This dissertation report discusses characterizing oil and gas reservoirs using open hole wireline logging tools and techniques. It provides background on reservoir properties that can be measured using logs like resistivity, porosity, and saturation. It also describes the various electrical, radioactive, sonic, and other open hole wireline logging tools and their measurement principles.

1. A DISSERTATION RERORT
On
CHARACTERIZING THE RESERVOIR BY OPEN HOLE - WIRELINE LOGGING
At
WELL LOGGING SERVICES,
OIL AND NATURAL GAS CORPORATION LIMITED,
RAJAHMUNDRY ASSET, RAJAHMUNDRY.
under the supervision of
SHRI K.S.MURTHY
Deputy General Manager (Wells)
Well Logging Services -O.N.G.C,
Rajahmundry Asset, Rajahmundry.
under the guidance of
Dr. B.A.RAO
Chief Geophysicist (Wells)
Well Logging Services -O.N.G.C,
Rajahmundry Asset, Rajahmundry.
submitted by:
A.P.V.V.S.S. DILEEP, P.RAMANA MURTHY, K.CHINNABABU
M.Sc. (TECH.) GEOPHYSICS
(2007-2010)
DEPARTMENT OF GEOPHYSICS
COLLEGE OF SCIENCE &TECHNOLOGY
ANDHRA UNIVERSITY
VISAKHAPATNAM -03

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CERTIFICATE
This is to certify that this dissertation/summer training program entitled
“CHARACTERIZING THE RESERVOIR BY OPEN HOLE WIRELINE
LOGGING” is the bonafide work of students submitted to the
Geophysics Department, Andhra University in partial fulfillment of the
M.Sc.(Tech)degree in Geophysics.
(Proff N.V.B.S.S.PRASAD)
Head –Department of Geophysics,
Andhra University, Visakhapatnam.
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ACKNOWLEDGEMENT
Its my privilege to thank Proff: N.V.B.S.S.Prasad, Head -
Department of Geophysics, Andhra University, to all the pains he has taken
in providing an opportunity to undergo training in a prestigious organization
like Well Logging Services –O.N.G.C., Rajahmundry Asset, and I am very
grateful to him for his valuable guidance before and after the training for his
critical discussion in bring out this field training report.
I special thank to K.S. Prasad, G.M (W) and K.S.Murthy D.G.M
(W), Well Logging Services –ONGC. Rajahmundry Asset, for granting
permission to undergo project training.
I would like to acknowledge the contributions of Dr.B.A.Rao, C.G
(W) of Well Logging Services –O.N.G.C., in reviewing some of the material
in this document and esteemed guidance, kind cooperation during the entire
period of training.
I wish to express my gratitude to the staff members of Department
of Geophysics and my colleagues for their cooperation and encouragement
in the training program.
While I can not possibly mention the names of all of those who
contributed to text in this document, I thank all of them for their time and
efforts.
(A.P.V.V.S.S.DILEEP)
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CONTENTS
Chapter 1:………………………………………………… 05
INTRODUCTION
Wire line logging
Chapter 2:………………………………………………… 12
PROPERTIES OF RESERVOIR
Properties of rocks and fluids
Chapter 3:………………………………………………… 22
OPEN HOLE TOOLS and
INSTRUMENTATION
Electrical, Radio Active, sonic and other tools
Measurement Principles
Chapter 4:…………………………………………………. 39
OPEN HOLE LOGGING AND LOGS
Logging operation
Porosity, Lithology, Resistivity logs,
Chapter 5:………………………………………………… 55
INTERPREATION and
CHARACTERIZATION OF RESERVOIR
Calculation of R w, ø eff, V sh, S w
Preparation of Para log
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Chapter 1
INTRODUCTION
Wire line logging
The complete evaluation of petroleum bearing reservoir includes
data from several sources – 3D-seismics, V.S.P., mud logging, coring,
M.W.D-L.W.D, wire line logging, pressure tests, and sampling.
The science of petroleum bearing reservoir evaluation encompasses a
general knowledge of all these disciplines, while certain individuals may be
specialized in a specific discipline such as seismic interpretation ,log
analysis ,core analysis ………
In this connection research and development programs of an oil &
gas company are devoted to the investigation of the properties of reservoir
rocks and fluids and how they related to measurable properties.
Wire line well logging operations provide measurements of bore hole
and formation properties at accurately measured depth.
With a few exceptions, petroleum company personnel (i.e., geologist,
geochemist, geophysicist…etc.) are interested only in how wire line
measurements are related to information they need: physical & chemical
properties of reservoir not the tool functions.
For locating the petroleum bearing reservoir require an
understanding of nature of the subsurface sedimentary formations, and well
logs are important method of acquiring such information. Wire line well
logs are particularly useful in describing and characterizing reservoirs.
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Log measurements can define or at least infer these formation
properties such as porosity, shale volume, litho logy, and water, oil, or gas
saturation. Estimation of permeability, prediction of water cut, selection of
over pressure zones and calculation of residual oil can also be made. Log
analysis is primarily used to describe formation properties in a single well.
Quite normally, log and core data are often compared and used in
conjunction to define reservoir properties. When cores are not available, log
data are often used as extension from core analysis and log comparisons on
other wells.
