Well logging summary and Interpretation

1
Well logging
Well logging summary Compiled by : P.Eng. Mahmoud Jad
1.1 Terminology
Symbol Definition Unit
Φd Density porosity log Fraction
Φn Neutron porosity log Fraction
(Φd)sh Density porosity log for shale Fraction
(Φn)sh Neutron porosity log for shale Fraction
Φd corrected Corrected density porosity log Fraction
Φn corrected Corrected neutron porosity log Fraction
Φavg Average porosity Fraction
F Formation factor Dimensionless
Rw Water resistivity Ohm .m
Rwa Apparent water resistivity Ohm .m
Sw Water saturation Fraction
A Lithology factor / archie’s constant -
M Cementation factor -
Φls Apparent porosity of lime stone %
Φss Apparent porosity of sandstone %
Φs Porosity from sonic log %
Φnc1 Corrected porosity from neutron log for shale %
Φnc2 Corrected porosity from neutron log for hydrocarbon %
Φdc1 Corrected porosity from density log for shale %
Φdc2
Corrected porosity from density log for
hydrocarbon
%
Formation bulk density gm/cc
Matrix density gm/cc
Fluid density gm/cc
Hydrocarbon density gm/cc
tf Transit time of fluid sec/ft
tma Transit time of matrix sec/ft
tl Transit time from attached log sec/ft
Gamma ray reading from attached log API unit
Gamma ray reading opposite shale formation API unit
Gamma ray reading opposite sand formation API unit
Ish Shale index -
Vsh Shale volume %
Sw Residual hydrocarbon saturation %
Sh Formation water saturation %
So Formation oil saturation %
P Formation water salinity ppm

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Well logging
Formation evaluation is the process of using borehole measurements to evaluate
the characteristics of subsurface formation. These measurements may be grouped into
four categories:
1. Drilling operation logs
2. core analysis
3. wireline well logs
4. productivity tests
The continuous recording of a geophysical parameter along a borehole produces a
geophysical well log. The value of the measurement is plotted continuously against
depth in the well. Well logging plays a central role in the successful development of a
hydrocarbon reservoir. Its measurements occupy a position of central importance in the
life of a well, between two milestones: the surface seismic survey, which has influenced
the decision for the well location, and the production testing.
Log is a graphic representation of the variations versus depth of other parameters.
Wire line logs are measurements of physical parameter in the formations penetrated by
borehole; they are run while drilling has been stopped i.e. after the drill string has been
pulled out from the borehole. Some types of geophysical well logs can be done during
any phase of a well's history: drilling, completing, producing, or abandoning.
It is called also wireline logging due to the wireline cable which carries at its end the
instruments & lower it into the well.
1.2 Introduction

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Well logging
The measured well log consists of:
 Log header: includes all information about the well logged and information
necessary to describe the environment the measurement has been informed in
(e.g. drilling mud parameters). Tool sketches and remarks informing about
specific events during the logging operation complete the header.
 Main log: main display of measurement performed.
 Log trailer: includes tool/computation parameter table and calibration records.
Wireline cables consist mainly of two layers:
 Outer Wire rope: to provide strength to cable to carry the
instruments.
 Inner Wire: to provide electric power to downhole equipment
& for data telemetry.

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Well logging
1.3 History of well logging
 In 9191 Conrad Schlumberger gave the idea of using electrical measurements to
map subsurface rock bodies .
 In 1919 Conrad Schlumberger and his brother marcel begin work on well logs .
 The first electrical log was introduced in 1927 in France using stationed resistivity
method.
 The first commercial electrical resistivity tool in 1929 was used in Venezuela, USA
and Indonesia.
 Sp was run along with resistivity first time in 1931
 Schlumberger developed the first continuous recording in 1931
 and neutron logs was started in 1941 Micro-resistivity array dipmeter and
lateralog were first time introduced in 1950's
 The first induction tool was used in 1956 followed by formation tester in 1957,
fomation density in 1960's, electromagnetic tool in 1978 and most of imaging
logs were developed in 1980's Advanced formation tester was commercialized in
early 1990's

