Estimation of Petrophysical parameters of well log data

A thesis Submitted in partial fulfillment of the requirement

For the award of degree of

Master of Science in

Geophysics

By MANISH KUMAR SINGH

M.Sc. Geophysics (07EX4002)

Under the guidance of Dr. A. K. BHATTACHARYA

Associate Professor

Department of Geology and Geophysics Indian Institute of Technology

Kharagpur-721302 2009

Acknowledgement I am very much grateful to respected Dr. A. K. Bhattacharya who helped me

through this endeavour and providing me all possible facilities from time to time.

This work wouldn’t have been completed without his support.

I am thankful to Dr.A.K. Gupta,Head of the Department of Geology and

Geophysics, IIT Kharagpur for providing me the necessary facilities for pursuing

this work.

Literally words are unable to express my heartful sentiments to my parents without

whom I would have never been able to accomplish my academic pursuit and

whose blessings guide me through all thick and thin.

Last but not the least I would like to thank my friends who have helped in one or

the other way in making this project successful.

MANISH KUMAR SINGH

M.Sc.(Geophysics)

Abstract The main objective of this project is to evaluate the petrophysical parameters of a

well data for hydrocarbon bearing zones. This well data is of Assam basin having

the reservoir in Tipam formation.

Gamma ray log, Spontaneous potential log, Resistivity log (LLD, LLS and

MSFL), Neutron log and Density log have been used here for evaluating the

reservoirs. This petrophysical evaluation is done by available formulae using

Archie’s Equation.

Shale amount is less in the reservoir zones except zone one. Resistivity values are

quite high for two zones which are probably due to the presence of gas. All the

reservoir zones are having low bulk density along with neutron-density crossover

which indicates that all of these are gas zone.

The interpretation result shows five main prospective zones having good porosity

and high movable hydrocarbon saturation.

CONTENTS CHAPTER 1- Introduction

1.1 Need of well logging

1.2 Recent developments in well logging

CHAPTER 2- Data availability and Scale

CHAPTER 3- Geology of Assam Basin

3.1 Introduction

3.2 General Geology

3.3 Tectonics and Depositional History

CHAPTER 4- Basic Principle of Different Logs

4.1 Spontaneous potential log

4.2 Resistivity log

4.3 Gamma Ray Log

4.4 Caliper Log

4.5 Neutron porosity log

4.6 Density Log

CHAPTER 5- Processing steps used for Petrophysical Analysis

CHAPTER 6- Petrophysical Analysis

CHAPTER 7- Discussion and Conclusion

References

Chapter:1 Introduction NEED OF WELL LOGGING: There is no instrument available for indicating the presence of oil underground. Thus, geological and geophysical methods have been found to the most satisfactory for locating oil, but these are indirect methods .The techniques of well logging developed to record the physical properties of formation with depth. These physical properties are those, which are used for petroleum exploration. The first log was recorded in 1927 in a well in small oil field of Pecelbuonn, in Alsace, a province in northeastern France. This was a simple electrical resisitivity log where taken intermittently and was hand plotted. Since that time the science of well logging has advanced with long strides and has taken the form of huge industry today .As the science of well logging advanced, the art of interpreting the data also advanced today, the total and detailed analysis of carefully chosen suit of wire line services provides a method of deriving or inferring accurate values for hydrocarbon and water saturation .The porosity, the permeability index and the lithology of the reservoir rocks. Today, millions of dollars are spent on wireline logging by the oil and gas industries since it serves as an eye inside the oil well. It provides subsurface information, which no other technique is able to provide.Wireline logging is a standard practice and a suite of logs is run in all exploration and development of production wells. Application of wireline log data may be summarized as follows: (1) Identification of porous and permeable beds and quantification of porosity

and permeability. (2) Identification of the pore fluids and the calculation of their saturation. (3) Correction of subsurface strata. (4) Facies analysis: determination of grain size profiles, diagnosis of

depositional environments and the prediction of the trend of the porous and permeable beds in the subsurface. However, facies analysis should always be undertaken in conjunction with sedimentological observations and core descriptions, and not on the basis of wireline log responses alone.

In all cases acquisition of logs involves lowering a special instrument down the well. This instrument is attached to a calibrated wireline and is slowly pulled up providing a continuous record of the rock characteristics which the device is designed to detect.To minimize costs; a number of logs are recorded simultaneously. A logging string is typically 3.5 inch in diameter and 25 to 60 ft long consisting of several tools .Logging speed is kept constant during individual surveys.

Fig1.1 (Well logging operation) Here,

1. Data recording truck 2. Conveyance system 3. Survey instruments

RECENT DEVELOPMENTS IN WELL LOGGING:

Horizontal and multilateral wells and their interpretation, these are preferred to normal well due to their superior economics and better productivity.

Improvement in data handling capacity of logging tools. Most development took place in 1990s and some in 1980s, among them

were Array Tools, Microprocessors Technology, Digital Telemetry and Imaging Techniques.

Array tool design improved vertical resolution and multiple depth investigation.

Resolution and coverage of borehole increased in electrical devices as well as in acoustic devices.

New generation nuclear devices like NMR CMR MRI which measures the amount of bound fluids or inversely mobile fluids and grain size have developed.

Device has developed which has made it possible to measurer water saturation in producing wells through tubing on regular basis. This helps in better monitoring of these producing reservoirs.

Measurement of resistivity through cased holes, which also gives water saturation in producing wells and help in reservoir monitoring.

LWD: logging while drilling saves a lot of time and gives real time data for quick and cost saving decisions.

