This article is based on
Chapter 3 of "The Log Analysis Handbook" by E. R. Crain, P.Eng., published by Pennwell
Books 1986 Republished as "Crain's Logging Tool
Theory" in 2004 and updated annually through 2016.
Updated Dec 2018.
webpage version is the copyrighted intellectual
property of the author.
Do not copy or distribute in any form without explicit
NEUTRON LOG BASICS
first commercial neutron logs were run in 1945 by Lane Wells. They
are based on particle physics
concepts. Neutron logs emit fast neutrons from a source at the bottom
of the tool. The effect of interactions with the rocks and fluids on
the neutron flux is measured by detectors above the source
on the logging tool. Hydrogen has by far the largest impact,
so the tool can be calibrated to represent hydrogen index,
which is highly correlated with porosity. So neutron logs
are considered to be porosity indicating tools.
There are three basic neutron logging instruments
with a chemical neutron source. Each instrument is classified
according to the energy level of the detected particles. Fast
neutrons have energies greater than 100 KeV; epithermal
neutrons have energies above 0.025 eV up to 100 KeV; thermal
neutrons have an energy of approximately 0.025 eV (at 25 C).
The three tool types are generically called neutron - gamma, neutron
- epithermal neutron, and neutron - neutron devices. The first is
represented by the original neutron log that emitted fast neutrons
and measured gamma rays of capture emitted by the nuclei of the
rocks when a neutron was captured. They were called neutron-gamma or
gamma ray neutron logs. The latter was widely used as a descriptive
term but was confusing as it suggested that a gamma ray log was also
being run with the neutron log. While both logs were often run
together, the description of how the neutron log response was
measured got lost in the process.
The second type emitted fast neutrons and measured the number of
epithermal neutrons as they lost energy after colliding with atoms
in the formation. These are represented by the sidewall neutron log
The third type uses detectors sensitive to thermal neutrons. These
have dual detectors that allow for some borehole compensation,
represented by compensated neutron logging tools (CNL).
A fourth type uses an accelerator source instead of a chemical
source of neutrons. It measures both thermal and epithermal
neutrons. The tools are more delicate and more expensive but
are safer and reduce paperwork if the tool is lost downhole.
GRN and CNL log types can be run in air, oil, or mud filled open or
cased holes. SNP logs were not calibrated for use in cased holes.
1. Radioactive Well Log Interpretation
J.P. Campbell, A.B. Winter,
Lane Wells, 1946
2. Experimental Basis for Neutron Log
J.T. Dewan, L.A. Allard,
Petroleum Engineer, 1953
3. Sidewall Epithermal Neutron Porosity Log
J. Tittman, H. Sherman, W.A. Nagel, R.P. Alger,
4. Dual Spacing Neutron Log - CNLi
R.P. Alger, S. Locke, W.A. Nagel, H. Sherman,
GAMMA RAY NEUTRON LOG (GRN, GNT)
In older tools
run from 1945 into the mid 1960's, fast thermal neutrons are sent out,
which are captured by hydrogen atoms. Gamma rays of capture are
emitted to balance the energy. The number of
gamma rays returning to the detector is inversely proportional
to the number of hydrogen atoms, which is highly related to the
porosity of the rock. These logs record the gamma ray
count rates in counts per second. Conversion to hydrogen index
or porosity was made by using a semi-logarithmic transform. This
tool is obsolete.
The logging tool produced a single curve in counts per second or API
neutron units. It was often run with a gamma ray log, so the GR
curve in ug-Ra equivalent/ton or API gamma ray units would be
presented in Track 1.
The emitted neutrons come from a radium – beryllium
or an americium – beryllium source. Radium and americium
are natural alpha particle emitters and the alphas eject fast
neutrons from the beryllium. With its 433 years half life,
the AmBe source output is considered very stable. Approximately
40 x 10^7 neutrons/sec at 4.5 MeV average energy are emitted
by the source.
When the neutrons are sufficiently
slowed down by collisions with the formation, they are captured
by the nuclei and a high energy gamma ray of capture is emitted.
The gamma ray count rate at the detector is inversely proportional
to the hydrogen content of the formation, in a semi-logarithmic
relationship. Borehole size and mud weight corrections and
calibration to porosity was done by the log analyst. Detectors
were Geiger-Mueller gamma ray counters.
The source to detector spacing on
the older tools was 15.5 inches, giving good statistical accuracy.
