Applied geophysics Well logging Part 3

Applied geophysics

Well logging

Part 3

edited by P. Vass

for Petroleum Engineer MSc Students

Caliper logging tools measure the diameter of the borehole as a function

of depth.

Caliper logging can be run in either open holes or cased holes filled with

any type of fluid (e.g. water-based mud, oil-based mud, salt water, air).

For the mechanical caliper tools the measurement of the borehole size is

carried out by means of articulated arms or bow springs pressed against

the borehole wall. These movable component parts of a caliper logging

tool are mostly in pairs facing each other.

Their positions in the plane, which is perpendicular to the borehole axis,

may change and follow the variations in the borehole diameter. The

distance between the opposite arms or bow springs, which gives the

diameter of borehole in a single direction, is measured by the help of an

electric circuit.

The electric circuit contains a potentiometer (variable resistor) whose

slider contact is moving according to the movement of arms. In fact, the

arms are mechanically coupled to the slider contact of the potentiometer,

so they are moving in phase.

Caliper logging

The movement of the slider contact changes the resistance of the circuit,

which in turn causes variations in the electrical voltage of the circuit.

Actually, the electrical voltage is measured and recorded directly.

The diameter of the borehole is derived (in inch) from the voltage by

means of a relationship based on an appropriate calibration method.

The computation of diameter is automatic (a program executes it), so the

log curve displayed on the screen of the surface data acquisition and pre-

processing system already shows in real time how the diameter is

changing in the borehole during the logging operation.

More about potentiometers:

https://www.electronics-tutorials.ws/resistor/potentiometer.html

Caliper logging

Slider Potentiometer

A simple sketch of the electric circuit applied in a mechanical caliper tool

Caliper logging

The figure represents a three-arm bow

spring caliper.

It is often used together with an acoustic

logging tool to provide the centralized

position of the acoustic logging tool in the

borehole.

A cylindrical spring tries to keep the flexible

but strong steel bow springs as open as

possible. But the diameter of borehole will

determine the positions of the bow springs.

As they are moving, the lower axial spring

is lengthening or shortening.

The disadvantage of bow spring caliper

devices is that the spring actuation quite

often gets plugged with drilling mud or

cuttings blocking the sliding mechanism in

the central part of the caliper.

Mechanical caliper measurement

Baker Hughes Inc., Introduction to

Wireline Log Analysis

Density logging instruments is also able to measure the distance between

the skid face and the backup arm (or backup shoe).

The massive skid contains the radiation source and the detectors.

The arm forces the skid face against the borehole wall with relatively high

pressure to provide as good contact with the formation as possible.

The movement of the arm during the logging operation is used for the

measurement of borehole diameter.


http://www.gowellpetro.com/product/litho-density-logging-tool-ldlt.html

Although the logging tool

is primarily designed for

measuring the bulk density

of rock formations, its

construction enables the

borehole diameter to be

measured.

Microresistivity devices are also able

to measure the borehole diameter by

means of their arms.

Two-point microresistivity caliper

tools provide the best indications of

the mud cake thickness opposite

permeable beds, because of the

lower force applied to the pad (which

does not damage the mud cake

significantly).

Remark

The electrodes of a microresistivity

device are built in a wear-proof

rubber or plastic pad adapted to the

end of an arm. Water-base mud is

required for the resistivity

measurement.


Baker Hughes Inc., Introduction to Wireline Log Analysis

A four-arm dipmeter tool provides diameter measurements from the two

pairs of opposite arms.

The planes of these pairs of arms subtends a right angle.

In such a way two independent borehole diameter curves can be recorded

in perpendicular directions (CALX, CALY).

Each arm has a pad with built in electrodes to measure microresistivity

curves in different directions. The tool also contains a device (gyroscopic

orientation equipment) for measuring the deviation and azimuth (bearing) of

the reference arm in the borehole.

The figure shows a pad assembly with a gauge ring calibrator having known

diameter.



Caliper log presentation

Daniel A. Krygowski: Guide to Petrophysical Interpretation

A caliper log curve is printed in track 1

(on the left side) with a dashed line

style.

The more frequently used mnemonics

for a caliper curve are CAL, CALI (I

means inch), CALX, CALY (X and Y

identify the different but perpendicular

azimuthal directions).

The horizontal scale is linear and

graduated in inches of diameter.

Generally, a reference line with a

constant value of the bit size (BIT) is

also presented in the same track to

facilitate recognizing the deviations of

a caliper curve from the bit size.

