Electrical Well Logs: SP and Resistivity Logs Explained

Science Benha
Faculty ofscience
Benha University
Geophysicsdepartment
Fourthlevel
2013
Electrical well logging
Resistivity and Spontaneous Potential
Abdel Aziz Hamed

Electrical Well Logs
AbdelAzizHamedElectrical Well Logs
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Content Page No
What is the logging…………………….………………..……….……………..….. (2)
What is the wire line logging……………………………..….…………………. (2-3)
Why log…………………………………………………………………..………… (5- 7)
Uses of logs……………………………………………………………..……………(8)
Pre face of electrical logs
SP…………………………………………………………………..………..… (9-10)
Resistivity……………………………….…………………………..……… (11-14)
Spontaneous Potential Log
Uses of SP Log……………………………………………………....……(15-16)
Applications of SP Log………………………....………………………. (17-23)
Instrumentation……………………….……………………………………… (24)
Calibration and standardization……………………………………………(24)
Radius of Investigation ……………………………………...….........….… (25)
Unwanted Effects ……………………………………...……….…...….... (26-27)
Factors affected on SP Deflection …………………………………………(28)
Quantitative Uses of SP ……………………………………………….... (29-30)
Limitation ……………………………………………...……………...……… (31)
ResistivityLog
Resistivity……………………………………………..…………...…..... (32-33)
Formation Factor ……………………………………….....………...…. (33-34)
Application of Resistivity……………………………………………….... (34)
Zone of Invasion and Resistivity ……………………………………..….. (35)
Factors affecting on Resistivity…………………………………….…. (36-37)
ResistivityTools………………………………………….………….……(37-43)
Quantitative Uses of ResistivityLog …..……………………….…….. (44-45)
Qualitative Uses of ResistivityLog …………………......................... (45, 47)
References………………………………………………………...………………... (48)

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What is Logging?
The birth of logging can be dated to the first recorded event [1] at
Pechelbronn on September 5, 1927 where H. Doll and the Schlumberger
brothers (and a few others) made a semi continuous resistivity
measurement in that tired old field in Alsace.
The operation was performed with a rudimentary device (a sonde)
consisting of a Bakelite cylinder with a couple of metallic electrodes on
its exterior. Connecting the device to the surface was a cable/wire, thus
providing us with the term wire line Logging. Wire line refers to the
armored cable by which the measuring devices are lowered and retrieved
from the well and, by a number of shielded insulated wires in the interior
of the cable, provide for the electrical power of the device and a means
for the transmission of data to the surface. More recently, the devices
have been encapsulated in a drill collar, and the transmission effected
through the mud column.
This procedureis known as logging while drilling (LWD).
What is Wireline Logging?
The process oflogging involves a number of elements, which are
schematically illustrated in Fig. 1.1. Our primary interest is the
measurement device, or sonde. Currently, over fifty different types of
these logging tools exist in order to meet various information needs and
functions. Some of them are passive measurement devices; others exert
fsome influence on the formation being traversed. Their measurements
are transmitted to the surface by means of the wire line.
Much of what follows in succeeding chapters is devoted to the basic
principles exploited by the measurement sondes, without much regard to
details of the actual devices. It is worthwhile to mention a few general
points regarding the construction of the measurement sondes.
Superficially, they all resemble one another. They are generally
cylindrical devices with an outside diameter on the order of 4 in. or less;
this is to accommodateoperation in boreholes as small as 6 inch. in
diameter. Their length varies depending on the sensorarray used and the
complexity of associated electronics required. It is possible to connect
a number of devices concurrently, forming tool Strings as long as 100 ft.
Some sondes are designed to be operated in a centralized position in the
borehole.
This operation is achieved by the use of bow-springs attached to the
exterior, or by more sophisticated hydraulically actuated “arms.” Some
measurements require that the sensor package (in this casecalled a pad)
be in intimate contact with the formation.

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This is also achieved by the use of a hydraulically actuated back-up arm.
Figure 1.2 illustrates the measurement portion of four different sondes.
On the right is an example of a centralized device which uses four
actuated arms. There is a measurement pad at the extremity of each arm.
Second from the right is a more sophisticated pad device, showing the
actuated back-up arm in its fully
extended position. Third from the right
is an example of a toolwhich is
generally kept centered in the borehole
by external bow-springs, which are not
shown in the photo. The toolon the
left is similar to the first device but has
an additional sensorpad which is kept
in close contact with the formation
being measured.
These specially designed instruments,
which are sensitive to one or more
formation parameters of interest, are
lowered into a borehole by a surface
instrumentation truck.
Fig. 1.1 The elements of well logging: a measurement
sonde in a borehole, the wire line,
and a mobile laboratory. Courtesy of Schlumberger.
This mobile laboratory provides the downhole power to the instrument
package. It provides the cable and winch for the lowering and raising of
the sonde, and is equipped with computers for data processing,
interpretation of measurements, and permanent storage of the data.
Most of the measurements which will be discussed in succeeding chapters
are continuous measurements. They are made as the toolis slowly raised
toward the surface.
The actual logging speeds vary depending on the nature of the device.
Measurements which are subject to statistical precision errors or require
mechanical contactbetween sensor and formation tend to be run more
slowly between 600 ft and 1800 ft/h – newer tools run as fast as 3600ft/h.
Some acoustic and electrical devices can be withdrawn from the well,
while recording their measurements, at much greater speeds.

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The traditional sampling provides one averaged measurement for every
6inch. of tool travel. Forsome devices that have good vertical resolution,
the sampling interval is 1.2 inch. There are special devices with
geological applications (such as the determination of depositional
environment) which have a much smaller vertical resolution;
their data are sampled so as to resolve details on the scale of millimeters.
In the narrowest sense, logging is an alternate or supplement to the
analysis of cores, side-wall samples, and cuttings. Although often
preferred because of the possibility of continuous analysis of the rock
formation over a given interval, economic and
technical problems limit the use of cores.∗ Side-wall cores obtained from
another phase of wire line operations give the possibility of obtaining
samples at discrete depths after drilling has been completed. Side-wall
cores have the disadvantage of returning small sample sizes, as well as
the problem of discontinuous sampling.
Cuttings, extracted from the drilling mud return, are one of the largest
sources of subsurfacesampling. However, the reconstitution of the
lithological sequence from cuttings is
imprecise due to the problem of
associating a depth with any given
sample.
Although well logging techniques (with
the exception of side-wall sampling) do
not give direct access to the physical rock
specimens, they do, through indirect
means, supplement the knowledge gained
from the three preceding techniques.
Well logs provide continuous, in situ
measurements of parameters related to
porosity, lithology, presence of
hydrocarbons, and other rock properties
of interest.
Fig. 1.2 Examples of four logging tools. The dipmeter, on theleft, has sensors on four
actuated arms, which are shown in their fully extended position. Attached to the bottomof one
of its four arms is an additional electrode array embedded in a rubber “pad.”It is followed
by a sonic logging tool, characterized by a slotted housing, and then a density device with its
hydraulically activated back-up arm fully extended. The toolon the extreme right is another
version of a dipmeter with multiple electrodes on each pad. Courtesy of Schlumberger.