Fig 1.2 (Symbols used in log interpretation)

Fig 1.3 (Resolution and depth of investigation for different logging tools)

Chapter: 2 Data Availability and Scale WELL INFORMATION: The well being analyzed is a vertical well drilled with the use of water based mud and the drill bit is of 8.50 inches diameter. The data available is for the depth range of 3025m-3925m (900m). DATA AVAILABILITY: The well data available for the given depth range is a composite of following logs:

• Caliper Log

• SP log

• Gamma ray log

• Resistivity logs: (LLD, MSFL, LLS)

• Neutron-porosity log

• Density log Resistivity of mud filtrate at surface temperature =3.125 ohm-m Mud density = 1.12 gm/cc DATA QUALITY: Data quality can be commented upon on the basis of deviation of caliper from the bit size. In our data, the quality commonly shows a good trend however it is poor at certain regions. The borehole enlargement in these regions may be due to washout. LOG SCALE: SP log In mV GR log In API Bite Size In Inches Caliper log In Inches Resistivity log In ohm-meter Neutron-porosity log In percentage Density log In gram/cc

Fig 2.1 (Scale of well log data)

Chapter 3 Geology of Assam Basin INTRODUCTION: The Upper Assam Basin is situated in a unique geotectonic framework in the northeast part of India and is bounded by the Main Boundary Fault in the north, the Disang Thrust System in the south-east and the Mishmi Thrust in the north-east. The basin has been affected by three phases of tectonic deformations during inter-plate convergence between Indian and Eurasian plates. A study for maximum horizontal direction in various parts of the basin reflects its complex tectonic setup. However, the predominant NE-SW direction appears to be the reflection of speculated direction of movement of the Indian plate. A study of earthquake focal mechanism also indicates similar trends of compression direction. GENERAL GEOLOGY: The Upper Assam Basin is one of the most important petroliferous basin of India and encompasses parts of the Indo-Burma range and shelf areas to the west. The Indo-Burma range is a geologically complex tectonic belt which extends in north-south direction along the geographical boundary of India and Burma (presently Myanmar). It is characterized by association of a number of thrust/overthrust, ophiolitic rocks, high degree of metamorphism, pelagic sediments etc. On the other hand, the shelf area is comparatively free from the thrust tectonics and is characterized by occurrences of normal faults down to basement. Sediments ranging in thickness from 3500 m to more than 7000 m was deposited over granitic basement. The age of sediments ranges from Upper Cretaceous through Paleogene to Neogene times. One important structural feature is the area known as “Belt of Schuppen” which is a series of thrusts and over-thrusts trending in the northeast-southwest direction and flanks the eastern part of the shelf area of the basin. The thickness of sediments increases towards the eastern thrust belts as well as to the northeast. Presence of commercial hydrocarbon has been established in clastic sediments of both Paleogene and Neogene age. TECTONICS AND DEPOSITIONAL HISTORY: The Upper Assam Basin is a foreland basin located at the boundaries of two convergent plates namely the India and Eurasia. According to paleo-geographic reconstruction, the area originally appears to have occupied an intra-continental position in the greater Gondwanaland. After separation from Gondwanaland, the Indian continental mass moved northward. During its north-ward journey, the Upper Assam shelf faced an open sea (Tethys) ahead. All the cretaceous and early Tertiary shelf sediments were deposited during this period. Gradual thinning of the Tethys sea took place with the northward movement of the India and simultaneous under-thrusting of the oceanic crust below Eurasian continental crust and its consumption. The destruction of the Tethys was complete by early Eocene era and

collision pattern changed from continent-oceanic crust to continent-continent. It was assumed that, India first collided with Eurasia in the northernmost point, then took a anti-clockwise rotational movement together with an east-northeast directed motion that resulted in another collision zone in the northeast affecting the Upper Assam shelf. By then, the depositional environment of this disturbed shelf margin had changed from predominantly shallow marine (Disang/Jaintia Group) through fluvio-deltaic (Barail Group) to fluviatile (Tipam group). The area took position at the corner between east-west trending Indo-Tibetian and north-south trending Indo-Burman hill ranges which developed along the convergent plate boundaries. As a result, the basin is structurally wrapped by the Main Bounndary Fault to the north, Mishmi Thrust to the northeast and the Naga-Dishang Thrust System of the Indo-Burman range to the east and southeast. To the west, occupies the Shillong-Mikir Hill massif which is the northeastern extension of Pre-Cambrians of Indian Peninsula. On the basis of fault types and settings, the tectonic phases can be divided into three groups. The oldest group of faults is the NE-SW striking normal faults which extends down to basement. These faults are due to development of extensional tectonics at the upper part of the continental crust due to down-wrapping during subduction. The second group of faults includes the Naga-Disang thrust belt (Belt of Shuppen) along the eastern and southeastern margin of the basin and also the Main Boundary Fault in the northern side. These faults developed during collision and further convergence between north-east drifting Indian and Asian continental crusts and associated crustal shortening. A third fault system, which affected these earlier thrusts and delimited them as well as the Upper Assam basin in the northeast direction the Mishmi thrust which trends in the NW-SE direction and exposes the Pre-Cambrian rocks. The basin is still tectonically active and is subjected to intense tectonic forces which are evident from frequent earthquakes occurring in this area.

STRATIGRAPHY:

The tertiary succession of the Upper Assam Shelf Sediment is given below:

Age Lithostratigraphic Nomenclature Thickness

(m)

Major Lithology

Pliestocene Alluvium

------------------------------- Unconformity -----------

--------------------

Pliocene Dihing

Group

Dhekiajuli Fm.

1300-

2000

Unconsolidated sand with

thin clay bands

-----------------------Unconformity --------------------

Dupitila

Group

Namsang Fm. 0-1000 Poorly consolidated

sandstone with clay and

lignite bands

Girujan Fm 400-900 Mottled clay with

sandstone lenses

Upper

Middle

Miocene

Tipam

group

Tipam

Fm.

