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technical training 2007
Module FE1
Wireline Logs
&
LWD Interpretation
Stag Geological Services Ltd.
Reading
UK
Revision J
February 2007
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WIreline Logs & LWD Interpretation
Chapter 1 Introduction
Chapter 2 Spontaneous Potential Logs
Chapter 3 Gamma Ray Logs
Chapter 4 Resistivity Logs
Chapter 5 Bulk Density Logs
Chapter 6 Neutron Porosity Logs
Chapter 7 Sonic Logs
Chapter 8 Lithology Determination
Chapter 9 Reservoir Evaluation
Chapter 10 Shaly Sand Analysis
Chapter 11 MWD Overview
Chapter 12 LWD Imaging Logs
Chapter 13 Log Witnessing
Appendix A Vendor Brochures
Appendix B Log Interpretation Charts
Figure : Table of Contents
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Introduction
Locating the presence of oil and gas deposits underground is a complex process
spanning many months of preliminary research followed by exploration and
development drilling. Potential sites for exploration are identified from seismic
studies but full evaluation can only be made by drilling wells to see what is
actually there.
Advances in seismic data collection and interpretation techniques are leading to
less uncertainty and greater chances of locating commercial reserves, but the
results of the drilling process are ultimately only as good as the interpretation
techniques used in the evaluation process.
Formation Evaluation can be grouped into four major categories:
• Before Drilling
Seismic Interpretation
Offset Data
• During Drilling
Mud Logs and Wellsite Geology
Measurement While Drilling (MWD)
Coring
• Post - Drilling
Wireline Logs
Production Tests
Whilst advances in seismic processing have been remarkable in recent times the
process is still best suited to large scale exploration and field evaluation. Wellsite
geology and mudlogging provide geological data while drilling the well but the
drilling process and the inefficiences of the hole cleaning process only allows for
a largely qualitative and subjective approach. Coring does produce whole rock
from which detailed petrophysical analysis and quantitative measurements of porosity, permeability, fluid saturation may be made but cores are normally only
taken over short intervals in reservoir rocks leaving the majority of the section
un-sampled.
Petrophysical logging enables large sections of exposed (and sometimes cased)
hole to be scanned and variety of geological and reservoir data to be obtained;
quantitative analysis can be performed on the data to supplement other informa-
tion. Historically, petrophysical logging has been called “Wireline Logging”, or
even “Electric Logging” but neither of theses terms adequately describe the
current range of logging tools or conveyance methods.
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Formation Evaluation
The objectives of logging are multiple and varied; depending on the type of well
being drilled and the information required. However, we might try and list some
of the required information as follows:
• Geological Correlation
Identification of lithology for correlation between wells or to assist
general geological evaluation in the current well. Different logging
runs taken over the same interval need to be depth matched in order
to ensure that we are comparing like with like. Perforating, taking
sidewall cores or obtaining pressure information and fluid samples
all require accurate internal depth correlation using logs.
• Petrology
Logs can help to identify lithology, mineral assemblages and pick
out features such as bedding, lamination, porosity, permeability, ce-
mentation, fractures and facies and depositional environments.
• Reservoir Parameters
Logs can identify permeable zones, measure porosity and permeabil-
ity, identify fluid types and provide information to calculate satura-
tion levels, differentiate between water, oil and gas and determinefluid contact points. Reservoir pressure can be measured and fluids
obtained for analysis.
• Rock Mechanics
Rock strength and the tectonic forces acting upon rocks at depth can
be evaluated from logging tools and the information used to help un-
derstand drilling and borehole problems.
• Geosteering Applications
When MWD and LWD tools are used the information obtained, at
the time of drilling, may be used to help drill the well to the required geological target and indeed navigate the reservoir.
Wireline Logs
In September 1927, Marcel and Conrad Schlumberger, with Henri Doll, recorded
the first electrical resistivity log at Pechelbronn, France. This log was actually
called a “carottage electrique” or electrical core since it was a quantitative
recording of rock properties. The log was hand plotted from point-by-point resis-
tivity measurements. Since then, more than fifty geophysical-type well logs have
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been introduced to record the various electrical, nuclear, acoustical, thermal,chemical and mechanical properties of the earth.
Figure 1: First “Electric” Log
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Figure 2: First Schlumberger Log
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Without interpretation, the measurements provided by the various logs are not particularly useful. It takes time, knowledge, and experience to convert the raw
data into meaningful and practical information often using sophisticated
computer software; the input data consisting of raw well log data, and the output
being porosity, hydrocarbon type, fluid saturations, and lithology.
Logging tools are conveyed into and out of the borehole in a number of ways.
Traditionally wireline conveyed tools log boreholes after they have been drilled;
the wireline not only conveying the instruments but also providing the means of
data transmission from the tool to the surface equipment.
However, borehole conditions often make the use of wireline tools very difficult.High inclinations, high pressures and temperature and unstable borehole condi-
tions can provide severe limitations on the use of wireline tools. Attaching the
instruments to jointed drillpipe or tubing can overcome some of these issues and
the hole is logged whilst tripping the pipe to the surface. A cable attached to the
logging tool is strapped to the pipe and reeled in as the string is tripped. Whilst
this process does allow high angle and unstable boreholes to be logged the
process is very time consuming and, therefore, expensive. The use of coiled
tubing can significantly reduce costs as tripping speeds are much higher and the
conductive cable can be threaded internally through the coiled tubing eliminating
handling time.
The use of MWD and LWD logging tools overcomes many of these issues and
also enables the hole to be logged very shortly after drilling minimising invasion
and other interpretation issues.
The Wireline Logging Process
The logging company provides the tools, surface equipment and a team of expe-
rienced engineers to perform the logging operation, which may take anything
from a few hours to many days, depending on the nature of the work. The surface
logging unit comprises the control functions, surface computer systems, cable
drum and winch. The logging tools, which may be up to 30m long are attached to the cable, which is used both for suspension and data transfer, and lowered to
the bottom of the borehole.
The cable is then pulled out of the hole and the various rock properties are con-
tinuously measured. Pulling speeds are dependent on the type of tool being run
but are typically around 1800 feet per hour (600m/hr) when radioactive tools
such as a gamma ray log are present and can be as much as 6000 ft/hr (1800m/
hr). During the logging process the data is recorded at surface, correlated for
depth and corrected for borehole and mud conditions.
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Figure 3: Wireline Logging Schematic
Surface Data
Acquis it ion System
Mechanical
Winching
Drum
Digital
Data
Transmission
Loggingcable
Downhole
Logging Tool
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Logging Runs
A logging run is typically made at the end of each drilled section, immediately
prior to casing being installed. Whilst some tools can make measurements
through steel it is beneficial to record basic information over the open-hole
section in order to maximise data quality and minimise interpretation difficulties.
Figure 4: Wireline Unit
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Each logging run is identified by a suitable alpha-numeric system to record thetype of instrument being used and the actual tools that were run. This is important
for calibration and cost management reasons.
Data Interpretation
Data processing is almost always done by computer, typically in town but
increasingly using modern high powered computers at the wellsite. Basic infor-
mation can be derived by hand using Quick-Look or Shaly Sand methods or by
using relatively simple spreadsheets or other processing software.
Types of Logs
Many different types of logs, measuring various rock properties may be run at
each casing point. Generally the first and intermediate logging runs are per-
formed for lithological evaluation and stratigraphic correlation purposes. Minor
hydrocarbon bearing zones may also be identified, together with possible source
rock information.
Over the main reservoir section the amount of information required is much
greater and a full suite of logs covering lithology, porosity, permeability and
fluid saturations are required.