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?Why log
Exploratory drill holes or wells are the only means of direct
access to the subsurface.Drilling is an expensive means of access
to the lithosphere, and sampling of the rocks and fluids penetrated
and geophysicalwell logging are the only ways information can be
derived from these holes. Valid well logs, correctlyinterpreted, can
be used to reduce future drilling costs by guiding the location,
properdrilling, and construction of test holes and productionor
disposalwells. Welllogging also enables the vertical and
horizontal extrapolation of data derived from drill holes.
,the lithologyGeophysicallogs can be interpreted to determine
,bulk density,resistivity factor-formation,resistivity,geometry
yield ofspecific, andmoisture content,permeability,porosity
, andmovement,to define the source, andbearing rocks-water
. Quantitativechemical and physical characteristics of water
interpretation of logs will provide numerical values for some of the
rock characteristics necessaryto designanalog or digital models
of ground-water systems.Log data aids in the testing and
economic developmentof ground-water supplies and of recharge
and disposalsystems and can be of considerable value in the
designand interpretation of surface geophysicalsurveys. Stallman
(1967)pointed out that if the following pretestinformation is not
obtained , failure of pumping tests is invited. Hydraulic conditions
along the well bore ; storage characteristics of the aquifer; and
depth to, and thickness of, the aquifer being tested,as well as
changes in either within the area of the test. He also suggested
that changes in transmissivity should be mapped.Estimates of
pertinent hydraulic properties of the aquifer can be provided by
borehole,geophysicalstudies. , Geophysical well logging can
provide continuous objective records with values that are
consistentfrom well to well and from time to time, if the equipment
is properlycalibrated and standardized. In contrast, the widely
used geologist’s or driller’s log of cuttings is subjective, greatly
dependentupon personal skills and terminology, and is limited to
the characteristics being sought.

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Geophysicallogs can be reinterpreted in a postmortem
investigation of some geologicor hydrologic factor that was not
considered while the hole was being drilled. Serendipity -the gift of
finding agreeable or valuable things accidentally-has resulted in
the discoveryof uranium, phosphate,potash, and other minerals
from the interpretation of well logs. Each year many more wells are
drilled for water than for petroleum. Although mostof the water
wells are shallow, each is a valuable sample of the geologic
environment, and logs of these holes can aid in the definition and
developmentof water supplies,sand and gravel deposits,other
nonmetallic and metallic mineral deposits,petroleum,and waste
storage or disposaland artificial-recharge sites and can provide
the engineering data necessary for construction. In contrast to
uninterrupted geophysicallogs, samples of rock or fluid almost
never provide continuous data. Even if a hole is entirely cored,with
loo-percentrecovery, laboratory analysis of the core involves the
selectionof point samples.Continuous coring and subsequent
analysis of enough samples to be statistically meaningful costs
much more than most geophysical-logging programs.In addition,
the volume of material investigated by mostlogging sondes may
be more than 100 times as large as the volume of most core
samples extracted from the hole. Although geophysicallogging
should partly supplant routine sampling of every drill hole, some
samples,properly taken and analyzed, are essential to the
interpretation of logs in each new geologic environment. Sidewall
sampling techniques are available for poorly consolidated
sediments and can be utilized after logs have been run, to provide
the most representative samples (Morrison, 1969).Sidewall
samples can also be taken in hard rocks by commerciallogging
service companies; however, they are relatively expensive.One
well, adequately sampled and logged,can serve as a guide for the
horizontal and vertical extrapolation of data through borehole
geophysics.Furthermore, well logging provides the only means for
obtaining information from existing wells for which there is no data
and from wells where casing prevents sampling.

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Geophysicallogs can be digitized in the field or office by
commercialservice companies and are then amenable to
computeranalysis and the collation of many logs.They can be
very economically stored on, and retrieved from, magnetic tape.
Digitized geophysicallogs of oil wells are transmitted by radio and
telephone for interpretation by log analysts in response to the need
for rapid answers at the well site. Logging techniques also permit
time-lapse measurements to observe changes in a dynamic
system. Changes in both fluid and rock characteristics and well
construction caused by pumping or injection can be determined by
periodic logging.Radiation logs and, under some conditions,
acoustic logs are unique in providing data on aquifers through
casing. This permits logging at any time during or after the
reestablishmentof native fluids behind the pipe. The graphic
presentation of geophysicallogs allows rapid visual interpretation
and comparisonat the well site. Decisions on where to set screen
and on testing procedurescan be made immediately, rather than
after time-consuming sample study or laboratory analyses.
1. Plan the logging program on the basis of the data needed.
2. Carry out drilling operations in a manner that produces the most
uniform hole and the least disturbance of the environment.
3. Take representative formation and water samples where
necessary,using logs as a guide, if possible.
4. Insist on quality logs made with calibrated and standardized
equipment.
5. Logs should be interpreted collectively, on the basis of a
thorough understanding of the principles and limitations of each
type of logging technique, and some knowledge of the
geohydrologicenvironment under study.

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USES OF LOGS
A set of logs run on a well will usually mean differentthings to
different People.Letus examine the questions asked–and/or
answers sought by a variety of people.
The Geophysicist:
As a Geophysicistwhat do you look for?
'' Are the tops where you predicted?
'' Are the potential zones porous as you have assumed from
seismic data?
'' What does a synthetic seismic sectionshow?
The Geologist:
The Geologistmay ask:
'' What depths are the formation tops?
'' Is the environment suitable for accumulation of Hydrocarbons?
'' Is there evidence of Hydrocarbon in this well?
'' What type of Hydrocarbon?
'' Are Hydrocarbons presentin commercialquantities?
'' How good a well is it?
'' What are the reserves?
'' Could the formation be commercialin an offsetwell?
The Drilling Engineer:
"What is the hole volume for cementing?
"Are there any Key-Seats or severe Dog-legs in the well?
"Where can you get a good packer seat for testing?
"Where is the bestplace to set a Whip stock?
The Reservoir Engineer:
The ReservoirEngineer needs to know:
"How thick is the pay zone?
"How Homogeneous is the section?
"What is the volume of Hydrocarbon per cubic meter?
"Will the well pay-out?
"How long will it take?
The Production Engineer:
The ProductionEngineer is more concerned with:
"Where should the well be completed (in what zone(s))?
"What kind of productionrate can be expected?
"Will there be any water production?
"How should the well be completed?
"Is the potential pay zone hydraulically isolated?

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Spontaneous potential& Resistivity Log
1) Spontaneous potential: (SP)
The spontaneouspotential log (SP) measures the natural or
spontaneouspotential difference (sometimes called self-potential) that
exists between the borehole and the surface in the absence of any
artificially applied current.
It is a very simple log that requires only an electrode in the borehole and
a reference electrode at the surface. These spontaneous potentials arise
from the different access that different formations provide for charge
carriers in the borehole and formation fluids, which lead to a spontaneous
current flow, and hence to a spontaneous potential difference.
The spontaneous potential log is given the generic acronym SP.