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Well logging
1.4 Field operations
Wire line electrical logging is done from a logging truck as a " mobile laboratory" the
truck carries the down measurement instruments .the electrical cable and winch
needed to lower the instruments into the bore hole, surface instrumentation needs to
power the down hole instruments and to receive and process their signal .measurement
instruments are usually composed of tow components :-
 The cartridge: - contains electronics that power the sensors.
 The snode: contains sensors used in making the measurement. It runs into the
bore hole at the end of a wire line connects it to recorders on the surface located
in the logging unit.
Parameters that are measured-:
There are tow types of phenomena that can be studied by wire line logging :-
1. Natural ones “temperature, spontaneous radioactivity, etc “that are measured
by a sensor or a receiver with no signal transmitted beforehand.
2. Induced ones "radioactivity, electric logs, wave travel time, etc ”generated by a
transmitter and measured by one or more receivers, the unit is mounted on a
truck that parked near the well for operations on shore.
Well logging is performed in boreholes drilled for the oil and gas, groundwater,
mineral and geothermal exploration, as well as part of environmental and
geotechnical studies.

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Well logging
Wireline Unit:
The cabin that contains the surface
hardware needed to make wireline
logging measurements. The logging unit
contains at the minimum the surface
instrumentation, a winch, a depth
recording system and a data recorder.
The surface instrumentation
controls the logging tool, processes the
data received and records the results
digitally and on hard copy. The winch
lowers and raises the cable in the well.
A depth wheel drives the depth
recording system. The data recorder
includes a digital recorder and a printer.
1. Onshore:
The logging company sends Truck
Logging Unit which contains the
computers, winch and recorders.
2. Offshore:
The logging unit is stored as small
house on the rig.
Logging While Drilling (LWD):
In the 1980s, a new technique, logging while drilling (LWD), was introduced which
provided similar information about the well. Instead of sensors being lowered into the
well at the end of wireline cable, the sensors are integrated into the drill string and the
measurements are made while the well is being drilled. While wireline well logging
occurs after the drill string is removed from the well, LWD measures geological
parameters while the well is being drilled.
However, because there are no wires to the surface, data are recorded
downhole and retrieved when the drill string is removed from the hole. A small subset
of the measured data can also be transmitted to the surface in real time via pressure
pulses in the well's mud fluid column. This mud telemetry method provides a bandwidth
of much less than 100 bits per second, although, as drilling through rock is a fairly slow
process, data compression techniques mean that this is an ample bandwidth for real-
time delivery of information.

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Well logging
Measuring While Drilling (MWD):
It is the evaluation of physical properties, usually including pressure, temperature
and wellbore trajectory in three-dimensional space, while extending a wellbore. MWD is
now standard practice in offshore directional wells. The measurements are made
downhole, stored in solid-state memory for some time and later transmitted to the
surface. Data transmission methods vary from company to company, but usually involve
digitally encoding data and transmitting to the surface as pressure pulses in the mud
system.
Some MWD tools have the ability to store the measurements for later retrieval with
wireline or when the tool is tripped out of the hole if the data transmission link fails.
1.5 Objectives of well logging
 Determination the nature and amount
of fluids contained in the rocks.
 Estimation of accurate values of
hydrocarbons saturation.
 Estimation of accurate values of water
saturation.
 Estimation of hydrocarbon in place
 Estimation of recoverable hydrocarbon
 Determination of the lithology of the
reservoir rock.
 Determination of permeability index.
 Determination of porosity.
 Fracture detection
 Determination of water salinity
 Reservoir pressure determination
 Water flood feasibility
 Reservoir quality mapping
 Identify potential reservoir rocks and
cap rocks.
 Analyze sediment deposition conditions.
 Locate “woc” & “goc “.
 Correlation of offset wells.