Lower

600-900

Medium grained

arenaceous sandstone with

shale bands and some coal

streaks

------------------------------- Unconformity ---------------------

---------------

Argillaceous Unit

TER

TIA

RY

Oligocene

Barail

Group

Arenaceous Unit

500-1200 Upper part is

predominantly

mudstone/shale with fine

grained sandstone and

coal while the lower part

is sandstone & shale

Upper

Middle

Kopili

Fm

Lower

300-500 Splintery shale and fine

grained sandstone

Prang M

Narpuh M

Eocene

Sylhet

Fm

Lakadong M +

Therria

350-450

Splintery shale with

sandstone and limestone

bands

Paleocene

Jaintia

group

Langpar Fm

20-120

Quartzitic sandstone with

minor claystone &

limestone

-----------------------Unconformity --------------------

Pre-cambrian

/

Proterozoic

Basement Granitic rocks

Fm = Formation M = Member

Chapter:4 Basic Principle of Different Logs SPONTENEOUS POTENTIAL LOG: The spontaneous potential log (SP) measures the natural or spontaneous potential difference (sometimes called self-potential) that exists between the borehole and the surface in the absence of any artificially applied current. It is a very simple log that requires only an electrode in the borehole and a reference electrode at the surface. These spontaneous potentials arise from the different access that different formations provide for charge carriers in the borehole and formation fluids, which lead to a spontaneous current flow, and hence to a spontaneous potential difference. The spontaneous potential log is given the generic acronym SP. The SP log has four main uses: • The detection of permeable beds. • The determination of Rw. • The indication of the shaliness of a formation. • Correlation. It is very important to recognize that this log has no absolute scale – only relative changes in the SP log are important. There are three requirements for the existence of an SP current: • A conductive borehole fluid (i.e., a water based mud). • A sandwich of a porous and permeable bed between low porosity an impermeable formations. • A difference in salinity between the borehole fluid and the formation fluid, which are the mud filtrate and the formation fluid in most cases. Note, however, that in some special cases an SP current can be set-up when there is no difference in salinity, but where a difference in fluid pressures occurs.

ELECTRO-CHEMICAL COMPONENTS: These components arise from the electrochemical interaction of ions in the mud filtrate and formation fluids. The electrochemical contribution, itself, consists of two effects: The diffusion potential (sometimes called the liquid-junction potential). This potential exists at the junction between the invaded and the non-invaded zone, and is the direct result of the difference in salinity between the mud filtrate and the formation fluid. Assume that the formation fluid is more saline than the mud filtrate for a moment, and that the only dissolved ions in the system are Na+ and Cl-, as NaCl. The chloride ions have a higher mobility than the sodium ions. When

the two fluids come into contact across the interface between the invaded and non-invaded zones, diffusion will occur. Ions from the high salinity mud filtrate will diffuse into the invaded zone to try to balance the salinities out. The chloride ions are more mobile and so more of them diffuse into the invaded zone than sodiums. The net result is a flow of negative charge into the invaded zone, which sets up a charge imbalance (potential difference), called the diffusion potential. The diffusion potential causes a current to flow (from negative to positive) from the invaded zone into the non-invaded zone.

The membrane potential (sometimes called the shale potential). This potential exists at the junction between the non-invaded zone and the shale (or other impermeable rock) sandwiching the permeable bed. These beds are usually shale, and the argument that follows applies mainly to shales, but is also valid to a less extent for other low permeability rocks. Shales have the property that they can preferentially retard the passage of anions. This is called anionic permselectivity or electronegative permselectivity and is a property of membranes. It is due to an electrical double layer that exists at the rock-fluid interface, and that has the ability to exclude anions from the smaller pores in the rock (sometimes called anion exclusion). The strength of this effect depends upon the shale mineralogy, the fluid concentration and the fluid pH. Most other rocks exhibit the same behaviour but to a lower degree for geologically feasible fluid concentrations and pHs, but cationic permselectivity is possible, if rare. Most subsurface shales are such efficient anionic permselecting membranes that they repel almost all anions (say, chloride ions). This results in the shale being more positive than the non-invaded zone, and hence there is an electrical membrane potential, which causes current to flow from the invaded zone into the shale.

Fig 4.1 (SP potential for formation water more saline than mud filtrate)

Ec= Em +Ej = -Klog (Rmf/Rw) = 65+0.24T (T in oC)

= 61+0.133T (T in oF)

Where, Ec=Electrochemical potential Em= Membrane potential Ej= Liquid junction potential Rmf=Resisitivity of mud filtrate Rw = Resisitivity of water T= temperature in C K =Constant If the self potential is negative then it means that formation is more saline than the

mud filtrate and vice versa.

The SP tool has a poor resolution. Although it can be used for correlation, it is best not to rely solely upon it. If it has to be used for defining a bed boundary, it is best to take the inflexion point in the SP change as the boundary depth. Bed resolution is bad, and one would not expect it to show beds less than about 20 times the borehole diameter. Several factors govern the amplitude of the SP deflection opposite a permeable bed. This is because the size of the deflection and the change in the SP curve between beds depends upon the distribution of the current flux and the potential drops taking place in each part of the formation. The following parameters are important: • The thickness of the permeable bed, h. • The true resistivity of the permeable bed, Rt. • The diameter of the invaded zone, di. • The resistivity of the invaded zone, RXO. • The resistivity of the bounding formations. • The resistivity of the mud, Rm. • The diameter of the borehole, dh. • The relative salinities of the mud filtrate and the formation fluids.

Fig 4.2 (The SP deflection and its Rmf-Rw Dependency)

RESISTIVITY LOG:

The resistivity of a formation is a key parameter in determining hydrocarbon saturation. Electricity can pass through a formation only because of the conductive water it contains. With a few rare exceptions, such as metallic sulfide and graphite, dry rock is a good electrical insulator. Moreover, perfectly dry rocks are seldom found. Therefore, subsurface formations have finite, measurable resistivities because of the water in their pores or absorbed in their interstitial clay.