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Origin of SP current:
• Electrochemical components
• Electro kinetic components
Tool Operation:
The tool is extremely simple, consisting of a single electrode that is
connected to a good surface earth point via a galvanometer for the
measurement of DC potential. A small 1.5 V battery is also included
commonly to ensure that the overall signal is measured on the correct
scale.
Uses of SP:
• The detection of permeable beds
• The determination of Rw
• The indication of the shale lines of a formation
• Correlation.
Notes:
• The SP tool has a poorresolution. So it can be used for correlation.
• The drilling mud salinity will affect the strength of the electromotive
forces (EMF) which give the SP deflections. If the salinity of the mud is
similar to the formation water then the SP curve may give little or no
responseoppositea permeable formation; if the mud is more saline, then
the curve has a positive voltage with respect to the baseline opposite
permeable formations; if it is less, the voltage deflection is negative. In
rare cases the baseline of the SP can shift suddenly if the salinity of the
mud changes part way down hole.
• Mud invasion into the permeable formation can cause the deflections in
the SP curve to be rounded off and to reduce the amplitude of thin beds.
• A larger wellbore will cause, like a mud filtrate invasion, the deflections
on the SP Curve to be rounded off and decrease the amplitude opposite
thin beds, while a smaller diameter wellbore has the opposite
effect.

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2) ResistivityTools:
Resistivity logging is a method of well logging
that works by characterizing the rock or Sediment
in a borehole by measuring its electrical resistivity.
Resistivity is a fundamental material property
which represents how strongly a Material opposes
the flow of electric current.
The log must run in holes containing electrically
conductive mud or water.
1-Electrode tools:
Modern Resistivity Log:
Laterologs: (LL)
It is a type of modern electrodes which
have a number of electrodes.
• LL3 has 3 current emitting electrodes
(vertical resolution is (1ft).
• LL7 has 7 current emitting electrodes
(vertical resolution is (3ft).
• LL8 is similar to the LL7, but has the
current return electrode
(Vertical resolution is 1ft).
Dual Laterologs:(DLL)
It is the latest version of the later log. As its name implies, it is
a combination of two tools, and can be run in a deep penetration (LLd)
and shallow penetration (LLs) mode.
These are now commonly run simultaneously and together with an
additional very shallow penetration device. The tool has 9 electrodes.
Both modes of the Dual later log have a bed resolution of 2 feet.
The resistivity readings from this toolcan and should be corrected for
borehole effects and thin beds, and invasion corrections can be
applied.
The dual later log is equipped with centralizes to reduce the borehole
effect on the LLs. A micro resistivity device, usually the MSFL, is
mounted on one of the four pads of the lower of the two centralists.

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NOTE: Separation of the LLs and LLd. from each other and from the
MSFL is indicating the presence of a permeable formation with
hydrocarbons.
Spherically FocusedLog:(SFL)
The spherically focused log (SFL)
has an electrode arrangement that
ensures the current is focused quasi-
spherically. It is useful as it is
sensitive only to the resistivity of the
invaded zone.
Micro-Resistivity Logs
Micro Log: (ML)
It is a rubber pad with three button electrodes
placed in a line with 1 inch spacing The result from
this toolis two logs called the 2”normal curve (ML)
&the1½“inverse curve (MIV).
The difference between the two curves is an
indicator of mud cake (so it is used in making sand
counts).
Micro laterolog:(MLL)
It is the micro-scale version of the laterolog.
The toolis pad mounted, and has a central button
current electrode.
The depth of investigation of the MLL is about
4 inches.

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Proximity Log: (PL)
This toolwas developed from the MLL.
It is used to measure RXO. It has a depth of
penetration of 1½ ft., and is not affected by mud
cake. It may, however, be affected by (Rt) when
the invasion depth is small.
Micro Spherically FocusedLog:(MSFL)
It is commonly run with the DLL on one of
its stabilizing pads for the purposeof measuring
RXO.
It is based on the premise that the best resistivity
data is obtained when the current flow is spherical
around the current emitting electrode.
The current beam emitted by this device is
initially very narrow (1”), but rapidly diverges.
It has a depth of penetration of about 4” (similar
to the MLL).
2- Induction Tools:
These logs were originally designed for use
in boreholes where the drilling fluid was very
resistive (oil-based muds or even gas). It can,
however, be used reasonably also in water-based
muds of high salinity, but has found its greatest
use in wells drilled with fresh water-based muds.
The sondeconsists of 2 wire coils, a transmitter
(Tx) and a receiver (Rx).
High frequency alternating current (20 kHz) of
constant amplitude is applied to the transmitter
coil.
This gives rise to an alternating magnetic field
around the sonde that induces secondary currents
in the formation.

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These currents flow in coaxial loops around the sonde, and in turn create
their own alternating magnetic field, which induces currents in the
receiver coil of the sonde. The received signal is measured, and its size is
proportional to the conductivity of the formation.
Calibration:
Induction logs are calibrated at the well site in air (zero conductivity)
and using a 400ms test loop that is placed around the sonde. The
calibration is subsequently checked in the well oppositezero conductivity
formations (e.g., anhydrite), if available.
1- The 6FF40 Induction-ElectricalSurvey Log (IES-40)
It is a 6 coil device with a nominal 40 inch Tx-Rx distance, a 16 inch
short normal device and an SP electrode.
It is a smaller scale version of the IES-40. It is a 6 coil device with
a nominal 28 inch Tx-Rx distance, a 16 inch short normal device and an
SP electrode.
3- The Dual Induction-Laterolog (DIL)
It has several parts:
(i) a deep penetrating induction log (ILd) that is similar to the IES-40.
(ii) a medium penetration induction log (ILm).
(iii) a shallow investigation laterolog (LLs).
and an SP electrode. The ILm has a vertical resolution about the same as
the ILd (and the IES-40), but about half the penetration depth.
4- The Induction SphericallyFocusedLog (ISF)
It combines (i) IES-40, (ii) a SFL, and (iii) an SP electrode. It is often run
in combination with a sonic log.
5- Array Induction Tools (AIS, HDIL)
It consists of one Tx and four Rx coils.
Intensive mathematical reconstruction of the signal enables the resistivity
at a range of penetration depths to be calculated, which allows the
complete invasion profile to be mapped.

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Spontaneous-potentiallogs
are records of the natural
potentials developed betweenthe
borehole fluid and the surrounding
rock materials.
(No artificial currents are applied)
The spontaneous potential is used
chiefly for:
1) Geologic correlation(indicate
Facies).
2) Determination of bed thickness.
3) Separating nonporous from
porous rocks in shale-sandstone
and shale-carbonate sequences
(impermeable zones such as Shale
,and permeable zones such as Sand)
maximum deflectionis clean Sand and
minimum is shale .
4) Determination Formation water
Resistivity (Rw) in salt and fresh mud.
5) Calculation the volume of Shale in
permeable beds.
Because the electric log is a
measure of natural potentials and
resistivities, it can be run only in open
(uncased) holes that are filled with a
conducting fluid such as mud or water.
Presentation of an SP curve in
a Sand-Shale Sequence.