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Well logging
1.6 Advantages and Limitations of Well Logging
Advantages: Limitations:
o Continuous measurements
o Easy and quick to work with Short
time acquisition
o Better resolution than seismic data
o Economical
o More advanced tools
o Better depth control
o Only choice available (certain tools).
o More certain on data quality
o Indirect measurements
o Limited by tool specification
o Affected by environment
o Varying resolution
o Invasion effect
o Hole condition dependent.
o Unable to log in high angle wells (>60
deg )
o Acquired after drilling, more rig time
o More uncertainty in getting data or
good data in problem prone wells
1.7 Types of well logging
There are three major types of well logs:
1. The logs used by geologists and reservoir engineers to evaluate the
characteristics of the formations and fluids and quantify them.
2. The logs used by drillers that provide technical information.
3. The logs used by production staff to study fluid and fluid flow phenomena.

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Well logging
1.8 Logging tools
These are electronic devices that records data over depth. The tool is attached to
the end of wireline cable & lowered to the borehole. There are many types of tools such
BHC tool, GR tool, Density tool and many others. Usually, these tools are integrated as
measurement sensors in one tool called Sonde.
Cartridge: The section of a wireline logging tool that contains the telemetry, the
electronics and power supplies for the measurement, as distinct from the sonde that
contains the measurement sensors.

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Well logging
Caliper Tool:
The Caliper Tool is a 3 armed device that measures the internal diameter (I.D.) of
casing or open borehole completions. This information is crucial to all types of
production logging. The caliper probe provides a “first look” at borehole conditions in
preparation for additional logging. It uses a tool which has 2, 4, or more extendable
arms. The caliper is a useful first log to determine the borehole conditions before
running more costly probes or those containing radioactive sources.
The log is used to: "Interpretation Goals"
 measure borehole diameter,
 Location of cracks, fissures, caving, faulting,
casing breaks.
 assess borehole quality and stability
 For calculation of pore volume for pile
construction.
 Input for environmental corrections for other
measurements.
 Qualitative indication of permeability.
 Correlation.
 Correction of other logs affected by borehole
diameter
 Provide information on build-up of mudcake
adjacent to permeable zones.
 Locate packer seats in open hole.
Notes
 Increasing in diameter of borehole indicates
about Wash out Process (ex: Shale).
 Decreasing in diameter of borehole indicates
about Invasion process (ex: Porous Sand).