Two caliper curves of a four-arm (or dual-

caliper) tool are presented on the right side

of a well log. The plotted diameter data

measured different but perpendicular

azimuthal directions.

Here, the two curves (C13, C24) are

displayed in tracks 2 and 3 with opposite

scale directions.

In such a way the places of wash-outs can

be emphasized.

In track 1 the average hole diameter (CAL) is

displayed.

An integrated hole volume (VOL) is also

added as horizontal ticks on the left margin

of the track. The computation of hole volume

is based on the diameter data and executed

by a program.

The abbreviation TEN means cable tension.

Caliper log presentation


Acoustic pulse-echo imaging tools provide the complete (360 degree)

circumferential coverage of the borehole size and shape.

Acoustic caliper measurement

In such a device (e.g. BoreHole

TeleViewer, BHTV), a centralized

ultrasonic transmitter-receiver unit

rotates rapidly while the tool is being

pulled up slowly in the borehole.

As a result of this spiral movement of

the transmitter-receiver unit, detailed

acoustic images of the borehole wall

are obtained from the detected

amplitude and travel time of reflected

acoustic waves. The travel time values

are used for computing the distance

(radius) of borehole wall from the tool

axis as a function of depth and azimuth

(the velocity of ultrasonic wave in the

mud is a known value). O. & L. Serra, 2004: Well Logging Data

Acquisition and Applications, Serra Log

A spiral plot of the acoustic radius

gives the 3D image of a section of

the hole.

Acoustic caliper measurement

Schlumberger: UBI (Ultrasonic

Borehole Imager) brochure

The time elapses between the

emission of an ultrasonic wave

packet having dominant frequency of

MHz or greater and its arrival at the

receiver after reflecting off the

borehole wall gives the so-called

travel time.

r(z,)=t(z,)v

r(z,): the radius of borehole wall as

a function of depth (z) and azimuth

()

t(z,): travel time of ultrasonic

wave packet as a function of depth

(z) and azimuth ()

v: velocity of ultrasonic wave in the

mud

Interpretation goals:

• separation of permeable and impermeable beds on the logs,

• computation of borehole volume,

• input data for environmental corrections of other measurements (e.g.

resistivity logging methods),

• correlation,

• log quality control.

Separation of permeable and impermeable beds on the logs

A hole section is on gauge when its diameter is the same or nearly the

same as the bit size. It indicates the presence of impermeable massive

rocks (resistant to mechanical effects of drilling and mud circulation, e.g.

limestone, granite).

A hole section is caved or washed out when its diameter is larger than the

bit size. Wash-outs are typical of shale beds.

Rocks in pieces can also be removed by drilling and mud circulation when

the rock formation is fractured or under-consolidated.

If the caliper log curve has a smooth profile with smaller diameter than the

bit size, a mud-cake build-up is indicated opposite a permeable bed.

Interpretation of caliper logs

Mudcake build-up

Malcolm Rider: The Geological Interpretation of Well Logs

A mud cake is an extremely

useful indicator of permeability,

because mud cakes are formed

only opposite permeable beds.

Mud cake thickness (hmc) can

also be estimated from the

caliper log curve:

ℎ𝑚𝑐 =𝐵𝐼𝑇 − 𝐶𝐴𝐿

2

BIT: bit size (inch)

CAL: borehole diameter (inch)

Some typical caliper log responses

Malcolm Rider: The Geological

Interpretation of Well Logs

An irregular (not smooth) profile of

a caliper curve with smaller

diameter than the bit size indicates

a so-called ‘tight spot‘ in the

borehole.

A frequent cause of tight spots is

the dominance of smectites in the

clay mineral composition of shale

or clay beds. Smectite is a group

of clay minerals. Such clays are

able to take water from the drilling

mud, expands, sloughs off the

formation and collapses into the

hole (swelling clays).

Smectite rich tight spots can be

very dangerous from the point of

view of well logging, because

logging tools can get stuck in these

sections.

Tight spots in a shale sequence


Three main types of elliptical borehole cross-sections can be recognized

on multi-arm caliper log curves:

• keyseat,

• washout,

• and breakout.


Malcolm Rider: The Geological Interpretation

of Well Logs

Wash-out develops especially in shaly zones.

On caliper logs, a washed out interval has a considerable

vertical extent and all the calipers are larger than the drill bit

size.

One of them is often much larger than the other(s).The

borehole shape changes gradually along the interval affected

by washing out.

Keyseat is an asymmetric oval portion of the hole.

Caliper curves show a difference from the bit size only in one

direction.