However, when a suite of logs in run is several wells representative
of a specific geological area, it can be used as a geological tool to
understanding subsurface formations by describing local geology,
stratigraphy, environment of deposition and reservoir geometry in our
present conditions.
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Chapter 2:
PROPERTIES OF RESERVOIR
Properties of rocks and fluids
The characterization of reservoir requires reliable knowledge of certain
fundamental reservoir properties. Log measurements can define or at
least infer these properties: resistivity, porosity, shale volume, litho logy,
and water, oil, or gas saturation and permeability.
1) Resistivity: The opposition to flow of electrical current offered by a
material 1m long, with a cross sectional area of 1sq m
Denoted by:R
Units :ohm m.
In combination with record depth, resistivity was the first formation
parameter measured by wireline logging technique. The log -resistivity
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measurements are the function of compaction (amount of porosity) of
rocks and fluid conduction (salinity).
For ex: 1) Sand stone with low porosity : very high resistivity
(high compaction )
2) Sand stone with high porosity :
(low compaction)
pores filled with gas/oil : very high resistivity
pores filled with fresh water : high resistivity
pores filled with saline water : very low resistivity
Then we can imagine the other Lithology conditions how affect the log
measurement. The high resistivity is an indication of oil/gas or high
compacted rocks.
True resistivity: the resistivity of the true /un invaded zone that is
beyond the transaction zone.
It is denoted by Rt.
Formation water resistivity: true resistivity of the formation water in
the un invaded water bearing zone. it is also true resistivity
It is denoted by Rw.
Invaded zone resistivity: the resistivity of invaded zone between the
mud cake and the transaction zone.
It is denoted by Rxo.
Filtrate resistivity: the resistivity of mud filtrate .that is in the invaded
zone.
It is denoted by Rmf.
In general the resistivity means true resistivity. Information about
formation water resistivity, invaded zone resistivity, filtrate resistivity is
useful in the calculation of true resistivity.
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2)Porosity: It is defined as total volume of sample that is occupied by pores or
voids
Or pore volume per unit volume of the formation.
It denoted by ø. Its units are API units .It is measured as percentage (%).
Porosities are classified according to the physical arrangement of the
material that surround the pores and to the distribution and shape of the
pores.
1) Primary porosity: The pore space exist between individual grains of
rock matrix during deposition is called intergranular, or matrix porosity/
primary porosity.
2) Secondary porosity: The pore space created by the action of
formation water or tectonic forces on the rock matrix after deposition is
called facture porosity/ secondary porosity.
These is another classification of porosity is effective porosity ,isolated
or non effective porosity.it is shown in the figure 1
Fig1 Fig 2
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Unit cells of two systematic packings of uniform spheres are shown in Fig.
2The porosity for cubical packing (the least compact arrangement) is
47.6% and for rhombohedra packing (the most compact arrangement) is
25.96%
Effective porosity: the porosity that is due to inter connected pores in the
formation.
Non effective or isolate porosity: the porosity that is due to isolate /closed
pores in the formation.
3)Permeability: It is defined as measure of the ease with which fluids can
flow through a formation.
It is denoted by K. Its units are Darcys; which is very large, the
millidarcy (md) is generally used.
In order to permeable rock must have some interconnected pores,
capillaries or fractures. Hence there exist some rough relation ship between
porosity and permeability. Greater permeability in general, corresponds to
greater porosity, but this is far from being an absolute rule.
Shales and some sands have high porosities ,but the grains so small
that the paths available for the moment of fluid are quite restricted and
their permeability may be very low. Other formations ,such as lime stone
composed of dense rock broken by a few small fissuies or fractures of great
extent .The porosity of a such a formations can be low ,but the permeability
of a fracture can be enormous.
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Permeability
It is simply a measure of the capacity of a reservoir rock to transmit
fluids
Absolute permeability
It refers to the permeability where in a reservoir rock only single fluid
is present.
Effective permeability
This is defined as the permeability to one fluid in a multi fluid system
i.e. the permeability to a fluid when its saturation is less than 100%.
Relative permeability
Relative permeability indicate the ease with which one fluid of the two
or more fluids present will flow through connecting pore spaces in the
presence of each other as compared to the ease with which one fluid will
flow when it alone is presen
4)Saturation: It is defined as fraction of its pore volume occupied by the fluid.
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Water saturation : It is a fraction of pore volume that contains formation water.
The symbol for saturation is Sw
Hydrocarbon saturation: it is a fraction of pore volume that contains hydrocarbons.
The symbol for saturation is Sh
Oil or gas saturation is a fraction of its pore volume that contains Oil or gas.
The pores must be saturated with some fluid .thus the summation of all saturations in a
given formation rock must total to 100%.
Sh, =( 1- Sw ).
The water saturation is the most important parameter in cauterizing the reservoir for
estimating hydrocarbon saturation.
5)Shale volume: the volume of Shale(mixture clay minerals) present in the
reservoir rock is the shale volune.
Typical shale consists of 50% clay minerals, 25% silica, 10% feldspar, 10% carbonates,
3% iron oxide, 1% organic material and also 2-40% water by volume.