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Well logging
Conventional Geophysical Well Logs [Adapted and Expanded from Keys (1990)]
Type of log
Varieties and
related
techniques
Properties
measured
Potential
application
Required hole
conditions
Other limitations
Spontaneous
potential.
Electric potential
caused by salinity
differences in
borehole and
interstitial fluids.
Lithology, shale
content, water
quality.
Uncased hole filled
with conductive
fluid.
Salinity difference
needed between
borehole fluid and
interstitial fluids.
Single-point
resistance.
Conventional,
differential.
Resistance of
rock, saturating
fluid, and
borehole fluid.
High-resolution
lithology. Fracture
location by
differential probe.
Uncased hole filled
with conductive
fluid.
Not quantitative; hole
diameter effects
significant.
Multi-
electrode
resistivity.
Normal, focused,
or lateral.
Resistivity, in
ohm-meters, of
rock and
saturating fluids.
Quantitative data
on salinity of
interstitial water;
lithology.
Uncased hole filled
with conductive
fluid.
Normals provide
incorrect values and
thicknesses in thin
beds.
Electrical
induction.
Deep, shallow,
and focused.
Resistivity, in
ohm-meters, of
rock and
saturating fluids.
Quantitative data
on salinity of
interstitial water;
lithology.
Open hole with
plastic casing.
Skin effect correction
for highly conductive
formation.
Natural
gamma.
Gamma spectral.
Gamma radiation
from natural or
artificial
radioisotopes.
Lithology-may be
related to clay
and silt content
and permeability;
spectral identifies
radioiso-topes.
Any hole
conditions, except
very large, or
several strings of
casing and cement.
Very high counts
need to be corrected
for dead-time.
Gamma-
gamma.
Compensated
(dual detector).
Electron density.
Bulk density,
porosity, moisture
content, lithology.
Optimum results in
uncased; qualitative
through casing or
drill stem.
Severe hole-diameter
effects.
Neutron.
Epithermal,
thermal,
compensated
activation, pulsed.
Hydrogen
content.
Saturated
porosity, moisture
content,
activation
analysis, lithology.
Optimum results in
uncased; can be
calibrated for
casing.
Hole-diameter and
chemical effects.
Acoustic
velocity.
Compensated
wave form.
Compressional
wave velocity.
Porosity,
lithology,
fracture location,
and character,
cement bond.
Fluid-filled, 3 to 16
inch diameter.
Does not see secondary
porosity.
Acoustic
televiewer.
Acoustic caliper.
Acoustic
reflectivity of
borehole wall.
Location,
orientation, and
character of
fractures and
solution openings,
strike and dip of
bedding, casing
inspection.
Fluid-filled.
Heavy mud or mud
cake attenuate signal;
very slow log.
Caliper.
Oriented, 4-arm
high-resolution
bow spring.
Hole or casing
diameter.
Hole-diameter
corrections to
other logs,
lithology,
fractures, hole
volume for
cementing.
Any conditions.
Deviated holes limit
some.

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Well logging
Fluid column
temperature.
Differential.
Temperature of
fluid near sensor.
Geothermal
gradient, in-hole
flow, location of
injected water,
correction of
other logs, curing
cement.
Fluid-filled.
Accuracy and resolution
of tools varies.
Fluid column
conductivity.
Resistivity.
Most measure
resistivity of fluid
in hole.
Quality of
borehole fluid, in-
hole flow,
location of
contaminant
plumes.
Fluid-filled.
Accuracy varies,
requires temperature
correction.
Flow.
Spinner, tracer,
thermal pulse,
electromagnetic.
Velocity of net
flow in borehole.
In-hole flow,
location, and
apparent
Hydraulic
conductivity of
permeable
interval.
Fluid-filled.
Spinners require higher
velocities.
Needs to be
centralized.
Some of Special logging Tools
 Resistivity Based Imaging Tool
- Pad device on 4 to 6 arm caliper, few mm resolution
- Application: Thin bed Evaluation, Dip meter, Paleostream direction, fracture
evaluation, stratigraphy.
 Nuclear Magnetic Resonance
- Using Permanent magnet to realign hydrogen protons to new magnetic field, a
Lithology dependence porosity, saturation and permeability estimation
 Dipole Shear Sonic
- Shear measurement, AVO and Rock mechanics applications
 Borehole sonic imaging
- Acoustic based borehole imaging for 360 coverage, lower resolution than
resistivity based imaging tools.
 Modular Formation Test
- Very robust formation tester with the capability to take unlimited pressure
tests, pump the fluid into the borehole, identify the fluid type before sampling
 Wellbore Seismic
- VSP: Vertical seismic profile surface guns, wellbore detectors
- SAT: Seismic acquisition tool
- WST: Well seismic tool
- DSA: Downhole seismic array tool (3 axis geophones)

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Well logging
Some of modern logging tools