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The chart paper commonlyused for the electric log is divided
into two vertical columns called tracks. Under the API (American
Petroleum Institute) system, the left-hand track is 2.5inches wide,
with four divisions per inch and major divisions at 1.25-inch
increments across the width of the track. The right-hand track is 5
inches wide, with four divisions per inch, and also has major
divisions at 1.25Lb-inchintervals across the width of the track.
The footage scale is divided into 10 divisions per inch, with major
divisions at 0.5inch and 2.5-inch intervals. The API chart paper
allows for a wide choice in SP, resistivity, and footage scales.
The electric log usually includes the spontaneous potential in the
left-hand track and one or more resistivity curves in the right-hand
track. Some of the commonlyused resistivity curves are the single
point, short normal, long normal, lateral devices, micro log, micro
focused log,and the guard, or laterolog. Each of these devices has
its application, depending on the lithology, depth of mud invasion,
and other borehole conditions.Table 2 shows the applicability of
various electric logging methods to the solution of typical
hydrologic problems.

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Principles and applications
The spontaneous-potentiallog is a graphic plot of the small
differences in voltage, measured in millivolts that develop at the
contacts between the borehole fluid, the shaleor clay, and the water in
the aquifer.
:Two sources of potential are recognized
The firstsource, and least important to the magnitude of SP, is the
phenomena. Thiselectro kineticstreaming potential caused by
electromotive force(emf) develops when an electrolyte moves through
a permeable medium. The emf appears in the borehole at places where
mud is being forced into permeable beds, although in water wells,
streaming potentials may be generated in zones gaining or losing water.
Streaming potentials can sometimes be detected on the SP curveby
sudden oscillations or by departures fromthe more typical responsein a
particular environment. (See section on “Extraneous Effects “) A
discussion of the streaming potential and its effects on SP was given by
Gondouin and Scala (1958).
.in theThe second and most importantsourceof SP arises
emf produced at the junction of dissimilar materials inElectrochemical
the borehole.
The junctions are between the following materials:
(Mud-mud filtrate), (mud filtrate-formation water), (formation water-
shale), and (shale-mud). Becausethe mud filtrate is derived fromthe
mud, it generally has a similar electrochemical activity, and any emf
developed across this junction will be minimal and can be disregarded.
The Potential developed across the junction from formation water to
shale to mud is called the membrane potential, and the potential
developed across the junction frommud filtrate to formation water is
.liquid junction potentialthecalled

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The potentials arising fromthese junctions causea currentto flow
near shale-aquifer boundaries in the mud column in the borehole. Figure
9 is a schematic diagram of the circulation of currentacross the various
junctions and through the borehole (Doll, 1948). When theformation
water is much more saline than the mud, the currentfollows the paths
shown by the arrows, entering the mud column fromthe shale and
moving into the sandstone. As the SP electrode moves upward through
the bottom shale (fig. 9), it senses a decreasing potential becausethe
currentflows parallel to the well bore. At the bed boundary the current
density is maximum and the SP curveexhibits an inflection at this level.
As the SP electrode moves toward the midpoint of the sandstonebed,
the currentdensity decreases, the curvatureof the SP curveis reversed,
and the SP attains its maximum negative potential at midpoint in the
sand. As the SP electrode moves beyond the midpoint of the sandstone,
the curverecorded is a mirror image of the lower half if the beds are
uniform. The SP log, therefore, is a measureof the potential drop that
occurs in the mud, and only approaches the static spontaneous potential
(SSP) under favorableconditions. (See “Glossary” for definitions.) If, on
the other hand, the formation water is fresh compared with the mud,
the polarity of the SP curveis reversed, and the reciprocalof the log in
figure 9 is produced. The SP is thereforemore positive oppositethe
sands and is more negative opposite the shales. This condition occurs in
hydrologic regimes where ground water contains very few dissolved
solids and results in an electric log on which both the SP and the
resistivity deflect in the samedirection, opposite the sand and shale
beds. An example of this is illustrated in figure 10. The water in the
formations above500 feet contain dissolved solids of the drilling mud lie
somewhereFormations below 800 feet havea dissolved-solids content
of about 250 mg/l. The dissolved solids of the drilling mud lie
somewherebetween these two extremes. The resistivities above450
feet are all off-scaleto the right, and lie between 100 and 200 ohm-
meters.

19
Spontaneous-potentialdeflections are recorded on the left-hand
track of the electric log, with deflections to the left considered as
negative and those to the right as positive. In a sand-shalesequence
containing formation water that is more saline than the mud, the
greatest positive deflections can be expected oppositethe shales, and
the greatestnegative deflections cart be expected opposite the sands.
A shale line can be constructed to fit through as many of the extreme
positive deflections as possible, and a sand line through as many of the
extreme negative deflections as possible. If the ionic concentration of
the borehole fluid and the aquifer water are constantthroughoutthe
length of the borehole, the shale line and the sand line will generally be
parallel to the vertical axis of the log.
, and all potentialhe shale line is considered to be the baselineT
measurements are made perpendicular to this line using the scale of SP
in millivolts on the log heading. When carefully applied this method can
be used to estimate sand-shaleratios and is applicable to aquifers with
water of high salinity and, in somecases, to fresh-water aquifers.
Unfortunately, the method fails in many water wells in sand-shale
formations becausethe SP deflections are not necessarily negative
opposite the sands. If the borehole fluid has a very low resistivity
compared with the sand beds, the SP deflections opposite the sands may
actually be more positive than the SP deflections opposite the shales.
Accordingly, the sand-shaleratios become meaningless for formations
that do not contain shaleor clay beds, even though large positiveSP
deflections can be seen. Perhaps the most significant, but often
misapplied, useof the SP in ground-water hydrology is the
determination of water quality fromSP deflections. In petroleum
exploration, whereNaCl is dominant.

