Neutron Methods in Well Logging - uni-miskolc.hugeofiz/Oktatok/vass/Well_logging_Neutron... · Neutron Methods in Well Logging edited by P. Vass For Petroleum Engineer & Geoengineer

Neutron Methods

in Well Logging

edited by P. Vass

For Petroleum Engineer & Geoengineer MSc Students

The first neutron device was developed shortly after World

War II.

In the beginning neutron logging was used to determine

formation porosity.

Later additional methods were invented to produce a limited

chemical analysis of the formation by means of analysing the

absorption rate of the emitted neutrons, and detecting

neutron-induced gamma rays.

In order to understand the tool responses of different neutron

methods, the knowledge of neutron interactions is of key

importance.

Introduction

A free neutron is very rarely produced in nature mostly due to

spontaneous nuclear fission.

In addition, a free neutron is not a stable particle.

Its mean lifetime is about 14 minutes, 42 seconds (half life is

about 10 minutes, 11 seconds) in vacuum.

The beta decay of the neutron:

free neutron proton + electron + antineutriono

Sometimes additional energy is also emitted in the form of

gamma ray.

When a neutron penetrates into a medium its lifetime

significantly shortens.

Interactions of neutrons with matter

The most important forms of neutron interactions is the

collision of neutrons with nuclei of atoms.

There are several ways of interactions of neutrons with nuclei.

Each of them can be characterized by its own microscopic

cross section.

The microscopic cross section is related to the probability of

occurrence and mainly depends on

• the energy of incident neutron

• and the type of target isotope.

Basically, the more important interactions can be divided into

two main groups:

• scattering interactions,

• absorption interactions.


The energy range of neutrons which is relevant to logging

applications covers about nine decades.

The more important ranges and values:

• source neutrons 5–15 MeV,

• fast neutron range above 10 eV

• epithermal neutrons in the range of 0.2–10 eV

• thermal neutrons around 0.025 eV.

The relationship between neutron energy and its associated

velocity (v):

𝑣 = 0.22 ∙𝐸

0.025

where the unit of v is cm/μs.


Energy and associated velocity ranges for different classes of

neutrons


Neutron type Energy range Associated velocity

High energy > 10 MeV > 4.4×107m/s

Fast 10 keV - 10 MeV 1.39×106 - 4.4×107m/s

Intermediate 100 eV - 10 keV 1.39×105 - 1.39×106 m/s

Slow 10 eV - 100 eV 44000 - 1.39×105 m/s

Epithermal 0.2 eV - 10 eV 6223 - 44000 m/s

Thermal 0.025 eV 2200 m/s

Relationship between neutron energy and associated velocity.


O. Serra, L. Serra: Well Logging, Data Acquisition and Applications

As a result of several types of neutron interactions different

particles and/or gamma ray are produced.

The type of interaction can be denoted by the following symbolism:

(symbol of incident particle, symbol of produced particle(s))

where the symbols of most frequent particles

n: incident neutron

n’: scattered neutron

: alpha particle

: beta particle

p: proton

: gamma ray

For example: (n, ) represents a neutron absorption which

produces a gamma photon with a characteristic energy of the given

target isotope.


The neutrons emitted by the source have high energy.

During their propagation they interact with the atomic nuclei in

the formation, so their energy gradually decrease until the

absorption or detection.

The time interval between the emission and absorption is

called lifetime.

The lifetime of the neutrons can be divided into four phases:

• fast neutron,

• slowing down,

• diffusion,

• and capture.


The typical interactions of the fast neutron phase:

• inelastic scattering (dominant),

• nuclear reactions,

• and radioactivation.

Inelastic scattering (n, n’, )

In the case of inelastic scattering a neutron (with an energy above 1

MeV) collides with a nucleus.

The incident neutron transmits some portion of its energy to the

nucleus, which goes from ground state to excited state (excitation).

This energy transmission reduces the energy of the incident

neutron.

The excited state lasts a very short time (less than a microsecond)

then the nucleus return to its ground state by emitting one or more

gamma photons (de-excitation).


The energies of these prompt gamma rays are unique to the target

nucleus.


O. Serra, L. Serra: Well Logging, Data

Acquisition and Applications

A nucleus may have different excited

states.

The number of possible excited states

and their discrete energy levels

depend on the type of the nucleus.

As a result of a given interaction a

nucleus can get into different excited

states.

Each excited state has its own

probability of occurrence.

When a nucleus returns to its ground

state, the energy of the emitted

gamma photon corresponds to the

energy difference between the actual

excited state and ground state.

Example of carbon:

612𝐶 𝑛, 𝑛′, 𝛾 → 6

12𝐶

the half-life of the excitation is 3.8 10-14s

energy of emitted gamma ray is 4.44 MeV

Example of oxygen:

816𝑂 𝑛, 𝑛′, 𝛾 → 8

16𝑂

the half-life of the excitation is 1.7 10-11s

the principal energy of emitted gamma ray is 6.13 MeV (additional

inelastic energies of the oxygen spectrum are 6.92 and 7.12 MeV).


This type of reaction requires an energy greater than a threshold.

The value of the threshold energy depends on the type of target

isotope (corresponds to the ground state of the nucleus) and the

interaction will not occur below it.

The ground state energy fundamentally decreases with the mass

number (the instability of the nucleus increases), so the occurrence

of inelastic scattering increases with not only the neutron energy

but also the mass number.

The spectral measurement of the prompt gamma rays coming from

inelastic scattering is used to determine the relative concentration

of carbon and oxygen in the formation.

The carbon-to-oxygen ratio is applicable to indicating the presence

of water and oil as well as to estimating the water saturation in oil-

bearing formations.


Nuclear reactions

In the case of nuclear reactions the high energy neutron collides

with a nucleus to produce one or more nucleus which is different

from the initial one.

Additional particles (alpha, beta, proton, neutron) and prompt

gamma ray can also be emitted.

For example: (n, ), (n, , )

These reactions belong to the group of neutron absorption.

A nuclear reaction results in a transformation of at least one

nucleus to another.

On the contrary, the nuclear scattering does not change the nature

of any nucleus.

Several types of nuclear reactions are known, but their probabilities

of occurrence are very small compared to those of the interactions

of interest in well logging.


