WELLOG LOG INTERPRETATION
© 2003 - 2007 WELLOG
All Rights Reserved
SHORT DISCLAIMER:
Interpretation of data
from well logs is many times subjective. Depending on the accuracy of the log data
and the experience, proficiency, and care taken by the observer in the process
of interpreting that data, the possibility for error is very great.
Different approaches to log interpretation many times produce different
conclusions based on the use of more data or less data including more or less
“good” data. The following information is for informational purposes only. Any
application of the following information including equations is the sole
responsibility of the user. No representation is made to the accuracy and/or
completeness of this information. If errors are found, the reader is
encouraged to contact support@wellog.com
MEASUREMENT UNCERTAINTY:
When measurements of
physical properties are made, a certain amount of uncertainty prevails. Learn
more about uncertainty of measurements at http://www.physics.nist.gov/cuu/Uncertainty/index.html.
Learn more about uncertainty in well log
analysis with
Here’s a link on the
subject of units of
measurement.
Links to Petrophysical
consultants:
Ron Zittel www.rjzpetrophysics.com
WEBINAR:
WELLOG provides a free online Seminar on log interpretation.
It’s called a webinar. NEW Material added every
day!
INTRODUCTION:
The purpose of any
Geophysical Log is to provide meaningful information about the geological and
physical conditions in and around a borehole. Many books have been
written on the subject of log interpretation. Fundamental Log Interpretation
has not changed in decades and will probably not change.
Today, logging is most
often performed using digital data acquisition platforms. The data stored in a
data file may have extensive statistical computation applied to it. Intelligent
systems apply the same and sometimes better algorithms than their human
counterparts once did. The result is faster and often times “smarter”
interpretation. Taking the additional steps required to apply corrections to
raw data and perform ‘sanity’ checks on results adds confidence to any
interpretation.
A WORD ABOUT
CALIBRATION:
EVERY LOG should
contain calibration
information. Interpretation can only be based on accurate and true
measurements having a verifiable reference. Ideally, calibration should be
performed before and after each log.
ELECTRIC LOG (E-LOG)
INTERPRETATION:
Possibly the most
important log that can be obtained is an
E-log. A properly calibrated E-log will provide important information about
formation Electrical Resistivity. In addition to resistivity,
Spontaneous Potential (SP) is obtained. SP shows lithology and type
of lithology in terms of sand/carbonate or shale/clay and relative proportion
of each.
Electric Log operation is based
on ohms law.
Ohms law
states: Resistance = V/I
Apparent Resistivity (ra) takes into account the electrode geometry as follows:
ra = V/I x G
Where:
G = Geometric Factor (4pAM) AM is the distance
measured (in meters) from A to M electrodes.
V = Measured Voltage
I = Applied Current
View resistivity [Model].
View an E-log tool: [E-log tool]
Learn more about [electric log]
applications.
Resistivity is usually
measured in units of ohms – meter2 / meter or; “ohm-meters”.
Electrical Resistivity
provides information about the fluid that is in the pore spaces within the rock
matrix in oil and water wells. Because electrical resistivity is
controlled by ion flow in liquids, the E-log will provide confirmation of the existence of water, water quality, and/or hydrocarbon content of the rock matrix. The electrode spacing (A to M)
used on the E-log tool is directly
related to the depth of measurement. When multiple spacings are used,
resistivities of different depths are measured. It is possible to form
conclusions on invasion and permeability based on resistivity measurements made
at two or more different depths into the formation. See a tornado chart. If no
invasion has occurred, then both shallow and deep curves will read the same
resistivity. If invasion has occurred, then the shallow resistivity will
reflect the resistivity of the invading mud filtrate and the deep resistivity
will reflect the formation fluid resistivity. Resistivity curves should
read the same and depart only where invasion occurs.
In a
water well, higher resistivity in a saturated zone implies higher
quality water. Total Dissolved Solids
in water is related to the resistivity of water. Although certain conditions
apply, as Total Dissolved Solids decrease, water resistivity increases.
(Turcan, 1966)
In wells having
hydrocarbons, increasing resistivity in sandstone or carbonate zones may be an
indication of increasing hydrocarbon content.