The rotating drill pipe string forms this shape when it is

bended (not straight) under some overweight.


Breakout is a stress-induced enlargement of the borehole

cross-section.

Typical characteristics of this phenomenon:

• the caliper tool stops rotating when enter a breakout zone

(a breakout impedes the axial rotation of a caliper logging

tool).

• the caliper curves show different diameters which indicates

an oval hole.

• the larger hole diameter exceeds the bit size, the smaller

diameter fits to the bit size.

• the direction of larger diameter does not consistently

coincide with the azimuth of borehole deviation.

The direction of larger diameter depends on the directional

mechanical stress prevalent in the rock formation.


The borehole volume is computed by integrating the hole volume of

each interval between the neighbouring caliper data.

The total borehole volume:

𝑉ℎ𝑜𝑙𝑒 =

𝑖=1

𝑁𝑑𝑖2 ∙ 𝜋

4∙ ∆𝑧

where di is the hole diameter (value of the caliper curve) at depth zi,

z is the sampling interval (depth interval between the neighbouring

depth levels at which caliper data are measured) and N is the

number of measured caliper data.


Approximation of borehole volume by means of flat cylinders having

uniform height on the left side of the figure.

Bar chart of borehole diameter data on the right side.


The blue segmented

line (representing a

linear interpolation

between neighbouring

data) shows where the

approximation over- or

underestimates a

portion of the hole.

For a longer depth

interval these contrary

effects compensate

each other.

The borehole volume is very useful information for well completion engineers,

because the volume of annular space between the borehole wall and the

outer surface of planned steel casing string can be determined by subtracting

the volume of the cased hole from the borehole volume (the volume of the

open hole section).


The necessary amount of Portland

cement is calculated so that the

cement slurry (mixture of cement

and water) safely fills the annular

space.

Vcement=C (Vhole – Vcased_hole)

C is a coefficient whose value

depends on the planned cement-

water ratio and the type of

cement.

After hardening the cement slurry

a cement sheath comes into

being, which binds the casing

string to the rock formations.

𝑉𝑐𝑎𝑠𝑒𝑑_ℎ𝑜𝑙𝑒 =𝑑𝑐2 ∙ 𝜋

4∙ 𝑁 ∙ ∆𝑧

Correlation based on caliper logs

Finding similar patterns on the caliper curves coming from neighbouring

wells may help in the identification of formation boundaries.



Some formations

consistently show

wash outs on the log

curves in a

particular

geographic area

(regardless of the

applied mud

program), so the

changes in their

position both

vertically and

horizontally can be

traced.

Environmental corrections for other measurements

The distance between a logging tool and the formation and the volume of

mud around a logging tool highly depends on the borehole size and

shape.

These factors have significant effects on most of the measurements.

As the borehole size increases, the effect of drilling mud on the logging

tool reading also increases in general.

Mud cake can influence the measured value as well.

Therefore, the hole diameter and mud cake thickness are used in various

charts for correcting the effects of these parameters on Density, Neutron,

Resistivity, and Induction logs.

Log quality control

When the caliper log indicates rough borehole with serious caverns,

the reliability of measurements can be query for the logging tools which

require as direct contact with the formation as possible (such as Density,

Neutron, and micro-resistivity).

These problematic intervals are called bad holes.


Spontaneous potential logging

A log curve of spontaneous potential (SP) is a recording of potential

difference between a movable electrode (M) in the borehole and a fixed

surface electrode (N) connected to the ground (grounded electrode).

L. Serra, O. Serra, 2004: Well

Logging – Data Acquisition and

Application

SP logging is a passive method because the electrical potential difference

is natural, it is not generated by the logging tool (artificial source is not

used). This is the simplest and cheapest logging method as regards the

technical implementation.

It helps in the lithological identification of beds and the delimitation of

porous permeable beds in clastic sedimentary sequences.

An SP log curve shows typical deflections opposite porous and permeable

beds if the following conditions are fulfilled:

• open hole portion is measured (no steel casing and cement sheet

separating the formations from the hole),

• water-based mud is used (freshwater-based mud is better),

• the drilling mud is at rest (no circulation and infiltration process),

• permeable beds having high water saturation,

• shale or clay beds separate the permeable ones,

• difference between the salinity of mud filtrate and formation water (that

results in difference between the resistivity of mud filtrate and formation

water).


The shape of an SP curve (SP response) reflects the effect of natural

electric currents. These currents driven by natural electric potential

differences circulate through the borehole, the adjacent shale beds and the

porous permeable rock formation.