Example of the neutron-induced alpha emission from oxygen:

816𝑂 𝑛, 𝛼 → 6

13𝐶

the produced carbon isotope is in ground state (there is no

gamma ray emission right after the nuclear transformation).

816𝑂 𝑛, 𝛼, 𝛾 → 6

13𝐶

the produced carbon isotope is in excited state, and it emits a

prompt gamma ray (possible energy levels with different

probabilities are 3.09, 3.68, and 3.86 MeV).


Radioactivation

In the case of radioactivation, a high energy neutron collides with

the target nucleus, which changes into an unstable isotope.

This intermediate product decays with a relatively long half-life to

the final nucleus.

If the end-product is in an excited state, it emits a prompt gamma

ray to return to the ground-state.

The possible energy levels of emitted gamma rays are

characteristic of the final nucleus.


Schematic representation of radioactivation.



Example of oxygen activation:

816𝑂 𝑛, 𝑝 → 7

16𝑁 → 816𝑂 𝛾

the half-life of the unstable nitrogen isotope is 7.2 sec

it decays by -emission to 16O

the 68% of emitted prompt gamma rays have an energy of 6.13

MeV.

Example of silicon activation:

1428𝑆𝑖 𝑛, 𝑝 → 13

28𝐴𝑙 → 1428𝑆𝑖 𝛾

the half-life of the unstable aluminium isotope is 2.3 min.

the emitted prompt gamma ray has an energy of 1.779 MeV.

Other activations are connected to iron, aluminium and iodine.


The spectral measurement of

prompt gamma rays (inelastic

spectrometry) coming from

inelastic scattering, nuclear

reactions and radioactivation

can be used in the chemical

analysis of the formation.

The elements which are

commonly analysed: carbon,

oxygen, silicon, calcium, iron

and sulphur.


The figure illustrates the

energy levels of emitted

gamma rays and their

probabilities of occurrence

for the oxygen activation.

O. Serra, L. Serra: Well Logging, Data Acquisition

and Applications

Slowing down phase

In the fast neutron phase the average energy of neutrons gradually

reduces below 10 keV principally due to the inelastic scattering .

(The neutron flux is reduced by other neutron interactions

absorbing free neutrons)

At lower energies the cross sections of common elements for the

fast neutron interactions significantly decrease.

It means that their probabilities are actually negligible.

In the next phase another interaction called elastic scattering

becomes dominant, which causes intensive loss of neutron energy.

Since the energy reduction entails the velocity reduction of

neutrons, this phase is called slowing down phase.


Elastic scattering (n, n’)

In the case of elastic scattering the incident neutron collides with a

nucleus and transmits some part of its kinetic energy to the target

nucleus.

The amount of decrease in the energy of neutron is the same as

that of increase in the energy of target.

Thus, the energy of the system (includes the neutron and target

nucleus) does not change. Only the division of energy between the

participants will change.

During this interaction the energy of the neutron is not enough to

excite the target nucleus, so the interaction does not result in the

emission of prompt gamma ray.

But the energy of neutron is to high for the target nucleus to absorb

the neutron, consequently it leaves the space of the nucleus with

reduced energy and in modified direction.


The loss of neutron energy

depends on the angle of incidence

and the mass of target nucleus.

The most effective reduction of

energy occurs when the neutron

collides with a nucleus having the

same or very similar mass as that

of neutron.

So, hydrogen atoms have the

highest slowing down power for

intermediate energies.

Theoretically, a single head-on

collision (=180°) of a neutron

with a hydrogen nucleus may

result in the loss of entire neutron

energy.




The ratio of the neutron energy after

elastic scattering E’ to the neutron

energy before scattering E0:

𝐸′

𝐸0=𝐴2 + 2𝐴𝑐𝑜𝑠Θ + 1

(𝐴 + 1)2

where A is the mass of the nucleus,

and is the angle change in the initial

direction of the neutron.

Consequently, the neutron slowing

down power of a rock formation is

very dependent of the

concentration of hydrogen.

The porosity estimation of

reservoir formations is based on

this fact (since the water-filled

pore space abounds in hydrogen).

The figure shows how the range of

neutron energy reduction on a

single elastic collision depends on

the mass of target nuclei.

For the heavy elements the

energy reduction is less than 20%

while it may reach 100 % for

hydrogen.




By the end of slowing down phase the kinetic energy of a neutron

reduces to the vibration energy of the atoms in thermal equilibrium.

The thermal energy of a neutron is 0.025 eV at 25 °C, which corresponds

to a mean velocity of 2200 m/s.

The time interval of slowing down phase ranges from n x 10 –

n x 100 s depending on the lithology, porosity, fluid saturations etc.

Parameters used for characterizing the slowing down power of materials:

• slowing down length,

• average logarithmic energy loss,

• average number of collisions to slow down.


Slowing down length (Ls).

It gives how far a neutron with an initial energy of 4.2 MeV is able to stand

away from the point source averagely in an infinite homogeneous medium

until its energy reduces to the epithermal energy of about 0.4 eV (close to

the lower limit of epithermal energy range).

This average straight line starts from the source and determines a circle

beyond which the neutrons are mostly in thermal phase.

The value of slowing down length depends on the material composition of

the medium.

The higher the slowing down power of a medium is, the shorter its

slowing down length is.


Average logarithmic energy loss (or energy decrement) (ξ)

The logarithmic energy loss of a single elastic collision:

𝜉 = −𝑙𝑛𝐸′

𝐸0

E’ : neutron energy after elastic scattering,

E0 : neutron energy before scattering.

So that the energy of neutron may reduce from an initial energy Ei to

some lower energy E, a sequence of collisions is required.

Each collision in the sequence has its own logarithmic energy loss.

The average logarithmic energy loss estimates the arithmetic mean of

these individual logarithmic energy losses.

For the energy degradation from 4.2 MeV to 0.4 eV the highest value

of average logarithmic loss is unity, and it belongs to hydrogen.


The average logarithmic energy loss for head-on collisions:

𝜉 = 1 −𝐴 − 1 2

2𝐴𝑙𝑛

𝐴 + 1

𝐴 − 1

For large values of atomic mass (A)

𝜉 ≈2

𝐴 + 2/3

The heavy elements have low average logarithmic energy loss.


