Download natural gamma ray logs

‫بسم هللا الرحمن الرحيم‬
Detecting The Radioactive
Minerals Using Well Logs
Methods
Asst. Professor Dr. Mahmoud A. AL-Mufarji
Asst. Professor Dr. Jawad R. AL-Assal
Asst. Lecturer Adnan A. Abed AL-Jabbory
Abstract
Radioactivity is accompanied by the emission of alpha
or beta particles, gamma rays. Gamma rays are
electromagnetic radiations emitted from an atomic
nucleus during radioactive decay. The clay minerals are
primarily responsible for two sources of radioactivity,
potassium, and thorium, associated with most shales.
There are two types of GR logs. One, the standard GR
log, measures only the total radioactivity. The other,
the NGS (Natural Gamma Ray Spectrometry) log,
measures the total radioactivity and the concentrations
of potassium, thorium, and uranium producing the
radioactivity.
We apply the gamma ray log in some wells in south Iraqi
field normally reflects the shale content of the
formations. This is because the radioactive elements
tend to concentrate in clays and shales. Thus, as shale
content increases the gamma ray log response increases
also.
When we are correlating density log and gamma ray log
for two wells in this study and Showing for maximum
gamma ray reading as a maximum reading in density
log because The original density tool consisted of a
high-energy gamma ray emitting source coupled with a
shielded gamma ray detector.
GAMMA –RADIATION
Gamma-radiation may be considered as an
electromagnetic wave similar to visible light or X-rays,
or as a particle or photon. Gamma rays are
electromagnetic radiations emitted from an atomic
nucleus during radioactive decay. These radiations are
characterized by wave lengths in the range of 1 0 -9 - 1 0
-11 cm, equivalent to frequencies ranging from 1019 to
1021 sec -1.
The energy is expressed in electron-volts (eV). The
energies of gamma rays are of the order of the MeV
(l06 eV). Usually, the α, β and γ-emissions are
simultaneous, figure (1).
β- and α -particles do not penetrate far enough to be
detected by logging techniques. The gamma rays have
a very high power of penetration and can be detected
and recorded in present-day hole conditions. For that
reason, they are the basis of several important logging
techniques.
Potassium-Bearing Minerals and Rocks
The source minerals of potassium are the feldspars
and micas plus a large number of minerals of minor
importance.
During alteration, some silicates such as the feldspars
are completely dissolved; the potassium is thus
liberated in ionic form and transported in solution
Micas, on the other hand, may lose only part of their
potassium during alteration, the remainder staying in
the crystal lattice.
Uranium-Bearing Minerals and Rocks
The source minerals are in igneous rocks of acid
origin. Table (1) lists the average content (in ppm) of
uranium and thorium in several rock types. These are
averages; actual values can vary considerably.
Thorium-Bearing Minerals and Rocks
Thorium originates from igneous rocks of the acid and
acido-basic types (granites, pegmatites, syenites,
nepheline syenites).
Table (1) lists the average concentrations (in ppm)
encountered.
NATURAL GAMMA RAY LOGS
The natural gamma ray (GR) log is a recording of the
natural radioactivity of the formation. There are two
types of GR logs. The standard GR log which measure
only the total radioactivity. and NGS (Natural Gamma
Ray Spectrometry) log which measure the total
radioactivity and the concentrations of Potassium,
Thorium, and Uranium producing the radioactivity.
METHODOLOGY AND RESULTS
The results are shown in Fig. (4) for some well in the xx field,
using Petrel program to read the maximum value of
Gamma ray. As noticed, gamma ray reading reached
maximum value in six depth interval:
Depth interval: 2288.66-2306.5 m and max. GR value=27 API
Depth interval: 2310.67-2320.8 m and max.GR value=25 API
Depth interval: 2375.88-2421.16m and max.GR value=23 API
Depth interval : 2178.2-2183.37m and max. GR value=26 API
Depth interval:2221.2-2233.8m and max. GR value=21 API
Depth interval :2330.16-2338.94 m and max.GR value=51 API
From these intervals the gamma ray log normally reflects the
shale content of the formations. This is because the
radioactive elements tend to concentrate in clays and
shales. Thus, as shale content increases the gamma ray log
response increases also.
Fig.(5) showed the correlation between gamma log and
the density log, it appeared that the maximum reading
of the gamma ray corresponding to maximum reading
of the density log at the same depth because the
original density tool consisted of a high-energy gamma
ray emitting source coupled with a shielded gamma ray
detector.
RECCOMEDATION
Using the most sophisticated software in interpretation
in gamma ray reading to detect the radioactive
minerals
Using the natural gamma ray spectral (NGS log) rather
than standard gamma ray.
In oil well drilling it is recommended to examine the
cutting from radioactivity.
Thank you