Additionally there are many other types of tools available for specific purposes,and of helping with the evaluation of cement jobs and other completion opera-
tions. The major logs used for routine evaluation of open hole sections are:
• Lithology Logs
Gamma Ray
Spontaneous Potential
• Resistivity (Saturation) Logs
Laterologs
Induction LogsWave Propagation Logs
• Porosity Logs
Formation Density Log
Neutron Porosity Log
Sonic Log
• Miscellaneous
Caliper
Dipmeter
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Repeat Formation Tester Sidewall Cores
Cement Bond Logs
Measurement While Drilling
Measurement while drilling services have been available since the early 1980s
and provide a means of obtaining petrophysical data in real time during the
course of drilling the well. This can be of significant benefit when compared to
wireline data which is often only available weeks after drilling a particular
section. MWD data is very useful in providing additional geological information
for the wellsite geologist and helping with geosteering applications in particular.
Figure 5: Example Log
Gamma Ray
Caliper
IN10 20
Bit Size
IN10 20
Gamma Ray
API0 150
FEET Resistivity
Induction Deep
OHMM0.2 200
Induction Medium
OHMM0.2 200
Sonic
Sonic Transit Time
US/F140 40
Porosity
DRHO
G/C3-0.75 0.25
Neutron Porosity
PU0.45 -0.15
PEF
0 20
Bulk Density
G/C31.95 2.95
5600
5700
5800
5900
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The logging tools are installed inside special drill collar sections located in theBHA. Powered by downhole turbines or batteries they measure rock properties
whilst the well is being drilled and transmit the data to surface by mud pulse
telemetry. This data is decoded and interpreted at surface on the wellsite and is
available to the drilling engineers and geologists at the same time (and often
earlier) as other drill returns logging information.
The range of MWD applications has been significantly extended and enhanced
over the years and now includes:
• Gamma Ray
• Resistivity
• Density
• Neutron Porosity
• Sonic
In addition MWD tools also provide real time directional survey data and drillingdynamics information, both of which can be vitally important to the successful
drilling of the well.
Borehole Environment
Both Wireline Logging Operations and MWD tools have to be able to work
under a wide range of physical and chemical conditions in and around the bore-
hole. The depth of the hole, bit diameter, borehole erosion, hole deviation, for-
mation temperature, mud weight and type and formation pressures each cause
particular problems to the performance of logging tools. Calibration and correc-
tion for borehole environment variables must be carried out both during and after logging runs in order to ensure that the interpreted results are as accurate as pos-
sible. In most cases it is necessary to make multiple measurements with different
tools and cross-plot the results to try and minimise the various effects on partic-
ular tool response. Once allowance has been made for factors such as borehole
temperature and pressure, the key environment effects controlling interpretation
are:
• Drilling Mud Type
• Mud Invasion Profile
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• Relationship of Pore Water to Mud Filtrate
• Borehole Erosion
• Tool Depth of Investigation
Porosity
One of the most important pieces of reservoir information is porosity. That is, the
amount of void space present in the rock expressed as a percentage of total rock
volume.
N. B. When used in Quick Look calculations, porosity is expressed
as a number between 0 and 1.
For example:
Porosity (φ) = 20% use 0.20
= 8% use 0.08
Effective Porosity is the amount of porosity able to transmit fluid, and is of vital
importance in reservoir evaluation.
Maximum porosity of 48% is obtained in granular sedimentary rocks when per-
fectly spherical grains of the same grain size are packed in cubic mode. With
compaction due to burial grain packing becomes closer and porosities will bereduced to less than 30% in most cases. Where there is significant variation in
grain size and with the addition of matrix or cement, porosity values can be
further reduced.
Permeability
Permeability is the ability of the rock to transmit fluid. It is measured in darcy's
and usually given the notation k. One darcy is the permeability when a fluid of
viscosity 1 centipoise is passed through a 1 cm cube with a differential pressure
Porosity % =Pore Volume
Total Rock Volume-----------------------------------------------
⎝ ⎠⎛ ⎞ 100×
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of 1 atmosphere. Since this is a relatively large unit of permeability most oil field reservoir permeability is expressed in millidarcy's (one thousandth of a darcy).
For granular clastic rocks, grain size is also a key variable in determining rock
permeability along with grain shape and sorting. Larger pore throats will allow
fluid to pass more easily than smaller sized throats.
Both porosity and permeability in carbonates (limestones and dolomites) are less
uniform than in granular clastic rocks, being less to do with transportation and
grain erosion, and more a product of original sedimentary features (grain type
and matrix) and subsequent (often post-depositional) diagenesis. Dolomites are
formed by post-depositional percolation of magnesium bearing fluids whichcauses original calcite (CaCO3) to re-crystallise as dolomite [(Ca.Mg (CaCO3)].
This process normally results in enhanced porosity and is a key factor in the pro-
duction of carbonate reservoirs.
The other major control on porosity in carbonates is fracturing, particularly in
Chalks. Whilst primary porosity of Chalks may be very high, being composed
mainly of highly spherical calcareous grains, (microscopic coccoliths), permea-
bilities may be almost zero because of the very small pore throats. Enhancement
of both porosity and permeability is required for these rocks to become potential
reservoirs. This can be a problem for wireline and MWD interpretation since the
resulting secondary porosity may be too large to be evaluated by the logging tool.
The main controls on porosity in clastic rocks are:
• Size of available pores
• Connecting passages between them
Definitions of Permeability
Absolute Permeability
When the rock is 100% saturated with one fluid
Effective Permeability
The ability to transmit a fluid in the presence of another fluid when the two are
immiscible.
Relative Permeability
The ratio of effective to absolute permeability.
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Permeability from Log Data
Reservoir permeability is not normally available form direct measurement, either
from wireline or MWD tools. Values are computed using mathematical models
which use porosity and irreducible water saturation as a means of deriving the
permeability. Irreducible water saturation is the amount of porosity that remains
containing water in a hydrocarbon bearing zone. Such water is present in isolated
pores not connected to the main permeable flow paths, or left adhered to grains
by capillary action and is not able to be removed from the rock. In certain cases
permeability may be estimated from imaging tools such as NUMAR’s NMRIL,
(Nuclear Magnetic Imaging Log).
Permeability is usually defined from the Darcy formula:
Where:
Q = 1cc volumetric flowrate
µ = 1 centipoise viscosity of flowing fluid
A = 1cm2 cross-sectional area
∆ p = 1 atmosphere/cm pressure gradient
L = 1 cm length of section
A permeability of one darcy is usually much higher than that commonly found;
consequently, a more common unit is the millidarcy, where: 1 darcy = 1000 mil-
lidarcy's
A practical oil field rule of thumb for classifying permeability is:
• poor to fair = 1.0 to 15 md
• moderate = 15 to 50 md
• good = 50 to 250 md
• very good = 250 to 1000 md
p A
LQk
∆×××
=µ
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• excellent = 1 darcy
Reservoir permeability is a directional property. Horizontal permeability (kH) is
measured parallel to bedding planes. Vertical permeability (kV) across bedding
planes is usually lower than horizontal. The ratio kH/kV normally ranges from
1.5 to 3.
When only a single fluid flows through the rock, the term absolute permeability
is used. However, since petroleum reservoirs contain gas and/ or oil and water,
the effective permeability for given fluids in the presence of others must be con-
sidered. It should be noted that the sum of effective permeabilities will always beless than the absolute permeability. This is due to the mutual interference of the
several flowing fluids.
Reservoir Permeability from Log Data
Timur Equation
Morris and Biggs
Where C is a constant as follows:
Gas: 80
Oil: 250
2
441360
irr
md Sw
k .
. φ =
2
3
irr
md Sw
C k
φ =
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Irreducible Water Saturation
This state is reached in hydrocarbon bearing zones when the reservoir will not
produce any water. It depends upon the Bulk Volume Water (BVW) which is cal-
culated from water saturation and porosity:
BVW = Sw x φ
When a zone’s bulk volume water values are constant, then the zone is at Swirr .
This is normally computed from cross-plotting Sw and Porosity on charts which
have hyperbolic lines indicating constant BVW values.
Water Saturation
The fraction of the pore space containing water is known as the water saturation,
and is given the notation Sw. The remaining fraction that contains oil or gas is
known as the hydrocarbon saturation, Sh, and is determined by 1- Sw, where 1 =
100% f.