Average number of collisions to slow down (n)

It gives an estimation of the average number of collisions required to

reduce the energy of neutron from an initial energy (Ei ) to some lower

energy (E).

Frequently applied initial and moderated energies are 4.2 MeV

(source energy) and 0.4 eV (epithermal energy).


The table shows the average

logarithmic energy loss and

average number of collisions

necessary to reduce the energy

of neutrons from 4 MeV to 0.4

eV for different media.

Darwin V. Ellis, Julian M. Singer: Well Logging for Earth Sciences



The figure illustrates the relationship

between the average number of

collisions to slow down (n) and slowing

down length (Ls) for water (100 p.u.)

and limestone (0 p.u.).

High slowing down power is

characterized by short slowing down

length and few collisions to slow down.

The difference between slowing down

length and real paths of neutrons

(random walks) during their

propagation is also demonstrated.

Diffusion phase

Beyond the slowing down length the neutrons quickly reach the energy

level of thermal equilibrium.

The propagation of thermal neutrons is correspondent to the process of

diffusion.

It means that the direction and intensity of propagation are determined by

the concentration difference of thermal neutrons in the formation.

So, the population of thermal neutrons is gradually spreading outwards.

By the effect of elastic collisions they lose further energies until they are

captured by some nuclei of the medium.

The spatial interval of the medium within which the thermal neutrons exist

is characterized by the so-called thermal neutron diffusion length (Ld).

This parameter gives an average distance for the thermal neutrons, which

covers the interval between thermalization and absorption.

Similarly to slowing down length its value is dependent of the type of

material.


Computed porosity dependence of slowing down length and diffusion

length for clean limestone and sandstone formations.



Absorption phase

It follows the diffusion phase and means the last interval of free neutrons’

lifetime.

The energy of thermal neutrons is low enough for the nuclei of the

medium to absorb the neutrons.

Two interactions are typical in this phase:

• thermal neutron capture,

• thermal activation.

Thermal neutron capture (n, )

During this interaction the incident thermal neutron is totally absorbed by

the target nucleus, which becomes excited.

The excited state lasts a very short time because one or more gamma ray

is emitted almost at once so that the nucleus can return to its ground

state.

The energy of emitted gamma ray is characteristic of the target nucleus.


Thermal neutron capture (n, )

The tendency of elements to

thermal neutron capture is

characterized by the

(microscopic) thermal capture

cross section (barn/neutron).

The most effective elements:

• chlorine (its concentration

depends on the salinity of

formation water),

• gadolinium (uncommon

element in formation water),

• boron (often connected to

shales),

• lithium.




Although hydrogen is a moderate absorber, still its effect on

neutron capture is significant in the case of its high concentration

(formation water, oil).

The macroscopic (or volumetric) thermal neutron capture cross

section of the formations mainly depends on

• the lithology,

• porosity,

• salinity of formation water,

• water saturation,

• shale volume,

• and the types of clay minerals.

The concentration of following elements can be estimated by the

spectral analysis of prompt gamma rays:

silicon, iron, calcium, sulphur, chlorine and hydrogen.


Thermal activation

The process is similar to that of radioactivation in the fast neutron phase.

Some elements transform into unstable isotopes after capturing a thermal

neutron.

The unstable isotope decays to a stable isotope of another element.

Since the nucleus of produced isotope is in excited state, it emits energy

in the form of delayed gamma ray to return to the ground state.

So, the emission does not follow directly the event of neutron capture,

and the energy of delayed gamma ray is characteristic of the produced

stable isotope (not the absorber).

Example of aluminium activation:

1327𝐴𝑙 𝑛 → 13

28𝐴𝑙 → 1428𝑆𝑖 𝛾

the half-life of the unstable aluminium isotope is 2.3 min.

the emitted delayed gamma ray has an energy of 1.779 MeV.

This interaction is exploited in the field of bauxite exploration to estimate

the aluminium concentration of the formations (Aluminium Activation Clay

Tool, Schlumberger).


The figure represents the timescale of free neutrons.

Two characteristic time intervals can be assigned to the characteristic

distances (slowing-down length and diffusion length).


Darwin V. Ellis, Julian M. Singer:

Well Logging for Earth Sciences

The slowing-down time gives how much

time elapses averagely between the

emission of fast neutrons (~4 MeV) and

their slowing-down to the lower limit of

epithermal energies (∼0.4 eV).

It primarily depends on the hydrogen

concentration of the formation.

Its value is about 2 μs in water and about

12 μs in non-porous formations.

The diffusion time gives how much time

elapses averagely between the

thermalization of neutrons and their

thermal capture.

It depends on the macroscopic thermal

absorption cross section of the medium.

It is of the order of 100 s.

The spatial distribution of neutrons

and gamma rays of thermal neutron

capture around a point source in a

homogeneous infinite medium.


Malcolm Rider: The Geological Interpretation of Well Logs

The probability of each neutron interaction is characterized by a

microscopic and macroscopic cross section.

While a microscopic cross section gives an effective area of target nucleus

which is provided for the interaction with an incident neutron.

The larger the microscopic cross section is, the higher the probability of

interaction to occur is.

The macroscopic cross section expresses the probability of interaction in

the unit volume of a medium.


The cross section of each interaction depends on the energy of neutrons

and the type of material.

The total cross section of neutron interactions is composed of the cross

sections of particular neutron interactions.

It expresses the probability of neutron interactions, independently of their

types, for a given material.


The figure shows the energy dependency of

most important cross sections for a material.

At a given energy level more than one type of

neutron interaction may occur simultaneously.

The occurrence of elastic scattering is the

least energy dependent.

The inelastic scattering is bound to an energy

threshold.

The thermal capture is limited to lower

energies.

The total cross section is the resultant of the

cross sections of different neutron interactions.


Microscopic cross sections of different nuclear interactions for a uranium

isotope.

https://www.nuclear-power.net/nuclear-power/reactor-physics/nuclear-engineering-fundamentals/neutron-

nuclear-reactions/microscopic-cross-section/

Neutron sources

Since free neutrons are rarely produced by natural processes, artificial

isotopes and devices are used for generating free neutrons.