The amount of fluid
contained in a formation is directly related to porosity. Porosity affects formation resistivity. In water filled pore spaces, as the volume of water increases, the capacity
for more ions increases. More ions mean more conductivity.
Conductivity and
Resistivity are inversely related.
Conductivity is
expressed in units of micro-mhos per centimeter.
Conductivity ( C, in micro-mhos/cm) = 10,000/
Resistivity (in ohm-meters)
In the SI system of
units, Siemens are used to replace mhos. 1 Siemens = 1 Mho. Learn more about [Siemens and Mhos].
Formation resistivity
is affected by three factors: Salt Concentration, Temperature, Pore volume
(porosity).
Formation Resistivity
Factor (F) is a fundamental concept in log interpretation and analysis. The formation resistivity
factor is defined as the ratio of the electrical resistivity of a rock 100
percent saturated with water to the resistivity of the water with which it is
saturated, (Archie, 1942).
The equation
is: F =
Ro/Rw
(Referred to as Archie’s Equation)
Given Rw = .05,
If Ro = 5.0 then F =
100
If Ro = 1.25 then F =
25
If Ro = .55 then F = 11
POROSITY FROM
RESISTIVITY:
Archie found a relation
of Formation
Resistivity Factor (F) to Porosity (f) as follows:
F = a / fm
The constants (a) and
(m) are related to lithology.
Cementation factor (m)
in a consolidated sandstone or a porous limestone is
1.8 to 2.0. In a clean unconsolidated sandstone values for (m) may be as low as
1.3 and the constant (a) is equal to 1.0.
An empirical formula
based on studies of core data from numerous localities has resulted in the equation:
Porosity of 10 percent
results in a Formation resistivity Factor of 100
Porosity of 20 percent
results in a Formation resistivity Factor of 25
Porosity of 30 percent
results in a Formation resistivity Factor of 11
Notice these three
Formation Resistivity factors are the same as previously calculated with F =
Ro/Rw above.
Therefore:
Rearranging:
f =
(1/Ro/Rw)1/2
Requirements for
this method are 100 percent water saturation, Rw is known and mineral
conduction is not present.
Using Shallow Resistivity from a pad mounted
measurement:
Given Resistivity
of the flushed zone, Rxo and Resistivity of the mud filtrate, Rmf, porosity may
be obtained a follows:
f =
(a Rmf/Rxo)1/m
where a = .62, m = 2.15 (From Winsauer et al.,
1952)
PERMEABILITY FROM RESISTIVITY:
(From Alger 1966,
Croft 1971, USGS)
A conclusion may be
made that if deep and shallow measurements are the same, that no invasion has
taken place. If deep and shallow measurements are different, then
invasion has taken place. Invasion is an indication that a rock matrix is
permeable. It is because of the ability of the E-log to measure fluid
content, fluid quality, lithology, and indirectly permeability, porosity and
formation factor that make an E-log potentially the most useful logging
tool.
See the page on permeability.
CONSIDERATIONS:
All logging methods
have limitations to consider.
Bed thickness effect:
The curves produced by the normal devices are affected by bed thickness
and resistivity (Lynch 1962).
Where the resistive bed
is more than
Although the radius of
investigation increases as the electrode spacing increases, the use of AM
spacing greater than 64 inches is not practical because thinner beds are not
only shown at less than true resistivity but may be recorded as conductive beds
if their thickness is less than or equal to the AM spacing. Focused
resistivity tools overcome this limitation.
INVERSION METHODS:
Recently, software
has been developed for improving resistivity log interpretation. Old logs and
new are being subjected to inversion processing that removes the effect of
surrounding formations. These techniques will make electrical resistivity a
more accurate viable logging method well into the future.
OTHER RESISTIVITY
METHODS:
The discussion thus far
has been related to resistivity using a “normal” electrode array.
Several other tools are
available for the purpose of measuring resistivity. Each tool is designed to
provide an accurate determination of formation resistivity in various borehole
environments.