The measured resistivity of a formation depends on - resistivity of the formation water - amount of water present - pore structure geometry

The resistivity (specific resistance) of a substance is the resistance measured between opposite faces of a unit cube of that substance at a specified temperature. The meter is the unit of length and the ohm is the unit of electrical resistance. In abbreviated form, resistivity is R = r A/L, Where, R is resistivity in ohm-meters, r is resistance in ohms, A is area in square meters, and L is length in meters.

The units of resistivity are ohm-meters squared per meter, or simply ohm-meters (Ohm-m).

THE DUAL LATEROLOG: The dual laterolog (DLL) is the latest version of the laterolog. As its name implies, it is a combination f two tools, and can be run in a deep penetration (LLd) and shallow penetration (LLs) mode. These are now commonly run simultaneously and together with an additional very shallow penetration device. The tool has 9 electrodes. In the LLd mode, the tool operates just like a LL7 tool but with the same bucking currents that are emitted from the A1 electrodes also being emitted from the additional farthest electrodes, A2. The result of this is to focus the current from the central electrode even more than was the case for the LL7. In the LLs mode, the A1 electrodes emit a bucking current as they did in the LL7 device, but the A2 electrodes are set to sink this current (i.e., the bucking current comes out of A1 and into A2 electrodes).

This means that the bucking current must veer away from the pathway into the formation, and back towards the tool A2 electrodes, and hence cannot constrain (focus) the current being emitted from the central electrode as much. The overall result is that the central electrode current penetrates less far into he formation before it dies away. Both modes of the dual laterolog have a bed resolution of 2 feet, and a sensitivity of 0.2 to 20,000 Ωm. o achieve this sensitivity both the current and voltage are varied during the measurement, keeping heir product (the power) constant. The dual laterolog is equipped with centralizes to reduce the borehole effect on the LLs. A micro-resistivity device, usually the MSFL, is mounted on one of the four pads of the lower of the two centralists. Hence, this tool combination examines the resistivity of the formation at three depths of penetration (deep, shallow, and very shallow).

Fig 4.3 (DLL electrode configuration in both LLS and LLD modes) THE SPHERICALLY FOCUSSED LOG: The spherically focussed log (SFL) has an electrode arrangement that ensures the current is focussed quasi-spherically. It is useful as it is sensitive only to the resistivity of the invaded zone.

Fig 4.4 (The SFL electrode configuration)

GAMMA RAY LOG:

The gamma ray log measures the total natural gamma radiation emanating from a formation. This gamma radiation originates from potassium-40 and the isotopes of the Uranium-Radium and Thorium series. The gamma ray log is commonly given the symbol GR. Once the gamma rays are emitted from an isotope in the formation, they progressively reduce in energy as the result of collisions with other atoms in the rock (Compton scattering).Compton scattering occurs until the gamma ray is of such a low energy that it is completely absorbed by the formation. Hence, the gamma ray intensity that the log measures is a function of: • The initial intensity of gamma ray emission, which is a property of the elemental composition of the rock. • The amount of Compton scattering that the gamma rays encounter, which is related to the distance between the gamma emission and the detector and the density of the intervening material. The tool consists simply of a highly sensitive gamma ray detector in the form of a scintillation counter. The scintillation counter is composed of a thalium activated single sodium iodide crystal backed by a photomultiplier. When a gamma ray strikes the crystal a small flash of light is produced. This flash is too small to be measured using conventional electronics. Instead, it is amplified by a photomultiplier, which consists of a photocathode and a series of anodes held at progressively higher electrical potentials, all of which are arranged serially in a high vacuum. The flash of light hits the photocathode and causes a number of primary electrons to be produced. These few electrons still represent too small a signal to be measured. The primary electrons are accelerated towards the first anode. For every electron that hits the anode, a number of secondary electrons are emitted (between 4 and 8 usually). These electrons are accelerated towards the next anode, where each of the secondary electrons produces even more secondary electrons. This process is repeated for each of say 10 anodes. If 6 electrons are emitted at each anode for each incident electron, we can see that a single incident gamma ray ultimately produces 610 = 60,466,176 electrons, which represents a current that can be amplified further by conventional amplifiers. The whole process takes an extremely short time, but during this time the photomultiplier is saturated and is insensitive to further gamma rays. However, the small incident flux of gamma rays produced from rocks ensures that saturation is rarely if ever reached for down-hole tools. Since the flash of light and the number of primary electrons is proportional to the energy of the gamma ray, the final current from the scintillation counter is also proportional to the energy of the incident gamma ray. Hence, there will be a threshold gamma ray energy below which the scintillation counter is insensitive to gamma rays.

The total gamma ray log is usually recorded in track 1 with the caliper log, bit size and SP log. Although the API scale goes from 0 to 200 API, it is more common to see 0 to 100 API and 0 to 150 API used in log presentations There are three factors governing the vertical resolution: • The size of the detector which is quite small (about 5-10 cm diameter). • The effect of the time constant as described in Section 10.6. For conventional logging, with the product of logging speed and time constant set to 1 foot, the contribution to degradation in the vertical resolution from his cause is 1 foot. • The hemispherical zone of sensitivity of the sensor. As the sensor is sensitive to gamma rays from a hemispherical zone, and its approximate depth of investigation of about 30 cm (1 foot) for formations of average density, we can see that the degradation in vertical resolution from this source will be about 2 foot. Hence, the vertical resolution of the tool is just over 3 foot (90 cm). This is quite a high vertical resolution for an open-hole tool, and so the gamma ray tool is good at defining thin beds, for fine correlation, and for depth matching between logging runs. DETERMINATION OF LITHOLOGY: The gamma ray log is an extremely useful tool for discrimination of different lithologies. While it cannot uniquely define any lithology, the information it provides is invaluable when combined with information from other logs.

