, the detector is electronically actuated for two independent preselected intervals. A comparison of the counting rates during these two intervals gives a measure of the rate of decay of the slow neutrons and of the associated gamma radiation. The average neutron lifetime in most earth formations is in the range from 50 to 500 microsec. It can be measured during a continuous logging operation at conventional logging speeds. The design of the logging instrument is described and the results of tests are compared with theoretical predictions. Formulas are developed which give the relationship between log response and formation properties. It is shown that the method is particularly sensitive to formation fluid salinity, and that salt water saturation can be measured accurately in either cased or open hole. The measurement can be made independent of borehole size, fluid type, casing and tool position in the hole by properly selecting the intervals during which the measurements are made. The results of tests with a prototype logging tool are given. Introduction A new nuclear logging system has been developed which employs the Accelatron,* an accelerator-type neutron source, and accurately measures formation brine saturation in an entirely new way. It has produced a type of formation log with better sensitivity, greater sampling depth and simpler quantitative interpretation than any other nuclear log thus far suggested. The new Neutron Lifetime Log* employs a pulsed electromechanical neutron source and a synchronously gated radiation detector. A prototype instrument has been field tested during recent months to demonstrate the operability of the apparatus and the feasibility of the method. Tests in wells and simulated boreholes have confirmed theoretical predictions and have shown that formation parameters can be measured independent of casing and other borehole parameters. Preliminary results of field tests have indicated that the log may have important and widespread applications. BASIC PRINCIPLE OF NEUTRON LIFETIME LOG The Neutron Lifetime Log is based on the fact that neutrons emitted by a source in a well are rapidly but not instantly captured by the material around the source. Their capture is a matter of statistical probability; the greater the number of capturing nuclei and the greater the "capture cross section", the greater is the probability that a neutron will be captured quickly. The average life of a thermal neutron in vacuum is about 13 minutes, but in common earth materials, the average neutron life ranges between extremes of about 5 microsec for rock salt and perhaps 900 microsec for quartzite. The Neutron Lifetime Log responds to variations in this average neutron life. The theoretical basis for a log of this general type has been well understood by nuclear logging experts in many laboratories both in America and in Russia, and developmental work along these lines has been in progress for many years. The Russian literature has reported both theoretical and experimental work but in this country there have been no published reports of progress toward a practical logging instrument. The logging instrument is designed to measure radiation produced by slow neutrons during selected intervals when no neutrons are being emitted by the source. The source is arranged to emit neutrons in bursts or pulses. During the quiescent interval between the pulses, it is possible to observe the exponential "decay" of the neutrons and the neutron-induced radiation as the individual neutrons progressively disappear due to capture by atoms in the formation or the borehole. When a short pulse of 14 mev neutrons is emitted by a source in a borehole, the individual neutrons are slowed to thermal energy within a few microsec. Thus, a cloud of "slow" neutrons is formed around the source within 10 to 50 microsec after the pulse. This cloud is most dense within a few inches of the source, and is progressively less dense out to a radius of about 3 ft, where radiation from the source is practically undetectable. JPT P. 319ˆ

The Neutron Lifetime Log (NLL)* has gained industrywide acceptance as a superior cased-hole logging technique. Quantitative evaluation of hydrocarbon deposits has been proved to be feasible where conditions are favorable. Under less favorable conditions, where very small contrasts are observed, it is usually possible to make useful qualitative judgment of formation fluid content. Experience gained from studying many Neutron Lifetime Logs has provided a better knowledge of the accuracy and reliability of interpretive techniques. The expected range of magnitude of ~R and ~w in various areas and conditions will be discussed. The API neutron test pits have been logged and ~R determined for the three limestone blocks in that facility. Studies of the chemical composition of rocks and fluids, including the effects of trace elements having very high thermal neutron capture cross-section, have added to the understanding of variations in ~R' Activation logging for silicon content is a new technique which is made possible by the NLL instrumentation. Lithologic differences due to variations in sand; shale and carbonate content can be recognized by this means. This log has proven in many cases to be a valuable companion interpretation tool when used with the lifetime log. Other companion logs previously commercially available for simultaneous recording with the lifetime log are the gamma ray or epithermal neutron logs. Use of the log as a means for reservoir monitoring has improved the accuracy and precision with which oil! water/ gas interfaces can be followed during the producing life of a reservoir. When used in this fashion, the log provides data which improve knowledge and understanding of the mechanism of production and the problems related thereto. Introdm,,1ionThe NLL, introduced as a commercial logging service over 2lh years ago, has already gained industry-wide acceptance as the most outstanding cased-hole logging service available today. Advances in both logging capabilities and interpretive techniques, combined with the sheer bulk of logs run during this period, have been continuously increasing the value of the service. In addition to the United