Modes of Occurrence shale
Shale or clays minerals occur mainly as:
Laminated Shale
Dispersed Shale,
Structural Shale
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Laminated Shale
Laminated shale refers to thin lamination of clay minerals of an inch to many inches in
thicknesses that are inter-bedded with clean sand (Figure 2.1). The effective porosity and
the permeability of the shales are essentially zero so the overall porosity and
permeability of the reservoir rocks are reduced in proportion to the fractional volume of
the shale
Dispersed Shale
Dispersed clay occurs as disseminated particles in the pore spaces of the sand and
replaces the pore fluid. This type of distribution is very damaging to the reservoir quality
as it chokes the pores and reducing the effective porosity and permeability of the
reservoir unit (Figure 2.2).
Structural Shale
Here aggregates of the clay particles occurs and they takes the place of the sand
grains that is occur as framework grains of the reservoir along with the sand grains
(Figure 2.3). Here the porosity and permeability of the reservoir rock is affected very
little
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Chapter 3:
OPEN HOLE TOOLS and
INSTRUMENTATION
Electrical, Radio Active, sonic and other tools
Measurement Principles
In the field operation wire line logging is done a mobile
laboratory, logging truck .It carries the down hole measurement
instruments, the electrical cable and winch needed to lower the instruments
in to the bore hole, the surface instrumentation is needed to power the down
hole instruments and to receive and process their signals, and the equipment
needed to make a permanent recording of the Log.
LOGGING UNIT:
Logging service companies utilize a variety of logging units,
depending on the location (onshore or offshore) and requirements of the
logging run. Each unit will contain the following components:
Logging cable
Winch to raise and lower the cable in the well
Self-contained 120-volt AC generator
Set of surface control panels
Set of down hole tools (sondes and cartridges)
Digital recording system
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Well logging is a wire line operation where the physical
parameters of various formations encountered in well, are measured by
lowering the logging tools as a function of depth.
These measurements help to understand the well/reservoir formations
behavior.
Finally
 Well log is a continuous record of measurement made in bore hole
respond to variation in some physical properties of rocks through
which the bore hole is drilled.
 Traditionally Logs are display on girded papers shown in figure.
 Now a day the log may be taken as films, images, and in digital
format.
The down hole measurement instruments are composed of two
components. One component contains the sensor, called sonde. The
component of the down hole tool is cartridge, contain the electronics that
powers the sensors, process and transmit signals to the truck. The down hole
tool is attached to an electrical cable that is used to lower the tool into and
remove from the well. the cable contain seven insulated copper conductors
or a fiber optic conductor along with six conductors.
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LOGGING TOOLS:
In practice the open hole wire line logging tools are used to
measure the various parameters that influences the porosity, permeability
and saturation of the formation. Various types of logging methods are used
to determine the formation properties.
The logging tools are classified along with logging methods based on the
measurement principle. There are four principle logging methods are in use:
Electrical logging,
Radio Active logging,
Sonic logging, and
Miscellaneous logging.
ELECTRICAL LOGGING TOOLS:
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In the electrical logging tools SP, Focused resistivity &micro resistivity
and induction tools are in use.
The SP tool together with Normal and Lateral devices is called
conventional ES tools. The focused / non focused a resistivity &micro
resistivity and induction tools are called non conventional ES tools.
Conventional Electrical Survey Tools:
SP Tool
Tool principle:
The SP tool the records the naturally occurring electrical potential
(voltage) which is produced by the interaction of formation connate water,
conductive drilling fluid and certain ion-selective rocks (shale). It measures
the potential difference between the movable electrode (A) in the borehole
and the fixed surface /reference electrode (B).
It cannot record the potential difference in holes filled with
nonconductive muds because such muds do not provide electrical continuity
between the SP electrode and the formation. Furthermore, if the resistivities
of the mud filtrate and formation water are about equal it will record the SP
with less significant features
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Normal Device:
In a normal device, current is passed between the two electrodes A
and B (Figure 3). The resultant potential difference is measured between
the two potential electrodes M and N. Electrodes A and M are on the sonde.
Electrodes B and N are located at and infinite distance theoretically on the
surface.
The distance AM is called spacing of the tool which is 16 inches
(0.40 m) for short normal and 64 inches (1.62 m) for long normal. The point
of the reference is O, which is located at the center of the electrodes A and
M.
Lateral Device:
In a lateral device, current is passed between the electrodes A and B.
the resulting potential difference is measured between the electrodes M and
N (Figure 4). These potential electrodes are located on the sonde. Here the
point of inscription is O, which is the mid point of electrodes M and N. The
spacing AO is 18 ft. 8 inches (5.7 m).
This device differs from the normal device in the sense that here the
position of the current and potential electrodes has been changed. Generally
longer the spacing the deeper the radius of investigation of the tool.
Therefore the lateral device has deeper depth of investigation than of the
normal device.
Non Conventional Electrical Survey Tools:
Focused resistivity tools:
The response of the conventional electrical logging systems can
be greatly affected by the borehole and adjacent formations. A family of
resistivity tools that uses focusing currents to control the path taken by the
measure current minimizes these influences. These current are emitted from
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special electrodes on the sondes. The focusing electrode tools include the
Laterolog (LL) and Spherically Focused Logs (SFL).
