well logging project report_dileep p allavarapu

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.

Figure 1.4 Spherically Focused Log

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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, ρb,

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 (aw 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,Rt)

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.