Fig 4.5 (Effect of different lithologies on Gamma ray log)

CALIPER LOG: The Caliper Log is a tool for measuring the diameter and shape of a borehole. It uses a tool which has 2, 4, or more extendable arms. The arms can move in and out as the tool is withdrawn from the borehole, and the movement is converted into an electrical signal by a potentiometer. In the two arm tool, the borehole diameter is measured. This is shown in track 1 of the master log together with the bit size for reference. Borehole diameters larger and smaller than the bit size are possible. Many boreholes can attain an oval shape after drilling. This is due to the effect of the pressures in the crust being different in different directions as a result of tectonic forces. In oval holes, the two arm caliper will lock into the long axis of the oval cross-section, giving larger values of borehole diameter than expected. In this case tools with more arms are required. In the four arm (dual caliper) tool, the two opposite pairs work together to give the borehole diameter in two perpendicular directions. An example of a four arm tool is the Borehole Geometry Tool (BGT). This has 4 arms that can be opened to 30 inches (40 inches as a special modification), and give two independent perpendicular caliper readings. The tool also calculates and integrates the volume of the borehole and includes sensors that measure the direction (azimuth) and dip of the borehole, which is useful in plotting the trajectory of the borehole. In the multi-arm tools, up to 30 arms are arranged around the tool allowing the detailed shape of the borehole to be measured. The caliper logs are plotted in track 1 with the drilling bit size for comparison, or as a differential caliper reading, where the reading represents the caliper value minus the drill bit diameter. The scale is generally given in inches, which is standard for measuring bit sizes. USES OF CALIPER LOG: The commoner uses of the caliper log are as follows: • Contributory information for lithological assessment. • Indicator of good permeability and porosity zones (reservoir rock) due to the development of mudcake in association with gamma ray log. • Calculation of mudcake thickness. • Indication of hole quality for the assessment of the likely quality of other logs whose data quality is degraded by holes that are out of gauge. Other log data can often be corrected for bad hole conditions using the caliper readings, but the larger the correction, the less reliable the final data will be. Centralized tools are designed to be about 4 inches in diameter for a standard 8.5 inch hole, and they are designed to work with 2.25 inches of drilling mud between them and the formation.

If the hole caves to 14 inches, which is not uncommon, the distance to the formation becomes 5 inches and the tool responses are degraded. This can be corrected for to some extent if the caliper value is known. Tools that work by being pressed up against the side of the borehole wall have even greater problems because the irregularity of the borehole wall makes it impossible to obtain reliable readings. • Selection of consolidated formations for wireline pressure tests, recovery of fluid samples, for packer seating for well testing purposes, and for determining casing setting depths.

Fig 4.6 (Typical caliper response to various lithologies)

NEUTRON LOG: The neutron log is sensitive mainly to the amount of hydrogen atoms in a formation. Its main use is in the determination of the porosity of a formation. Neutron log are used for the delineation of porous formation and determination of their porosity .they respond to the amount of hydrogen in the formation. Neutrons are electrically neural particles, each having a mass almost equivalent to the mass of a hydrogen atom; high energy neutrons are continuously emitted from the source in the sonde. These neutrons collide with the nuclei of the formation materials in what may be thought as elastic “billiard ball collisions. Collision with the heavy nuclei does not slow the neutron very much. This is called moderation; energy is lost at every collision. There is greatest loss of energy of the neutron when it collides with similar mass. Thus hydrogen of the formation largely slows down to neutrons. Thus within a few second s the neutrons are slowed down to energy corresponding to thermal velocity around 0.025MeV.atoms of chlorine or hydrogen itself captures this slowed down neutrons. The capturing nucleus becomes intensely excited and emit high energy capture gamma ray . These are two kinds of detector on the sonde. One may count the capture gamma rays; other may count the neutron itself. The tool operates by bombarding the formation with high energy neutrons. These neutrons undergo scattering in the formation, losing energy and producing high energy gamma rays. The scattering reactions occur most efficiently with hydrogen atoms. The resulting low energy neutrons or gamma rays can be detected, and their count rate is related to the amount of hydrogen atoms in the formation. In formations with a large amount of hydrogen atoms, the neutrons are slowed down and absorbed very quickly and in a short distance. The count rate of slow neutrons or capture gamma rays is low in the tool. Hence, the count rate will be low in high porosity rocks. In formations with a small amount of hydrogen atoms, the neutrons are slowed down and absorbed more slowly and travel further through the rock before being absorbed. The count rate of slow neutrons or capture gamma rays in the tool is therefore higher. Hence, the count rate will be higher in low porosity rocks. THEORY: In neutron logging there are three processes of interest: neutron emission, neutron scattering and neutron absorption. NEUTRON EMISSON: The neutron tool emits high energy (4.5 MeV) neutrons from a radioactive source. They move very fast, and their energy is related to their speed. They are called fast

neutrons. The neutron sources used in logging are a mixture of two elements (i) a source of alpha radiation such as radium, plutonium or americium, and (ii) beryllium-9. The alpha particles from the radium, plutonium or americium interact with the beryllium-9 in an atomic reaction that produces carbon-12, a fast neutron and gamma rays. NEUTRON SCATTERING: The fast neutrons interact with the nuclei of atoms within the formation. The interaction is a form of elastic scattering involving the neutrally charged neutron and a stationary positively charged nucleus. At each interaction (collision) the neutron looses some energy and slows down, and the nucleus of the atom in the formation material gains energy. Such collisions occur with nuclei of ALL elements. However, the process of energy transfer (i.e., energy loss from the neutron) is most efficient when the masses of the neutron and the nucleus are the same, and becomes much less efficient when the nuclei of the formation material are more massive than the neutron. The neutron has approximately the same mass as hydrogen nuclei (the lightest element). Hence the neutrons lose energy by elastic scattering most efficiently by interaction with hydrogen nuclei, and lose energy much less efficiently by interaction with more massive nuclei such as silicon or oxygen. Figure 15.2 shows the efficiency of several elements in slowing down fast neutrons. The initially fast neutrons (>0.5 MeV) quickly lose their energy and become slower passing through stages called intermediate neutrons (102 to 105 eV), epithermal neutrons (0.1 to 100 eV), and finally thermal neutrons (<0.1 eV). In solid materials containing reasonable amounts of low atomic mass elements, this process can happen very quickly for a given neutron (of the order of microseconds). However, the time taken to slow down to a given energy will vary from neutron to neutron, depending on the chance collisions with nuclei. Thermal neutrons are so called because they have energies which are those that a particle has as a result of it existing a room temperatures. In other words, they only have the small energies associated with the random kinetic motion associated with room temperatures. When the neutrons attain epithermal or thermal energies, collisions occur much less frequently because the neutrons are moving from nucleus to nucleus much more slowly. Within a few microseconds of being exposed to the fast neutron source, the formation has slowed the incoming neutrons down to epithermal and thermal levels, and a cloud of these thermal neutrons exists in the formation surrounding the source. Collisions continue resulting in little further loss of energy and the slow diffusion of the thermal neutrons from the zone around the detector. During this process the neutrons are absorbed by the formation nuclei.