14
Well logging
1.9 Selection of the Tools to run
It depends on what type of information you are about to get and the cost you are
willing to spend.
Ability to Define Your Need:
 Geological
 Geophysical
 Reservoir
 Petrophysical
 Mechanical
Type of Information to Acquire
Geology Geophysics Petro-physics Reservoir
Rock
Mechanics
 Sand
development and
sand thickness
 Stratigraphic
information
 Lateral continuity
 Hydrocarbon
source
 Velocity
uncertainty
 Well to seismic
tie
 Seismic and
fluids/lithology
correlation
 Porosity
 Water
saturation
 Permeability
 Mineralogy
 Compartment
 Fluid properties
 Reservoir
pressure
 Reservoir
monitoring
 Stress
direction
 Pressure
profile
 Fracture
orientation
1.10 Well Log Interpretation
The three most important questions to be answered by well site interpretation are:
1. Does the formation contain hydrocarbons, and if so at what depth and are they
Oil or gas?
2. If so, what is the quantity present?
3. Are the hydrocarbons recoverable?
1.10.1 Interpretation procedure
The basic logs, which are required for the adequate formation evaluation, are:
1. Permeable zone logs (SP, GR, Caliper)
2. Resistivity logs (MFSL, Shallow and Deep resistivity logs)
3. Porosity logs (Density, Neutron and Sonic).

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Well logging
Generally, the permeable zone logs are presented in track one, the resistivity logs are
run in track two and porosity logs on track three.
Using such a set of logs, a log interpreter has to solve the following problems,
(I). Where are the potential producing hydrocarbons zones?
(II). How much hydrocarbons (oil or gas) do they contain?
First step:
The first step in the log interpretation is to locate the permeable zones. Scanning
the log in track one and it has a base line on the right, which is called the shale base line.
This base line indicates shale i.e., impermeable zones and swings to the left indicate
clean zones- e.g., sand, limestone etc. The interpreter focuses his attention immediately
on these permeable zones.
Next step:
To scan the resistivity logs in track 2 to see which of the zones of interest gives high
resistivity readings. High resistivity reflects either hydrocarbons in the pores or low
porosity.
Next step:
Scan the porosity logs on the track 3 to see which of the zones have good porosity
against the high resistivity zones. Discard the tight formations. Select the interesting
zones for the formation evaluation.
1.10.2 Quantitative interpretation Procedure:
1. Ensure that the logs are “on depth” relative to each other by taking a “marker “which is
an anomaly or a distinctive response that appear on all logs in the same depth, so the
logs are all “on depth”.
2. Take the readings from the attached logs (if there are any corrections, make them
carefully).
3. Calculate porosity from sonic log and density log.
Calculate the shale correction for neutron and density log.
Calculate the hydrocarbon correction for neutron and density log.
Calculate the effective porosity
Calculate the formation factor
Calculate water saturation and oil saturation.
1.10.2.1 Estimation of shale content (Vsh):
 First we should estimate shale / radioactivity index (Ish):-
From gamma ray log:
Where:
= gamma ray response in the zone of interest
=the average gamma ray response in the clean sand formation
= the average gamma ray response in the cleanest shale formation
Vsh =
𝜸𝒍𝒐𝒈− 𝜸 𝒎𝒊𝒏
𝜸 𝒎𝒂𝒙− 𝜸 𝒎𝒊𝒏

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Well logging
Empirical correlations relating shale content to gamma ray shale index, Ish
 Then The shale volume can be calculated from one of the following methods
01 For tertiary rocks, the larionov equation is:
02 The stieber equation is: −
03 The clavier et al equation is: √
04 For older rocks, the larionov equation is:
Note: Tertiary model gives the least value as shown in figure and will be used in the analysis.
05 From resistivity log √
Where:
Rt = is the reading from resistivity log
Rtsh= is the resistivity opposite to the cleanest shale zone
06 from neutron porosity log:
Where:
Øn = neutron porosity log reading at zone of interest
(Øn)sh= neutron porosity log reading opposite to the cleanest shale zone