Lateral resistivity measurements are used when it is necessary to
obtain deep formation resistivity measurements. Deep formation resistivity is a
close approximation of true resistivity where invasion is small. In cases of
deep invasion, interpretation must include a correction for the invading
borehole fluid. Note: Due to the larger spacing of electrodes used in this
method, thin formations are less noticeable on the log.
Focused electrode resistivity tools are used in boreholes that have low
resistivity mud or other drilling fluids. Normal and lateral logging tools tend
to conduct current thru the borehole fluid in this case. Focused
electrode systems are designed to reduce or eliminate borehole fluid
conduction. The current emanating from the tool therefore flows into the
surrounding formation and provides a more accurate measurement of formation
resistivity.
Micro electrode [wall]
resistivity tools have small electrodes attached to a non conductive pad that
is pressed against the borehole
wall while logging. These tools are designed to measure the resistivity of the
combined mud filtrate (Rmf) and resistivity of the flushed zone (Rxo). The
objective is to obtain information about formation porosity and permeability.
The small spacing used in the electrodes make this tool very accurate in
establishing bed boundaries.
Induction resistivity tools use electromagnetic induction as a
method of measuring formation resistivity. It is important to know that all
other resistivity measurements require fluid in the borehole. Induction logging
tools provide resistivity measurements in oil/water and air.
Corrections are applied
to all of the above resistivity methods.
ACOUSTIC LOG
INTEPRETATION:
An Acoustic Log
(sometimes referred to as a sonic log) when properly calibrated, will provide
important information about the physical structure of a rock matrix. The ability of sound to travel
within and through rock or sand and gravel depends on the physical structure of
the matrix. The amplitude, speed and phase relationships of a transmitted
sound wave that returns to an acoustic receiver is a function of all of the
combined matrix densities, interconnections, cementation, fracturing, and
porosities within the matrix.
Because the total
transit time from the transmitter to the receiver includes the path thru the
borehole fluid + formation + borehole fluid, Borehole
compensated (two or four receiver) logging tools are used. Borehole
compensation is accomplished mathematically by subtracting the borehole
transit time.
Acoustic waveforms
provide information related to transit time (density) and amplitude (interconnection) of the material comprising the rock
matrix. Surface Geophysics has for many years used seismic reflection and
refraction for determination of subsurface structure. Transit time (Dt) through sandstone,
limestone, water, and other materials have been determined in the
laboratory. Relationships between porosity and transit time are
known. It is possible to determine porosity of a given matrix if the
transit time is known. Beginning with velocity;
The bulk velocity is
the sum of the fluid velocity and the matrix velocity.
The relationship
between bulk velocity (vb) and fluid velocity (Vf)
combined with matrix velocity (Vma) becomes:
Given an equation
referred to as the Wyllie “time average equation”:
1/vb = f/vf + 1-f/vma
Transit time (Dt) is the reciprocal of
velocity.
The equation for
porosity (f)
obtained from transit time (Dt)
is:
f = (Dtlog – Dtma) / (Dtf – Dtma)
Where Dtlog = Measured Dt, Dtf = fluid Dt, Dtma = assumed matrix Dt.
Fluid Dt is usually considered
200 microseconds per ft. (Note some sources use 188 microseconds per ft.)
COMMON MATRIX VELOCITIES:
(microseconds per ft.)
MATRIX: VELOCITY:
Sandstone,
unconsolidated 58.8 or more
Sandstone,
semi-consolidated 55.6
Sandstone, consolidated 52.6
Sandstone, shaly 57 to 70
Limestone 47.6
Dolomite 43.5
Shale 62.5
to 167
Calcite 45.5
Granite 50.0
Gypsum 52.6
Quartz 55.6
Salt 66.7
Areas having fractures
including unconsolidated matrix can be inferred from an Acoustic Log.
CEMENT BOND LOG
INTERPRETATION:
Acoustic logging is
also used for determination of cement bond in cased wells. This type of log is most often referred to
as a Cement Bond Log (CBL).
Acoustic signals
propagated in steel casing are observed to have large amplitude in free casing
because much of the energy is retained in the casing. Whereas the opposite
effect is found in casing that is in contact with a solid such as cement. The
casing signal is much smaller because the energy is coupled into the
surrounding cement and formation.