NEUTRON ABSORPTION: Thermal (and to some extent epithermal) neutrons can be absorbed by the nuclei of the formation atoms. The efficiency of neutron absorption varies from element to element. The only elements which exhibit significant neutron absorbing behaviour and exist is reasonable quantities in rocks are hydrogen and chlorine. TOOL OPERATION: There are three main types of neutron tool, which are: • The Gamma Ray/Neutron Tool (GNT) • The Sidewall Neutron Porosity Tool (SNP) • The Compensated Neutron Log (CNL) Now a days compensated neutron tool are used. These use Americium-beryllium source to provide neutrons with initial energy. Short distance detector measures the response of the borehole while the other measures the reaction of the formation. Count is taken as ratio of the long spacing to short spacing. More the count, lesser the porosity. All neutrons tools are calibrated with respect to limestone.

Fig 4.7(The CNL tool)

DENSITY LOG: INTRODUCTION: The formation density log measures the bulk density of the formation. Its main use is to derive a value for the total porosity of the formation. It is also useful in the detection of gas-bearing formations and in the recognition of evaporites. The formation density tools are induced radiation tools. They bombard the formation with radiation and measure how much radiation returns to a sensor. THEORY: The tool consists of: • A radioactive source: This is usually caesium-137 or cobalt-60, and emits gamma rays of medium energy (in the range 0.2 – 2 MeV). For example, caesium-137 emits gamma rays with a energy of 0.662 MeV. • A short range detector: This detector is very similar to the detectors used in the natural gamma ray tools, and is placed 7 inches from the source. • A long range detector: This detector is identical to the short range detector, and is placed 16 inches from the source. The gamma rays enter the formation and undergo compton scattering by interaction with the electrons in the atoms composing the formation, as described in Section 9.3. Compton scattering reduces the energy of the gamma rays in a step-wise manner, and scatters the gamma rays in all directions. When the energy of the gamma rays is less than 0.5 MeV they may undergo photo-electric absorption by interaction with the atomic electrons. The flux of gamma rays that reach each of the two detectors is therefore attenuated by the formation, and the amount of attenuation is dependent upon the density of electrons in the formation. • A formation with a high bulk density has a high number density of electrons. It attenuates the gamma rays significantly, and hence a low gamma ray count rate is recorded at the sensors. • A formation with a low bulk density has a low number density of electrons. It attenuates the gamma rays less than a high density formation, and hence a higher gamma ray count rate is recorded at the sensors. The density of electrons in a formation is described by a parameter called the electron number density, ne. For a pure substance, number density is directly related to bulk density, and we can derive the relationship in the following way. • The number of atoms in one mole of a material is defined as equal to Avagadro’s number N (N≈6.02×1023). • The number of electrons in a mole of a material is therefore equal to NZ, where Z is the atomic number (i.e., the number of protons, and therefore electrons per atom).

• Since the atomic mass number A is the weight of one mole of a substance, the number of electrons per gram is equal to NZ/A. • However, we want the number of electrons per unit volume, and we can obtain this from the number of electrons per gram by multiplying by the bulk density of the substance, ρb. Hence, the electron number density is Ne=(NZ/A) ρb Where, Ne = the number density of electrons in the substance (electrons/cm3) N = Avagadro’s number (≈6.02×1023) Z = Atomic number (no units) A = Atomic weight (g/mole) ρb = the bulk density of the material (g/cm3).

Fig 4.8(Gamma ray energy spectra) OPERATION: A radiation emitter and one detector are all that is necessary for a simple measurement. The early tools had only one detector, which was pressed against the borehole wall by a spring-loaded arm. Unfortunately, this type of tool was extremely inaccurate because it was unable to compensate for mudcake of varying thicknesses and densities through which the gamma rays have to pass if a measurement of the true formation is to be achieved. All the newer tools have two detectors to help compensate for the mudcake problem. The method of compensation is described in subsequent sections. The

newer two detector tools are called compensated formation density logs, an example of which is Schlumberger’s FDC (formation density compensated) tool. Compensated formation density tools have one focussed (collimated) radiation source, one short spacing detector at 7 inches from the source, and one long spacing detector 16 inches from the source. The source and both detectors are heavily shielded (collimated) to ensure that the radiation only goes into the mudcake and formation, and that detected gamma rays only come from the mudcake or formation. The leading edge of the shield is fashioned into a plough which removes part of the mudcake as the tool is pulled up the well. The tool is pressed against one side of the borehole using a servo-operated arm with a force of 800 pounds force. Under this pressure and the pulling power of the wireline winch the plough can make a deep impression in the mudcake. The large eccentering force also means that there is much wear of the surface of the tool that is pressed against the borehole wall. The heavy shielding also doubles as a skid and a wear plate that protects the source and detectors and can be replaced easily and cheaply when worn down.