These tools are much superior than the conventional logs and for
also highly resistive adjacent formations. They are also better for resolution
point of view for thin to moderately thick beds. Focusing electrode systems
are available with deep, medium and shallow depths of investigation.
Focused electrical logging tool (DLL)
The deep-reading devices include the Laterolog 7, the Laterolog 3,
and the deep Laterolog and Dual Laterolog tool (DLL). The medium- to
shallow-reading devices are the Laterolog 8 of the Dual Induction-
Laterolog tool (DIL), the shallow Laterolog of the DLL tool.
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Non -focused micro-resistivity tool:
Micro resistivity devices are used to measure resistivity of the flushed
zone, Rxo, and to delineate permeable beds by detecting the presence of mud
cake. The example of such tool is micro log.
Focused micro-resistivity tool:
These are also used to measure resistivity of the flushed zone, Rxo,
and to delineate permeable beds by detecting the presence of mud cake. The
example of such tool is micro spherically focused logging (MSFL).The
Micro SFL is a pad-mounted spherically focused logging device that has
replaced the micro laterolog and Proximity tools.
It has two distinct advantages over the other Rxo devices. The first is
its combinability with other logging tools, including the DIL and DLL tools.
This eliminates the need for a separate logging run. The chief limitation of
the micro laterolog measurement is its sensitivity limitation to mud cake.
The proximity logs are insensitive to the mud cake.
Tools principle:
In order to minimizes mud cake effect select the electrodes spacing
and bucking-current control. The surveying current flows outward from a
central electrode, Ao. Bucking currents, passing between the electrodes, Ao
and A1, flow in the mud cake and, to some extent, in the formation. The
measuring current, Io, is there by confined to a path directly into the
formation, where it quickly “bells” out and returns to a remote electrode, B.
To achieve this, the bucking current is adjusted to make the monitor voltage
equal to zero.
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By forcing the measure current to flow directly into the formation, the effect
of mud cake resistivity on tool response is minimized; yet, the tool still has a
very shallow depth of investigation
Induction tool:
The induction-logging tool was originally measure formation resistivity in
boreholes containing oil- base muds and in air-drilled boreholes. Induction
tools have many transmitter and receiver coils. The principle can be
understood by considering a sonde with only one transmitter coil and one
receiver coil .
Tools principle:
The high frequency current is sent through a transmitter coil and an
a.c magnetic field created induces currents in the formation surrounding
the borehole. These currents flow in circular ground loops coaxial with the
transmitter coil and create, in turn, a magnetic field that induces a voltage
in the receiver coil. Because the alternating current in the transmitter coil is
of constant frequency and amplitude, the ground loop currents are directly
proportional to the formation conductivity.
The voltage induced in the receiver coil is proportional to the ground
loop currents and, therefore, to the conductivity of the formation. There is
also a direct coupling between the transmitter and receiver coils. Using
“bucking” coils eliminates the signal originating from this coupling.
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The induction tool also works well when the borehole contains
conductive mud unless the mud is too salty, the formations are too resistive,
or the borehole diameter is too large.
RADIO ACTIVE LOGGING TOOLS:
The GR together with Neutron and Density tools are called Conventional Radio Active
Tools. These tools detect either Neutron or Gamma ray.
Scintillation counter:
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GR TOOL:
It measures the natural radioactivity of the formations. In sedimentary
formations its reading reflects the shale content of the formations. The most
radioactive elements tend to concentrate in clays and shales not in clean
formation.
Tool principle:
The gamma rays are emitted by the radioactive elements like U, Th, K
in the formations, and detected by the suitable gamma ray sensor (typically
scintillation detector, 8 to 12 inches in active length). The detector gives a
discrete electrical pulse for each gamma ray detected. The parameter
recorded is the number of pulses recorded per unit of time by the detector.
DENSITY TOOL:
Tool principal:
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In this technique a radioactive source is applied to emit medium-energy
gamma rays into the formations. These gamma rays may be thought of as
high-velocity particles that collide with the electrons in the formation. This
type of interaction is known as Compton scattering.
The scattered gamma rays reaching the detector, at a fixed distance
from the source, are translated in terms of formation density. The number of
Compton-scattering collisions is related directly to the number of electrons
in the formation. . Electron density is related to the true bulk density, ρ b,
which, in turn, depends upon the density of the rock matrix and density of
the fluids and the density of the formation.
The depth of investigation of density tool is quite shallow. Most of the
density tool signal comes from a region less than 8” from the borehole wall.
The CNL tool, gathers most of its signal from the region within 12” of the
borehole wall. Thus, the density tool less affected by light hydrocarbons
than the CNL tool.
NEUTRON TOOL:
Tool principal:
Neutron tool respond primarily to the amount of hydrogen in the
formation. Neutrons are electrically neutral particles, each having a mass
almost identical to the mass of a hydrogen atom. High-energy (fast)
neutrons are continuously emitted from a source in the sonde. These
neutrons collide with nuclei of the formation materials in what may be
thought of as elastic “billiard-ball” collisions. With each collision, the
neutron loses some of its energy.
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The amount of energy lost per collision depends on the relative mass of the
nucleus with which the neutron collides. The greater energy loss occurs
when the neutron strikes a nucleus of practically equal mass - i.e., a
hydrogen nucleus. Collisions with heavy nuclei do not slow the neutron very
much. Thus, the slowing of neutrons depends largely on the amount of
hydrogen in the formation.