Longer 18.5” spacing
tools became popular because they were more sensitive to porosity
but suffered from higher statistical variations. These tools
are obsolete and no longer available, but many thousands exist
in well files waiting for the serious petrophysicist to use
for finding bypassed oil and gas.
large number of charts for specific tools, spacings, borehole
conditions and rock types were available from service companies,
such as the one shown below. These may no longer be
easily found today, and the semi-logarithmic approach described
below works well except in very low porosity .
GNT-F or G neutron porosity interpretation chart. Hundreds
of such charts exist for dozens of tools for a large range of
hole sizes, mud weights, and casing sizes. most are not
contained in conventional chart books. Some are available on the
Denver Well Log Society CD set
no appropriate chart exists, or if you don't believe in them, it is expedient to use the "High
porosity- Low porosity" method.
Select a high porosity point on the log, usually a shale,
and assign it a porosity based on offset wells with scaled
logs or a local compaction curve. This is PHIHI.
2. Pick the count rate on the neutron log at
this point - this is CPSHI, even though it is a low numerical value.
a low porosity point on the log. Assign this a porosity value,
again based on offset scaled porosity logs or core porosity. This is PHILO.
Tight lime stringers or anhydrite are best but you need some
imagination if there are no truly low porosity streaks.
the corresponding count rate on the log. This CPSLO, even though
it is a larger number than CPSHI.
5. Plot these points on semi-log graph paper as shown
below. Read porosity for any other count rate from the graph.
Example of Porosity from Neutron Counts per
Second - no shale correction
To use this plot in a calculator or computer
instead of on a graph:
1: SLOPE = (log (PHIHI / PHILO)) / (CPSHI - CPSLO)
2: INTCPT = PHIHI / (10 ^ (CPSHI * SLOPE))
3: PHIn = INTCPT * 10 ^ (SLOPE * NCPS)
SIDEWALL NEUTRON LOG (SNP)
A second type of logging system responds
primarily to epithermal neutrons and is referred to as a neutron-epithermal
(N-EN) log or a Sidewall Epithermal Neutron Log (SNP). The
neutron detector counts the slow epithermal neutron density,
which is largely determined by the amount of intervening hydrogen
between the source and detector. The detector is a lithium
iodide crystal with suitable shielding to eliminate thermal
neutrons. This tool had a short lifespan from the mid 1960's
to the early 1970's and was replaced by the CNL.
The source and detector are on a skid identical to the density
logging tool. The skid is pressed against the borehole walls,
eliminating most borehole and mudcake effects, except for very
rugose hole conditions.
Detector count rates from the SNP
systems are converted by a computer in the logging truck to porosity units, on a
sandstone, limestone, or dolomite scale, depending on the assumed
or known mineralogy of the formation. There is little borehole
effect because the tool is a sidewall pad device similar to
the density log skid. However, it sees a relatively small volume
of rock, and has been generally superseded by the compensated
neutron log (CNL).
graph below shows the relationship between count rates at the detector
of a sidewall neutron porosity (SNP) tool and porosity for the
three primary lithologies. These results were obtained in the
laboratory with quarried rock samples, and constitute the basic
data for calibrating the tool in terms of porosity.
Response of SNP neutron log: Count rate vs slowing down
length (left) and count rate vs porosity (right)
above is the same count rate data plotted against the
calculated slowing down length Ls. A linear best fit is used to
describe the correspondence between count rates and slowing down
length. At low porosities, there is some lithological effect.
Different rocks with the same count rate have different slowing
down lengths. Also for some reason, the water point does not fit
the line. Fortunately this is not serious as logs are seldom run
in zones with more than 45% water.
we know, or can calculate, the slowing down length for neutrons
of epithermal energy, in particular rocks, such as calculated
in the previous Section, we can enter this value in the graph
obtain the apparent porosity reading for a real SNP type tool.
The usual log presentation is a single porosity curve, with GR
in Track1, and possibly the epithermal neutron count rate.
COMPENSATED NEUTRON LOG (CNL)
The most commonly run neutron log
today is the compensated neutron log. It is an eccentered dual
detector log that can be run in both open and cased boreholes.
This log measures the rate of decrease of neutron density with
distance from a source and converts it to a calibrated apparent
porosity value. The rate of decrease, represented by the ratio
of the near to far count rates, is primarily due to the
hydrogen content of the formation.
Most CNL tools are neutron – thermal neutron
(N – N)
tools but some have additional detectors for epithermal neutrons
(N – EN) measurements.
Helium-3 detectors are used in
the small diameter CNL instrument whereas the large diameter instrument
utilizes lithium iodide crystals.