20
The following equation is usefulto calculate the quality of the formation
water:
𝑆𝑃 = −𝐾 𝑙𝑜𝑔
𝑅𝑚
𝑅𝑤
Where
SP = log deflection, in millivolts
K = 60 +0.133T
T = borehole temperature, in degrees Fahrenheit
Rm = resistivity of borehole fluid, in ohm-meters
Rw = resistivity of formation water, in ohm-meters.
There are three cases of SP deflection:
1) Rm =Rw there is no deflection(shale base line).
2) Rm >Rw (–ve) deflectionand the deflectionwill be to the left of
the shale line (shale free).
At thick, water bearing sand free shale (static SP) & at thin beds
shale or gas (pseudo SP)
3) Rm <Rw (+ve) deflectionand the deflectionwill be to the right of
the shale line.
In actual practice the SP deflection oppositea sand bed is read from
the log and the Rm is measured with a mud kit or a fluid-resistivity tool.
Inserting thesevalues in the equation determines the formation fluid,
Rw which can be related back to milligrams per liter of NaCl from
salinity-resistivity charts. Theequation is predicated on the following
important assumptions, which may or may not hold true in water wells:
(1) Both the borehole fluid and the formation water are sodiumchloride
solutions.
(2) The shale formations are ideal ion-selective membranes, and the

21
sand formations haveno ion-sieving properties (no clayey sand or sandy
clays in the zone of investigation).
(3) The borehole fluid has a much greater resistivity than the combined
resistivity of the sand and the shale.
In petroleum logging the sand formations are generally saturated with
brines of low resistivity, and the conditions in assumption 3, above, are
generally satisfied.
In hydrologic logging where the sand formations are saturated with
fresh water, the resistivity of the sands may be many times that of the
borehole fluids, and the conditions in assumption 3, above, may no
longer be valid. However, the combined resistivity of the sands and
shales can be less than that of the borehole fluid if the fresh water used
in the drilling fluid has a lower ionic concentration than the formation
water. The divalent ions Ca (+2) and Mg (+2) commonly found in fresh
water havea different effect on the SP than Na (+1). The calcium and
magnesiumsolutions affect the SP as though the water weresaltier than
its resistivity indicates. Alger (1966) described a method for finding the
.)m/RwK log (R-SP =of fresh waters by useof the equationwR
In order to correlate SP deflections with water resistivity and ionic
concentration fromsalinity-resistivity charts, hefound it necessary to
convertall anions and cations in the water to an equivalent sodium
chloride solution. Alger assumed that, within one ground-water regime,
the relative ionic concentrations and ratios are approximately constant.
The relations between SP, Rw and total dissolved solids - once
determined froma borehole having an electric log and water analyses -
were extrapolated by Alger to other bore-holes in the regime exclusively
fromthe SP deflections. In an unpublished paper by HubertGuyod,
Consultant, entitled “Fundamentals of Electrical Logging and their
Application to Water Wells,” presented at the Second Advanced Seminar
on Borehole Geophysics, U.S. Geological Survey, Denver, Colo.,
December 1968.

22
can be usedRw)Rm/log (k-SP=Guyod pointed out that the formula
only when the following conditions aresimultaneously satisfied:
(1) The formation water is very saline.
(2) NaCl is the predominantsalt.
(3) The mud is relatively fresh and contains no unusualadditives.
Guyod further stated that “the three conditions specified aboveare
probably never met by fresh water aquifers. Therefore, it is not possible
to determine, not even estimate, from the SP curvethe resistivity of
formation water, that is, the total dissolved solids in fresh water sands.”
Patten and Bennett (1963) also emphasized the unreliability of the SP
formula for determining dissolved solids. Considering the unreliability of
the method, especially for dissolved solids of less than about
10,000 mg/l, the method probably should not be used in the analyses of
fresh-water-bearing aquifers. Vonhoff (1966)) however, found thata
“workableempirical relationship exists be-tween the spontaneous-
potential deflection on the electric log and the water quality in the
glacial aquifers.” His conclusions arebased on a study of test wells in
Saskatchewan, Canada, in which the ionic composition of the drilling
fluid and formation water are similar, and the resistivity of the drilling
fluid is much greater than that of the formation water. The actual
dissolved solids in the test wells ranged from1,191 to 3,700 mg/l.
Figure(9)

23
Figure (10)
Figure (11)

24
Instrumentation
In its simplest form an SP logging device consists of a movablelead
electrode which traverses the borehole on an insulated wirea ground
electrode also made of lead and a device for measuring potential such as
a millivolt meter. The movable electrode senses the ohmic-potential
drop caused by the currents flowing into the mud column near the
shale-aquifer boundaries. Becausepotential at the ground electrode
remains constant the potential read fromthe meter represents the
change in potential in the mud between the shales and the permeable
formations. Figure11 shows theSP measuring circuit and, also, the
electrical equivalent. In this diagramthe upper Ek represents the shale
streaming potential, the lower Ek is the streaming potential developed
across the sand, Ep is electrochemical potential developed at the liquid
junction (mud filtrate-formation water), and Es is the membrane or
shale potential. The terms Rsh, Rss, and Rm are the resistances of the
shale, sandstone, and mud, respectively.
Calibrationandstandardization
Spontaneous potential is measured in millivolts, and the millivolts per
horizontalchart inch, or full scale of the chartpaper (sensitivity), is
shown on the log heading of the left track. The smallsingle-conductor
cable loggers used in ground-water studies commonly haveSP
sensitivities of 10, 25, 100, and 200 mv per inch. Some loggers are
furnished with an auxiliary SP calibrator. The main purposeof the
calibrator is to check logger function on all ranges of SP sensitivity. The
calibrator contains a battery and resistors so as to providea
predetermined potential, in millivolts, which is applied to the SP
electrodes of the logger. Each rangeon the calibrator should produce a
1-inch deflection of the recorder pens when the SP sensitivity control on
the logger panel is set in the corresponding range. Calibrators supplied
with the single-conductor loggers arenot precision devices, and
deflections on corresponding recorder ranges may or may not exactly
equal 1 inch (or one division). Accuracies aregenerally on the order of
±10 percent.

25
Radiusof investigation
The potential drop along the mud column results from currents that
originate away fromthe borehole along formation boundaries.
For example, in a lithologic section, such as that in figure 12
(Schlumberger Well Surveying Corp., 1958),the currents tend to flow
fromthe borehole into the permeable beds until a sufficient cross-
sectional area of the compact (very resistive) formation is encountered
to carry the current. The currentthen flows across thelarge area of
resistiveformation until an
impervious conductiveshale
bed is intersected. The
currents then travel with
greater density along the
conductive beds until the
borehole is intersected
again. Therefore, the radius
of investigation is highly
variable. Also, the SP may
not relate directly to the
permeable beds becausethe
effect of these beds spreads
the SP above and below the
bed boundaries, as shown by
the SP curveon the left-hand
side in figure 12.

26
(Unwanted logging effects)Extraneous effects
connected to a surface earth to workThe SP tool must be
.effectively
nd an iron probe canthis causes no problem a:on land wellsFor
directly into soilbe pushed
the SP will not bewithout an effective earth:offshore wellsFor
recorded
Other unwanted SP potentials:
(Heavy rain _ Noise_ Logging drum and
Sheave Magnetism _Disruption to the
ground reference).
SP noise can be defined as any spurious
or unwanted signals that are not correlated
with the actual SP in the borehole. Noise and
anomalous potentials are relatively common
problems in SP logs. Some of the early model
loggers used insulated cable with a single
conductor, and the SP was relatively free
fromnoise. With the introduction of steel
armored cable, noise became troublesome.
Steel is electrochemically active, and when
the cable is immersed in an electrolyte
(water or drilling mud), a battery effect
develops along its entire wetted length.
If the cable remains motionless in the drill hole, the batteries become
polarized, and their output remains constant. This small current
impresses on the SP electrode a potential that merely shifts the SP curve
left or right. When the cable begins to movein the hole, however, the
polarization film is wiped off intermittently, and the currentoutput from
the battery effect is thereforevaried. The varying potentials resulting
fromcable motion are thus impressed on the SP to producenoise.