The current flow lines concentrate near the boundaries between beds and

the borehole.

Takács, 1978


Takács, 1978


Static SP (SSP) is a theoretical maximum of SP opposite thick and clean

(non-shaly) permeable beds, which could develop if SP currents were not

able to flow through the boundaries. Its value primarily depends on the

ratio of mud filtrate and formation water salinities as well as the

temperature.

In practice, the flow of currents through the bed boundaries and borehole

results in some decrease in the natural potential difference, so the

measured SP never attains the value of SSP. But, it can approach this

maximum in the middle of a thick, clean, water-bearing permeable interval.

The systematic appearance of SP deflections opposite permeable beds is

owing to a natural electric potential difference called electrochemical

potential.

The electrochemical potential (Ec) is the sum of two potential differences

coming from different phenomena:

• diffusion potential,

• and membrane potential.

They are regarded as SP components.

L. Serra, O. Serra: Well Logging – Data Acquisition and Application, 2004


The diffusion potential (Ed), also known as

liquid-junction potential, is produced by an

electrochemical process taking place at the

contact of virgin and invaded zones.

Here the mud filtrate and formation water

meet. If the two solutions have different ion

concentrations, diffusion of ions begins from

high to low concentration.

Anions have typically greater ionic mobility in

dilute solutions than cations, because of the

smaller diameters of hydration shells. The

significant difference in mobility is particularly

true for Cl- anions and Na+ cations.

Due to the different mobility, the anion

concentration becomes higher in the solution

of originally lower concentration after some

time.

In the other solution an excess of cations will

occur.

Thus, a charge separation comes into being,

which causes an electric potential difference

called diffusion potential between the two

sides of the liquid contact.

The magnitude of this potential primarily

depends on the ratio of ion concentrations

and the temperature.

L. Serra, O. Serra: Well Logging –

Data Acquisition and Application,

2004


The membrane potential (Em) also develops

due to the different salinities of formation

water and mud filtrate, but the diffusion is

taking place through the adjacent shale beds

of a permeable formation.

Because the surface of clay particles are in

excess of negative charges, the shale and

clay beds behave as a semi-permeable

membrane which permit only cations from

high to low concentration and impede the

overpass of anions.

As a result, the cation concentration will be

higher in the solution of originally lower salt

concentration after some time.

This charge separation results in the

membrane potential.

The magnitude of this potential also depends

on the ratio of ion concentrations and the

temperature.

L. Serra, O. Serra: Well Logging –

Data Acquisition and Application,

2004


Additional, not electrochemical, electric potential may also exist in the

borehole environment, if filtrate from the mud is seeping into the permeable

and/or its adjacent shale beds during an SP logging operation.

This potential is called electrokinetic or streaming potential.

It has no direct geological information content, so it is regarded as a

measurement noise. Its magnitude depends on rather the physical and

physicochemical parameters of the borehole environment.

Fortunately, its effect is minimal after the mud invasion has been stopped

by a fully developed mud cake.

Very small rates of simultaneous filtration into both permeable and shale

beds produce electrokinetic potential components having opposite signs,

so they usually compensate (eliminate) the effects of each other.

In special cases (e.g. very low permeability formations k< 5 md, special

muds etc.), however, it may cause problematic distortions in certain parts

of the measurements.


Presentation of SP log curves


An SP log curve is displayed and printed

in track 1 (on the left side with a solid

line style) together with the caliper

(CAL), bit size (BIT) and natural gamma

ray (GR) curves.

The horizontal scale is linear and

graduated in millivolts.

Since there is no absolute reference of

electric potential in practice, the

measurement does not require a

calibration. The zero value is adjusted by

the logging operator before logging.

The lack of calibration does not cause

problem, because not the absolute value

of SP but the SP difference from the

shale baseline is used in the evaluation

of SP curves. The difference, in turn,

does not depend on the selection of local

zero potential.

SP deflection means the systematic (not

stochastic) deviation of SP log curve from a

basic trend line called shale baseline. It has

a geological reason.

It usually occurs opposite porous permeable

beds with high water saturation, so it is

regarded as the indication of porous

permeable beds in clastic sedimentary

sequences.

Shale baseline is a drawn line which fits well

to SP log readings opposite impermeable

shale or clay beds.


For shorter intervals of a formation the shale baseline is typically vertical,

because the SP values of shales or clays can be approximated by a

constant.

For longer intervals the baseline is often a linear trend line because of the

gradual increase in temperature and electrode polarization.