SONIC LOGGING TOOLS:
SONIC TOOL:
A sonic tool consists of a transmitter that emits a sound-pulse and a
receiver that picks up and records the pulse as it passes through the
formation and reaches to receiver.
Sonic- Tools:
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Tool principal: A transmitter sends compressional or longitudinal waves
into the borehole fluid (mud). The compressional wave incident on wall gets
refracted into the formation. Acoustic pulse sent into the borehole mud
received by the receiver is measured. These waves do not reach the receivers
at the same time, but at different times depending upon the path traversed
and the velocity of the medium. One path is 2ft longer than the other.
Array sonic tool :It contains an array of 8 piezoelectric receivers 2
piezoelectric transmitters ,receivers are spaced 6in apart with the closest
receiver 8 or 11.5feet from the upper transmitter. Two of these receivers,
1and 5th spaced 2feet apart ,can be used for slandered LONG SPACED
SONIC(LSS) ,three of these receivers ,6,7and 8th spaced 2 feet apart ,can be
used for DEPTH DERIVED BHC (DDBHC).
Sonic- Tool
It record full wave form with the help of 8 receiver array.
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CALIPER
The wire line caliper measures continuously the diameter of the
boreholes drilled. There are many types of caliper devices and design varies
widely. Some calipers are exclusively run for recording diameter. But quite
often it is run in combination with other tools, especially with micro
resistivity tools.
The diameter of the borehole is very useful parameter required to
 Detect caving and decrease in hole size, so that amount of cement
required for the casing can be calculated.
 To select depth at which packer of DST tool can be set.
 For log interpretation, mud cake thickness is important parameter
required for correcting micro resistivity logs, sidewall neutron
porosity log etc.,
 Detection of mud cake thickness itself is a good indication of
permeable nature of beds quite often this may be only way of detecting
permeable beds
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Checking the correction of caliper logs:
This can be done best by recording the diameter inside the casing
present at the top part of the well. Reading given by different calipers in the
same hole may be different depending on the caliper design combined with
the hole cross-section
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Chapter 4:
OPEN HOLE LOGGING AND LOGS
Logging operation
Porosity, Lithology, Resistivity logs,
What is logging? What is a log?
Logging: The process of continuous recording of measurements corresponds
to different properties of formation in a well
To obtain comprehensive information about the formation, some
electrical instrument will be lowered into the well.
Log: The record of comprehensive information about the formation in a
well during logging process.
Also print of all the data acquired in his well.
The prestigious organization Well Logging Services –O.N.G.C.
provides a wide range of service and information, allowing their ASSET to
define, reduces and manages their risk operations.
It provides the following services
Open hole logging services.
Cased hole logging services.
Production logging services.
Data Processing and Interpretation.
There are three suits in Open hole logging services.
Suit 1: Rt, Gamma Ray, SP, Caliper.
Suit 2: Sonic, Neutron, Density, Gamma Ray.
Suit 3: R.F.T, Side wall sampler, Dip meter.
These three suits provide the information about Lithology, Porosity,
and Resistivity of the Formations in the Bore hole.
In these logs GR and SP provides the information about litho logy. So these
are called as litho logy logs. Some other logs infer the porosity and
resistivity.
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Litho logy logs: GR and SP.
Porosity logs: Neutron, Formation density, Sonic.
Resistivity logs: Conventional, Focused, Induction, Micro resistivity logs.
LOGGING
Wireline logging operations provide measurements of bore hole and
formation properties at accurately measured depths. The measurements are
made under pseudo-dynamic conditions: bore hole fluid is static during
logging operations, the measuring device is ascending the bore hole while
the measurements are being recorded. there are few exceptions: some tool
are held stationary while measurements made ,some tools are moving while
formation fluids are enter or exit the bore hole and some tools are
descending the bore hole while the measurements are made.
LOG
The Log must be containing the data acquired in well and all the
necessary information to make an accurate interpretation of reservoir.It
must be in the form of standard presentation, known tool configuration,
environmental conditionsIt must be contain corrected tool response in
known conditions and calibrated for specifications
It must be documented correct operating procedures and deviations from
that.
Components of log: log header , log tool sketch, log remarks ,log
calibration ,log tail
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.
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.
The content of a Log Header is all general information about the well and
job such as Company name, Well name, Tools run, Interval logged Mud
record Deviation data…
Log Tool sketch is that which tool has been used to record the data
Must have tool numbers Sensor offsets…
The content of a Log Main log is a record of the required data vs.
depth over the whole well.
Color codes & scales defined by client, presented in several scale
(1/500 and 1/200)
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The content of a Log Remarks is disclaimer and additional information
that will help for the interpretation
The content of a Log Calibrations is a proof that all operational checks
have been performed before running in hole and after pulling out of hole.
Tool was reading correctly in air. Last time we check the tool at the base in
known conditions, it was reading within tolerance
The content of a Tech Log is tool performance while logging and it tells the
engineer ,if the tool was working fine.
Monitoring of hardware (voltages, current) Green is good, Red is bad!
