Classification of logs

BASIC WELL LOGGING (PET 407)
CLASSIFICATION OF LOGS
(Section 3)
Compiled by: Dr. Ohia

Logs are classified according to the technology that they
use or according to their functions.
By their technology:
They are classified as Open and Closed hole logs.
By their functions or properties:
Electrical logs (Resistivity log, Borehole Imaging), Porosity
logs (Density, Neutron Porosity, Sonic), Lithology logs
(Gamma ray, Self/spontaneous potential), Nuclear logging
(Gamma ray logs, Spectral gamma ray logs, Density logging,
Neutron porosity logs, Pulsed neutron lifetime logs, Carbon
oxygen logs, Geochemical logs), Caliper, Nuclear magnetic
resonance.

OPEN HOLE LOGS
Open-hole logging refers to logging operations that are
performed on a well before the wellbore has been cased
and cemented. In other words, the logging is done through
the bare rock sides of the formation.
This is the most common type of logging method because
the measurements are not obstructed and it's done during
or after the well has been drilled. The following are the
types and logs of open hole tools: Correlation and lithology
(Gamma ray, Photoelectric effect, Spontaneous potential),
Resistivity (Induction, Laterolog, Microresistivity), Porosity
and lithology (density, sonic, Compensated Neutron), etc.

CASED HOLE LOGS
On the other hand, cased-hole logging involves retrieving logging
measurements through the well casing, or the metal piping that is
inserted into the well during completion operations. Cased-hole
logging is performed more rarely but still provides valuable
information about the well.
Cased-hole logging is used to help operators obtain additional
information from a well or reservoir that has already been
completed. For example, the well may have already started
production and a cased-hole log could help determine what has
hampered flow. In some cases, the decision must be made to plug
and abandon the well or recomplete it, and the cased-hole log will
help identify what lies beyond the casing of the well.

CASED HOLE LOGS
Cased-hole logging can be used to evaluate the formation and
completion of the well, as well as determine the state of the
cement, corrosion and perforation. Most cased hole logs have a GR
and casing collar log (CCL) for depth control. Both gamma ray and
neutron porosity logs can be run through the casing of a well, and
better ideas of thermal decay and interval transit time can be
achieved through porosity, hydrocarbon saturation and
producibility measurements.
Examples of cased hole logs are: Formation density logs, formation
resistivity logs, reservoir saturation logs, porosity logs, gamma ray
logs, spectra gamma logs, Cement Bond logs, Casing Collar Logs,
etc.

Over time, many different types of logs have been developed to collect
data about wellbores and subsurface formations. We shall try to provide
an overview of the various types of well logs corresponding to reservoir
characteristics.
LITHOLOGY LOGS
• Gamma ray
• Self/spontaneous potential
•LITHOLOGY LOG
ELECTRICAL LOGS
• Resistivity log
• Borehole Imaging

POROSITY LOGS
• Density
• Neutron porosity
• Sonic
Nuclear logging
• Gamma ray logs
• Spectral gamma ray logs
• Density logging
• Neutron porosity logs
• Pulsed neutron lifetime logs
• Carbon oxygen logs
• Geochemical logs
OTHERS
• Acoustic logs
• Caliper logs

LITHOLOGY LOGS
These are logs that record the structure and content of soils and rocks during a
drilling or excavating operation. The best logs for lithological purposes are those
that are (1) most influenced by rock properties and (2) least influenced by fluid
properties.
(1) Gamma ray logs:
Gamma ray logging is a method of measuring naturally occurring gamma radiation
to characterize the rock or sediment in a borehole or drill hole. It is a wireline
logging method used in mining, mineral exploration, water-well drilling, for
formation evaluation in oil and gas well drilling and for other related purposes. Its
a log of the natural radioactivity of the formation along the borehole, measured in
API units, particularly useful for distinguishing between sands and shales in a
siliclastic environment. This is because sandstones are usually nonradioactive
quartz, whereas shales are naturally radioactive due to potassium isotopes in
clays, and adsorbed uranium and thorium.

Gamma ray logs:
Different types of rock emit different amounts and
different spectra of natural gamma radiation. In
particular, shales usually emit more gamma rays than
other sedimentary rocks, such as sandstone, limestone
etc. because radioactive potassium is a common
component in their clay content, and because the
cation exchange capacity of clay causes them to absorb
uranium and thorium.
This difference in radioactivity between shales and
sandstones/carbonate rocks allows the gamma tool to
distinguish between shales and non-shales.