Sw can be calculated from log interpretation, normally using a combination of
resistivity and porosity data.
Figure 6: Bulk Volume Water
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Formation Temperature
The resistivity of saline solutions is affected by temperature, so that corrections
must be made to raw data whenever the temperature has varied between data col-
lection points. This is particularly true when using Rmf or Rm information in sat-
uration or Rw calculations.
In order to determine formation temperature at any point the Geothermal
Gradient must be known. Unless known to be otherwise, this gradient is
normally assumed to be linear, and is computed from knowledge of Surface
Temperature and Bottom Hole Temperature as recorded from the logging tools.
Surface Temperature
This is an estimated value from offset data or general knowledge of the area. The
following rules of thumb can be applied in the absence of better data:
Offshore (1m beneath sea bed): 35°F: (1.5° C )
Onshore (3m deep): 50°F: (10° C )
Bottom Hole Temperature
BHT is calculated from the results of maximum temperature data obtained
during the logging runs. Since the actual formation temperature is disturbed by
the drilling process and the invasion of mud filtrate into the rock pores, the
maximum measured values may not be accurate. Over time, however, the mud
in the borehole and the invaded zone will tend to equalise to true formation tem-
perature. If this increase in temperature can be measured, (by looking at BHT
values obtained from successive logging runs), the rate of change of temperature
with time can be extrapolated to infinite time and an interpreted true BHT value
can be obtained.
There are many mathematical models available for this interpretation but the
most widely used method is an adaptation of the Horner Plot which was devel-
oped to interpret pressure buildup during formation testing operations.
Geothermal Gradient
Once estimates of Surface Temperature and Bottom Hole temperature have been
made, a geothermal gradient can be established as follows:
BHT Ts –
TVD S – ----------------------- 100× °F /100ft=
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Where:
BHT = Bottom Hole Temperature °F
Ts = Surface temperature °F
TVD = True vertical depth
S = Surface Depth
Invasion Effects
During drilling the mud pressure in the annulus is maintained at a higher level tothe pore fluid pressure in order to prevent fluid incursions and wellbore instabil-
ity. When drilling through permeable formations this means that, with water-
based muds, liquid from the mud passes into the formation displacing original
pore fluids. The solid particles in the mud are left behind and eventually form an
impermeable mud cake which seals the rock and prevents further invasion. The
amount of fluid invasion that occurs is dependent on many factors including mud
properties and rheology, flow rates, differential pressure and rock permeability.
The net result though is to produce an annulus in the rock around the borehole
which contains predominately mud filtrate rather than original pore fluids.
Log interpretation techniques must take this invasion into account, particularlywhen using resistivity tools to locate hydrocarbon bearing zones. If the tool does
not penetrate deeply enough into the rock only mud filtrate may be seen and sub-
stantial hydrocarbon reservoirs may not be recognised. MWD tools can have a
significant advantage in this respect since they log the formation very shortly
after it has been drilled and before invasion has fully developed, whereas
wireline tools may be run weeks after drilling, allowing invasion to run its full
course.
Proceeding outwards from the borehole the following profile is normally estab-
lished:
• Flushed Zone
Formation pore space has been predominately flushed by mud fil-
trate. Irreducible water or hydrocarbons remain in isolated pores or
by capillary action. Water displaces medium gravity oil quite well,
but low gravity oil or light gas quite poorly. In gas reservoirs there-
fore, residual hydrocarbon content in the flushed zone can be quite
high.
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• Transition ZoneSome of the original pore water and hydrocarbons, if present, have
been replaced by mud filtrate but significant quantities remain. The
ratio of mud filtrate to original fluids decreases away from the bore-
hole.
• Uninvaded Zone
This zone is furthest from the borehole and remains undisturbed by
mud filtrate invasion. Pore fluids are 100% original water or hydro-
carbons.
Resistivity Log Profiles
Resistivity Logs with multiple depths of investigation such as Dual Laterologs
or Dual Induction Logs will show variable resistivity profiles across the flushed and invaded zones depending on the relationship of mud water (Rmf) to pore
water (Rw) resistivity.
• Where Rw is greater than Rmf
(salty mud and fresh water pore fluids) the flushed zone will show
lower resistivity values than the invaded and uninvaded zones when
no hydrocarbons are present.
• Where Rw is less than Rmf
Figure 7: Invasion Profiles
R e s i s t i v i t y
Borehole Wall
Rxo
Ro
Step Profi le
dj
Distance
R e s
i s t i v i t y
Borehole Wall
Rxo
Ro
Transition Profile
dj
Distance
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(fresh mud and salty formation water), the flushed zone will showhigher resistivities than the invaded and uninvaded zones when no
hydrocarbons are present.
This invasion profile is normally considered to be a simple step profile for quick
look analysis, but in reality is more complex since the three zones will have tran-
sitional not sharp boundaries. However, assuming a step profile means that three
tools with different depths of investigation are required for full evaluation, in
order to identify and make corrections for the mud filtrate invasion. Figure 1-4
shows different Resistivity Log profiles and also includes the Annulus Profile
which may occur for a short time when hydrocarbons are present. In this casewater may be flushed more easily than the oil or gas and subsequently dumped
ahead of them as a ring or annulus of low resistivity, between the flushed and
uninvaded zones. If present this phenomenon is short lived and the fluids quickly
find equilibrium.
Log Presentation
Wireline Log data is presented as a series of curves representing the continuous
measurement of various parameters. Logs are usually presented as a combinationof several individual tools. Traditional logs might be, for example:
• ISF - Sonic:
Gamma Ray
Deep Induction Resistivity
Spherically Focused Resistivity
Sonic
• Dual Laterolog:
Gamma Ray
Deep Laterolog Resistivity
Shallow Laterolog Resistivity
Micro Spherically Focused Resistivity
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Log Types
There are two major types of logs:
• Acquisition Logs
These logs contain the raw data as measured by the tool. It is often
referred to as the "Field Print " and is an unmodified wellsite log.
• Processed LogsThese are edited logs, subjected to computer processing to correct
for borehole conditions, invasion etc., and may contain the results of
Quick Look Interpretation.
Figure 8: ISF-Sonic Log
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Figure 9: High Resolution Laterolog
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API Presentation
The traditional API presentation of field prints has three tracks separated by a
depth column.
Track 1, to the left, is linear and normally contains Gamma ray, S.P. and Caliper
log data.
Track 2, to the right of the depth column, is usually a 3 or 4-cycle logarithmic
scale used for plotting resistivity data. This might cover the complete width of
the sheet or their may be a third track.
Track 3, on the right, is usually a linear scale and is used for porosity, sonic and
density data.
Log Heading
A log heading is attached to the top of each paper log or film. It includes infor-
mation about the location, rig type, mud properties, calibration and tool type.
Depth Scales
Logs are plotted according to customer requirements and to maintain compati-
bility with other data. Typically they are plotted on a 1:500 or 1:1000 scale,
although this can be varied and detailed sections may be required at scales of 1:200. Indeed with modern computer processing it is possible to generate any
scale for any section of log very easily.
Logging Speeds
The ultimate quality of log data is very much related to logging speed. This is
particularly true for nuclear devices where statistical data is used. If the tool is
pulled too fast not enough data will be recorded to provide accurate information,
especially for thin beds. Normal logging speeds for tools containing nuclear
devices are around 1800 ft/hour (600m/hr).
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Figure 10: Log Header
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Logging Tool Combinations
Early logging tools were required to be run independently and, of course, during
the 1920s and 1930s there were fewer of them to run. By the 1950s and 1960s
the Gamma Ray, S.P. and basic resistivity tools were being supplemented by
Induction and Laterolog devices, sonic, density and neutron porosity tools. Still,
however, only certain combinations were possible and into the 1970s it was usual
to run at least two suites of logs, (resistivity and porosity) to obtain the basic
information followed by sidewall coring and pressure testing and fluid samplingtools.