Significant change in the salinity of formation water or in the clay mineral

composition of impermeable beds breaks or shifts the shale baseline.

Example of shifted shale baselines (red line segments in the figure) along

a depth interval which includes an unconformity separating different

formations.


The magnitude and polarity of SP

deflection depends on several factors.

For thick and clean water-bearing beds the

difference between the salinities of mud

filtrate and formation water has a crucial

influence.

A negative SP (also called normal SP)

deflection appears if the salinity of mud

filtrate is lower than that of the formation

water.

In such a case the mud filtrate resistivity

(Rmf ) is higher than the formation water

resistivity (Rw).

This case often occurs when freshwater-

base mud is used and the formation is

located deep enough to have saline

formation water.


The greater the difference between the salinities (or the resistivities) is the

higher the magnitude of SP deflection is.

On the contrary, a positive SP (or reversed SP) deflection appears , if the

mud filtrate resistivity (Rmf ) is less than the formation water resistivity (Rw)

(that is the salinity of formation water is less than that of mud filtrate

fresh water bearing formation/ saltwater-based mud).

Where the shape of an SP deflection finely

indicates a permeable bed, the inflection

points of the deflection are used for

positioning the bed boundaries.

When the mud filtrate resistivity and

formation water resistivity are

approximately the same (Rmf Rw), the SP

curve is flat opposite the porous permeable

bed.

In such a case the natural potential

difference cannot develop, so the SP

deflection does not appear.


Some typical SP log responses


1. Shale baseline

2. Negative or normal SP deflection

3. Positive or reversed SP

deflection

4. Impermeable beds,

independently of their lithology, do

not result SP deflections.

5. The shale or clay content of a

permeable bed reduces the

magnitude of SP deflection.

The higher the shale or clay

content is the lower the magnitude

of SP deflection is.

2

1

3

4

5

SP log curves are used for not

only the indication of

permeable beds and the

determination of their

boundaries but also the

correlation of these beds

among the neighbouring

boreholes of an oil-field.

The positions of the same beds

can be identified along the

different boreholes by means of

finding the similarities in the

shape of SP log curves.

Beside the SP log curves

additional log curves sensitive

to the lithology (e.g. natural

gamma, resistivity, caliper) are

also used for executing the

correlation.


Telford et.al.: Applied Geophysics, Cambridge

University Press 1990

One of the quantitative applications of SP log

curves is the determination of shale or clay

volume fraction in permeable beds.

Its estimation is based on the following empirical

formula:

Vsh=1 – (SPdeflection / SSP ) ( 100 %)

At first, we have to find the shale baseline and the

clean sand line on the SP log curve.

The shale baseline represents the shale or clay

volume of 1 (or 100%). The SP value of the clean

sand line can be obtained from the SP deflection

of a thick non-shaly water-bearing bed. It

corresponds to the shale or clay volume of 0.

The difference between the SP value of the clean sand and the shale

baseline gives the approximation of SSP value.

A linear relationship is primarily assumed between the value of SP

deflection (SPdeflection) and the shale volume.


Example of the

determination of

shale baseline and

sand line.


Ferenczy L., Kiss B.: Szénhidrogén-tárolók

mélyfúrási geofizikai értelmezése

Example of typical

SP log responses

Factors influencing the SP deflection

George Asquith and Daniel Krygowski: Basic Well Log Analysis

The thickness of the permeable bed

also has an influence on the SP

deflection.

The thinner the bed is the lower the

magnitude of SP deflection is.

The hydrocarbon content of a

permeable bed decreases the

magnitude of SP deflection, because

it reduces the development of SP

components.

The higher the hydrocarbon

saturation is the lower the magnitude

of SP deflection is.

Opposite permeable but not clean

(shaly) beds the SP deflection never

even approximate SSP, so the

maximum of these deflections is

called pseudo-static SP (PSP).

Effect of bed thickness on the SP curve

O. Serra: Fundamentals of well log-

interpretation

The alternation of very thin permeable and

impermeable beds cannot be indicated by

an SP log curve because of its limited

minimum bed resolution. In such a case the

SP log curve shows a thicker virtual bed

whose deflection depends on the integrated

effects of individual beds.

Effect of shale or clay content on the SP curve

Zaki Bassiouni:Theory, measurement and

interpretation of well logs


Mineral composition of the rock matrix

Only special minerals and materials in the rock matrix have effect on the

natural electric potential field.

SP deflections occur opposite coal beds and rocks having higher

concentration of metallic sulphides and other conductive minerals.