Spectral gamma ray logs:
The latest sophistication on the gamma ray log is the spectral gamma ray
log. The technique of measuring the spectrum, or number and energy, of
gamma rays emitted as natural radioactivity by the formation. There are
three sources of natural radioactivity in the Earth: 40K, 232Th and 238U,
or potassium, thorium and uranium. These radioactive isotopes emit
gamma rays that have characteristic energy levels. Shales often contain
potassium as part of their clay content, and tend to absorb uranium and
thorium as well.
A common gamma-ray log records the total radiation and cannot
distinguish between the radioactive elements, while a spectral gamma ray
log. Spectral gamma ray logs are used to identify specific clay types, like
Kaolinite or Illite. This can be used for environmental interpretation as
Kaolinite forms from Feldspars in tropic soils by leaching of Potassium;
and low Potassium readings may thus indicate paleosols.

Radiation Detectors
Two-Step Process
Gamma rays interact with detector material
i) Rays transfer energy to electron(s) via
Photoelectric, Compton or Pair-production
• Material chosen for high probability of
absorption
ii) Electron converted to observable signal
• Direct electrical signal with gaseous or solid-
state detector
• Conversion (to light):
Scintillator/photomultiplier most common

(2) Self/spontaneous potential:
The Spontaneous Potential (SP) log measures the
natural or spontaneous potential difference between
the borehole and the surface, without any applied
current. It was one of the first wireline logs to be
developed, found when a single potential electrode
was lowered into a well and a potential was measured
relative to a fixed reference electrode at the surface.
The most useful component of this potential difference
is the electrochemical potential because it can cause a
significant deflection in the SP response opposite
permeable beds. The magnitude of this deflection
depends mainly on the salinity contrast between the
drilling mud and the formation water, and the clay
content of the permeable bed. Therefore, the SP log is
commonly used to detect permeable beds and to
estimate clay content and formation water salinity. The
SP log can be used to distinguish between
impermeable shale and permeable shale and porous
sands.

ELECTRICAL LOGS (E-Log)
Whether drilling for water, oil, minerals, environmental monitoring or exploration, the E-Log
is a fundamental tool for resistivity logging to obtain important information on what is in the
earth below. In many applications, it is the only log needed! A resistivity log is the first log
ran on a new well and is considered the base log on which further logging and engineering is
based. As new resistivity methods are introduced, they will never make this fundamental
method obsolete! Electrical resistivity is popular because it is a simple, low cost and efficient
method. It is without doubt the most practical, cost-effective logging method available
today.
There are two types of Electrical logging, namely: Resistivity logging and Borehole imaging.
(a) Resistivity logging: Resistivity logging measures the subsurface electrical resistivity,
which is the ability to impede the flow of electric current. This helps to differentiate between
formations filled with salty waters (good conductors of electricity) and those filled with
hydrocarbons (poor conductors of electricity). Resistivity and porosity measurements are
used to calculate water saturation. Resistivity is expressed in ohms or ohms/meter, and is
frequently charted on a logarithm scale versus depth because of the large range of resistivity.
The distance from the borehole penetrated by the current varies with the tool, from a few
centimeters to one meter.

Resistivity logging

ELECTRICAL LOGS
There are two types of resistivity logs: (i) Electrode based logs (Laterolog)
(ii) Electromagnetic logs (Induction log, Electromagnetic Wave resistivity
(EWR)
(i) Electrode based log (Laterolog): This is a type of a resistivity log
that is preferred for salt water mud only because they require conductive
muds like the water based mud (WBM). It is an electrode based mud.
(ii) Electromagnetic Log: An Electromagnetic Log, sometimes called
an "EM Log", measures the speed of a vessel through water. It operates
on the principle that: 1) when a conductor (such as water) passes through
an electromagnetic field, a voltage is created and 2) the amount of
voltage created increases as the speed of the conductor increases.
The EM Log creates an electromagnetic field. It measures the voltage
created and translates this into the vessel's speed through water.

Resistivity logging
Induction log was originally developed to measure formation resistivity in
boreholes containing oil-based and fresh water-based drilling muds.
Electrode devices (conventional electric logs) do not function in non-
conductive mud systems.
Induction logging devices are focused to minimize influence of borehole
and surrounding formations.
Designed for deep investigation to determine Rt.
New induction log devices are being developed using improved
electronics, telemetry, and computer processing.

Applications of Resistivity Well Log Data
---- Estimate water/moveable hydrocarbon
saturations.
---- Establish permeable zones.
---- Estimate porosity (based on resistivity).
---- Discriminate hydrocarbon versus water
saturated zones.
---- Correlate strata areally.

POROSITY LOGS
(1)Density:
Density logging is a well logging tool that can provide a
continuous record of a formation's bulk density along the
length of a borehole. In geology, bulk density is a function
of the density of the minerals forming a rock (i.e. matrix)
and the fluid enclosed in the pore spaces.
The density log measures the bulk density of a formation
by bombarding it with a radioactive source and measuring
the resulting gamma ray count after the effects of Compton
Scattering and Photoelectric absorption. This bulk density
can then be used to determine porosity.