With the development of Schlumberger’s triple combination tool, and similar
devices from the other leading service providers, it became possible to obtain
resistivity, porosity and gamma ray data from one logging run. The triple combo
tool though, at 90ft long and weighing around 1200 lbs was somewhat unwieldy
and less useful in tough logging conditions of high borehole inclination, severe
doglegs and sticky holes.
Figure 11: Log Presentation
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In the early 1990s Schlumberger developed the Platform Express Service which provides, in a tool only 38ft long, all the data from the old triple combo but using
better, modern sensors and electronics.
The following is a summary of the Baker Atlas and Schlumberger combination
tools:
Baker Atlas
FOCUS , from Baker Atlas, is the latest in high efficiency premium open hole
logging systems. All of the downhole instruments have been redesigned, incor-
porating advanced downhole sensor technology, into shorter, lighter, more
reliable logging instruments, capable of providing formation evaluation meas-
urements with the same precision and accuracy as the industry’s highest quality
sensors, at much higher logging speeds. Logging speeds are up to twice the
speed of conventional triple-combo and quad combo logging tool strings. The
logging system consists of the four standard major open hole measurements
(resistivity, density, neutron, acoustic) plus auxiliary services.
Service Application
• Array Resistivity (FOCUS HDIL) - includes real time 1-D radial inversion
processing for more accurate measurements of Rxo and Rt.
• Nuclear Porosity (FOCUS ZDL & FOCUS CN) - design changes improved
detector response and efficiency at high logging speeds of conventional
instruments, and enable production of a real time nuclear porosity cross-
plot log.
• Acoustic Slowness (FOCUS DAL) - offers an improved monopole signal
resulting in accurate compressional slowness values (Delta t) using a depth
derived borehole compensation technique.
• Auxiliary Measurements - Correlation Gamma Ray (GR), Borehole Temper-
ature, Downhole Tension, Mud Resistivity, Accelerometer (TTRmA), Two
Arm Caliper (TAC).
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Schlumberger
The Platform Express system is less than half as long as a triple-combo and
weighs about half as much, yet it gives you better, quicker and more accurate
answers—in real time. The use of integrated sensors, flex joints that improve pad
contact and other innovative technologies upgrade and expand traditional resis-
tivity and porosity measurements to include high-resolution micro-resistivity
and imaging measurements, plus tool movement measurements for speed correc-
tion and depth matching.
Figure 12: Baker Atlas Focus Log
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Introduction
Wireline Logs LWD Interpretation 1-27
Resistivity measurements are made with either the AIT* Array Induction Imager Tool or the High- Resolution Azimuthal Laterolog Sonde (HALS), both with a 12-
in. maximum vertical resolution.
Sensors for the Three-Detector Lithology Density (TLD) and Micro- Cylindri-
cally Focused Log (MCFL) measurements are integrated in the single pad of the
High-Resolution Mechanical Sonde (HRMS), which presses against the forma-
tion. The TLD log is a backscatter-type density measurement with 16-, 8- or 2-
in. vertical resolution. The MCFL Micro-resistivity measurement, which investi-
gates the same volume of the formation as the density measurement, has 2-in.
vertical resolution. Flex joints greatly improve pad application in rough holes.
The Highly Integrated Gamma Ray Neutron Sonde (HGNS) provides gamma ray
and neutron porosity measurements with a standard vertical resolution of 24 in.
Alpha processing is available to achieve 12-in. vertical resolution of the neutron
log.
Real-time speed correction and automatic depth matching of all measurements
are provided by an accelerometer for much faster turnaround on wellsite
processing.
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Introduction
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Figure 13: Platform Express
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Spontaneous Potential (S.P.) Logs
Wireline Logs LWD Interpretation 2-1
Introduction
The S.P. Log is a measurement of the electrical potential difference between a
moveable electrode in the borehole and a fixed electrode at the surface. It is used
to identify permeable zones and can be a very useful geological correlation tool
under the right conditions. To obtain meaningful results, the log must be run in
a water based mud borehole with a significant variation in mud filtrate and pore
water resistivity. The moveable electrode is attached to the cable, lowered to the
bottom of the borehole and pulled to the surface. Where there is no permeability,
no electrical potential exists between the rock and borehole and nothing is meas-
ured.
Origin of the S.P. Curve
At the bed boundary between a permeable and an impermeable rock, the mud
water and pore water are in contact via two interfaces. Along the permeable bed
the two waters are in direct contact. If there is a difference in salinity between the
two fluids chemical diffusion can take place across the interface. This is the dif-
fusion potential. If the mud water is less saline than the pore water then the +ve
sodium ions will tend to flow more freely to the higher concentration pore fluid
Figure 1: S.P Log
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from the mud, leaving a greater concentration of -ve chlorine ions behind in the borehole.
Across the bed boundary alongside the impermeable shale the shale potential is
effective. Here the chlorine ions are more mobile through the semi-permeable
membrane and tend to leave a higher concentration of sodium ions behind in the
mud. Figure 3-1 illustrates this process, and shows that in this case there are four
quadrants around the bed boundary having opposite electrical charges which
creates the potential for an electrical current to flow. Note that the electrical
potential only exists at the bed boundary and that the current is focussed at the
bed junction. The same situation exists in reverse at the base of the permeable
bed.
The electrical potential at the boundaries between permeable and non-permeable
beds is measured on a millivolt gauge. If the mud water is less saline than the
pore water the reading will be a negative value on the millivolt gauge, and the
deflection across the bed boundary will show as a movement to the left on the
log. If the mud water is more saline than the pore fluid then the movement will
be to the right on the log, indicating a positive deflection. Where the two fluids
have the same salinity, no electrical potential will be measured and no deflection
will be seen on the log curves.
The value in millivolts has no absolute meaning but merely represents a changein electrical potential across the bed boundary. The logging engineer sets the
shale baseline either to the right or to the left of the track depending on the
relative salinities of the mud filtrate and formation water.
Log Presentation
The S.P. data is normally recorded on Track 1 of the log. The track is scaled in
millivolts, usually shown as mv/chart division. Sometimes there may be a full
scale shown such as -140 to +60. In this case there are 200 mv across the full
scale. Movement to the left from the shale baseline is a -ve movement, and
movement to the right is +ve.
Any deflection of the curve away from the shale baseline indicates rock perme-
ability. It is not possible to calculate the actual amount of permeability in Darcys,
nor does the S.P. deflection indicate the amount of permeability. However the
log will show interbedded sections of permeable and impermeable rocks, pick
out bed boundaries and formation tops and enable calculations of bed thickness
to be made.
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Corrections
Corrections need to made to raw data before any quantitative interpretation of
S.P. data is done. In particular thin beds and the presence of hydrocarbons will
cause the S.P. deflection to be under-developed. Also, since a current flows
around the bed boundary, the amount of energy stored in the system is dimin-
ished, resulting in lower S.P. deflections than might otherwise be the case. In practice, corrections to bed thickness should be made for sections less than 10ft
(3m) thick.
Log Characteristics
The ideal response would be a sharp, histogram type, curve as the change from
permeable to non-permeable beds was recognised. However, the tool is moving
and a current is flowing, both of which contributing to a spreading of the current
patterns and a diffusion of the curve. Bed boundaries are normally attributed to
Figure 2: Origins of the S.P curve
+++
+++------
Relative excesscharge
+
Formationwater
Mudfiltrate
Lower salinityHigher salinity
Shale
Sandstone
Formation Borehole
millivolts- +
S.P. Log
-
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the inflexion point of the curve. That is the straight part of the curve as the con-cavity changes direction.
The amount of deflection is reduced from its ideal response, the Static Spontane-
ous Potential (or S.S.P.), by the current flow and also in thin beds less than about
10ft (3m) thick. Algorithms and charts are available to make corrections for these
effects when performing quantitative analysis. The presence of hydrocarbons
will also reduce the current potential.