The content of a Log Tail is end of the log
In this document our study limited to very few logs: GR, SP, Neutron,
Formation density, Sonic,, Focused, Induction, Micro resistivity logs and
Caliper.
Understanding the Log Measurements:
The logging instrumentation responds mostly to pore materials
and the chemical makeup (composition) of the rock matrix. As a result a
chemical rock classification is most suitable for use in log analysis. Rocks in
a well have rather unique log responses that are usually identified easily.
In order to understand log measurements and the methods of
obtaining these data, their must also be a general knowledge of other date
associated with logs.
The log is a record of the events leading up to and during the
drilling and completion of a bore hole. Some of the information of the log
header ,…….. is not a data from measurements taken by the wire line
logging tool.
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It is often useful in determining;
Why some log responses are questionable,
Why the logging instruments could not reach total driller depth
or
Why a logging instrument became stuck at a certain depth.
Some of the information is measured at the surface by the
logging crew and can have great importance in formation evaluation. The
information should be acquired and reported accurately.
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LITHO LOGY LOGS
Spontaneous Potential (SP) log
Gamma Ray (GR)log
Introduction:
The lithology identification logs, Spontaneous Potential (SP) and Gamma
Ray (GR) are recordings of naturally occurring phenomena in in-situ
conditions recorded as track 1 .
SP curve records-the electrical potential produced by the interaction
of formation connate water, conductive drilling fluid, and certain ion –
selective rocks (shale).
Natural GR Log indicates –the total natural radioactivity of the
formations.
THE SPONTANEOUS POTENTIAL (SP) LOG:
It is a permeability indicator; the magnitude of the SP deflection and
permeability of a formation have no direct relationship. however ,when the
mud is saline than connate formation water, permeable beds are often
delimited by negative SP excursions.
R mf > R w or R xo > R t implies Negative SP.
SP deflection normally occurs only if permeability exist to allow
ion migration between the mud and formation.
In many cases, a good value of Rw can easily be found from the SP-
curve recorded in clean water bearing formations.
From the SP Curve: SSP = -K log (Rmf / Rw)
Uses of SP:
 Detect the permeable beds
 Locate the bed boundaries and to permit correlation of such beds.
 Determine Rw
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 Give qualitative indication of bed shaleness
THE GAMMA RAY (GR) LOG
Introduction:
It is a good shale indicator, it measures total gamma ray emissions form the
formations. It is scaled in API units.It is also affected sometimes due to
borehole conditions.
The volume of shale (Vsh)in the reservoir rock can be estimated from the
deflection of the gamma ray curve.
In cased holes, gamma ray log is used as a depth control. Also it is used to
position the formation testers and sidewall core guns.
It is also used in radioactive tracer operations to locate pipe leaks,
channeling behind casing.
Uses:
.
 Correlation
 Evaluation of the shale content analyses
 Mineral analyses.
Main use of SP &GR:
- To differentiate porous and permeable reservoir rock (SST, LST,
and DOLOMITE) from non permeable clays and shales.
- Define bed boundaries and permit correlation of beds.
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POROSITY LOGS
Density log
Neutron log
Sonic log
Introduction:
Rock porosity can be obtained from the sonic log, the density log or
the neutron log. Here the tool response is affected by formation porosity,
fluid and matrix. If the fluid and matrix effects can be determined the tool
response can be related to porosity. Here the depth of investigation is only a
few inches generally with in the flushed zone.
DENSITY LOG
The density log is a measurement of scattered gamma rays reaching
the detector, at a fixed distance from the source, are translated in terms of
formation density.
The number of Compton-scattering collisions is related directly to the
number of electrons in the formation. Consequently, the response is related
to the true bulk density, ρb, which, in turn, depends upon the density of the
rock matrix and density of the fluids and the density of the formation.
Uses:
.
 Calculation of porosity
 Identification of hydrocarbon bearing zones
 Estimation of bulk density of formation
Bulk density: ρ b = Φ ρ f + (l - Φ) ρ ma
Porosity:
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NEUTRON LOG
Introduction:
The Neutron log measurement of the slowdown neutron counts. The
neutron collides with formation, after sufficient number of collisions the
neutron will reach a lower energy state where upon they are captured by
formation nuclei. When a nucleus captures a thermal neutron, it dissipates
the energy and slows down.
The neutron tool responds to porosity but they are also influenced by
other parameters and certain environmental effects: borehole fluid type,
density, salinity, borehole size, mud cake, stand off temperature and
pressure.
Uses:
 Identification of gas bearing formations
 Estimation porosity (mainly liquid filled)
 Determination of formation fluid type
 Determination of lithology
Special application:
In cased hole it is used for correlation and depth control for perforation
Depending on the device, these measurements may be made either in open
or cased holes.
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SONIC LOG
Introduction:
The sonic log is a measurement the speed of sound waves in formations or
the interval transit time. These measurements are useful for a number of
reasons in many professional
The interval transit time for a formation depends upon its litho logy
and porosity. This dependence upon porosity, when the litho logy is known,
makes the sonic log very useful as a porosity log. Integrated sonic transit
times are also helpful in interpreting seismic records.