POROSITY LOGS
(2) Neutron Porosity:
The neutron porosity log works by bombarding a formation with high
energy epithermal neutrons that lose energy through elastic scattering to
near thermal levels before being absorbed by the nuclei of the formation
atoms. Depending on the particular type of neutron logging tool, either
the gamma ray of capture, scattered thermal neutrons or scattered,
higher energy epithermal neutrons are detected.
The neutron porosity log is predominantly sensitive to the quantity of
hydrogen atoms in a particular formation, which generally corresponds to
rock porosity. Boron is known to cause anomalously low neutron tool
count rates due to it having a high capture cross section for thermal
neutron absorption. An increase in hydrogen concentration in clay
minerals has a similar effect on the count rate.

(3) Sonic:
A sonic log provides a formation
interval transit time, which
typically a function of lithology
and rock texture but particularly
porosity.
The logging tool consists of a
piezoelectric transmitter and
receiver and the time taken to for
the sound wave to travel the fixed
distance between the two is
recorded as an interval transit
time.
POROSITY LOGS

(3) Sonic:
POROSITY LOGS

NUCLEAR LOGGING
Nuclear tools measure the interactions between radiation emitted from logging
tools and the formation, as well as naturally occurring radiation. Nuclear logging
includes all techniques that either detect the presence of unstable isotopes, or
that create such isotopes in the vicinity of a borehole. Nuclear logs are unique
because the penetrating capability of the particles and photons permits their
detection through casing and annular materials, and they can be used regardless
of the type of fluid in the borehole.
Nuclear-logging techniques include: gamma, gamma spectrometry, gamma-
gamma, and several different kinds of neutron logs. Radioactivity is measured by
converting the particles or photons to electronic pulses, which then can be
counted and sorted as a function of their energy. Nuclear-logging tools exploit
only two types of radiation at cleverly positioned detectors: gamma ray logs
(spectral gamma ray logs, density logging) and neutron logs (neutron porosity
logs, pulsed-neutron-lifetime (PNL) logs. The detection of radiation is based on
ionization that is directly or indirectly produced in the medium through which it
passes.

NUCLEAR LOGGING
Three types of detectors presently are used for nuclear logging:
scintillation crystals, Geiger-Mueller tubes, and proportional counters.
Scintillation detectors are laboratory-grown crystals that produce a flash
of light or scintillation when traversed by radiation. The Geiger–Müller
tube or G–M tube is the sensing element of the Geiger counter
instrument used for the detection of ionizing radiation.
It is a gaseous ionization detector and uses the Townsend avalanche
phenomenon to produce an easily detectable electronic pulse from as
little as a single ionising event due to a radiation particle. The
proportional counter is a type of gaseous ionization detector device used
to measure particles of ionizing radiation. The key feature is its ability to
measure the energy of incident radiation, by producing a detector output
that is proportional to the radiation energy; hence the detector's name.

NUCLEAR LOGGING

NUCLEAR LOGGING
PEX Density Principals of Measurement

NUCLEAR LOGGGING
(1) Pulsed neutron lifetime(PNL) logs:
PNL logs measure the die-away time of a short-lived neutron pulse. They
probe the formation with neutrons but detect gamma rays. Chlorine has a
particularly large capture cross section for thermal neutrons. If the
chlorine in the formation brine dominates the total neutron capture
losses, a neutron-lifetime log will track chlorine concentration and, thus,
the bulk volume of water in the formation. For constant porosity, the log
will track water saturation, Sw. The neutrons are little affected by steel
casing, so this is the standard cased-hole saturation tool. Like other
nuclear tools, modern PNL tools incorporate two detectors for borehole
compensation.
They are used to determine residual oil saturation, they may be used as a
test of well integrity and zonal isolation, they are used to evaluate gravel-
pack quality, and they have been applied in nonconventional ways to solve
several production-logging problems.

NUCLEAR LOGGGING
(2) Carbon oxygen logs:
Recompletion of existing wells and the search for bypassed oil in
established fields require knowledge of the current oil saturation
behind pipe. Carbon/oxygen (C/O) logging was developed for fields
with low salinity where a pulsed neutron lifetime log is not useful.
This tools exploit inelastic scattering of high-energy neutrons off
carbon and oxygen to induce gamma rays. To obtain a lithology
compensation, most C/O tools also record neutron capture spectra
in which elements such as calcium, silicon, and iron reveal
themselves. Neutron capture only occurs shortly after the neutrons
have slowed down to thermal energies. Buffer timing separates
inelastic C/O spectra (during the neutron burst) from capture
spectra (slightly after the burst).