Quantitative Analysis
The S.P. Log is mainly used for qualitative interpretation of geology and for
inter-well correlation. The curves are generally very repeatable across the same
sequence and provide a tool similar in scope to the Gamma Ray Log. One major
quantitative use however, is in the calculation of Rw (formation water resistiv-
ity). This value must be known in order to make saturation calculations. It can be
measured from RFT samples or calculated from log analysis. The S.P. data
provides a means of performing this calculation, and can act as useful back-up
data if other methods are not available.
The amount of movement, in mv, of S.P. deflection away from the shale baselineis directly related to the difference in resistivity between the mud filtrate and the
pore water. Since the deflection can be read from the log and a value for Rmf,
(resistivity of mud filtrate) can be measured from a mud filtrate sample, the cor-
responding value of Rw can be calculated. Calculations of Rw are made in a zone
100% saturated with water, i.e where Sw = 1.0, as near as possible to the hydro-
carbon bearing zone being investigated. Rw is assumed to be constant through-
out the reservoir section.
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Figure 3: Schlumberger Chart SP-1
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Figure 4: Schlumberger Chart SP-2
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Gamma Ray Logs
Wireline Logs LWD Interpretation 3-1
Gamma Ray Log
The gamma ray log is a measurement of the natural radioactivity of a formation,
and is most often used as a shale indicator and for general geological correlation.
It is also used for depth matching of different suites of logs run at one casing
point.
The spectral gamma is used to provide more petrological information including
mineral suites, radioactive volumes and depositional environments.
On typical field prints, the Gamma Ray curve is located in Track #1, with scale
deflections in standard API units on a linear grid.
Most vendors use the mnemonic GR to represent the standard tool, though with
some variation.
Schlumberger:
• NGT: Natural Gamma Ray Tool
• NGS: Natural Gamma Ray Spectrometry
• HGNS: Highly Integrated Gamma Neutron Sonde (Platform Express)
Halliburton:
• HNGR: Hostile environment Natural Gamma Ray
• CSNG: Compensated Spectral Gamma
• PSG: Pulsed Spectral Gamma Ray
Baker Atlas:
• GR: Gamma Ray
• Focus-GR: Focus service Gamma Ray
Natural Gamma Ray
This log measures and records the natural radioactivity within a formation. Some
rocks are naturally radioactive because of the unstable elements contained in the
formation. Generally, three elements contribute the major portion of the radia-
tion observed in sedimentary rocks: the uranium series, the thorium series and
the potassium-40 isotope. The Gamma Ray log usually reflects the clay content
of sedimentary formations. Clean sands and carbonates normally exhibit a low
level of natural radioactivity, while shales tend show higher radioactivity.
However, not all shales are radioactive and not all radioactivity represents shales.
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Figure 1: Gamma Ray Log (Reeves Wireline)
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Natural Gamma Ray Spectral Log
The spectral log breaks the natural radioactivity of the formation into the differ-
ent types of radioactive material: thorium, potassium or uranium.
This can be used for stratigraphic correlation, facies identification, reservoir sha-
liness determination and sometimes for fracture identification.
Advantages of the Gamma Ray Log
• It is useful as a correlation tool
• It is used for depth control
• The major tool used for shale content calculations
• It may be run in casing, empty holes and in all kinds of drilling fluids.
Figure 2: Spectral Gamma ray Log
(Reeves Wireline)
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Limitations of the Gamma Ray Log
• Traditionally the GR tool must be logged at relatively low speeds
(1800 ft/hr) to give accurate bed definitions. Some newer tools are
extending this to nearer 3600 ft/hr.
Radioactivity
Radioactivity is a spontaneous disintegration of atomic nuclei by the emission of
subatomic particles:
• alpha particles
• beta particles
or of electromagnetic rays
• X rays
• Gamma rays
Gamma Rays
The phenomenon was discovered in 1896 by the French physicist Antoine Henri
Becquerel when he observed that the element uranium can blacken a photo-
graphic plate, although separated from it by glass or black paper.
In 1898 the French chemists Marie Curie and Pierre Curie deduced that radioac-
tivity is a phenomenon associated with atoms, independent of their physical or
chemical state.
The Curies measured the heat associated with the decay of radium and estab-
lished that:
• 1 g (0.035 oz) of radium gives off about 100 cal of energy
every hour
This heating effect continues hour after hour and year after year. The complete
combustion of one gram of coal results in the production of a total of only about
8000 cal of energy.
Embedded in a nucleus, a neutron is usually stable—that is, it will not decay into
a proton and an electron. The nucleus itself is then stable. However, if the nuclear
conditions are not optimal, for example if the nucleus has too many neutrons, one
or more of the neutrons may decay to produce gamma rays.
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Carbon 14
Every carbon atom contains six positively charged particles, (protons), in its
nucleus and six or more neutral particles, (neutrons).
The carbon atom's nucleus is surrounded by six negatively charged electrons.
The number of neutrons in a carbon atom's nucleus determines its isotope: atoms
of the same element that have different numbers of neutrons in the nucleus.
Carbon Dating
Three different isotopes of carbon exist naturally:
• Carbon-12 contains six protons and six neutrons and represents98.89% of all carbon
• Carbon-13 contains six protons and seven neutrons and represents
1.11% of all carbon
• Carbon-14 contains six protons and eight neutrons and represents a
negligibleamount of all carbon.
Carbon-14 is in a constant state of decay but, as long as an organism is alive,
ingesting more carbon, the balance between carbon-12 and carbon-14 remains
stable. When the organism dies, however, new carbon is not being taken in, and
so, as the carbon-14 decays, the ratio of carbon-12 to carbon-14 changes. Thehalf-life of carbon-14 is 5,730 years. This means that, after 5,730 years, half of
the carbon-14 will have gone. Therefore, the year of death of an organism can be
calculated from the proportion of carbon-14 left in a sample taken from its
remains. Although the proportion of carbon-14 has varied significantly during
the history of the Earth, correction tables have been developed to compensate for
this. In samples older than about 50,000 years, there will be insufficient carbon-
14 left to provide reliable results, and, conversely, recent samples will show too
little decay to provide reliable results.
Sources of Gamma radiation
As mentioned above, natural radiation from rocks comes from three sources, K,
U and Th. Whilst potassium (K) is the most abundant of the three elements in
rocks it produces less radiation than U or Th which, in relation to their weights,
produce more.
Gamma emission is usually found in association with alpha and beta emission.
Gamma rays possess no charge or mass, thus emission of gamma rays by a
nucleus does not result in a change in chemical properties of the nucleus but
merely in the loss of a certain amount of radiant energy. The emission of gamma
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rays is a compensation by the atomic nucleus for the unstable state that followsalpha and beta processes in the nucleus.
The energy emissions occur in the range of 0-3 MeV, and the elemental origins
are determined by their peak frequencies within this range. The radiation from40K is distinct at 1.46 MeV. Thorium and Uranium produce radiation over a
wider spectrum but Th has a distinct peak at 2.62 MeV and U at 1.7 MeV. This
is the methodology used in spectral analysis to identify the source of radiations.
Radiation Detectors
The Gamma Ray Tool, which was introduced into the oil field in 1939, measuresnatural radioactivity of formations penetrated by the wellbore. Detection is
accomplished by the ability of gamma rays to produce tiny flashes of light in
certain crystals, which are then converted into electrical pulses. The pulse size is
dependent on amount of energy absorbed from the gamma ray.
The main types of detector are:
• Ionization Chamber
• Geiger-Mueller Tube
• Scintillation Counter
Ionization Chamber
This is a gas filled chamber with an anode maintained at approximately 100 volts
positive with respect to the housing. The case is filled with high pressured gas.
An incoming gamma ray interacts with the detector wall material and/or gas
which releases an electron. The freed electron moves toward the anode through
the dense gas. Electron interactions with gas atoms release additional electrons
(the ionization process). As the free electrons are drawn to the anode, a minute
current is produced, making the gamma ray influx into the borehole proportional
to the amount and magnitude of current pulses produced at the anode.