The response of the Neutron Lifetime Log TM is studied in viewof the diffusion of thermal neutrons in a wellbore geometry. Qualitative theoretical arguments are given for the diffusioneffects of a log-inject-log (LIL) procedure. Experimentalverification that diffusion exactly cancels out the LILprocedure is presented. Introduction One application of pulsed neutron logging has been todistinguish qualitatively among gas-, oil- and saltwaterbearing formations. The parameter ordinarily measuredis the macroscopic, thermal neutron-absorptioncrosssection, which is sensitive to the chlorinecontent of the formation and can sense the saltwatersaturation of reservoir rock. A particularly usefuladvantage of pulsed neutron logs is that they can measurecased wells quantitatively. One presumed theoreticaldeficiency has been much discussed and contended. Neutron diffusion, a well known physical phenomenon, has been recognized or at least proposed - as a possiblesource of error when measuring cased wells. Certainlythis phenomenon sullies the otherwise simple relationship between the inherent nuclear properties of a formationand, measured by commercial instruments. This expectedinfluence of thermal neutron diffusion on the NeutronLifetime Log TM (NLL) for a particular case was investigated experimentally and is discussed in the first part of thispaper. Another application of pulsed neutron logs hasreceived considerable attention in recent years.Various people have tried to use pulsed neutron capture logs in alog-inject-log (LIL) procedure to ascertain residual oil saturation quantitatively in formations that have beendepleted by secondary recovery operations. It has beenargued that the effect of neutron diffusion on themeasured must be accounted for to attain the limitingaccuracy of the method. On the other hand, it has beenrecognized that the LIL procedure involves the comparisonof two separate measurements, each of which is influenced equally by diffusion effects (if any exist). This beingthe case, the net result of neutron diffusion is zero.Demonstration of this cancellation is verifiedexperimentally and discussed in the last pan of our paper. Theory of Measurement Thermal neutrons are not detected directly in the systemthat produces the NLL. Instead, gamma rays with energygreater than 2.2 million electron volts (MeV) aremeasured. This procedure effectively samples the thermal neutron population in the formations surrounding theborehole. The NLL instrument emits cyclical bursts of14-MeV neutrons, which suffer subsequent elastic andinelastic collisions and slow down to thermal velocities. Since protons are the best moderators of fast neutrons, any fluid-filled porosity results in a rapid slowdown.Typical sand formations saturated with water haveslowdown times of 10 to 20 seconds for 14-MeV neutrons. Thus, waiting an appropriate time after the fast neutronburst assures that observed gamma rays are caused bythermal neutron reactions. The detection of gamma rays generally is conceded tobe superior to neutron detection. This advantage isprimarily because most gamma rays can penetrate fartherto reach the detector than thermal neutrons. Furthermore, gamma rays travel with the speed of light, while neutronsdiffuse slowly, especially through the highly absorbingborehole fluids that come between the formation and thedetector. JPT P. 1788^

The theory of activation logging with an accelerator-type neutron source as a generator of neutrons by the deuteriumtritium (D,T) reaction is briefly reviewed and the instrumentation is described which has been employed recently in tests in field wells. Factors affecting the ability of the oxygen log to identify oil in situ are discussed both from the standpoint of instrumentation requirements and. with regard to borehole conditions and operating procedures. Examples of field logging results are given and compared with the expected results based on theory and on laboratory experiments. The role of borehole fluid in the logging process is explained, and the method of determining the optimum logging speed is discussed. Recent experience with the accelerator neutron source and instrumentation for activation logging is summarized and problems relating to the future applications for this equipment are presented. New nuclear logs which can be run in conjunction with the oxygen log are described and the advantages of a 14 Mev neutron source for conventional neutron logging are explained. Introduction Although activation logging was first proposed nearly 20 years ago the method has been of little more than academic interest until recent years due to the instrumentation difficulties involved. Apparatus has now been developed which is capable of making logs of this type and a field test of the equipment has been underway for several months. In view of the success attained to date it is confidently expected that activation logs of commercial quality will soon become available. The present paper is intended as an introduction to the operating principles of the logging equipment and a preliminary report on the results of logging tests. "Activation Analysis" is the term which has been used, since the development of the chain reacting pile, to denote the very sensitive analytical technique for detecting trace elements in samples by means of neutron irradiation. To analyze a sample of material by this technique it is placed in an atomic pile where it is irradiated by an intense flux of neutrons, after which it is removed and examined by various techniques to determine what "activation" was induced by the irradiation. Most of the earth's elements can be made radioactive by pile bombardment, and even very tiny quantities of certain elements may become sufficiently radioactive that their characteristic radiation, which may be beta rays, positrons, or gamma rays can often be identified with ease, particularly if the sample is dissolved and chemically separated into fractions.