There are two main types of neutron sources:

• chemical (or encapsulated) sources,

• neutron generators.

Chemical sources

The common property of chemical sources is that they are continuously

emit neutrons.

So, the neutron emission can not be switched off and on.

The application of neutron sources is very dangerous. They must be

stored in special containers filled with hydrogen rich materiel (e.g.

paraffin).

There are two ways of generating free neutrons by chemical sources:

• producing the interaction of particles with the nuclei of an appropriate

element,

• applying a special isotope whose nuclei spontaneously fissure.

Neutron sources

Interaction of particles with a target element

A mixture of two elements is encapsulated into a small metal sheath.

One of these elements is radioactive and emits particles ( emitter).

The particles interact with the nuclei of other element (target).

A target nucleus captures an particle, so it transforms into the nucleus of

another element, and emits a free neutron.

The target is a light element which has a large microscopic cross section

for the (α, n) interaction (e.g. beryllium, boron and lithium).

Mostly beryllium is used as a target element.

The process can be formulated as follows:

24𝐻𝑒2+ + 4

9𝐵𝑒 → 612𝐶 + 𝑛

The group of applicable emitters includes:

• plutonium (94Pu),

• radium (88Ra),

• polonium (84Po),

• americium (95Am).

Neutron sources

The most frequently used emitter is the americium, which is a

synthetic element, and is produced in small quantities in nuclear

reactors.

It has no stable isotopes. Its most important isotope is Am 241 with

a half-life of 432.2 year.

In practice , Am-Be chemical sources are applied principally.

The peak of emitted neutron spectrum is around 4.2 MeV (mean

energy of emitted neutrons).

The activity of source depends on the amount of components.

Typical activities used in production logging:

111 GBq (3 Ci), 185 GBq (5 Ci), 296 GBq (8 Ci), 370 GBq (10 Ci).

The sources used in open hole logging are stronger:

592 GBq (16 Ci), 666 GBq (18 Ci), 703 GBq (19 Ci).

Neutron sources

A sample of americium under

microscope

https://en.wikipedia.org/wiki/Americium

A capsule of Am–Be neutron

source

https://www.researchgate.net/figure/Capsule-of-the-241-Am-Be-

neutron-source-X14-code-AMN24-assembly_fig1_283665544

Neutron sources

Californium source

Californium (98Cf) is a synthetic element, which has 20 known

unstable isotopes.

The most expensive material in the world. Its 1 gram costs about

27 million dollars (the second most expensive material is the

diamond with the price of around 55 000 dollars/ gram).

Its production is very complex (implemented in nuclear reactors

and particle accelerators), and only a few countries have

permission to produce it.

One of its isotope with a mass number of 252 has significant

practical applications.

Its a very dangerous and strong neutron emitter with a half-life of

2.6 year.

Neutron sources

Californium source

During its disintegration, two different events may occur with

different probabilities.

The probability of decay to isotope of curium-248 is much higher

(96.9%) than that of spontaneous nuclear fissure (3.1%).

The neutron emission is associated with the nuclear fissure (3.7

neutrons per fissure on the average).

The mean energy of emitted neutrons is 2.35 MeV.

The activity of 1 g of Cf-252 is 2300 GBq (~62 Ci).

Because of its dangerousness, its application is not permitted in all

countries.

Due to the relatively low energies of neutrons, it is used mainly in

the spectrometry of thermal neutron capture and activation (e.g.

aluminium activation).

Neutron sources

Neutron generators

The working principle of neutron generators is the same as that of a

linear charged particle accelerator.

In such a device, ions of deuterium are created (𝐷+ = 12𝐻+) at first,

then they are accelerated by high voltage electrodes.

The charged particles impact with a target which contains ions of

tritium (𝑇+ = 13𝐻+).

The target may be metal hydride or carbon impregnated with

tritium.

As a result of the nuclear fusion of a deuterium and tritium ion a

helium ion and a neutron are produced.

𝐷+ + 𝑇+ → 24𝐻𝑒2+ + 𝑛

The kinetic energy of the free neutron is around 14.1 MeV.

Neutron sources

A special system of electrodes with cylindrical symmetry is used for

focusing the beam of deuterium ions.

A typical voltage by which the ions can be accelerated and focused

is about 125 kV.

The main advantages of neutron generators (compared to chemical

sources):

• much higher neutron flux can be generated,

• the magnitude of neutron flux can be modified,

• the energy of emitted neutrons is higher (which is useful for

producing various fast neutron interactions in the formation),

• the emission of neutron can be controlled (switched on and off),

• safe application (there is no radiation after switching off)

• repeated sequence of neutron impulses can be formed to detect

the temporal decay of neutron energy and flux between the

neighbouring impulses.

Neutron sources

Schematic structure of a neutron generator

https://www.researchgate.net/figure/Schematic-diagram-

of-neutron-generator_fig1_230903221

A photo of a neutron

generator



Neutron logging methods

The table summaries some important characteristics of the different

applications of neutron interactions in well logging.

Type of

neutron

interaction

Type of

neutron

source

Energy of

emitted

neutrons

Detected ray

or particle

end its

energy

Elements of

crucial

importance

Derived

information

Samples of

logging

tools

inelastic

scattering

neutron

generator14 MeV

ray, 1.5-7.5

MeVC, O water saturation

GST, RST,

MSI, PSG,

RMT

elastic

scattering

Am-Be~4.35 MeV

n0 , 0.025 eV

n0 , 0.1-10 eVH porosity

CNL, CN,

DSNT, SNP neutron

generator14 MeV

thermal-

neutron

absorption

neutron

generator14 MeV

ray, 1.5-7.5

MeV

Ca, Fe, Si, S,

Ti, Gd, Cl, H

mineralogical

composition

GST-C, GRA,

PSG, ECS,

MST, RST

thermal

activationCf 2.35 MeV

ray, 181

keV-2 MeVAl

aluminium

concentrationAACT

thermal

neutron

decay time

neutron

generator14 MeV

ray, > 50

keVCl water saturation

TDT, RST,

RMT

Neutron logging methods

GST-I Neutron Induced Gamma Ray Spectroscopy – Inelastic Mode

RST Reservoir Saturation Tool (Schlumberger)

