PERMFILE49521

• Permit No: M1999002
• IBM Index Class Name: Permit File
• Doc Name: Reference: NUCLEAR WELL LOGGING
• Media Type: D
• Archive: No

IS2 NUCLEV.R WELL LOGGING
4.2. NATURAL GAMMA-RAY LOGS
Natural radiation is due to the disintegration of the radioactive nuclei contained in the
ground. The leading ones are potassium 40, the elements in the thorium family and the
elements in the uranium family. The energy associated with each radiation is expressed in
iv1eV (millions of electron volts). The principal radiations are:
(a) For potassium, 1.46 hteV.
(b) For thorium, 0.63 MeV.
(c) For uranium, 1.12 and 0.93 t~teV.
Therefore, either an overall measurement of this radiation or a spectroscopy can be
performed to identify the amount of each radioactive element present.
4.2.1. Conventional Gamma-Ray log
[n this log, a counter indiscriminately detects all the eamma rays (GR) coming from a
formation. The energy o(these rays decreases rapidly with the distance covered from their
emission by a given atom. A measurement is thus made of the overall level of radioactivity
coming from a formation tine extending no farther than 15 to 2S cm (6 to 10")around the
well. This level depends on the specific gravity of the formation, the existence of casing and
the density of the mud. Therefore it is difficult to make a quantitative interpretation of the
log, but charts are provided by the service companies.
The sonde is made up of a radiation detector which is generally a scintillation crystal
because it is much more efficient than the Geieer counters that were used previously. The
number ofimpulses picked up as the gamma rays strike the counter is averaged in a circuit
with a "time constant" of )', Z, 4 or 6 s depending on radiation intensity. Naturally, the
lareer the time constant is the more slowly the sonde must be pulled up the well.
Figure 4.1 shows an example of a GR log. This los resembles the SP loe. The
radioactivity, which increases toward the rieht, increases with the clay content of
formations. Clean formations such as sand, limestone and dolomite eenerally have weak
radioactive radiation. The existence of volcanic ash or of disinte_arated eranite can cause
high radioactivity in non-areillaceous layers.
An estimate of the clay content is given b}' the equation:
GR - GRm;,,
f °R - GRm., - GRmio (4.1)
where
GR =radioactivity read on loo.
GRm;,, =radioactivity read in clean formations,
GRm„ =radioactivity read in shale or clay.