Geiger-Mueller Tube
The Geiger-Mueller counter is similar to the ionization chamber, but has much
higher voltages and a lower gas pressure. The initial reaction is much the same
as that of the ionization chamber; however, the high positive voltage (1,000
volts) at the anode causes the free electron to be fast moving as it collides with a
gas atom, discharging additional electrons. The secondary electrons are drawn
rapidly toward the positive wire which causes additional collisions resulting in
many more electrons reaching the anode in pulses which are more easily
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detected. This ionization must be stopped or quenched because the cumulativeelectron showers can damage the detector. Quenching is achieved by lowering
the anode voltage.
Scintillation Counter
The most modern logging detector is the scintillation counter. It has two basic
components, a scintillating crystal and a photo multiplier tube. The transparentsodium-iodide crystal (NaI) will give off a minute burst of light when struck by
a gamma ray. The light energy strikes a photo sensitive cell or cathode which
causes electron emission. The electrons so produced are drawn to an anode
which, upon impact, releases additional electrons which are directed to another
anode. There are several stages of such amplification which finally give a suffi-
cient flow of electrons to be easily measured and recorded as an indication of the
gamma radiation penetrating the detector.
Figure 3: Geiger-Mueller Tube
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Radius of Investigation
Ninety percent of the measured gamma rays originate with the first six inches of
the formation being investigated. The addition of another medium (i.e., cement
or casing) reduces the total quantity of gamma rays, but does not detract from the
usable information. With the proper speed and time constants, adequate resolu-
tion can be achieved in formations as little as three feet thick.
Formation boundaries are located at the mid-point of the recorded curve.
Units of Measurement
Gamma radiation is measured from the various detectors as discreet pulses of
electricity representing individual gamma ray “hits”. These are counted and
Figure 4: Scintillation Counter
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averaged over a time period and may be reported in a number of ways includingBecquerels and Curies. However in borehole logging API Gamma Ray Units are
mostly used.
This relates to a test borehole at the University of Houston, Texas. The well is
surrounded by special high and low radioactive concrete. One API unit is 1/200th
of the difference in radioactivity measured in the two sections of concrete. “Reg-
ular” shales having a radioactive content of about 2.7% will exhibit values of
around 100 API units assuming the same operating conditions, (8½” hole, water
based mud etc.) are used. Obviously this varies with changing tool and borehole
environmental conditions and formation mineralogy.
Uses of the Gamma Ray Log
As discussed above, the gamma ray tool is used to:
• Identifying lithologies
• For correlation and depth matching
• For calculating shale volume
Lithology Determination
Radioactive isotopes of K, Th and U are the source of the gamma rays. These are
present in various minerals, particularly clay minerals. However, some evapor-
ites, for example, are also rich in K, and igneous and metamorphic rocks are very
radioactive. For Th and U content. Sands and carbonates whilst lacking radioac-
tive minerals in their pure forms can have significant amounts of associated
gamma producing minerals.
The heavy radioactive elements tend to concentrate in clays and shales. Gamma
rays (bursts of high energy, electromagnetic waves) are statistical in nature. This
means that the number of gamma rays received by the detector will fluctuate,
even when the instrument is stationary in the hole. These statistical variations are
averaged out.
Occurrence of Potassium (K)
Clay Minerals:
Illite 5.20%
Glauconite 4.5%
Kaolinite 0.63%
Smectite 0.225%
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Evaporites:Sylvite 52.5%
Carnallite 14.1%
Polyhalite 12.90%
Muscovite Mica
Biotite Mica
Orthoclase Feldspar
Occurrence of Uranium
Origin: Acid Igneous Rocks
Preserved in: Reducing Conditions
Black Shales
Distribution: Erratic Peaks
Occurrence of Thorium
Origin: Acid and Intermediate Igneous Rocks
Preserved as: Detrital Grains
Zircon, Thorite, Epidote
Clay Minerals:Bauxite, Kaolinite, Illite, Smectite
The contribution to the overall radioactivity of the three elements is fundamen-
tally the same although, because of the variation in energy, a small quantity of
uranium has a large effect and a large quantity of potassium has a small effect.
The radiation from 40K has a single energy value of 1.46 MeV. Uranium and
thorium emit radiations over a wide spectrum but with some distinct peaks; 2.62
MeV for thorium and 1.7 MeV for uranium. As the gamma rays pass through the
formation, drilling mud and steel of the tool before hitting the detector their
energy levels will be degraded by Compton Scattering; however, the three peak
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values noted above are usually distinct and form the basis of the spectral gammaray detector.
Quantitative Interpretation of Gamma Ray Logs
The Gamma Ray Log can be sued to give a quantitative assessment of clay
content of sandstone reservoirs in order to aid porosity and saturation calcula-
tions. Neutron log porosity values will be incorrect where there is significant clay
content in a sandstone because of the contained hydrogen within some clay
minerals such as Smectite.
Shale volume (Vsh) calculations begin with determining the Gamma Ray Index(IGR ).
where:
IGR = Gamma Ray Index (dimensionless)
GR = Gamma Ray Reading of Formation
GRmin = Minimum Gamma Ray (clean sand or carbonate)
GRmax = Maximum Gamma Ray (shale)
The calculated IGR is then used on the appropriate chart or determined
mathematically using:
Consolidated - Older rocks
Unconsolidated - Tertiary Rocks
IGR GR GRmi n –
G Rm ax G Rm in – -------------------------------------------=
V Sh
0.33 22 I GR×( )
1.0 – [ ]=
V Sh
0.083 23.7 I
GR×( )
1.0 – [ ]=
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Resistivity Logs
Wireline Logs LWD Interpretation 4-1
Introduction
Resistivity logs were the first tools to be developed for wireline logging opera-
tions, and remain amongst the most important. They are also referred to as
Saturation Logs since their primary aim is to help with hydrocarbon evaluation.
The main uses of resistivity logs are:
• Identification of Hydrocarbon Bearing Zones
• Quantification of Hydrocarbon Saturation
• Identification of Permeable Zones
• Calculation of Diameter of Invasion
• Calculation of Porosity
Where Sw = 1.0
Resistivity tools measure how easy it is for an electrical signal to pass through
the formation. Rock grains and hydrocarbons are both insulators so the only
conductive part of the formation is salty water in the pore space. Hence, a porous
rock saturated with salty water will have low resistivity while the same rock
containing hydrocarbons will have a higher resistivity. High resistivity may also
indicate a low porosity rock, even if water saturated.
The log may also be used for geological correlation and, in association with other
petrophysical data, to help with lithological identification, environments of
deposition, facies analysis and overpressure detection.
Types of Resistivity Tools
The major types of resistivity tools are:
• Electrode Logs (conductive drilling fluids)
Normal Devices
Lateral Devices
Laterologs
Spherically Focused Logs
• Induction Logs (non-conductive drilling fluids)
• Micro Resistivity Logs
• Electromagnetic Wave Propagation LWD Tools
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Log Presentation
Most modern Resistivity Logs are plotted in track 2 on a typical field print under
a logarithmic scale. The units of measurement of resistance are ohms. Resistivity
is measured in ohm-m2/m (ohm-m). In order to accommodate a sufficient range
of values a logarithmic scale of 0.2 - 2000 ohm-m is normally used, with a back
up scale of x10, (2 - 20000). Older, Normal or Lateral Logs were displayed on a
linear scale plot.
Figure 1: Dual Laterolog
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Electrode Logs
Normal Tools
The first electric logs were called Normal Tools. A current is passed between two
electrodes (A & M) on the logging tool and the potential drop between them
indicates the resistivity.
Tool depth of investigation is a function of the distance between the two elec-
trodes on the tool. The larger the distance between electrodes, the deeper the
depth of investigation. Thus typical configurations were the 16" Short Normal
and the 24" Normal. The 16" Short normal was the basic tool and allowed inves-tigation of the invaded zone around the borehole.