27
The sourceand effect of this noise is shown in figure13
(Electra Technical Laboratories, 1959). Noisefromthe partof the
armored cable near the surfacein the borehole can also be coupled into
the SP ground electrode. If the well is cased to considerabledepth, the
casing may act as a shield around the wetted upper part of the cable to
effectively screen the SP ground electrode fromcable noise. This type of
noise is mosttroublesome wheresurfaceformations havehigh
resistivities. Corrective procedures to lessen cable noise caused from
battery effect are based on the electrical isolation of the SP electrode
and the SP ground electrode from the cable. Wrapping the armored
cable with insulating tape for 10 feet or more above the SP electrode
may displacethe battery effect far enough up hole to reduce the
magnitude of this sourceof potential. Moving the SP ground electrode
as far as possiblefrom the well head may also help. Most noise-
reduction procedures arelargely trial-and-error processes becausethe
sourceof noisegenerally is not evident. Other sources of noise and
anomalous SP are magnetization of armored cable, currents set up by
casing corrosion in the Logged well or in nearby wells, magnetic storms
fromsolar flares flow of ground water through the well bore, and
manmade effects. In this last category fall such influences as circulating
ground currents near electrical switch yards and transformer stations,
electrochemical action along buried pipelines, Cathodic protection of
buried pipelines, and potentials set up along large or long metallic
objects, such as railroad tracks. If theSP ground electrode is in the mud
pit, even the operation of the rig mud pumps, or generator, or the
dragline cleaning of the pits. Water moving into the hole may be located
by SP noise.
The oscillating SP between 700-750 feetin figure 14 is probably due to
streaming potentials caused by water moving into the well bore.

28
Factors affected on SP deflection
1) Bed thickness: SP decreases when bed thickness decreases
2) Bed resistivity: higher resistivity reduces the deflectionof the SP
curves
3) Borehole and invasion: the effectof borehole diameterand
invasion on the sp log is very small, in general can be ignored.
4) Shale content: the presence ofthe shale in a permeable
formation reduces the SP deflection.
5) Hydrocarbon content: in hydrocarbon bearing zones the SP
deflectionis reduced,this effectcalled (hydrocarbon suppression).
6) Salinity effect:(Rm/Rw) fresh mud (-ve) SP, saline mud (+ve)
SP.
7) Mud filtrate: the magnitude and direction of SP deflectionfrom
base line depends on Rm, Rw.
8) Permeable beds:depends on differencein salinity of mud and
formation where there is no deflectionin permeable beds when no
differencein salinity.

29
:Quantitative uses of SP
Rwwater resistivityFormation1)
𝑆𝑆𝑃 = −𝐾 𝑙𝑜𝑔
(𝑅𝑚) 𝑒
(𝑅𝑤) 𝑒
SSP: static spontaneous potential.
K: temperature dependentcoefficient.
(Rm)e: equivalent mud filtrate resistivity.
(Rw)e: equivalent formation resistivity.
1) Negative deflectionthe formation water more saline than mud
filtrate.
2) Positive deflectionthe formation water fresherthan mud filtrate.
of shale contentVolume2)
𝑉𝑠ℎ(%) = 1 − 100
𝑃𝑠𝑝
𝑆𝑠𝑝
Vsh: volume of shale.
Psp: pseudo static spontaneous potential. (Water bearing shaly
sand zone)
Ssp: static spontaneous potential. (Max SP in clean sand zone).

30
𝑉𝑠ℎ =
𝑆𝑃 − 𝑆𝑃𝑐𝑙
𝑆𝑃𝑠ℎ − 𝑆𝑃𝑐𝑙
Vsh: shale volume.
SP: SP log reading.
SPsh: SP log reading in shale zone.
SPcl: SP log reading in clean sand zone.
(FT)temperatureFormation3)
𝐹𝑇 = 𝑆𝑇 + (
𝐵𝐻𝑇 − 𝑆𝑇
𝑇𝐷
) 𝐹𝐷
FT: formation temperature.
ST: the mean annual
surface temperature.
BHT: bottom hole
temperature.
TD: total depth of the well.
FD: formation depth.

31
:Limitations
1) Borehole mud must be conductive.
2) Formation water must be water bearing and conductive.
3) A sequence of permeable and non-permeable zones must exist.
4) A small deflectionoccurs if Rm=Rw.
5) No fully developed infront of thin beds.

32
measurement of a formationa: the resistivity log isThe log
resistivity that is its resistance to the
passage of an electric current.
It’s measured by resistivity tools.
Conductivity tools measure formation
conductivity or its ability to conductive
an electric current.
It’s measured by induction tools.
Most rock materials are essentially
insulators, while there enclosed fluids
(except hydrocarbons) are conductors.
Conductivity is generally converted
directly and plotted as resistivity on log
plot.
&. Meterhm(OUnits of measurements are:Units and presentation
Mhos).
Presentation: tracks 2 and 3 on logarithmic scale.fig.1
Resistivity
The electrical resistivity of a rock depends on
physicalproperties of the rock and the fluids it
contains. Most sedimentary rocks are
composed of particles having a very high
resistanceto the flow of electrical current.
When these rocks aresaturated, the water
filling the pore spaces is relatively conductive
compared with the rock particles or matrix.
The resistivity of a rock, therefore, is a
function of the amount of fluid contained in the porespaces, the salinity
of that fluid, and how the porespaces are interconnected.

33
The main factor in pore geometry is probably tortuosity, which is
defined as the squareof the ratio of the actual path length of the
currentto the length of the sample through which it flows.
Resistivity is measured in ohm-meters, which is the resistanceof a cube
of material that is one meter on a side, and the resistance of that cube,
in ohms, is numerically equal to the resistivity, in ohm-meters.
The resistivity of a rock that is 100-percentsaturated with formation
water is Ro and the resistivity of the water is Rw True resistivity, Rt, is
distinguished from Ro be-causecorrection for partial saturation by
hydrocarbons is necessary in petroleumexploration. Correction for
partial saturation would also be necessary in hydrology if we made
resistivity logs above the water table. Using Ro fromlogs and F, it is
simple to calculate Rw which is a function of the temperature and
quality of water in an aquifer:
𝑅𝑤 =
𝑅𝑜
𝐹
Formation factor
The formation resistivity factor
(F) is de-fined as the ratio of the
electrical resistivity of a rock 100-
percent saturated with water to
the resistivity of the water with
which it is saturated, F=Ro/Rw
(Archie, 1942). Becausemost
rock grains have a very high
resistancerelative to water, the
formation factor is always greater
than 1.

