Prospecting by radioactivity logging methods

Contents
Introduction
Well Logging
Classification of well logging methods
Radioactivity logging methods
1) gamma ray log
2) neutron- gamma ray log
3) Pulsed neutron logging
4) Formation density log
 Case study
 Application
 conclusion
 References

Introduction
Geophysical exploration is the study of physical
properties by using appropriate instruments
Types of geophysical exploration:
1) electrical
2) magnetic
3) gravity,
4) seismic,
5) radioactivity
6) Airborne minerals surveys
7) Well logging

Well logging
Continuous recording of a physical parameter of the
formation with depth by sonde
First developed by Schlumberger brothers in 1927
To identify the productive zone of hydrocarbon
To define petrophysical parameters
To determine depth, thickness, formation
temperature and pressure of reservoir
To distinguish between oil, gas and water zones in a
reservoir
To measure hydrocarbon mobility

Classification of well logging methods
Well logging methods may be classified into
four main divisions:
1. electrical logging
2. radioactivity logging
3. sonic logging
4. miscellaneous

RADIOACTIVITY LOGGING METHODS
Radioactive logs are based on the nuclear radiations
of the formations (radioactive elements) and for
detecting possible productive horizon and evaluating
their reservior properties
Radioactivity logs are mainly of 3 types :
1) gamma ray log which measures the radioactivity of
the formations
2) the neutron gamma ray log which measures the
radioactivity induced artificially in the formations by
bombarding the rocks with neutrons from a neutron
source

3) The formation density log which is based on
the measurements of scattered gamma rays
from the formations , irradiated by a gamma
rays source.
 All these types of logging , the radioactivity is
measured in terms of gamma –ray emmision
with the help of either Geiger –Muller tubes or
scintillation counters

Gamma ray logs
Gamma ray logging is a method of measuring naturally
occurring gamma radiation to characterize the rock or
sediment in a borehole or drill hole
Different types of rock emit different
amounts and different spectra of natural
gamma radiation
shales usually emit more gamma rays
than other sedimentary rocks, such as
sandstone, or limestone because radioactive
potassium is a common component in their clay
Content and because the cation exchange
capacity of clay causes them to adsorb
uranium and thorium

It is useful for lithology detection and also for
local and regional correlation of wells sections.
 The simultaneous recording of the gamma ray log
with casing collar locator makes it possible to
spot precise intervals for firing shots with the gun
to make perforation in the casing pipes
 These perforation are necessary to facilitate flow
of formation fluids into the well bore
An advantage is that it works through the steel
and cement walls of cased boreholes

 Neutron gamma Ray Log
Principle:
A neutron source bombards the
formation and the resultant scattering
and neutron deceleration depend
largely on the formation of hydrogen
Content.
 This log is obtained by bombarding
the formation with neutrons from
radioactive source like radium, beryllium
mixture and measuring the induced
radioactivity

Fast neutrons emanating from the neutron
source(velocity about 10,000 km/sec) lose energy
rapidly as a result of collision with other light
atomic nuclei , mainly hydrogen nuclei present in the
media
The neutrons are slow down to thermal velocities
(2km/sec) they are readily absorbed by the atoms of
of Cl , H etc. resulting in the emission of high energy
capture gamma rays which are detected and
recorded
In water and oil , since hydrogen concentrations per
unit volume are about equal, the log reflects the
porosity. Detector in the tool is such that high
porosity

The neutron gamma ray tool is responsive to all
the water in the formation including water
chemically combined with shales and water of
crystallisation such as in gypsum. So Log will read the
higher porosities.
Neutron-neutron log in which intensity of thermal
/epithermal neutrons is recorded instead of the
induced gamma activity.
 The log response is the primarily a function of
hydrogen concentration and hence porosity, but
with reduced interference from chlorine atoms.

Pulsed neutron logging
Initially developed in U.S.S.R
A neutron generator which emits bursts of high energy
neutrons is employed. The bursts may last from 30 to
300 micro sec. and duration of the pause
 The neutron are rapidly slow down in the holes and in
the formation to thermal velocities and they are captured
by nuclei of the surrounding media with emission of
gamma rays
The recording of the capture gamma rays , instead of
thermal neutron is claimed to be superior for
determining the decay of the thermal neutrons in the
rocks .And this log is referred to as Thermal Decay
Time Log.

These pulsed neutron tools are effective in
distinguishing oil, water and gas horizons
particularly in cased wall. So , they can be used to
trace the movement of oil ,water contact in a
producing well.
Advantage is that the influence of bore hole fluid ,
casing and annulus mud and cement on the log is
practically nil, as the measurement are delayed till
the bore hole and other undesirable effects have
almost completely decayed

Formation density log
GR
API0 200
CALIX
IN6 16
CALIY
IN6 16
RHOB
G/C32 3
DRHO
G/C3-0.25 0.25
4100
4200
Principle:
Measurement of energy loss
between a radioactive
emitting source and return of
gamma rays. The loss of
energy is by collision of
gamma rays with electrons in
the formation, the electron
density being directly
proportional to the bulk
density of the formation.

The intensity of the scattered gamma rays measured
during logging , is a function of formation density
and this can be converted into porosity of the
lithology involved
Shales sections have a higher background
radioactivity may vitiate the density determination
to some extend
Gravimeter surveys in deep wells using filament and
transducers and electric circuits of high resolution
for determination of the force of gravity in
boreholes

Gravimeter is used for obtaining rock densities to
interpret gravity anomalies in surface surveys.
Data obtained in deep wells are very useful for
determining density and porosity of rocks
Gravity measurements in the wells possibly helps to
keep track of displacement of oil and gas by water
in producing fields

Application
Accurate lithologic records through casing and cement.
Applications consist in determining
1. sample lag,
2. making correlations and cross sections,
3. locating faults,
4. mapping subsurface structure for deeper drilling,
5. logging beds too thin for electric logs to record,
6. making detailed lithologic records of oil sands
for use in connection with the recovery of oil by
water
flooding,
7. surveying potash deposits in cased wells
8. and in solving problems of sedimentation.

Conclusion: Radioactivity logs has four types , they are
1) gamma ray log 2) the neutron gamma ray log
3) pulse neutron logging 4) the formation density log.
 Gamma ray log are very useful for lithology detection and
also for local and regional correlation
 The neutron gamma ray tool is responsive to all the water
in the formation . Neutron log is mainly used for porosity
evaluation and also most reliable porosity log.
 Advantage of Pulse neutron is that the influence of bore
hole fluid casing and annulus mud and cement on the log
is practically nil.
 Density Log helps in evaluation of formation density,
porosity, shale content and lithology identification.

REFERENCES:
Outlines of Geophysical Prospecting- A manual of
geologists by M.B. RAMACHANDRA RAO
page no. 352 to 355
 Course in Mining Geology By R.N.P .
AROGYASWAMY. Page no. 255
www. Wikipedia.com
www..kgs.ku.edu
www2.ggl.ulaval.ca
archives.datapages.com › ... › AAPG Bulletin› Year 1941
Issue May by WL Rusell- 1941

Prospecting by radioactivity logging methods

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