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t:GCLE.IR ti'F.LL LOGGLUG 163 <br /> 4.4.1.1. C} lindricai tools <br />t distance from the ~ <br /> These tools are non-directional. <br />n accelerator then Thz oldzst arz o(the GNT typz or the N-G or N-N typz. The dztector is sensitive to <br /> capture gamma rays (N-G) as cell as to slow neutrons (GNT) or else to slo++ neutrons <br />lei, which provide ~ alone CN-N). The sourer-dztector spacine can be chosen so as to obtain the best result <br /> depending on the porosity range o(thc formations encountered. The response of sondes <br /> can bz converted into "neutron" porosity by means of charts. Tools of the CNL <br /> (Compznsated Neutron Log) npe are instruments with two spacings in which thz <br /> ~ detectors are sensitive to thermal neutrons. The ratio of near-to-fardetzctorcount rates is <br />uch as plutonium- used to compute a "neutron" porosity with a given rock matrix (usually limestone). In a <br /> <br />rgy of 4 to 6 MeV I <br />first approximation, the environmental elizcts such as borehole fluid salinity, size, castns. <br />. <br />during successive ~ etc. tend to be cancelzd out leaving only residual etfzcts to be corrzcted usine a series of <br />- <br />~ng collisions with I departure curves. <br />tuns. After having I An examplzs of such a log is shown in Fig. 4.9. <br />d is captured by an The neutron-CNL has a greater investigation range than previous neutrons bzcause it is <br />noun[ of hydro en ~ equipped with a powerful radioactive source and the spacings are laree, usually 33 em and <br />" <br />° <br />" <br />" <br />rstitial Quids <br />For ) with a 30 <br />63 cm (l5 <br />). The investigation radius is about 25 cm (!0 <br />io salt-water <br />and 25 <br />. <br />:ht. It is great with (tllzd porosity. It can be used in either a cased or opzn hole, but onz that is filled with liquid. <br /> (t does not work as such in a aas-(tiled borehole. <br /> A nzw four-detector Dual Porosity CNL tool uses nvo epithermal nzutron detectors in <br /> addition to the nvo standard thermal neutron dztectors. <br /> Tao separate porosity measurements are thus obtainzd, one from each pair of <br />i detectors. In clean formations, thz porosities mzasured agrez. In shaly formations <br /> containine hieh neutron absorbers, the porosit}' derived +vith the epithermal detectors <br /> reads lower than and aerees more closely with thz densit}-derived porosiq'. <br /> In the tool, the epithermal detectors are spaced closer to the nzutron source than thz <br /> thermal neutron dztectors. A nz+v data-processing technique usine individual dztector <br /> count rates provides values ofporosity Izss sensitive to variations in borehole parameters. <br /> Figure 4.l0 shows an example ofa Dual Porosity CNL loe. SO,rJ. is theeffzctivestand- <br /> otT ofthe neutron sondes as computed with the new data-processing tzchnique. DCAL is <br /> the difference betwezn borzholz diameter and bit diameter. <br />4.4.1.2. Pad-tope tools <br />This type of tool includzs thz SNP (Sidewall Neutron Porosity). the SE\ ISidzwall <br />Epithermal Neutron) and the S\ L (Sidztvall Neutron Loe). The neutron detector counts <br />tmma lob, or thz solely the epithermal neutrons. <br />ron" log, or again Figure 4.1 ] shows an examplz of a nzutron-SNP loe. The "limestone" porosity is <br />~sured to obtain a recorded directly. The tool opzrates in uncased wells containing liquid or sas. <br />A comparison of CN L and S\ P neutron logs is eiven in Fie. 4.9. The caliper is recorded <br />ale, or else the tool ' simultaneously as is a natural samma-ray log. <br />as is done by the ~ The "density" and "neutron" porosities can bz combinzd to determine the lithology <br />and true porosity of formations. The investigation radius of the nzutron S\ P is about <br />20 cm (8") in a 30% porosity formation. <br />