Figure 2: Normal Electrode Logging Tool
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To penetrate deeper into the formation and have a greater chance of measuringthe true, undisturbed formation resistivity (Rt), the 18’ 8" (5.68m) Lateral Log
was used. This large distance between electrodes was achieved by varying the
position of them and providing guard, or bucking, electrodes to focus the current
and force it to travel laterally from the tool rather than in a spherical nature,
resulting an a far deeper depth of investigation.
Using two tools with different depths of investigation enables evaluation of the
invaded zone to determine the extent of mud filtrate invasion and its affect on
formation resistivity. If three tools are used with different depths of investigation
then the diameter of invasion can be determined and corrections made for calcu-
lating true formation resistivity, which may still not be measured correctly by thedeepest reading tool where the amount of flushing is very large.
Figure 3: Lateral Electrode Logging Tool
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Laterolog
The modern electrode log is called the Laterolog and is a refinement of the long
spaced Lateral log described earlier. It attempts to do the same job by further
refining the focused current with the use of even stronger guard electrodes to
ensure that the current is emitted laterally from the tool and penetrates far into
the formation.
One of the main reasons for the development of the Laterolog was to produce a
tool capable of giving good results in very saline water based systems. Obviously
in this case, the easiest route for the emitted current to take is to travel straight up
the borehole through the conductive drilling mud. No formation resistivity meas-
urements would be obtainable. The Laterolog minimises this process and results
in formation measurements being made.
Typically two Laterologs with different depths of investigation have been run
alongside each other. The LLD is a long spaced tool for measuring Rt, or close
to it depending on the extent of invasion. LLS is a medium spaced tool which
measures the resistivity of the invaded or transitional zone. These Dual Later-
ologs (DLL) are combined with a short spaced tool (Micro Resistivity) for
measuring the flushed zone. When the three readings are combined, full evalua-
tion may be made of the extent of fluid invasion and calculations made for
Diameter of Invasion and a correction factor for estimation of true formation
resistivity, Rt.
Modern laterolog tools have multiple transmitters and receivers to produce an
array of resistivity measurements with different depths of investigation and
vertical resolution.
Deeper investigating devices are usually centred in the borehole while the shal-
lowest reading tools designed to measure Rxo are mounted on a pad forced up to
and touching the borehole wall.
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Figure 4: Laterolog Tool
A2
A1
M2
M1 A0
M'1M'2 A'1
A'2
Rxo pad
28ft
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Baker Atlas HDLL
The Baker Atlas High Definition Laterolog provides up to eight resisitivity
measurements from 10”- 50” depth of investigation. This provides:
• More accurate formation resistivity, water saturation, and reserves estimates
• Better determination of movable fluids and recovery factor
• Improved evaluation of thinly bedded reservoirs
• Superior measurements in deeply invaded formations
• Detailed evaluation of the drilling fluid invasion profile
Schlumberger HRLA
The Schlumberger High Resolution Laterolog Array Tool provides five resisi-
tivity measurements together with a Micro-Cylindrically Focused Log (MCFL)
for flushed zone resistivity, Rxo for invasion profiling and Rt determination.
Figure 5: Schlumberger HRLA Log
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Induction Logs
Induction logs were developed to obtain readings in non-conductive drilling
fluids, such as fresh water or oil based muds.
Transmitter coils produce magnetic fields by passing an AC current around
them. These magnetic fields induce electrical currents to flow in the formation
which in turn produce secondary magnetic fields. These are detected by the
receiver coils, their strength being proportional to the induced current flowing in
the formation. In this way the non-conductive fluid is by-passed and normal
resistivity measurements can be made. In fact the primary measurement made by
the tool is conductivity, which is converted to resistivity for log presentation.
This does mean that in heterogeneous formations the tool tends to give a slightly
low apparent resistivity value since the induced current swill be travelling
through the most conductive part of the rock.
Several transmitter and receiver coils are used to focus the current and to provide
multiple depth of investigation curves. These are given notations such as 6FF40,
which refers to 6 coils and an effective tool spacing of 40". As with Laterologs,
the longer the spacing the deeper the depth of investigation.
In general, induction logs tend to saturate out at lower resistivity values than
laterologs so are less happy in high resistivity environments but tend to give
better estimates or Rt with deep invasion.
Figure 6: Induction Tool
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Schlumberger Array Induction Tool (AIT)
The Schlumberger AIT uses eight induction coil arrays operating at multiple
frequencies to produce a set if five resistivity logs with 1ft vertical resolution and
progressive radial investigations from 10”-90”.
Baker Atlas Focus High Definition Induction Log (HDIL)
The Baker Atlas Focus High Definition Induction Log also provides a set of five
resisitivity logs from 10”-90” depth of investigation, running at frequencies from
10-150 kHz.
Micro Resistivity Logs
Micro Resistivity Logs are special tools developed to measure the resistivity of
the flushed zone. They consequently have a very small depth of investigation,
usually a matter of centimetres, which is achieved by having very short spacing
between the electrodes. There are a number of different types of Micro Resis-
tivity tools; their use is dependent on the type of information required and their
compatibility with other tools. The following is a list of the most common types
of Micro Resistivity Logs although with modern tools such as the Schlumberger
Platform Xpress and Baker Atlas Focus service these are normally integrated
into the main suite of tools.
• Microlog (ML)
• Microlaterolog (MLL)
• Proximity Log (PL)
• Micro Spherically Focused Log (MSFL)
All of these logs have very short spaced electrodes for evaluation of the flushed
zone, but they are arranged in a slightly different manner. All of the micro logs
are pad mounted devices, which means that the array of electrodes are mounted
on a pad which is forced up to the side of the borehole by a spring loaded arm,making direct contact with the mud cake or borehole wall.
Microlog
The Microlog is unique in that it produces two curves which, whilst both only
penetrating the flushed zone, have slightly different depths of investigation. The
three electrodes are arranged so that there are two sets of spacing, a 1" and a 2"
set. The longer set enables a deeper penetration than the shorter.
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The two curves are called the micro normal (2") and the micro inverse (1"). Sinceone curve penetrates deeper in to the flushed zone than the other it is less affected
by the resistivity of the mud cake than the other.
The overall effect is that, in the presence of mud cake, the two curves show
different values of resistivity and the traces on the log move apart. Where there
is no mud cake present, the two curves will show the same values and overlay
each other.
Separation of the curves will always indicate the presence of rock permeability
since no mud cake build up will be seen alongside impermeable rocks and
therefore the two curves will overlay each other.
The Microlog is a very old tool however, and seldom run in modern applications.
Its primary use was in evaluating very thin interbedded sand/shale sequences
where the sand laminations and thin beds could be quantitatively measured from
the nature of the Microlog. The sand count is the overall amount of sand in the
reservoir section being evaluated. Most of this application was relevant to certain
plays in the Gulf Coast area of the USA.
Micro Spherically Focused Log
The MSFL is the only micro resistivity log that may be combined with other
resistivity tools and run at the same time. The other micro logs need to be run asindependent logs and are thus very expensive. The MSFL is usually the only
micro log that is used in modern logging operations.
Embedded in an articulated neoprene pad, pushed up against the borehole wall
by a spring loaded arm, are a series of concentric metal rings containing the elec-
trodes. The arrangement is similar to the Laterolog but the focusing ensures that
only a few cms depth of investigation is achieved. By comparing the MSFL with
the shallow and deep Laterolog or induction log the diameter of invasion can be
calculated and a correction factor for Rt established. Because of the influence of
the mud cake on the Microlog readings, true resistivity of the flushed zone (Rxo)
can only be obtained after mathematical correction for the effect the resistivityof the mud cake.