MSI Multiparameter Spectroscopy Instrument

PSG Pulsed Spectral Gamma-Ray Tool (Halliburton)

RMT Reservoir Monitor Tool (Halliburton)

CNL Compensated Neutron Log (Schlumberger)

CN Compensated Neutron Log (Baker Atlas)

DSNT Dual Spaced Neutron Tool

SNP Sidewall Neutron Porosity

GST-C Neutron Induced Gamma Ray Spectroscopy – Capture Mode

GRA Geochemical Reservoir Analyzer (Schlumberger)

PSG Pulsed Spectral Gamma (Halliburton)

ECS Elemental Capture Spectroscopy Sonde (Schlumberger)

AACT Aluminium Activation Clay Tool (Schlumberger)

TDT Thermal Decay Time (Schlumberger)

Neutron Porosity Measurements

The measurement of porosity is based on the high slowing-down

power of hydrogen.

If a hydrogen rich fluid (water, oil) fills the pore space, the energy

degradation of neutrons principally depends on the porosity.

But the hydrogen nuclei play an important role not only in the

slowing-down of neutrons but also in the thermal absorption of

neutrons at lower energies (in the diffusion phase).

If the concentration of elements with large cross section of thermal-

neutron absorption (chlorine, gadolinium, boron, lithium) can be

neglected in the formation, the rate of thermal-neutron absorption

mainly depends on the liquid filled porosity.

The probability of thermal neutron absorption to occur increases

with the number of thermal neutrons in the investigated volume.

Thus, the flux of gamma ray coming from thermal neutron capture

also increases with the number of thermal neutrons in the medium.


The relationship between the number of thermal neutrons per unit

volume (thermal neutron density [neutrons/cm3]) and the liquid filled

porosity (that is the hydrogen concentration) depends on the

source-detector spacing.

For short distances the thermal neutron density in the medium

increases with the porosity.

Beyond some interval the relationship becomes inverse.

The detectors applied in the logging tools are placed in the far zone

where the inverse relationship is valid.

Accordingly, the increase of porosity decreases both the flux of

thermal neutrons and capture gamma rays near the detector.

The increase of shale or clay content also reduces the count rate

because of the high bound water saturation.


The figure shows the distribution of

thermal neutron density as a

function of distance from the source

in a homogeneous medium.

The different curves of the graph

pertain to rocks with different

porosities in the range of 10 to 40%.

The thermal neutron density quickly

decreases with the distance.

The curves intersect each others at

about 20-25 cm from the source.

Here, the thermal neutron density is

just slightly dependent on the

porosity.

In the far zone, the thermal neutron

density decreases with the porosity.


So, the neutron porosity of rocks can be related to the flux of the

following particles and rays (measured at suitable distances from

the source):

• epithermal neutrons,

• thermal neutrons,

• and prompt gamma rays coming from thermal neutron capture.

Accordingly, three types of neutron porosity logging were

developed:

• neutron-epithermal neutron logging,

• neutron-thermal neutron logging,

• and neutron-gamma (ray) logging.


The temporal separation of different phases (slowing-down,

diffusion and absorption) develops the spatial separation of

neutron populations with different energy intervals in the formation.

Thus, a suitable source-detector spacing must be selected

depending on the type of particle or ray to be detected.

The difference in source-detector spacing for the three neutron

porosity methods (left: neutron-epithermal neutron, middle:

neutron-thermal neutron, right: neutron-gamma).



Qualitative comparison of some properties of the three neutron

porosity methods:

neutron-

epithermal

neutron

neutron-thermal

neutron

neutron-gamma

source-detector

spacing

short medium long

minimum bed

resolution

good medium bad

depth of

investigation

shallow medium deeper

sensitivity to

thermal neutron

absorbers

not sensitive sensitive very sensitive


The neutron-gamma logging method was developed at first, but it is

not used any longer.

Because of its high sensitivity to thermal neutron absorbers, the

relationship between the hydrogen concentration and detected

count rate of gamma ray is not quite unambiguous. The

background radiation of rocks also influences the detected gamma

count rate. So, the porosity estimation is less reliable.

Neutron-epithermal neutron tools are the least sensitive to thermal

neutron absorbers, but their radial depth of investigation is rather

shallow.

Its further disadvantage is the worse efficiency of epithermal

neutron detectors (the uncertainty of measured count rate is

higher).

Neutron-thermal neutron tools provide the best compromise

between the advantageous and disadvantageous properties, so

their usage is common in well logging.


The dual-detector system of neutron

– thermal neutron tool is called

compensated neutron logging tool

(CNL).

Due to the suitable selection of

detector positions the ratio of near

to far detector count rates primarily

depends on the slowing-down

length of the formation (which is

explicitly affected by the hydrogen

concentration).

Thus, the effects of thermal neutron

absorbers, mudcake and borehole

are significantly reduced.




The typical vertical resolution of a CNL tool is about 2 ft (~61 cm).

Some tools can provide a vertical resolution of 1 ft.

The radial depth of investigation decreases with the porosity.

Under average conditions the depth of investigation is about 10 in

(25.4 cm).

The sensitivity of neutron logging to the formation porosity

increases with the decrease of porosity in the range of 40 to 2%.

It decreases below 2% since the effect of rock matrix will

predominate over the effect of hydrogen with respect to the

slowing down of neutrons.

The porosity range of reliable application of neutron-thermal

neutron logging extends from 2% to 35%.

Neutron Logging in Cased Holes

Neutron-thermal neutron logging can be applied not only in open

holes but also in cased holes.

The slowing down power of casing and cement sheath is not too

significant, so the tool primarily responds to the amount of

hydrogen in the formation.

Accordingly, a neutron-thermal neutron log reflects the amount of

liquid-filled porosity in clean formations whose pore space is filled

with water or oil,.

A cased hole neutron log can also be used for depth identification

by means of its correlation with the open hole neutron log.

This application is very useful in zones whose variations do not

appear in the gamma ray logs (i.e. in thick, clean carbonate

formations).


A portion of a cased hole log with

gamma ray, neutron, casing collar

locator and cable tension curves.