Fig 4.9(Schematic diagram of a formation density tool) LOGGING SPEED: The typical logging speed for the tool is 1300 ft/hr (400 m/hr), although it is occasionally run at lower speeds to increase the vertical resolution. The log quality is not as affected by logging speed as the natural gamma ray logs because much higher count rates are obtained with the radioactive source on the tool. VERTICAL RESOLUTION: The vertical resolution at the typical logging speed (1300 ft/hr) is good (about 26 cm, 10 inches), which is defined by the distance between the two detectors. The measurement point is taken to be half way between the two detectors. Beds can be

resolved down to about 60 cm (2 ft) with the density tool reading the true density value of the bed. Even better resolutions are possible with slower logging speeds. Partial reaction of the logging tool to very thin beds of anomalously high or low density is sometimes encountered. For example, thin (5 – 10 cm thick) layers of calcareous nodules. The high vertical resolution means that the log is useful for defining formation boundaries. DETERMINATION OF POROSITY: The porosity f of a formation can be obtained from the bulk density if the mean density of the rock matrix and that of the fluids it contains are known. The bulk density ρb of a formation can be written as a linear contribution of the density of the rock matrix ρma and the fluid density ρf , with each present is proportions (1- Ф ) and Ф respectively : ρb = (1 − Ф) ρma + Ф ρf where, ρb = the bulk density of the formation ρma = the density of the rock matrix ρf = the density of the fluids occupying the porosity Ф = the porosity of the rock.

SONIC LOG: INTRODUCTION: The sonic or acoustic log measures the travel time of an elastic wave through the formation. This information can also be used to derive the velocity of elastic waves through the formation. Its main use is to provide information to support and calibrate seismic data and to derive the porosity of a formation. WAVE TYPES: The tool measures the time it takes for a pulse of “sound” (i.e., and elastic wave) to travel from a transmitter to a receiver, which are both mounted on the tool. The transmitted pulse is very short and of high amplitude. This travels through the rock in various different forms while undergoing dispersion (spreading of the wave energy in time and space) and attenuation (loss of energy through absorption of energy by the formations). When the sound energy arrives at the receiver, having passed through the rock, it does so at different times in the form of different types of wave. This is because the different types of wave travel with different velocities in the rock or take different pathways to the receiver. The transmitter fires at t = 0. It is not shown in the figure because it is masked from the received information by switching the receiver off for the short duration during which the pulse is transmitted. This is done to ensure that the received information is not too complicated, and to protect the sensitive receiver from the high amplitude pulse. After some time the first type of wave arrives. This is the compressional or longitudinal or pressure wave (P-wave). It is usually the fastest wave, and has small amplitude. The next wave, usually, to arrive is the transverse or shear wave (S wave). This is slower than the P-wave, but usually has higher amplitude. The shear wave cannot propagate in fluids, as fluids do not behave elastically under shear deformation. These are the most important two waves. After them come Rayleigh waves, Stoneley waves, and mud waves. The first two of these waves are associated with energy moving along the borehole wall, and the last is a pressure wave that travels through the mud in the borehole. They can be high amplitude, but always arrive after the main waves have arrived and are usually masked out of the data. There may also be unwanted P-waves and S-waves that travel through the body of the tool, but these are minimized by good tool design by (i) reducing their received amplitude by arranging damping along the tool, and (ii) delaying their arrival until the P-wave and S-wave have arrived by ensuring that the pathway along the tool is a long and complex one. In practice the sonic log data is not presented as a travel time, because different tools have different Tx-Rx spacings, so there would be an ambiguity. Nor is the data presented as a velocity. The data is presented as a slowness or the travel time

per foot traveled through the formation, which is called delta t (Δt or ΔT), and is usually measured in μs/ft. BOREHOLE COMPENSETED SONIC (BHC) TOOL: This tool compensates automatically for problems with tool misalignment and the varying size of the hole (to some extent) that were encountered with the dual receiver tools. It has two transmitters and four receivers, arranged in two dual receiver sets, but with one set inverted (i.e., in the opposite direction). Each of the transmitters is pulsed alternately, and Δt values are measured from alternate pairs of receivers. These two values of Δt are then averaged to compensate for tool misalignment, at to some extent for changes in the borehole size. A typical pulse for the BHC is 100 μs to 200 μs, with a gap of about 50 ms, giving about 20 pulses per second. There are four individual Tx-Rx readings needed per measurement, so 5 measurements can be made per second. At a typical logging speed of 1500 m/h (5000 ft/h), gives one reading per 8 cm (3 inches) of borehole. Several versions of the BHC are available with different Tx-Rx distances (3 ft.and 5 ft. being typical), and the Rx-Rx distance between pairs of receivers is usually 2 ft.

Fig 4.10 (Borehole compensated sonic tool)

LONG SPACING SONIC (LSS) TOOL: It was recognized that in some logging conditions a longer Tx-Rx distance could help. Hence Schlumberger developed the long spacing sonic (LSS), which has two Tx two feet apart, and two Tx also two feet apart but separated from the Tx by 8 feet. This tool gives two readings; a near reading with a 8-10 ft. spacing, and a far reading with a 10-12 ft. spacing.

Fig 4.11 (Long spacing sonic tool) DEPTH OF INVESTIGATION: In theory, the refracted wave travels along the borehole wall, and hence the depth of penetration is small (2.5 to 25 cm). It is independent of Tx- Rx spacing, but depends upon the wavelength of the elastic wave, with larger wavelengths giving larger penetrations. As wavelength = V/f (i.e., velocity divided by frequency), for any given tool frequency, the higher the velocity the formation has, the larger the wavelength and the deeper the penetration. VERTICAL RESOLUTION: The vertical resolution is equal to the Rx-Rx spacing, and hence is 2 ft. LOGGING SPEED: The typical logging speed for the tool is 5000 ft/hr (1500 m/hr), although it is occasionally run at lower speeds to increase the vertical resolution.