Porosity and permeability

There is no empirical relationships between SP deflections and these

important petrophysical properties.

A very small permeability (< 1 md) may be enough to provide the diffusion

of ions and to develop an SP deflection.

Rock texture

Gradual decrease of SP within an SP deflection may indicate a gradual

attenuation in dominant grain size (gradation) if it entails increasing volume

fraction of shale or clay.


Some other factors

SP deflections are smaller in borehole portions having larger diameter if

there is no additional change in the environment.

Greater diameter of invasion also reduces the amplitude of SP deflections.

The temperature linearly increases the value of SP.

The pressure difference directly influence the diameter of invasion and the

magnitude of electrokinetic potential, so it indirectly affects the SP.

As a summary, it can be stated that SP logging is the simplest well logging

method, which records the electric potential between a surface and a

measuring electrode. The causes of anomalous SP values are the

phenomena induced by the salinity difference between the mud filtrate and

formation water.

If there is no salinity difference , SP deflections ( flat SP) do not appear on

an SP log curve.

The measurement cannot be performed in cased holes or open holes filled

with non-conductive mud (oil-based mud or gaseous drilling fluid).

By this time, the significance of SP log has significantly reduced in

hydrocarbon exploration, because the special admixtures (e.g. KCl and

certain polymers) of water-based mud do not support the development of

spontaneous potential in the borehole. Thus, the SP curves measured in

such environments are featureless and not applicable to split a formation

into beds.

The main applications of SP log curves

• identification of permeable and impermeable beds,

• determination of permeable bed boundaries and the effective bed

thicknesses,

• correlation of beds among neighbouring boreholes,

• determination of the shale volume fraction in shaly permeable beds,

• estimation of formation water resistivity (Rw) in clean, water-bearing

formations (Rw is an important parameter of different water saturation

formulae).


Natural gamma ray (GR) logging

All the rocks produce some natural radioactivity. Some of them

radiate much more than others.

The natural radioactivity of rock formations is measured by the

(natural) gamma ray logging method (GR, NGR).

Natural radioactivity results from the spontaneous decays of

certain isotopes (instable isotopes).

During the atomic disintegrations (radioactive decays) the nuclei

of the radioactive isotopes may emit

• alpha particles (helium nuclei with two positive charges),

• beta particles (electrons or positrons),

• gamma rays,

• and heat rays.

Gamma rays and the electromagnetic spectrum

Gamma rays are high-energy

electromagnetic waves (with high

penetration power), which are emitted by

some radioactive elements.

These rays form the highest frequency

range of the electromagnetic spectrum.

The frequency band of gamma ray

covers (by definition) the range of 1019 to

1021 Hz

Gamma ray has dual nature similarly to

the light.

When it interacts with atoms, it behaves

like a particle (called photon) the energy

of which depends on its frequency:

E = f h,

where h is the Planck constant.

Gamma rays

After the nucleus of a radioactive isotope has emitted an alpha or

a beta particle, the nucleus is often left in an excited state (that is

it has some extra energy).

But the nucleus is not able to stay in the exited state, so it "calms

down" after a while by releasing this plus energy in the form of

gamma ray.

The energy of emitted gamma ray is a characteristic of the

emitting nucleus.

AreK 40

18

40

19

An example of the gamma ray

emission is the decay of potassium 40

isotope by electron capture.

The nucleus captures an electron from the closest electron shell

(K-shell) then it changes into argon 40 and emits gamma ray ()

as well as a neutrino ().

The energy of gamma ray emitted by the metastable argon is

equal to 1.46 MeV.

Natural radioactivity

The product of a decay (called daughter element) may be a stable or

an instable isotope.

An instable isotope undergoes further decays until a stable isotope is

finally produced.

A radioactive series identifies a group of isotopes which includes the

initial radioisotope and all its stable or unstable daughter elements.

Practically, three radioisotopes are responsible for the gamma ray

activity of rock formations:

• potassium 40 (40K),

• thorium 232 (232Th),

• and uranium 238 (238U)

The decay of 40K takes place in a single step and results in the

emission of a single characteristic gamma ray at an energy of 1.46

MeV. (In fact, only the 11 % of decays produce such gamma rays.)

However, both thorium and uranium isotopes have their own

radioactive series with several intermediate isotopes.

ATOMIC NUMBER

NU

MB

ER

of

NE

UT

RO

NS

Decay series of 238U (8 alpha, 6 beta disintegration)

The beta decay of 214Bi is accompanied by

a gamma radiation of 1.76 MeV.