34
Formation factor is roughly related to effective porosity in poorly
consolidated rocks as follows:
𝐹 = .62∅−2.15
.
Guyod (1966) showed how porosity and resistivity data fromlogs can be
used to determine the salinity of the interstitial water (fig. 3). The figure
is based on average values for clay-freegranular aquifers and is
therefore approximate when applied to a particular aquifer. Thus, if all
other factors are equal (fig. 3), the higher the porosity and salinity, the
lower the aquifer resistivity.
Principal uses (application of resistivity logs):
The resistivity logs wereused in the following (table 6.1):
1- To find hydrocarbons.
2- Petro physicalcalculations.
3- Give information about lithology, texture, facies, over pressureand
sourcerock aspects.
4- Determination the volume of shale.
5- Correlate between wells.

35
Zones of invasion and resistivity:
There are three zones:
There is small layer of mud cake on the wall of bore hole. And it’s the
firstlayer in the borehole.
1-Flushed zoneclosestthe borehole, behind the mud cake.
2-Transition zonebetween flushed zoneand UN invaded zone(virgin
formation).or called invaded zone.
3-Un invaded zone(virgin formation) where the essential target of
resistivity logging is that of the true resistivity of the formation (Rt).
Distribution of porefluids in zones around a well which initially
contained hydrocarbons.
From Dewan [2].

36
Factors affecting on resistivity of reservoirs:
1-Salinty: 1/R
2-Temprature: 1/R
3-Saturation: 1/R
4-Presence of Hydrocarbon: R
4-Lithology plays an indirect role for Controlling Resistivity
where:
- Most rock forming mls are insulator.
- Pore content oil or gas insulator.
- Pore content water conductor.
- Clay and shale conductors.
Rock resistivity decreaseswith:
- Increasing porosity or fracturing.
- Increasing water saturation and water salinity.
- Increasing shale and clay content.
g on resistivity logging:Factors affectin
1- Borehole Size.
2- Mud Cake type and thickness.
3- Invasion Diameter.
4- Bed thickness and tool Resolution.
5- Depth of Investigation.

37
rocesses:PeologicalGluenced byesistivity is infRRock
-Clay alteration (decrease).
-Dissolution (decrease).
Faulting (decrease).-
-Salt water intrusion (decrease).
-Shearing (decrease).
Weathering (decrease).-
-Metamorphism (increase or decrease).
-Induration (increase).
-Carbonate precipitation (increase).
-Silification (increase).
2) Resistivity Tools:
Resistivity logging is a method of well logging that
works by characterizing the rockor Sediment in a
borehole by measuring its electrical resistivity.
Resistivity is a fundamental material property which
represents how strongly a Material opposesthe flow
of electric current.
The log must run in holes containing electrically
conductive mud or water.
1-Electrode tools:
Modern ResistivityLog:
Laterologs:(LL)
It is a type of modern electrodes which have a number of electrodes.
• LL3 has 3 current emitting electrodes (vertical resolution is 1ft).
Two guard electrode to keep central (bucking current) more focused.
Potential of central electrode is measured relative to the potential at
infinity to give a potential difference.
From (potential difference & current) we can calculate the resistivity.
• LL7 has 7 current emitting electrodes (vertical resolution is 3ft).

38
Main current from central electrode, bucking current from far electrodes
(80) inch apart.
The main current focused in a thin disk far out into the formation.
The potential between one of the monitoring electrodes and the potential
at infinity measured and known resistivity from (potential difference and
current) to provide geo metrical factor of the arrangement.
Strongly focused beam a little affected with (hole size), and penetrates the
invaded zone, and measures (Rt) true resistivity.
• LL8 is similar to the LL7, but
has the current return electrode
(Vertical resolution is 1ft).
Measured (Rxo) rather than (Rt).
Dual Laterologs: (DLL)
It is the latest version of the later log. As its
name implies, it is a combination of two tools,
and can be run in a deep penetration (LLd).
And shallow penetration (LLs) mode.
These are now commonly run simultaneously
and together with an additional very shallow
penetration device.
The toolhas 9 electrodes (four potential and
five current).
Both modes of the dual later log have a bed
resolution of 2 feet.
The resistivity readings from this tool can and
should be corrected for borehole effects and thin
beds, and invasion corrections can be applied.
The dual later log is equipped with centralizes
to reduce the borehole effect on the LLs.
A micro resistivity device, usually the MSFL, is mounted on one of the
four pads of the lower of the two centralists.

39
The difference between (LLD)and (LLS)
LLD: the main currentemits a currentwith
intensity equal10,
And the five electrodes emits currentwith
the same polarity but differentin intensity to
focus main current.
LLS: only three currentelectrodes emit
current, this currentreturn to reversely
polarizedelectrodes.
The two measurements have differentdepth
of investigationare called (Rd) deep
resistivity,and (Rs) shallow resistivity.
NOTE: Separation of the LLs and LLd. from each other and from the
MSFL is indicating the presence of a permeable formation with
hydrocarbons.
Spherically FocusedLog:(SFL)
The spherically focused log (SFL) has an electrode arrangement that
ensures the current is focused quasi-spherically. It is useful as it is
sensitive only to the resistivity of the invaded zone (Ri).

40
Micro-Resistivity Logs
Micro Log: (ML)
It is a rubber pad with three button electrodes
placed in a line with 1 inch Spacing.
The result from this tool is two logs called the
2”normal curve (ML) &the1½“inverse curve (MIV).
The difference between the two curves is an indicator
of mud cake (so it is used in making sand counts).
A known current is emitted from electrode A, and the
potential differences between electrodes M1 and M2
and between M2 and surface electrode are measured.
Micro laterolog:(MLL)
It is the micro-scale version of the laterolog. The
tool is pad mounted, and has a central button
current electrode that emits known measurements
current surrounded coaxially by two rings shaped
monitoring electrodes, and a ring shaped guard
electrode that produces a bucking current as in
DLL. The depth of investigation of the MLL is
about 4 inches.
Proximity Log: (PL)
This tool was developed from the MLL. It is used to measure RXO.
It has a depth of penetration of 1½ ft., and
is not affected by mud cake. It may,
however, be affected by Rt when the
invasion depth is small.

41
Micro Spherically FocusedLog:(MSFL)
It is commonly run with the DLL on one of its stabilizing pads for the
purposeof measuring RXO.
It is based on the premise that the best resistivity
data is obtained when the current flow is spherical
around the current emitting electrode.
The current beam emitted by this device is
initially very narrow (1”), but rapidly diverges. It
has a depth of penetration of about 4” (similar to
the MLL).
2- Induction Tools:
These logs were originally designed for use in boreholes where the
drilling fluid was very resistive (oil-based
muds or even gas). It can, however, be used
reasonably also in water-based muds of high
salinity, but has found its greatest use in
wells drilled with fresh water-based muds.
The sondeconsists of 2 wire coils, a
transmitter (Tx) and a receiver (Rx). High
frequency alternating current (20 kHz) of
constant amplitude is applied to the
transmitter coil. This gives rise to an
alternating magnetic field around the sonde
that induces secondary currents in the
formation. These currents flow in coaxial
loops around the sonde, and in turn create
their own alternating magnetic field,
which induces currents in the receiver coil of
the sonde. The received signal is
measured, and its size is proportional to the
conductivity of the formation.
The induction tools are important because
they provide the only resistivity measurement in wells drilled with oil
base mud.