Porosity:
(Wyllie- Time Average Equation)
∆t log = Φ∆tf + (1-Φ) ∆tma
Φs = (Δtlog- Δtma)/ (Δtfl- Δtma)
Where Δtma is the travel time of the sonic wave through the rock matrix,
 For SST: 55.5μsec/ft
 For LST: 47.5μsec/ft
 For DOL: 43.5μsec/ft
 Δtfl is the travel time of the sonic wave through the fluid; it is
generally 189μsec/ft.
Uses:
 Estimation of porosity
 Identification of lithology and factures
 Integrated travel time and velocity for seismic interpretation
 Identification of Cement behind the casing.
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RESISTIVITY LOGS
Normal log
Lateral log
Latro logs
Introduction:
The resistivity log is a record of potential variation (or its equivalent,
apparent resistivity) versus depth. the resistivity is a function of measured
potential difference and sending current into the formation. It is sensitive to
rock properties such as porosity, shaliness, compaction or degree of
sedimentation, pore distribution and pore fluids.
In general the formations encountered in oil wells are poor conductors,
having resistivities in the range 0.2 to 1000 ohm-m.
The resistivity of the formation depends on:
 Resistivity of the formation water
 Amount of water present
 Pore structure geometry
 Fluid type
In addition to the deep responding resistivity tools, a number of
shallower responding resistivity devices are available for the measurements
of Rxo and Rt.
DUAL LATRO LOG (DLL):
he dual latro log is a set of record of resistivity, there are two records: DLL
(latro log deep) and LLS (latro log shallow). Its response is mostly
dependent upon the true formation resistivity.
However LLS reading is useful to get true resistivity from LLD reading, and
most of the times LLD is very close to the true resistivity.
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Uses:
 Estimation of true resistivity
 Identification of diameter of invasion.
MICROLATRO LOG (MLL):
The micro latro log is a record of measured resistivity of the flushed
zone, Rxo, and to delineate permeable beds by detecting the presence of mud
cake.
Response of MLL depends upon the Rxo/Rmc ratio as current is
prevented from flowing into mud cake. The depth of investigation of this tool
is three to five inches, so even if invasion is low or moderate, MLL responds
to invaded zone. Effect of mud cake is negligible up to cake thickness of 3/8
inches but increase rapidly with greater thickness of cake. MLL
measurements are not preferred where mud cake thickness is (greater than
3/8 inches).
Uses:
 Identification of permeable beds.
 Information of flushed zone resistivity.
MICRO SPHERICAL FOCUSED LOG (MSFL):
The micro spherical focused log is a record of measured resistivity of
the flushed zone, Rxo. is the shaping of the equipotent surface produced by
resistivity device to approximately spherical form. A careful selection of
electrode spacing achieves an optimum compromise between too much and
too little depth of investigation. MSFL gives near true Rxo value in thick
mud cake and low invasion conditions.
Uses:
 Information of flushed zone resistivity in low invasion conditions .
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Chapter 5:
INTERPREATION and
CHARACTERIZATION OF RESERVOIR
Calculation of R w, ø eff, V sh, S w
Preparation of Para log
Interpretation and characterization is an art of science through its
systematic application of rules based on past experience to assign and to
validate the geologic framework and composition to the reservoir.
Selection of control Parameters:
Before the well log data interpretation The chosen log data interval
select the control parameters, such as
 Rsh → Resistivity value against shale from Resistivity log
 Rlim → Maximum Resistivity observed on resistivity log
 Rw → Resistivity of formation water (to be estimated)
 ΦNs → Neutron porosity against shale from Neutron-Density log,
Near the zone of interpretation
 Rhob sh → Density value against shale from density log, near zone of
Interpretation.
 ΦNsh → Density porosity of the shale (calculated using empirical
formula)
 ΦS sh → sonic porosity of the shale (calculated using empirical
formulae)
 Rhobmat → Density of the matrix
 GR min→ Minimum gamma ray count in the GR log, (form clean bed)
 GR max → Maximum gamma ray count in the GR log (form shale bed)
 SSP → Minimum SP curve deflection in SP-log (clean)
 B.H.T → Borehole temperature at the bottom of the well.
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Determination of Rw from the SP:
Formation water, sometimes called connate water or interstitial
water, is the water uncontaminated by drilling mud that saturates the porous
formation rock. The Resistivity of this formation water is an important
interpretation parameter since it is required for the calculation of
saturation’s (water and/or hydrocarbons) from basic logs. There are several
sources for formation water Resistivity information. These include water
catalogs, the spontaneous potential (SP) curve, and various Resistivity –
porosity computations and cross-plots.
In many cases, a good value of Rw can easily be found from the SP-
curve recorded in clean water bearing formations. The static SP (SSP) value
in a clean formation is related to the chemical activities (a w and amf) of the
formation water and mud filtrate through the formula:
SSP = -K log (aw/amf)
For Nacl solution, K=71 at 77°f (25°C); k varies in direct proportion to
temperature:
K=61+0.133T°F
K=65=0.24 T°C
For pure Nacl solution that not too concentrated, resistivities are
inversely proportional to activities. However, this inverse proportionality
does not exactly at high concentrations or for all types of waters. Therefore
equivalent resistivities Rw and Rmf, which by definition are inversely
proportional to the activities, are used, Rw is the equivalent formation water
resistivity and Rmf is the equivalent mud filtrate Resistivity.