The thin plate velocity
of sound in steel is approximately 5300 meters per second (188 microseconds per
meter).
A receiver having 3 feet
spacing will receive the casing signal
(first arrival) at 177 microseconds plus a short additional period allowing for
transit time thru the borehole fluid.
A receiver signal “time
gate” is set at the time of the expected casing signal. The casing signal will
be the first arrival at the receiver in free casing. The signal amplitude is
recorded. A high signal amplitude indicates poor
cement bond. A relatively low signal amplitude
indicates good cement bond. Amplitude is normally presented on a scale of 0 to
100 percent amplitude. An area having no cement bond is represented by 100
percent amplitude. Due to the fact that well cemented pipe can never reduce the
signal to “zero”, a good reference for zero signal is the best cemented portion
of the cased hole. Using information obtained from a Variable Density
(waveform) display referred to as a VDL display, it is possible to
observe the entire receiver wave train. When cementation is complete ( good bond) from casing to cement to formation, it is
possible to observe waveform shift in delta- time in the later arrivals that
can be correlated to open-hole acoustic delta-time logs.
CBL ATTENUATION:
The measurement of
attenuation measured in decibels (dB) is obtained from the amplitude as
follows:
Attenuation = 20/D x Log10(A/Ao)
Where:
Attenuation is measured
in decibels.
Ao is the transmitter amplitude measured in millivolts.
A is the receiver amplitude
measured in millivolts.
D is the distance from
the transmitter to receiver (spacing) meters or feet as specified.
Note: Attenuation
refers to the reduction of amplitude. Therefore, attenuation is measured
in terms of -
dB.
OTHER CBL TOOLS:
Sector bond tools
(SBT), Radial bond tools (RBT), and Ultrasonic Imager Tools (USIT) are other
options available for Cement bond applications including casing inspection.
GAMMA LOG
INTERPRETATION:
Natural gamma
radiation occurs in rock formations in varying amounts. Uranium, Thorium,
Potassium, and other radioactive minerals are associated with different
depositional environments. Sedimentary sandstone and Carbonate environments are
low in gamma radiation. Clay and Shale formations exhibit greater amounts of
gamma radiation. A log of gamma radiation in “counts” or API units will give a
positive indication of the type of lithology. Interpretation of gamma log data
is done based on the relative low and high count rates associated with respective
“clean” and “dirty” environments. Composition of formations having more clay or
shale as indicated by higher gamma count rates generally are more tightly
compacted with fine particles and therefore have less porosity and
permeability. Formations having high gamma count rates even though they may
exhibit low water saturation are generally unfavorable for production in oil
and water well environments.
It is important to be
aware that certain areas are known to have sandstone formations with higher
than normal levels of radiation. These formations are sometimes erroneously
interpreted. Information from an SP log can be
used for correlation.
Coal formations
normally have very low (almost zero) gamma radiation and contrast quite well
with surrounding formations. Knowledge of local “exceptions” is an important
aspect of accurate interpretation.
GAMMA LOG CALIBRATION:
Gamma radiation is
detected differently in every logging tool. Due to variation in detector types,
tool design, detector efficiency and overall tool response, the American
Petroleum Institute (API) standard of API Units is commonly used for
calibration. A Test well located in
NEUTRON LOG
INTERPRETATION:
A Neutron Log when
properly calibrated (usually to an API standard) will provide important
information about the content of the pore spaces within a rock matrix. Neutrons emitted from a
neutron source are slowed down and eventually captured through interaction with
hydrogen atoms. Once captured, a gamma ray of capture is created.
Neutron Logging
tools are designed to respond to slow Thermal Neutrons or Gamma Rays of
Capture.
Since hydrocarbons and
water (H20)
contain hydrogen a neutron log will provide knowledge of the hydrogen in the
pore spaces of the matrix. When more hydrogen is present, more neutrons
are captured, and fewer neutrons reach the neutron detector. Conversely,
lower porosity, neutrons travel farther and reach the detector, increasing
neutrons counted at the detector. In other words, increased fluid filled porosity is indicated by lower neutron count.
