Compensated neutron logs have been around for manyyears, and have been used successfully for manypetrophysical applications. Although the basic designand general response characteristics of theseinstruments have remained relatively constant over theyears, service companies have refined the responsecharacterizations and have updated their hardware,providing evolutionary improvements. Unfortunately,the general understanding among petrophysicists ofhow compensated neutron instruments work and howbest to make use of the log data appears to have laggedor perhaps even worsened in recent years. This paperdiscusses several obstacles to understanding andinterpreting neutron logs and proposes some techniquesfor avoiding problems.One obstacle is a tendency to oversimplify the physicsof the measurement. Hydrogen plays a very importantrole in slowing fast neutrons to the epithermal orthermal levels at which compensated neutroninstruments detect them, and a hydrogen index (HI) hashistorically been used as an aid in discussing neutrontool responses and in interpreting neutron log data. Inmany cases, hydrogen plays a dominant role in neutronlogging, but it is not the only element that interacts withneutrons. Thinking of neutron log responses strictly interms of HI can lead to a misunderstanding of theresponse and to errors in log interpretation. A betterway to deal with neutron log responses is in terms ofthe neutron slowing, diffusion, and absorptionproperties of the formation.Another obstacle is a tendency to think of allcompensated neutron measurements as being the same.Although most compensated neutron instruments aresimilar in design, their response characteristics are notthe same. The differences show up most readily in themineralogy and environmental corrections for differentinstruments. A careful comparison of the correctioncharts for different tools shows this. Attempting tocorrect a neutron log using the charts or the underlyingalgorithms intended for a different tool can be a seriousmistake.Another issue is that different curve names may meandifferent things. It is very important to understand whatthe mnemonics mean and which curve to use for aspecific application.The issues of statistical precision, resolution, and datasampling rate are strongly interrelated for compensatedneutron tools. These issues are not difficult toovercome, but it is necessary to be aware of them andtake each into consideration when using the data.One final obstacle is a lack of understanding of theresponse differences in wireline and LWD instruments.Differences in detailed design parameters can result inresponse differences, but there are also basicdifferences in the way wireline and LWD data arerecorded and processed. These, too, can cause responsedifferences, some of which can be quite significant.Some of these basic differences will be consideredalong with the discussion of other issues.Although this paper is not an exhaustive treatment ofcompensated neutron logging issues, some of the moreimportant ones are discussed. Alternative ways oflooking at the data are presented, as are some ideas forfurther work.
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