Laserfiche WebLink
ISq NUCLEAR N'ELL LOGGING <br />Then according to local kno+vledee, we may have: <br />Vn = ~cR <br />or V,h = 0.33 (Z= `°" - I) (q.~) <br />where V,h =volumetric content of shale or clay. <br />The radioactivity login Fig. q.l +vas taken in a cased well, and the position of the casing <br />joints +vas recorded simultaneously by means of a maenetic sensor. By correlation, these <br />casing collars are spotted in relation to the SPand hence in relation to all the logs recorded <br />in an open hole. The casine is perforated by determinin_ the depths in relation to the <br />casing collars +chich in turn are positioned in relation to open-hole toes. in this way the <br />accuracy obtained is independent of the depth, and so thin layers can be made to produce. <br />An open-hole GR can also be correlated with acased-hole GR for the samz purpose. <br />To sum up, it can be said that: <br />(a) A GR log is particularly useful when the SP is defective as the result of salt mud or <br />non-conducting mud. <br />(b) This loe indicates the clay consent, i( not quantitatively at least qualitatively. <br />(c) This log can locate deposits of radioactive materials, potash or uranium ore. <br />(d) It can delimit non-radioactive layers such as coal seams. <br />(e( It enables casings to be perforated +vith great accuracy o(depth. <br />(() It enables radioactive tracers to be used. <br />(g) Lastly, it is capable oflocating various radioactive deposits in old water-producine <br />wells or in fractures in which a circulation of water-carrying potassium or <br />radioactive salts has occurred. <br />4.2.2. Spectral Gamma-Ray logs <br />This loe makes use of a very efficient tool which records the individual response to the <br />different radioactive minerals. These minerals include potassium 40 and the elements in <br />the uranium family as well as those in the thorium family. The GR spectrum emitted by <br />each element is made up of easily identifiable lines. As the result o(the Compton effect the <br />counter records a continuous spectrum. The presence of potassium, uranium and thorium <br />can be quantitatively evaluated only with the help of a computer which calculates in real <br />time the amounts present. The counter consists of a crystal 30 cm (12")long and S cm (2") <br />in diameter. This crystal is optically coupled to a photomultiplier. The radiation level is <br />measured in several energy windo++'s. These data are multiplexed via an electric conductor <br />in the cable leading to the surface computer. <br />Fieure 4.2 sho+vs an example of a spectral GR log. On the left track, SGR is the Total <br />GR count, and GRST is this total count minus the uranium count. <br />The main field applications of this log are: <br />Cla}• content evaluation: Some formations may contain non-clayey radio <br />materials. Then the curve GR-U or GR-K may give a better clay content. <br />