NUCLEAR LOGGGING
3) Geochemical logs:
Geochemical logging is still struggling to find applications.
Schlumberger’s latest incarnation is called the environmental
capture sonde (ECS). Applications lie primarily in rock and clay
typing for reservoir description. They capture the following
elements: Silicon, calcium, Iron, Sulfur, Gadolinium, Titanium,
Chlorine and Hydrogen.
Even with its accurate elemental abundances, conversion of those
numbers to mineralogy and petrophysical parameters such as
permeability still requires a locally calibrated interpretation model.
With limited goals and careful local calibration, geochemical logs do
provide useful information.

OTHERS
Acoustic logs:
Acoustic-log data provide a fundamental and
essential element of modern seismic reservoir
characterization. Acoustic-log data are
commonly calibrated using checkshot (velocity)
or vertical seismic profile (VSP) surveys prior to
use in geophysical applications. Data from these
surveys, which use downhole receivers and
surface acoustic sources, are used to adjust the
log data for drift and borehole conditions and
result in improved time-depth correlation.
Acoustic-log data are combined with density-
log data, to generate an impedance log that in
turn is used to produce a synthetic seismogram.

OTHERS
Acoustic logs:
Acoustic data acquired using modern
array tools can provide high-resolution
(0.5 m).
Acoustic logs do not measure cement
quality directly, rather, this value is
inferred from the degree of acoustic
coupling of the cement to the casing and
to the formation, but they also provide
the primary means for evaluating the
mechanical integrity and quality of the
cement bond using Cement bond logs
(CBL)

Acoustic logs

Acoustic logs:
a) Cement bond logs (CBL):
Properly run and interpreted, cement-
bond logs (CBL) provide highly reliable
estimates of well integrity and zone
isolation. Just as filtrate invasion and
formation alteration may produce
changes in formation acoustic properties,
and thus variation in acoustic logs over
time, so also cement-bond logs may vary
over time as the cement cures and its
properties change.
Some of the objetives of the CBL are that
it helps to locate the top of the cement, it
helps to assess cement quality in terms
of zonal isolation, and it helps to assess
the effectiveness of repairs (squeeze).

a) Cement bond logs (CBL)

(b) Borehole Imaging:
The term "borehole imaging" refers to those
logging and data-processing methods that are used
to produce centimeter-scale images of the
borehole wall and the rocks that make it up. The
context is, therefore, that of open hole, but some
of the tools are closely related to their cased-hole
equivalents. Borehole imaging has been one of the
most rapidly advancing technologies in wireline
well logging. The applications range from detailed
reservoir description through reservoir
performance to enhanced hydrocarbon recovery.
Specific applications are fracture identification,
analysis of small-scale sedimentological features,
evaluation of net pay in thinly bedded formations,
and the identification of breakouts (irregularities in
the borehole wall that are aligned with the
minimum horizontal stress and appear where
stresses around the wellbore exceed the
compressive strength of the rock).

(b) Borehole Imaging:

OTHERS
Caliper Logs:
Caliper logs are used to measure hole diameter along it’s depth and formation
stability. They Usually measure mechanically but a few uses sonic (acoustic/sound)
devices.
Since wellbore shapes are usually irregular, a tool that can measure several
locations along the wellbore wall is often used. This device is called a multi-finger
caliper. Sometimes a simple two arms caliper tool can be used.
Drilling Engineers use this log to understand the condition of the wellbore and the
effectiveness of the mud system – since drilling muds are used primarily to
maintain wellbore stability. It is a good indicator of good permeability and porosity
zones in the reservoir rock (which can be verified with the GR or SP log), since a
stable wellbore is indicative of a well developed mud cake. Thus it can be used for
lithology assessment – shally formation can be very unstable.

OTHERS
Caliper Logs:
They contribute information for lithological assessment – shaly formation can be
very unstable. They are used in the calculation of mud cake thickness, hmc = (dbit
– dh)/2, where h stands for the hole, in inches. They are used in the measurement
of borehole volume, Vh = (dh 2 /2)+1.2%, in litres per metre (L/m) and in the
measurement of required cement volume, Vcement = 0.5 x (dh2 – d2 casing) +
1%, in litres per metre (L/m).
They are used for the indication of hole quality for the assessment of the likely
quality of other logs whose data quality is degraded by holes that are out of
gauge.
Log data can often be corrected for bad hole conditions using the caliper readings,
but the larger the correction, the less reliable the final data will be. and they are
used in the selection of consolidated formations for wireline formation pressure
tests, recovery of fluid samples, for packer seating for well testing purposes, and
for determining casing shoe setting depths.

REFERENCES LINKS
http://petrowiki.org/Types_of_logs
https://en.wikipedia.org/wiki/Well_logging
http://pages.geo.wvu.edu/~jtoro/Petroleum/
15_logs.pdf
http://www.wellog.com/elog.htm

THIS IS THE END OF THIS SECTION

Classification of logs

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