Variations from standard
borehole conditions are compensated by means of the dual detector
system. The corrected apparent porosity values are derived
from the count rate ratio of near and far spaced detectors
by a computer program. Additional environmental corrections
may be required in hot, high salinity boreholes. The program
also compensates for casing and cement thickness in cased hole
graph at right
shows the relationship between near and far detector thermal
neutron count rate
ratio versus porosity for the three primary minerals, the data
again being made in laboratory formations. The middle graph shows
the same ratio data plotted against calculated migration length
- Lm. The fit is excellent and even takes in the water point.
Again the migration length of an arbitrary mineral or mixture
of minerals can be entered into the graph to obtain
the apparent porosity reading for a real thermal neutron CNL type
tool. Note that real tools may use function formers or computer
algorithms to modify the apparent tool response. This is shown
for the 1970'x vintage Schlumberger CNL tool in the bottom graph.
Considerable computer modeling of neutron response by service
companies has generated numerous revisions of these response curves.
Most of the effort has been directed at producing a linear porosity
scale in all lithologies. Consult specific service company
chartbooks for the era and tool designation.
petrophysical analysis programs use a generic lithology correction.
Some perform corrections for a specific, but unknown, tool design
that may be inappropriate for the the tool under investigation.
Still others offer many tool options, but not all possible tools
will be listed. The generic transforms are usually sufficient for
typical petrophysical analysis, except in very hot, very salty mud
systems, where accurate corrections may be needed.
The count rates from the two detectors can be displayed and
are often used as gas detection indicators.
In clays, micas, and zeolites, the apparent CNL (thermal)
porosity is consistently higher than the SNP (or CNL epithermal) porosity.
As a result, the CNL thermal measurement shows higher porosities in
shales than the SNP tool. The explanation for this effect is
that the SNP tool responds only to the slowing down of the
neutrons by hydrogen atoms, whereas the CNL measurement is
also affected by the neutron capture process, since the tool
measures both thermal and epithermal neutrons.
The log produces from 1 to 3 porosity curves based on different
lithology assumptions and the two count rate curves. It is usually
combined with the gamma ray, 1 or 2 caliper curves and either or both
the density and density porosity.
ACCELERATOR NEUTRON POROSITY SONDE (APS)
A fourth style of neutron log uses
a particle accelerator to create the fast neutrons, instead of a
chemical source. The accelerator forces deuterium and tritium
collisions at high energy levels to produce the neutrons. This tool
is derived from the concept of the pulsed
neutron (thermal decay time) tool widely used in cased hole
logging to measure reservoir properties. It produces an
epithermal neutron log.
The tool is called the accelerator porosity sonde (APS) by
Schlumberger. When combined with a litho-density and spectral
gamma ray logs, they call it an integrated porosity log (IPL).
Integrated porosity lithology service
delivers accurate formation porosity and lithology information
by acquiring neutron porosity, natural gamma ray spectrometry,
density, and photoelectric effect measurements using one modular
The accelerator porosity sonde, which uses an
electronic pulsed neutron source instead of a chemical source, is
the core of the tool.
The large yield of the neutron source enables epithermal neutron
detection, which in combination with borehole shielding obtains
porosity measurements that are only minimally affected by the
borehole environment and formation characteristics, such as
lithology and salinity.
Five detectors provide information for porosity evaluation, gas
detection, and shale evaluation with improved vertical resolution
and borehole correction.
The tool is usually combined with a litho-density
photoelectric logging tool and a natural gamma ray spectrometry
Log curves produced are similar to the CNL.
NEUTRON LOG CURVE NAMES
Gamma Ray Neutron (GRN)
Sidewall Neutron Porosity Log (SNP)
NPHI or PHIN
Compensated Dual-Spacing Neutron Log (CNL)
NPHI, NPOR, TNPH, or PHIN
neutron count rate ratio
neutron near count rate
neutron far count rate
Accelerator Porosity Sonde (APS)
APSC or PHIN
density, density porosity, PEF, sonic curves as requested
gamma ray, gamma ray spectrolog as requested
EXAMPLES OF NEUTRON LOGS
Old style GRN neutron log recorded in counts per
second (upper left), sidewall neutron SNP log
(upper right) and
modern compensated neutron CNL log (dashed curve) with density
(solid curve). All have gamma ray in Track 1 with caliper
for SNP and CNL-FDC displays.
Typical display format for
PE-density porosity-neutron porosity log on a sandstone scale. The
density correction curve may appear on the left or right side of the
wide track. A density scale of 1.65 to 2.65 gm/cc may be used
instead of the density porosity scale.