The prominent gamma ray emission of the

uranium series is due to an isotope of

bismuth.

ATOMIC NUMBER

NU

MB

ER

of

NE

UT

RO

NS

Decay series of 232Th (6 alpha, 4 beta disintegration)

The beta decay of 208Tl is accompanied by a

gamma radiation of 2.62 MeV.

The prominent gamma ray emission of

the thorium series is due to an isotope

of thallium.

Natural gamma ray logging (GR) is a passive measurement (similarly to SP

logging), because there is no need to use a man-made gamma ray source.

Actually, the rocks themselves are the sources of natural gamma radiation.

A GR logging tool includes a single detector.

In the past, Geiger-Müller counters were used as detectors.

Now, scintillation counters are used for GR measurement. They are much

more efficient than the Geiger-Müller counters.

The detector measures the number of impacted gamma photons per unit time

in counts per second (cps). This quantity is called count rate or counting rate in

practice.

In petroleum industry the GR logging tools are calibrated to a unit called GAPI

(G: gamma ray, API: American Petroleum Institute).

By means of the calibration, the effects of technical parameters depending on

the logging tools can be significantly reduced in the measurement.

So, the measured value in GAPI unit characterizes mostly the natural

radioactivity of the formation near the tool.

A GR logging tool can be run in either open holes or cased holes filled with any

type of fluid (e.g. water, water-based mud, oil-based mud).

Centralization is not absolutely necessary.

Measurement of gamma radiation

Two types of gamma ray logging are applied in practice:

• (total) gamma ray logging (GR),

• spectral gamma ray logging (SL, NGS).

Total gamma ray logging measures all the gamma radiation of the

formation independently of the energy levels of gamma rays.

So, the measured counting rate includes the effects of all the isotopes

emitting gamma ray in the formation near the logging tool.

Spectral gamma ray logging method (also known as natural gamma

spectrometry) is able to separately detect gamma rays falling into different

energy intervals. As a matter of course, the total gamma ray is also

measured.

The entire energy range of natural gamma rays is divided into intervals

called energy windows.

Three of these energy windows are fitted to the energy levels of

characteristic gamma radiations used for the identification of 40K, 232Th, and 238U. By means of this energy selective measurement, the concentrations

of potassium, thorium and uranium in the formation are estimated.


Log Interpretation Principles/Applications, Schlumberger 1989

The figure illustrates the natural

gamma ray spectra.

The peaks of the magnified part

of the spectrum correspond to

the energy levels of

characteristic gamma radiations

belonging to 40K, 232Th, and 238U.(dN/dE: the number of gamma

photons falling into an energy unit)

For the spectral gamma ray measurement, the high-energy part of the

spectrum is divided into three energy windows (W3, W4, and W5).

Each window includes one of the characteristic peaks of three naturally

occurring isotopes.

By the separated detection of gamma rays in each window, it is possible to

determine the concentrations of thorium 232, uranium 238, and potassium

40 in the formation.


Log Interpretation Principles/Applications, Schlumberger 1989

In order to reduce the statistical

variations of the measurements,

the low-energy part of the

spectrum is also detected in two

energy windows (W1 and W1).

This part of the spectrum is

mainly due to the scattering of

emitted gamma photons on

atomic electrons of the medium

(rock). (Compton scattering)

A spectral gamma ray tool measures:

• the total gamma ray (GR in GAPI),

• the uranium free gamma ray (KTH in GAPI),

• the concentration of potassium (K in %),

• the concentration of uranium (U in ppm),

• and the concentration of thorium (Th in ppm).


Spectral gamma ray tool diagram (Halliburton)


K Th U

relative

abundance in the

Earth’s crust

2.59 % =

25900 ppm

~ 12 ppm ~ 3 ppm

gamma rays per

unit weight

1 1300 3600

Although the relative radiation activity of potassium is very

small compared to the , it is the most frequent of the three

elements.

Abundance ratio and relative radiation activity of the

elements related to the natural radioactivity of rocks

40K/Ktotal (%) 232Th/Thtotal (%) 238U/Utotal (%)

Ratio of the

radioactive isotope

to the total amount

of element found

in nature (%)

0.0199 100 99.27

Although the ratio of potassium 40 to the total potassium and

the relative radiation activity of the potassium are very low,

the contribution of potassium 40 to the total gamma radiation

of rocks is about 50 % due to the high relative abundance of

potassium.

The contributions of the thorium and uranium to the total

gamma radiation are 29 % and 20 %, respectively.