A thick carbonate formation is

covered by a shale bed with high

gamma ray activity.

Here, the neutron log is not scaled in

porosity unit but in neutron API unit.

The API unit of a neutron logging

tool is obtained from the count rate

(cps) by means of a conversion

factor.

The gamma ray curve is featureless

in the carbonate formation so, it is

not applicable to any separation and

depth identification.

But the neutron curve reflects the

porosity variations.

Schlumberger (1989): Cased Hole Log Interpretation Principles/Applications


Gamma ray and single

detector neutron log

responses to different

formations.

The neutron API is a

proposed standard unit,

but it is not so widely

used in practice as

gamma API.

Several service company

apply their own units

based on their calibration

processes.

James J. Smolen, Ph.D., 1996: Cased Hole and Production

Log Evaluation


Identification of gas/liquid contacts in completed wells

Since the hydrogen concentrations of water and liquid

hydrocarbons are very similar, the oil/water contact can not be

indicated by the neutron-thermal neutron logging method.

However, the hydrogen concentration of gas is considerably low

than that of liquids.

Therefore, the detected count rate of thermal neutrons is

significantly higher when gas is present in the pore space, near

enough to the wellbore.

Thus, the neutron-thermal neutron logging is applicable to the

identification of gas/liquid contacts.

The dual-detector neutron-thermal neutron tools used in

completed wells work with chemical sources (Am-Be).

Their smaller diameter provides the passage through the

production pipe string.



The logging tool is not calibrated in porosity unit.

The thermal neutron indications of the two detectors are

displayed in the same track but with different scales.

The curves can be scaled by either in the unit of raw count rate

(cps) or in a converted unit based on a company standard.

The scale limits of the two curves are adjusted in such a way that

the curves fit well opposite impermeable shale and totally liquid

filled permeable zones.

If gas saturation appears opposite the tool, behind the cement

sheath, the curve of long spacing detector separates from the

other curve.

Increasing gas saturation increases the rate of separation.

Increasing shale content decreases the rate of separation.

The changes in the gas/liquid contacts and gas saturation can be

monitored by means of periodical neutron-thermal neutron

logging of the gas bearing reservoir zones.


gas bearing reservoir

probably gas bearing

reservoir

liquid bearing reservoir

with some residual

gas saturation

Thermal Neutron Die-away Logging

The technical implementation of Thermal Neutron Die-away, also

known as Thermal Decay Time (TDT) logging, is based on the

application of a neutron generator as a source of high energy

neutrons.

In this case the neutron generator is used in impulse mode.

By means of using TDT logging devices the macroscopic cross

section of thermal neutron capture (a or simply ) of the

formations can be determined.

The method is mostly applied in cased holes for separating the

oil- and water-bearing zones of the reservoirs.

Not only qualitative but also quantitative results can be obtained

with respect to the water saturation.


If a short-time impulse of high energy

neutrons is emitted from a neutron

generator, the neutrons penetrate into

the medium and their energy gradually

decrease with both the time elapsed

from the emission and distance from the

source.

The figure illustrates the spread of

neutrons coming from an impulsive

source in the formation, and the thermal

neutron density at a detector as a

function of time.

Here, t1 symbolizes the slowing-down

time, when most of the neutrons have

been termalized.

Time t2 is in the interval of diffusion

phase, when the thermal neutron

absorption decreases the neutron

density.




The slowing-down time of fast neutrons primarily depends on the

hydrogen concentration of the formation and it is typically less

than 15 s.

The diffusion time of thermal neutrons lasts a longer interval

depending on the concentration of hydrogen and other elements

with high thermal neutron capture cross section (e.g. chlorine) in

the rock.

The range of diffusion time extends from 5 s (rock salt) to 900 s

(quartzite).

So, the theory of TDT logging is based on the phenomenon that

the average lifetime of neutrons in rocks depends on their

composition and reservoir parameters.

Mainly the composition of fluid filling the pore space has an

important role in the log response.


The average lifetime of neutrons is closely related to the

macroscopic cross section of thermal neutron capture in the

medium.

The number of neutrons (N) in the medium, after time t has

elapsed from the emission of neutrons, can be determined by the

following relationship:

𝑁 = 𝑁0𝑒−𝑣Σ𝑎𝑡,

where

N0 is the number of neutrons emitted from the source and

penetrated into the medium at t0=0 s,

v is the velocity of neutrons,

a is the macroscopic cross section of thermal capture for the

neutrons propagating with velocity v in the medium.

The relationship assumes that all the neutrons are in thermal

equilibrium with the surrounding medium.


In order to implement the measurement of macroscopic cross section of

thermal capture in wellbores, a logging tool must include the following

essential components:

• a neutron generator working in impulse mode (periodically emits

impulses of neurons),

• and a detector whose operation can be controlled and limited to

definite counting gates (time intervals).

An example of the important working parameters of a TDT logging tool:

• the neutron generator evenly emits 1000 impulses of neutrons in 1 s,

• the time spacing between two neighbouring emissions of neutrons is

1 ms =1000 s,

• the length of each impulse is 30 s,

• the number of emitted fast neutrons in an impulse is about 105

• the length of a counting gate (an operating interval of the detector) is

100 s,


t1 is the time when the first counting gate starts within the time spacing of

neutron impulses (e.g. 400 s),

t2 is the time when the second counting gate starts (e.g. 700 or 800 s),

…

tn is the time when the nth counting gate starts.



By means of the repeated detections of thermal neutrons or

thermal capture gamma rays (within the time spacing of neutron

impulses), the rate of decrease in the thermal neutron density at

the detector can be determined.

The process can be characterized by a constant called neutron

lifetime, which is the half-time of neutrons.

It gives the time in which the initial number of neutrons in the

medium reduces to its half.

𝑁 =𝑁02= 𝑁0𝑒

−𝑣Σ𝑎𝑡 Τ1 2

If the velocity (v) of thermal neutrons is chosen to be 2200 m/s:

𝑡 Τ1 2=

3.15

Σ𝑎[s].

So, the neutron lifetime (t1/2) can be calculated in the knowledge

of macroscopic cross section of thermal neutron capture.


If the neutron lifetime is shorter in a rock, the decrease of neutron

density with time is faster, which is cased by a higher value of .