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Well logging
07 From cross-plot:
−
−
Where:
Øn = neutron porosity log reading at zone of interest
(Øn) sh= neutron porosity log reading opposite to the cleanest shale zone
Ød = density porosity log reading at zone of interest
(Ød) sh= density porosity log reading opposite to the cleanest shale zone
Calculation of minimum Vshale, (Vsh,min) is the smallest value of Vsh among Vsh values.
1.10.2.2 Determination of porosity from density log (фd):-
Firstly by using the attached (ρb -log) determine every reading at selected
interval then determine (ф d).
Where:
is the measured bulk density (from Lithe-Density tool)
is the density of the matrix (depends on lithology)
is the density of the fluid in pore volumes.
is the percent volume of pore space .
1.10.2.3 Determination of porosity from sonic log (фs):-
Firstly by using the attached ( t -log) determine every reading at selected
interval then determine (ф s).
Where:
tlog is the reading on the sonic log in µsec/m
tmat is the transit time of the matrix material
tf is the transit time of the saturating fluid
1.10.2.4 Corrections:
1. Correction for sandstone:
Since the neutron porosity log measures the porosity in l.S Units, three porosity
units are added to the apparent neutron porosity to obtain the corrected
neutron porosity for sandstone.
Ønc = øn + 3
𝑡 𝑙𝑜𝑔− 𝑡 𝑚𝑎𝑡
𝑡 𝑓−𝑡 𝑚𝑎𝑡

18
Well logging
2. Correction for shale:
a) For neutron porosity:
Where:
Ønc = corrected neutron porosity to shale.
Øn = apparent neutron porosity from log.
Ønsh = apparent neutron porosity opposite to the cleanest shale.
b) For density porosity:
Where:
Ødc = corrected density porosity to shale
Ød = apparent density porosity from log
Ødsh = apparent density porosity opposite to the cleanest shale
3. Correction for hydrocarbon effect:
Light oil or gas will cause the formation density (ρb) to decrease by an amount of
∆ρb & apparent porosity (ød & øn) to increase by an amount of ( ∆ød & ∆øn ) respectively .
[ ]
[ ]
(Ρb)corr. = ρb + ∆ ρb (Øn )corr. = øn + ∆øn (Ød)corr. = ød - ∆ød
Where:
Sor = residual hydrocarbon saturation (from relative permeability curves)
P = formation water salinity (ppm)
1.10.2.5 Determination of effective porosity:-
After determination of corrected porosity for shale and hydrocarbon:
√

19
Well logging
1.10.2.6 Determination of water resistivity (Rw):-
1. From bulk density V.s resistivity plot.
 By using the attached logs we should determine values of (Rt) & (ρb).
 Then draw “Hingle plot “which is a relation ship between (ρb) and (1/Rt
.5
)
 Connect the point which determines the matrix density with the most Upper
points in the chart this line will be (%100) Sw line. (Rt) value which be on this line
equal the value of (Ro).
Then determine the value of porosity from the (ρb) value which is in this line.
 The formation factor (F) equal (1/ ф2
).
2. From charts by using formation water salinity & reservoir temperature
3. From SP.
4. From Rxo & Rt
1.10.2.7 Calculation of water saturation:-
Sw% can be determined by the following equation:
√
Where:
F = formation factor
Rw = formation water resistivity
Rt = true formation resistivity
That equation is for clean sand but for shaly sand the equation becomes:
√
1.10.2.8 Determination of hydrocarbon saturation:-
Sh% can be calculated from the following equation
1.10.2.9 Determination the volume of:-
 Water :- =фeff *Sw
 Oil :- = фeff *Shyd
 Residual oil :- = фeff * Sor

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Well logging
1.11 References
 “Schlumberger log interpretation”,vol.9, 1 (9171-1989)
 Donald P. Helander“fundamental of formation evaluation”, oil & gas consultants,
international Inc., Tulsa.
 E.R.Crain,”the log analysis handbook”, vol. 9, Pennwell publishing co., Tulsa,
oklahoma, USA.
 “Theory, measurement, and interpretation of well logs”, Zaki Bassiouni, SPE
textbook series vol.4.
 Geology & Geophysics in Oil Exploration 2010 by: Geophysicist, Mahmoud
Ahmed Sroor.

Well logging summary and Interpretation

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