Natural abundance of the radioactive isotopes

Minerals:

alkali feldspars, micas, illite, montmorillonite (in the crystal

lattice), alkali evaporites (e.g. KCl, sylvin)

Potassium ions may be adsorbed onto the surfaces of clay

particles.

Rocks:

shale, clay and claystone, silt and siltstone, sandstones with

alkali feldspar content (e.g. arkose and graywacke).

Potassium-bearing minerals and rocks

The source minerals of uranium are connected to acidic

igneous rocks (granite, rhyolite, diorite etc.)

Uranium is very soluble, and is often transported in solutions

from its source location and accumulates in farther

environments.

Uranium may accumulate:

• on clay particles (adsorbed onto their surfaces),

• in phosphates,

• in very resistant minerals (such as zircon, titanite or

sphene, monazite, allanite, biotite, xenotine) occurring in

detrital, fluvial, lacustral or deltaic sediments,

• in organic matter (in low-oxygen environments).

Uranium-bearing minerals and rocks

Thorium originates from acidic and intermediate igneous rocks

(e.g. granites, pegmatites, syenites,nepheline syenites).

Thorium is practically an insoluble element, and thorium-bearing

minerals are very stable (resistant to chemical processes).

So, the thorium is transported mainly in suspension, and is a

common constituent of the detrital fraction of sediments.

Thorium is found principally:

• in clays of detrital origin (adsorbed onto its particles),

• in acidic and intermediate igneous rocks,

• in certain sands and sandstones contain resistive heavy

minerals (such as monazite, zircon, xenotime, allanite),

• in chemical compounds of thorium.

Thorium-bearing minerals and rocks

Interpretation goals:

• correlation of formations,

• determination of lithology,

• estimation of shale (or clay) content,

• identification of clay types,

• fracture identification,

• source rock identification.

Correlation of formations:

The gamma ray log curves are scanned for similarities in shape

and magnitude.

Interpretation of gamma ray logs

Correlation by means of gamma ray logs


Identification of permeable beds by means of GR

and SP curves


Determination of lithology

In general, the reservoir rocks are

less radioactive than the

impermeable shales and clays.

However, some sandstones and

dolomites can be radioactive.

shale beds

Some typical gamma ray log responses


Natural radioactivity in evaporites


Gamma ray characteristics of coal and organic

rich shale


Estimation of shale (or clay) content:

The magnitude of the gamma ray in the formation can usually be

related to the shale or clay content of the formation in clastic

sedimentary sequences of strata.

The relationship between the measured gamma ray and the shale

or clay volume fraction can be linear or non-linear. The applied

relationships are all empirical.

Gamma ray index, IGR:

𝐼𝐺𝑅 =𝐺𝑅𝑙𝑜𝑔 − 𝐺𝑅𝑐𝑙𝑒𝑎𝑛

𝐺𝑅𝑠ℎ𝑎𝑙𝑒 − 𝐺𝑅𝑐𝑙𝑒𝑎𝑛

IGR : describes a linear response to the shale or clay content,

GRlog : log reading at the depth of interest,

GRclean : Gamma Ray value of the clean bed,

GRshale : Gamma Ray value of the shale or clay bed.



Linear Gamma Ray – shale (clay) volume relationship:

Vshale = IGR

Non-linear Gamma Ray – shale (clay) volume relationships:

Steiber:

𝑉𝑠ℎ𝑎𝑙𝑒 =𝐼𝐺𝑅

3.0 − 2.0 ∙ 𝐼𝐺𝑅Clavier:

𝑉𝑠ℎ𝑎𝑙𝑒 = 1.7 − 3.38 − 𝐼𝐺𝑅 + 0.7 2

Larionov (for tertiary rocks):

𝑉𝑠ℎ𝑎𝑙𝑒 = 0.083 ∙ 23.7∙𝐼𝐺𝑅 − 1

Larionov (for older rocks):

𝑉𝑠ℎ𝑎𝑙𝑒 = 0.33 ∙ 22∙𝐼𝐺𝑅 − 1


Estimation of shale (clay) content:

All the previous relationships are empirical.

The choice of which to use is up to the user, and depends on other

information about the formation.

If no other information is known, the linear relationship is probably

the best choice, although it is the most pessimistic (that is, it

predicts the most shale or clay volume for a given GR response.

All the non-linear relationships predict less shale or clay volume

than the linear response of the gamma ray index.




George Asquith and Daniel Krygowski: Basic Well Log Analysis

Documents

Applied geophysics Well logging Part 3