42
Applications:
1- Reservoir delineation.
2-Determination of true formation resistivity.
3-Determination of water saturation.
4-Hydrocarbone identification.
5-Determination of movable hydrocarbon.
6-Invation profiling.
7-Thin bed analysis.

43
Calibration:
Induction logs are calibrated at the well site in air (zero conductivity)
and using a 400ms test loop that is placed around the sonde.
The calibration is subsequently checked in the well opposite
zero conductivity formations (e.g., anhydrite), if available.
It is a 6 coil device with a nominal 40 inch Tx-Rx distance, a 16 inch
short normal device and an SP electrode.
It is a smaller scale version of the IES-40. It is a 6 coil device with
a nominal 28 inch Tx-Rx distance, a 16 inch short normal device and an
SP electrode.
3- The Dual Induction-Laterolog (DIL)
It has several parts:
(i) Deep penetrating induction log (ILd). That is similar to the IES-40.
(ii) Medium penetration induction log (ILm).
(iii) Shallow investigation laterolog (LLs)
And an SP electrode. The ILm has a vertical resolution about the same as
the ILd (and the IES-40), but about half the penetration depth.
4- The Induction SphericallyFocusedLog (ISF)
It combines
(i) IES-40.
(ii) SFL.
(iii) SP electrode.
It is often run in combination with a sonic log.
5- Array Induction Tools (AIS, HDIL)
It consists of one Tx and four Rx coils.
Intensive mathematical reconstruction of the signal enables the resistivity
at a range of penetration depths to be calculated, which allows the
complete invasion profile to be mapped.

44
Quantitative uses of the resistivity logs:
The resistivity logs are used to give the volume of oil in a particular
reservoir, or in petro physical terms, to define the water saturation.
Sh+Sw=1, where, Sh=Sg+So
Sw: water saturation.
Sh: hydrocarbonsaturation.
So: oil saturation.
Sg: gas saturation.
Shr=1-Sxo
Shr: residual hydrocarbonsaturation.
Sxo: water saturation in invaded zone.
Shm=Sh-Shr, or, Shm=Sxo-Sw
Shm: movable hydrocarbonsaturation.
RF: Shr/Sh
RF: recovery factor.
The basic equation of petro physics (Archie equation)
Sw= ( 𝑎 𝑅 𝑤/∅ 𝑚
𝑅𝑡)1/n
Where:
SW:water saturation of un invaded zone.
Rw: formation water resistivity.
Rt: resistivity of un invaded zone.
∅: Formation porosity.
n: saturation exponent and its equal 2.
a&m: these values are varied according to the lithology and
equal to the formation factor.

45
Rsh: resistivity reading in front shale.
Rt: resistivityof un invaded zone.
b: constant which equal 1.
The ratio (Rsh/Rt) range from (0.5 to 1).
Qualitativeuses
:exturesT-1
The simplest expressionof resistivity variation with texture
is that variation of resistivity with porosity changes.
When porosity decreases the resistivity increase, these
changes indicate to change in water saturation and the
presence of hydrocarbons.
:ariationsVthologicalLi-2
Although resistivity logs do not allow the direct identification
of commonlithologies, they are very sensitive lithology
indicators.
The resistivity logs are in fact responding to two things
(changes in texture and changes in composition).
:Facies-3
Facies change can be followed on resistivity logs; one of the
principle uses of resistivity log in facies analysis is its ability to
registerchanges in (sand-shale) mixture.

46
:Correlation-4
The sensitivity of the resistivity logs to lithological changes is
the bases for their use in correlation.
Logs which correlate well are those which are more sensitive to
vertical changes than to lateral changes.
:Compaction, shale porosity and over pressure-5
There is relation between conductivity and shale porosity.
Where the resistivity increase with increasing of the compaction
of shale along the pore hole, and this relation plotted between
./m) and Depth (km)2
shale resistivity (Ωm
:Source rock investigation-6
The resistivity log may be used both qualitatively and
quantitatively to investigate source rocks, the effectof a source
rock on the resistivity log depends onthe maturity of the organic
matter, when the organic matter immature there is a little effect,
but when the organic matter is mature there is a large effect.
:ithologyLGross-7
Resistivity logs cannot be used for a first recognition of the
commonlithologies.There are no characteristic resistivity limits
for shale, or limestone or sandstone.The values depend on
many variables such as compaction,composition,and fluid
content ……., however, in any restricted zone, gross
characteristics tend to be constant and the resistivity log may be
used as a discriminator.
In certain specificcases the resistivity logs can be used to
indicate a lithology. These cases are clearly where certain
minerals have distinctive resistivity values (salts, anhydrite,
gypsum and coal) all have unusually high. But in case of
limestone and dolomite the resistivity usually high.

47
Ohm. mResistivityLithology /mineral
Variable(0.5-1000)
Depend on porosityand salinity
10000-infinity
10000-infinity
1000
10-1000000
0.0001-0.1
Moderate
Generally high
Generally high
Moderate-low
Very high
Very high
High
High
Very low
Shale
Limestone
Dolomite
Sandstone
Salt
Anhydrite
Gypsum
Coal
pyrite

48
REFERENCES
1-Dr M.Sharaf text Book (WellLogging)for Fourth Level Faculty
of Science, Benha University, Geophysics department.(2012)
2- WellLogging for Earth Scientists 2nd
Edition. Darwin V. Ellis and
Julian M. Singer.
3- An Introduction to GeophysicalExploration 2nd
Edition (Philip
Kearey and Michael Brooks).
4- Applicationof Borehole Geophysics to Water-Resources
investigations by W. ScottKeys and L. M. MacCary. (1985)
5- Applied Geophysics.(W.M.Telford,L.P.Geldart,R.E.Sheriff).
(1990)
6- Well Logging (Data Acquisitionand Applications) Oberto Serra
and Lorenzo Serra. (2004)
7- The geological Interpretation of Well Logs.Second Edition.
Rider (1996)
8- Geology& Geophysics in Oil Exploration. (2010)
9- An Introduction to GeophysicalExploration. Third Edition.
Philip Kearey, Michael Brooks and Ian Hill. (2002)
10- Applied Geophysics.Second Edition. (W.M.Telford,
L.P.Geldart, R.E.Sheriff).(2004)
By:
Geophysicist: Abdel Aziz hamed Abdel Aziz
Fourth level
Mob: 01144299361-01024719740.
E-mail: zezo0_1992@yahoo.com.

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