SSP = -K log (Rmfe / Rwe)
Knowing the formation temperature, the static SP value recorded
opposite a porous, permeable, nonshaly formation can be transformed into
the resistivity ratio (Rmf / Rw).
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The Rmf value at surface is given. The Rmf value at particular depth is
calculated from below formula. i.e.
Temp gradient =(Td -Ts)(100)/depth difference.
Where
Td=Temp in borehole at bottom depth
Ts=Temp at surface
Formula:-
Temp in particular depth=surface temp+(temp gradient*given depth)/100
Rmf at given depth =( (Ts+6.77)/(temp at given depth+6.77))*Rmf at
surface
For given SP value at particular depth plotted on SP chart-1 then we
find Rmfe/Rwe
After that Rmf value plotted on SP chart-2 at temp of given depth then we
can estimate corresponding Rmfe value
From this Rmfe value we can find Rwe by using Rmfe/Rwe value
After that Rwe value plotted on SP chart-2 at temp of given depth
corresponding Rw value is estimated
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Formation factor:
Archie’s experiment show the resistivity of a clean formation is
proportional to the resistivity of the brine saturating rock. The constant of
proportionality is known as formation resistivity factor (F).. Also his
experiment concludes an empirical relationship between formation factor
and porosity.
and
where
m=cementation exponent
a =Archie’s constant
The most widely used Archie’s relation between F and Φ for sands is,
And other relationships for sands HUMBLE formula, TIXIER formula
respectively below.
The formation factor in term of resistivity as follows:
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Determination of Rw from Hingle cross plot:
In the late 1950’s Hingle proposed a method based on resistivity &
porosity log data which allows the percent water saturation to be
determined directly from a cross plot. The method is based on the well
known archie’s equation, which in a rearranged form is plotted on special
grid type graph paper.
Plotting procedure is outlined as follows,
 Select proper cross plot paper
 Taking the x-axis in linear fashion for raw logging parameters (∆t, ρb
) and establish porosity scale. Porosity will be zero at the matrix
point and increases to the right. Taking Y-axes axis in logerthemic
fashion for raw log data (Deep resistivity,R t)
 Plot the resistivity (Rt)Vs (∆t, ρb, ΦN). The resistivity scale can be
changed by any order of magnitude to fit the log data. This is done
without changing the validity of the graph paper grid.
 The straight line drawn through the most north-westerly(clean)points
defines Sw=1. Extrapolate this to the intersection with X-axis( Φ =0 )
.
 At the intersection determine the matrix value (∆tma or ρma ) for a
proper porosity scaling of the X-axis.
 Calculate Rw from any corresponding set of Φ and Ro data along the
water line such as Rw =Ro/F.
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Estimation of water saturation:
Resistivity ratio method:
When a borehole is drilled, the formation close to the borehole is
invaded with mud filtrate. In an oil bearing zone, we will normally have a
zone of low resistivity close to the borehole, and one of higher resistivity
further away. Thus a comparison of a deep resistivity device with a shallow
resistivity device will detect hydrocarbons. Form the Archie’s equation we
can derive an expression for water saturation as a function of the ratio of
these two curves
The saturation in terms of porosity as follow :
The saturation of water in term s of resistivity as follows:
And
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Shale corrected water saturation:
(Indonesian equation)
Shale Volume Calculation: The natural gamma ray log can be
used to calculate volume of shale in porous reservoirs. The volume of shale
expressed as a decimal fraction or percentage is called Vshale.
Calculation of the gamma ray index is the first step needed to determine the
volume of shale from gamma ray log.
The gamma ray log has several nonlinear empirical responses as well a
linear responses. The non linear responses are based on geographic area or
formation age. All non linear relationships are more optimistic that is they
produce a shale volume value lower than that from the linear equation.
Linear response (Vshale = IGR) :
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There are several formula for shale volume calculations show in below:
Estimation of Effective Porosity:
A quick look estimate of porosity can be made simply by reading the
neutron log and Density log using the limestone porosity scale, and taking
the average of the readings.
For clean formation
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In the presence of shale
Where,
 Φe is effective porosity
 ΦN is neutron porosity
 ΦNsh shale corrected neutron porosity
 ΦD is density porosity
 ΦD sh shale corrected density
The minimum value of Rw is considered for Sw calculation based on the
assumption Rw values estimated above are on the higher side.
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Reservoir Characterization Parameters
The petroleum reservoir is that portion of the rock that contains the
pool of the petroleum. Each reservoir is unique in its details. In order to
characterize a reservoir, there are certain parameters which have to be
estimated. The main reservoir characterization parameters being porosity,
permeability and its saturation.
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Conclusion
Determination of Rw, Vsh, Φeff and Sw are estimated in water
bearing & hydrocarbon bearing formations taking sample data acquired in
well. The same sets of log data are processed using Geoframe software
package available with workstations in Well Logging Services, GOC (Gas
Oil Contact), OWC (Oil Water Contact) and GSC(Gas Shale Contact) where
ever observed are marked in the parameter logs.
The dissertation work on carried out during the period 19th May 2009
to 19th Jun 2009 gave an insight into the details of formation evolution and
this provided valuable knowledge & experience . This will be useful further
studies and job assignments.
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