Since the initial number of neutrons (N0) is not known exactly, the

thermal neutron density (or flux) must be measured at least two

different times (t1, t2) between two neighbouring neutron impulses.

𝑙𝑛𝑁2𝑁1

= −𝑣Σ𝑎 𝑡2 − 𝑡1

After converting the natural logarithm into common logarithm the

following expression is obtained for calculating the macroscopic

cross section of thermal neutron capture:

Σ𝑎 =10.5

Δ𝑡𝑙𝑔

𝑁1

𝑁2[1/cm],

where t is the difference between the beginnings of two counting

gates in the unit of microsecond.

The ratio of neutron densities can be replaced by the ratio of

measured count rates.


Since the value of a is constant for a given rock the ratio of

measured count rates increases with t.

In well logging and petrophysics the so-called capture unit (cu) is

used to measure the thermal neutron capture power of a material.

The capture unit is equal to 1000 times the unit of 1/cm3.

In practice, not the thermal neutrons are detected in the detector

counting gates, but the capture gamma rays.

The decrease of thermal neutron density entails the decrease of

thermal neutron capture, which causes the decrease of capture

gamma ray flux.

The decay of capture gamma ray flux is also the exponential

function of the time.

By means of detecting the capture gamma ray the radial

investigation depth of the logging tool can be improved (n x 10

cm), and the thermal capture gamma ray is less sensitive to the

effect of wellbore (fluid, casing, cement).


In order to reduce the effect of background gamma radiation

(owing to the radioactive isotopes and Compton scattering), only

the gamma rays having higher energy than a selected threshold

(e.g. 2.2 MeV) are generally detected.

Another solution of the problem is the separated measurement of

background radiation during the logging operation.

The effect of background radiation can be minimalized by means

of subtracting the measured background radiation from the

gamma ray count rates detected in the counting gates.

The background radiation must be measured in the time intervals

when all the neutrons have been captured, so there is no capture

gamma ray.

For example, a break is inserted after some periods of neutron

impulses, in which the neutron generator does not work and the

background radiation is detected and recorded.


The proposed logging speed is 6 m/min (10 cm/s).

The time spacing between the neutron impulses is typically 1 ms.

The tool takes about 0.1 mm in the wellbore within that time.

The displayed curves on the log:

• the two curves of thermal capture gamma rays scaled in cps,

• and the curve of computed macroscopic cross section of

thermal neutron capture.

The macroscopic cross section of thermal neutron capture is very

sensitive to the hydrogen and chlorine concentration of the

formation fluid.


The figure represents how the

number of thermal neutrons

decreases with the time elapsed

from the neutron impulse for

sandstone samples filled with

different fluids.

The behaviour of oil and fresh

water filled samples are the

same since the hydrogen

concentration of these fluids are

very similar, and there is no

chlorine content.

But the rate of thermal neutron

absorption is higher in the

sample filled with salt water due

to the presence of chlorine.

The difference in the rate of thermal

neutron absorption is reflected by the

different slopes of lines and neutron

lifetimes (t1/2).


The table includes the

macroscopic cross section of

thermal neutron capture and

neutron lifetime for some rocks

and materials.

The sigma of rock matrices

generally less than 10 cu

(because of the lack of

effective absorbers).

So, the sigma of a rock

formation primarily depends on

the porosity, water saturation

and salinity of formation water.

Material a [cu.] t1/2 [ms]

Limestone, ϕ=0 7 450

Water filled limestone,

ϕ=10% (10% NaCl)12.1 262

Water filled limestone,

ϕ=30% (10% NaCl)22.3 143

Sandstone, f=0 3.5 912

Water filled sandstone,

ϕ=10% (10% NaCl)8.9 354

Water filled sandstone,

ϕ=30% (10% NaCl)19.8 159

Clay 20 - 40 160 - 80

Anhydrite, ϕ=0 12.1 262

Dolomite, ϕ=0 6.8 533

Fresh water 22.2 142

Oil 22.2 142

Salt water (3% NaCl) 31.7 100

Salt Water (10% Nacl) 56 56

Halite (NaCl) 726 4.3

Portland cement ~ 13 ~ 240

Iron 200 15.7


The sigma of hydrocarbons is approximately the same as that of

fresh water.

Consequently, increasing oil saturation decreases the value of

sigma if the formation water is saltwater.

The range of sigma expected in well logging extends from 0 to

60 cu.

The graph shows how the sigma of

water depends on the salinity.


The most important applications:

• determination of oil/water contacts and indication of water

inflows,

• estimation of water saturation or change in the water saturation

between two different runs.

The contact between the oil and water phases appears on the log

when the formation water is salt water.

In the salt water filled zone the count rate of capture gamma ray

is less and the macroscopic cross section of thermal neutron

capture is greater.

If a well is monitored by the periodical repetition of TDT logging

(e.g. once or twice a year), the changes in the water saturation of

reservoir zones between two runs can be investigated.

A water inflow is a natural consequence of the production since

the production entails the raise of O/W contact.


The original level of

O/W contact is

indicated by the arrows

assigned to the symbol

A.

Ten years after the well

completion the capture

gamma ray curves of

two counting gates

show the new level of

O/W contact at the

symbol B.


Response equations for the quantitative evaluation

1. For clean porous formations filled with a single fluid phase:

Σ = 𝜙Σ𝑓𝑙𝑢𝑖𝑑 + 1 − 𝜙 Σ𝑚𝑎

comes from the TDT log,

ϕ comes from other logs (e.g. CNL, DEN, ACL),

fluid can be either measured on a fluid sample taken from the

reservoir, or derived from Rw and formation temperature

data for water (Rw,T equivalent salinity of formation

water w).

ma is generally unknown, but it can be calculated in a water-

bearing zone (Sw= 100%) if the values of other quantities

are known.

