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162 NUCLEAR w'ELL LOGGING <br />There are three main types of logs: <br />(a) Logs measuring the density of slowed-down neutrons at a certain distance from the <br />source. <br />(b) Logs analyzing the decrease rate of the neutrons emitted by an accelerator then <br />slowed down. <br />(c) Logs of the interaction between the neutrons and atomic nuclei, which provide <br />elemental analyses. <br />4.4.1. Conventional neutron logs <br />Neutrons are emitted b}' radioactive sources made up of mixtures such as plutonium- <br />bcnilium or americium-beryllium. These are fast neutrons with an enerev of 4 to 6 MeV. <br />Fieure 4.3 ewes a schematic diagram of the trajectory followed during successive <br />collisions with atom nuclei. Most of the enerey lost is done so durine collisions with <br />hydrogen nuclei which have an almost identical mass to that of neutrons. After having <br />traveled a certain distance. a neutron becomes "thermal"or"slow"and is captured by an <br />atom which emits a capture eamma ray. This distance varies with the amount of hydrogen <br />present, i.e. the porosity' of the medium and the nature of the interstitial Iluids. For <br />example, with high porosity filled ++'ith water or oil, this distance is slieht. It is great with <br />low porosity or in formation containing gas. <br />1 <br />Fig. 4.3. -Principle of the neutron-gamma sonde. <br />The capture gamma ray's c:tn be measured to obtain aneutron-gamma log, or the 4 <br />thermal neutrons can be measured [o obtain a "neutron-thermal-neutron" log, or again <br />the cpithermal neutrons (not }'ct completely slowed down) can be measured to obtain a <br />"neutron-epithermal-neutron" toe. <br />A centered or out-of-center cylindrical tool can be used in the borehole, or else the tool ; <br />can be equippcd with a pad applied against tltc wall in an open hole as is done by the <br />density tool. ~ <br />4.4.1.1. Cvlindri' <br />These tools are n <br />The oldest are of <br />capture gamma ray <br />alone (N-N). The sc <br />depending on the p~ <br />can be con+'erted i <br />(Compensated Neu <br />detectors are sensiti <br />used to compute a " <br />first approximation <br />etc. tend to be canc. <br />departure tunes. <br />An examples of s <br />The neutron-CN I <br />equipped with a pov <br />63 cm (15" and 2Y') <br />filed porosiq. It cat <br />It does not work a <br />A ne++' (our-detec <br />addition to the two <br />T+vo separate p~ <br />detectors. In clean <br />containing high ne <br />reads lower than a: <br />In the tool, the e. <br />thermal neutron de <br />count rates provide <br />Figure 4.10 show. <br />olio( the neutron sr <br />[he difference betw~ <br />4.4.1.2. Pad-typ <br />This t}'pe of tool <br />Epithermal Neutro <br />solely the epitherm <br />Figure 4.I1 shot <br />recorded directly. l <br />A comparison of <br />simultaneously as i <br />The "density" ai <br />and true porosity t <br />20 cm (3") in a 30" <br />