Neutron porosity is calculated based on neutron tool response
in known lithologies having known
porosity.
Tool
response is specified in terms of American Petroleum Institute (API) units. The standard unit for neutron logging
tools is the “API Neutron Unit”. 1000 API units is
assigned to any neutron tool in a water filled hole having 7 - 7/8 inch
diameter in
When a tool is
calibrated at the API test well, its response to a standard neutron calibrator
is also determined. The differential deflection produced by this two
environment device is compared to the API test well deflection representing
1000 API Units. A definite number of API units can then be assigned to a tools
calibrator deflection. This calibration figure must be determined for each
model or series of tool.
Each tool supplier develops
a transform from API units to porosity for the neutron tools they produce.
The general equation is: Porosity
(f) = natural log (API
Log counts * constant + constant)
Neutron Porosity is
based on a Limestone matrix (
A correction to obtain
porosity for a sandstone matrix is: Porosity
(fss) = 0.95 (f(n)) + .035
DENSITY LOG
INTERPRETATION:
A Density Log when
properly calibrated will provide reliable information about matrix bulk density. When
density is known and a specific matrix is assumed then porosity of the matrix
may be determined. A mathematical relationship exists between measured
density, assumed matrix density with no porosity and the density of the
material filling the pore space. Water has a density of 1 gram per cubic
centimeter. Sandstone with no porosity has a density of 2.65 grams per cubic
centimeter. If a sandstone matrix is assumed for example, then a given
density of 2.00 grams per cubic centimeter allows calculation of 40 percent
porosity.
The equation for
porosity (f)
obtained from bulk density is: f = (rma – rb) / (rma – rf)
Where rb = Measured bulk
density, rf = fluid density, rma = assumed matrix density.
For reference,
Sandstone has a density of 2.65 gm/cc, Limestone is 2.71 gm/cc, Dolomite 2.87
gm/cc.
NEUTRON – BULK DENSITY
CROSS-PLOT:
Combination of data
from a Neutron Porosity Log and Bulk Density log can be helpful in
identification of Lithology. A chart is used that has the known relationship
between Neutron Porosity and Bulk Density for three matrices; Sandstone,
Limestone, and Dolomite. It is possible to determine ratio of
Sandstone/Limestone and obtain a more accurate porosity using the cross-plot
chart. Results from the cross-plot chart should be correlated with known
lithological information.
Neutron porosity
and density porosity are often presented in an overlay on the same scale on a
log for shale and gas identification.
View a
neutron-density cross-plot chart.
Cross plot methods
are treated extensively in the WELLOG webinar.
AN ADDITIONAL
BENEFIT:
If the Lithology is
known to be a Sandstone and the cross-plot shows a Dolomite,
then it is possible one or both sets of log data are not properly calibrated.
If the cross-plot shows correlation,
then it provides a closed loop between logging tool response and lithology.
SHALE VOLUME CORRECTION:
Porosity data
should be corrected for shale content in the zone of interest. Porosity values are optimistic when
shale is present.
Depending on the value of Rmf/Rw, either the natural
gamma data or SP data is used to determine shale volume.
Correction for shale is covered extensively in the advanced pages
of the WELLOG webinar.
FORMATION EVALUATION:
After the
appropriate corrections are applied, a realistic Formation Evaluation can be
made. It should not be under-estimated that many corrections are required to
properly analyze a well log.
WATER SATURATION:
One objective in Log
Interpretation is the evaluation of a petroleum formation for water saturation
(Sw). If it assumed that only two types of fluid occur in the formation, for
example oil and water.
The calculation for
water saturation is as follows: Sw = (F * Rw / Rt)1/n
Where n is the
saturation exponent (usually a value of 2).
The oil saturation as a
percent of the pore space is simply: So = (1 – Sw)
WELLOG has an extensive log interpretation library and
personnel with experience in log interpretation.
WELLOG will provide answers to your log
interpretation questions free of charge!
WELLOG will provide training on Logging and
Log Interpretation.
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and additions occur every day.
Registration in the
webinar is voluntary. Email training@wellog.com
with Name, Company, and your interest in log interpretation.
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