Σ𝑚𝑎 =Σ − 𝜙Σ𝑤1 − 𝜙



2. For clean porous formations filled with two different fluid

phases (CH & W):

Σ = 𝜙𝑆𝑤Σ𝑤 + 𝜙 1 − 𝑆𝑤 Σ𝐶𝐻 + 1 − 𝜙 Σ𝑚𝑎


ϕ comes from other logs (e.g. CNL, DEN, ACL),

w can be either measured on a fluid sample taken from a water-

bearing zone of the reservoir, or derived from Rw and

formation temperature data (Rw, T equivalent salinity of

formation water w),

CH can be measured on fluid samples or obtained from tables,

ma can be calculated in a water-bearing zone (Sw= 100%) of

the same reservoir

𝑆𝑤 =Σ − 𝜙Σ𝐶𝐻 − (1 − 𝜙)Σ𝑚𝑎

𝜙(Σ𝑤 − Σ𝐶𝐻)



3. For shaly formations filled with two different fluid phases

(CH & W):

Σ = 𝜙𝑒𝑓𝑓𝑆𝑤Σ𝑤 + 𝜙𝑒𝑓𝑓 1 − 𝑆𝑤 Σ𝐶𝐻 + 𝑉𝑠ℎΣ𝑠ℎ + 1 − 𝜙𝑒𝑓𝑓 − 𝑉𝑠ℎ Σ𝑚𝑎


ϕeff and Vsh comes from the evaluation of other logs,

w can be either measured on a fluid sample taken from a water-

bearing zone of the reservoir, or derived from Rw and

formation temperature data (Rw, T equivalent salinity of

formation water w),

CH can be measured on fluid samples or obtained from tables,

ma can be calculated in a water-bearing zone (Sw= 100%) of

the same reservoir,

sh taken from the TDT log at the adjacent shale beds.


Determination of the change in water saturation between two runs

of TDT logging made in different times (reservoir monitoring)

Σ1 = 𝜙𝑆𝑤1Σ𝑤 + 𝜙 1 − 𝑆𝑤1 Σ𝐶𝐻 + 1 − 𝜙 Σ𝑚𝑎

Σ2 = 𝜙𝑆𝑤2Σ𝑤 + 𝜙 1 − 𝑆𝑤2 Σ𝐶𝐻 + 1 − 𝜙 Σ𝑚𝑎

Σ2 − Σ1 = 𝜙 𝑆𝑤2 − 𝑆𝑤1 Σ𝑤 − Σ𝐶𝐻

Δ𝑆𝑤 =ΔΣ

𝜙(Σ𝑤 − Σ𝐶𝐻)

1 comes from the first TDT log,

2 comes from the second TDT log made some months or years

after the previous one.

The last formula yielding the change in water saturation does not require

the knowledge of ma.

Minimum requirements for the reliable determination of water saturation

from TDT logging:

• salinity of formation water 100 000 ppm,

• formation porosity 15 %.


ExampleTwo zones can be separated within a reservoir. The lower zone (zone A)

is water-bearing without hydrocarbon content.

The upper zone (zone B) is oil-bearing, but it contains some amount of

water beside hydrocarbon.

TDT logging was made in the producing wellbore. The following data are

known from the cased and open hole logging operations as well as their

evaluation.

Zone A

The count rate of gamma ray in the first counting gate: N1=6600 cps

The count rate of gamma ray in the second counting gate:

N2=1300 cps

The time difference between the two counting gates: t=300 s

The level of background gamma ray: Nbk=400 cps

The porosity of zone: A=26 % 0.26

The resistivity of formation water: Rw=0.049 m @ T=68 °F


Zone A

The equivalent salinity of formation water is determined from the

resistivity and temperature data by means of an appropriate nomogram.

ceq_NaCl=210 000 ppm

The macroscopic cross section of thermal capture of formation water

(w) can be obtained from the salinity since the relationship is

determined by laboratory measurements.

w=98 cu

That value is also valid for zone B since both zones are in the same

reservoir.

Zone B

The count rate of gamma ray in the first counting gate: N1=12 000 cps

The count rate of gamma ray in the second counting gate:

N2=5150 cps

The time difference between the two counting gates: t=300 s

The level of background gamma ray: Nbk=400 cps

The porosity of zone: A=28 % 0.28

The macroscopic cross section of thermal capture of oil is also known:

CH=22.2 cu (the same as that of fresh water).


Task 1

Let us calculate the water saturation in zone B. Sw=?

In order to determine the water saturation in zone B, the knowledge of

sigma of rock matrix is needed. The value of this parameter can be

obtained from a totally water filled zone (zone A).

Step 1

Calculating the sigma of zone A

Σ𝐴 = 100010.5

Δ𝑡𝑙𝑔𝑁𝛾1𝑐𝑜𝑟𝑟

𝑁𝛾2𝑐𝑜𝑜𝑟= 29.3 𝑐𝑢

where N1corr and N2corr the count rates of counting gates corrected for

the background radiation: Ncorr = N-Nbk

The sigma of rock matrix can be calculated by means of the response

equation:

Σ𝑚𝑎 =Σ𝐴 − 𝜙Σ𝑤1 − 𝜙

= 5.2 𝑐𝑢


Task 1

Step 2

Calculating the sigma of zone B

Σ𝐵 = 100010.5



Now the water saturation can be calculated by means of the response

equation:

𝑆𝑤 =Σ𝐵 − 𝜙Σ𝐶𝐻 − (1 − 𝜙)Σ𝑚𝑎

𝜙(Σ𝑤 − Σ𝐶𝐻)= 0.17 → 17%

The reservoir was logged a year after, and the data of TDT logging for

zone B are the following:

N1=11 000 cps N2=5000 cps t=300 s

Nbk=400 cps.

Task 2

Let us calculate the change in water saturation between the two runs of

TDT logging.


Task 2

Step 1

Calculating the sigma of zone B for the second run

Σ𝐵2 = 100010.5



The change in sigma of zone B:

B = B1 - B2 = 0.9 cu

Step 2

Calculating the change in water saturation :

Δ𝑆𝑤 =ΔΣ𝐵

𝜙(Σ𝑤 − Σ𝐶𝐻)= 0.042 → 4.2%

So, the water saturation at the second logging operation is:

Sw2 = Sw1 + Sw = 21.2 %

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Neutron Methods in Well Logging - uni-miskolc.hugeofiz/Oktatok/vass/Well_logging_Neutron... · Neutron Methods in Well Logging edited by P. Vass For Petroleum Engineer & Geoengineer