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Logging While Drilling Tools (LWD)
The earliest LWD tools used version of the traditional 16” Short-Normal tool for
resistivity measurements. This was a simple, tried and trusted tool with a shallow
depth of investigation. Since LWD tools log the well within minutes of being
drilled it was thought that invasion would not be a significant factor and therefore
a deep reading tool would not necessarily be required. However, being an
electrode type device it will only work in conductive, salty water drilling fluids.
In actual fact invasion can be an issue even with LWD tools since invasion can
happen ahead of the bit even before the section has been drilled and, with resis-
tivity tools often many metres behind the bit, slow drilling can result in signifi-
cant invasion. Additionally the need to run LWD tools with oil based mud precludes the use of short-normal devices.
In order to overcome these issues LWD Electromagnetic Wave Propagation
resistivity tools have been developed. These are similar to the Induction tools
used in wireline logging but work at higher frequencies and are able to offer
multiple depths of investigation, including deep reading devices for estimates of
Rt and better vertical resolution. Typical wireline induction tools work at 20
kHz, for example.
Figure 7: Micro Spherically Focused Log
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The tool broadcasts a constant frequency propagation signal (either 2 MHz or 400 kHz) from the transmitting antennas into the formation. The signal travels
through the formation and is picked up by the receiving antennas. The resistivity
of the formation produces changes in the electromagnetic wave form: the wave
amplitude is attending and the phase is shifted as it passes through the rock. The
receiving antennas are able to measure these changes and the formation resis-
tivity is determined from both effects. EMR tools are able to work in all mud
types.
Wave propagation tools therefore provide, as a minimum, two resistivity curves:
• Amplitude Attenuation (Deep)
• Phase Shift (Shallow)
Figure 8: EMR Theory of Operation
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Some generalities regarding EMR measurements are:
• Tools measure more accurately in conductive media
• Improved vertical resolution in conductive media
• Depth of investigation increases with increasing formation
resistivity
• Depth of investigation is deeper for the 400 kHz resistivities
than the 2 MHz resistivities
• Depth of investigation for attenuation resistivities is deeperthan phase difference resistivities
• Depth of investigation for long spaced resistivities is deeper
than for short spaced resistivities
• Depth of investigation for ratio and difference resistivities
is deeper than for raw measurements
• Depth of investigation order is as follows:
400 kHz >Rat 2> MHz >Rat 400 kHz> Rpd > 2 MHz Rpd
long spaced > short spaced attenuation > far amplitude > near amplitude
phase difference > far phase > near phase
• • Vertical resolution is better for 2 MHz resistivities than
for 400 kHz resistivities.
• Vertical resolution is better for phase difference
resistivities than attenuation resistivities.
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Figure 9: 2 mHz Radial Response
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Interpretation Concepts
Lithology Determination
By themselves resistivity tools are unlikely to define lithology directly. However
the resistivity response can indicate certain features and curve styles can help
with facies and environmental analysis.
Shales tend to have low - medium resistivity values (depending on clay miner-
alogy), perhaps around 1-2 ohm-m. Non-porous rocks such as coal and evapor-
ites will have high resistivities.
Deeper reading tools have large spacing between the transmitters and receivers
and will only pick out gross formation characteristics. Shallower reading tools
and micro-resistivity devices will show more detail in finely bedded shaly sand
sections and may pick out other texture-related features.
Separation of array resistivity tools will indicate invasion and, therefore, perme-
ability. Non-separation of curves may indicate that the rock is tight or it may be
porous and have been invaded with similar fluid. For example in a water
saturated zone when Rw is similar to Rmf.
Figure 10: 400kHz radial Response
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Curve behaviour and trends may be useful for identifying grain size or clay-content variations where, again, the micro-tools will give more detail.
Fluid Saturation
In order to use the results of resistivity logs for quantitative saturation calcula-
tions the data must be combined with porosity and lithology information, since
this will also affect resistivity. Areas of high resistivity are possible hydrocarbon
bearing zones because oil and gas are effective insulators of electrical activity;
but only if the rock is porous.
If an increase in resistivity is caused only because of an increase in hydrocarbon
saturation then the amount of resistivity change can be used to estimate fluid
saturation. This is the basis of the quantitative analysis first proposed by Archie
in 1942 and used, albeit with modifications and enhancements, since.
Definitions
The overall, bulk rock, resistivity in the uninvaded zone is called Rt. It is
produced by the passive rock framework mineral and grain structure and by the
resistive or conductive pore fluids. Rt is derived from the deepest reading resis-
tivity tools but the apparent Rt values read directly from the log will often need correction for the effects of deep invasion by conductive drilling fluids.
The same bulk rock resistivity of the flushed zone is called Rxo and is measured
directly by the micro-resistivity tools.
The resistivity of the natural, or connate, water in a porous formation is called
Rw. This is determined by direct measurement of fluid samples obtained from
testing or by calculation from resistivity and porosity data.
The invaded zone primarily contains water from the drilling fluid, called mud
filtrate, and the resistivity of the zone is called Rmf .
When a formation is 100% water saturated with water of resistivity Rw its resis-
tivity, Rt , is termed Ro. The ratio of Ro/Rw is called the Formation Resistivity
Factor, F. The value of F, in water saturated formations, is independent of the
resistivity of the water with which it is saturated and varies only with porosity.
The value of Ro can be determined from:
Ro = F x Rw
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Geosteering Applications
Logging While Drilling (LWD) resistivity tools can be very useful in geosteering
applications. Near-bit resistivity measurements, such as the Schlumberger RAB
tool, can indicate lithology and fluid changes whilst the variable depths of inves-
tigation of MPR tools can indicate distance to bed or distance to fluid contacts
when drilling ERD or horizontal wells. Drilling pilot holes and detailing
modelling of expected resistivity responses will need to be done to make best use
of the technology.
Figure 11: EMR Log
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Introduction
The Formation Density Log provides information on bulk formation density,
(ρ b). The Litho-Density Log with additional photo-electric absorption curvegives information about matrix type which can be a valuable aid in geological
interpretation and correlation.
The log is used quantitatively as a porosity tool, but is also useful in formation
pressure evaluation and rock mechanics work. It can also provide, indirectly,
information about hydrocarbon density.
Principle of Operation
This nuclear device measures electron density from which bulk density is
derived. The data is plotted on a linear scale as gm/cc, with each chart division
normally representing 0.05 gm/cc.
Collimated Gamma Rays
Collimated Gamma Rays are emitted from a chemical source such as Caesium-
137, with a high energy level of around 1.5 Curie. This is one of the radioisotopes
of caesium with an atomic mass of 137 and a half-life of around 30 years. It is an
artificial radionuclide which was released into the stratosphere by the above
ground testing of thermo-nuclear weapons in the 1950s and 1960s and deposited
as fallout.
The emitted particles are interfered with by electrons in the formation and
gradually lose energy. The rate of energy loss is an indication of electron density,
and can be measured at different energy levels. After initial pair production,
Compton Scattering is the dominant energy reducing process. This is similar to
the interaction of snooker or pool balls colliding sequentially and losing energy
as they do so and represents the mid-range energy levels.
Eventually, at very low energy levels, remaining gamma rays are absorbed by
mineral particles in a process called Photo-electric Absorption, (Pe). Pe ismeasured in barns/electron and each mineral has a particular Pe coefficient,
which is very nearly unique. Analysis of Pe values, which are recorded on the
Litho-Density Log, can help in identification of rock matrix when cross-plotted
against sonic, density or neutron porosity data.
Compton Scattering
Some energy from the gamma ray is imparted to an orbital electron of the target
atom resulting in a freed electron and a gamma ray of reduced energy and change
of direction.
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The number of scattered gammas available for detection depends on the electrondensity, ρe, of the material through which they have passed and the ability of anatom to scatter gamma rays increases as the number of electrons in its orbital
shells (i.e. atomic number Z) increases. Since Z/A approximates to 1/2 for most
materials the electron density ρe can be estimated as:
The normal calibration standard is done using limestone and fresh water filled
porosity which then means that the estima