Chapter :5 Steps used for Petrophysical Analysis Step 1 Selection of ‘Zone of interest’ The primary objective of logging a well is to identify ‘zones of interest’ from hydrocarbon accumulation stand point of view by integrating log responses of different geophysical tools. Although this is not always true and some exceptions can occur but in general the characteristic log responses indicating possibility of presence of Hydrocarbon (Oil or Gas or both) are as follows- 1. High Resistivity value

2. Low Gamma ray value

3. Negative deflection of SP log

4. Mud cake formation

5. Low formation bulk density

Step 2 Calculation of Formation Temperature Formation temperature T2 is given by the formula, T2 = T1 + Geothermal Gradient × D T2: Formation temperature of zone of interest T1: Mean surface temperature D: Depth of interested zone (log depth)

Step 3 Calculation of Resistivity of Mud Filtrate at formation temperature (Rmf @ Tf) 1. Read Rmf from header file at surface temperature.

2. Determine the Rmf at formation temperature using the formula

T1 + 6.77 (temperature in Fahrenheit) Rm2 = Rm1 T2 + 6.77 T1 + 21.5 (temperature in degree Celsius) Rm2 = Rm1 T2 + 21.5 Here, T2 = Formation temperature and T1 = Mean surface temperature Rm1=Resistivity of mud filtrate at surface temperature Rm2 = Resistivity of mud filtrate at formation temperature Step 4 Calculation of Formation Water Resistivity (Rw) from SP log 1.Read value of SP deflection from log

2.Calculate Rmf at formation temperature

3.Convert Rmf at formation temperature to Rmfe

Rmfe ( T in degree Fahrenheit) SSP = -(65 + 0..133T)log

Rwe

a)Calculate Rmf @ 750 F using tha above formula. If Rmf @ 750 F is less than 0.1 ohm –m, then Rmfe = 0.85×Rmf (approximation) If Rmf @ 750 F is greater than 0.1 ohm –m ,then Rmfe =(146 Rmf -5)/(337 Rmf +77) b)Determine the value of Rwe @ 750 F If Rwe @ 750 F is greater than 0.12 ohm-m,then Rwe=-0.58+10(0.69.Rwe -0.24)

If Rwe @ 750 F is less than 0.12 ohm-m,then Rwe= (77.Rwe +5)/(146- 377.Rwe) Step 5 Calculation of Shale Volume (Vsh)

Shale volume present in the zone of interest or in the reservoir can be determined by GR log.

Vsh from GR log can be calculated as-

(GRlog - GRmin) IGR = (GRmax - GRmin) This Gamma Ray Index (IGR) and Vsh relation becomes non-linear for both structured clays and dispersed clays. Wide verities of non-linear relationships exit between IGR and Vsh. But none is universally accepted. A summary of this non-linear relationship is illustrated below. Linear Vsh = IGR Clavier Vsh = 1.7 – [3.38 – (IGR + 0.7)2]1/2

Steiber Vsh = 0.5× [IGR / (1.5 – IGR)]

Step 6 Determination of Neutron Porosity and Density Porosity

1. Effective porosity (Φe) is calculated by combining Neutron Porosity and Density porosity.

2. Neutron Porosity can be read directly from log.

3. Density porosity needs to be calculated from Density log using the formula as below-

(ρma – ρb) ΦD = (ρma – ρf) Here, ρma = density of matrix of the formation ρf = density of formation fluid in the vicinity of borehole (mud filtrate) ρb = bulk density of the formation Step 7 Correction of Neutron porosity and Density porosity for the presence of shale Neutron porosity as read from the log and density porosity as calculated need to be corrected for volume of shale present in the formation.

1. Corrected Neutron porosity

ΦNC = ΦN - VshΦNsh 2. Corrected Density porosity

ΦDC = ΦD - VshΦDsh

Step 8 Calculation of Effective Porosity (Φe) Effective porosity (Φe) is calculated by combining corrected neutron and density porosities using the formula,

ΦNC

2 + ΦDC2

Φe = √ 2 Step 9 Determination of Formation Factor (F) Formation factor “F” is calculated

F = a / Φm

Where, a = tortuosity factor m= cementation factor For sandstones , a = 0.62 , m = 2.15 Step 10 Determination of Water Saturation (Sw) For clean formation we can easily determine water saturation using “Archie’s Equation”

F*Rw Sw

n = Rt

Sw = water saturation n = saturation exponent (usually taken as 2) Rt = true resistivity, as read from the deep resistivity log F = formation factor

Step 11 Determination of Hydrocarbon Saturation (Shc) It is very easily determined from the relation, Shc = 1- Sw Where, Shc = Oil saturation of the zone of interest. Step 12 Determination of Movable Hydrocarbon Water saturation in the flushed zone (Sxo) is given by, F*Rmf Sxo = √ Rxo Rxo= Resistivity of the flushed zone given by Micro-logs (1 - Sxo) gives the hydrocarbon present within the flushed zone i.e. immovable hydrocarbon. This enables us calculating ‘movable hydrocarbon’ by subtracting the residual hydrocarbon. (1 – Sxo) from total hydrocarbon saturation Shc or (1 – Sw) Shcm = (1- Sw) – (1 – Sxo) = Sxo - Sw

Chapter: 6 Petrophysical Analysis Selection of ‘Zones of interest’: Zones of interests are selected by looking at the mud-cake formation and resistivity log that include many zones in this well. But only those zones are considered for further petrophysical analysis, which satisfy all the characteristic log response for hydrocarbon bearing zones. Here five hydrocarbon bearing zones have been selected for petrophysical analysis. Zones Depth Range (m) Thickness (m)

1 3685-3691 6

2 3692-3705 13

3 3748-3768 20

4 3796-3804 8

5 3807-3811 4 Surface temperature and Geothermal Gradient: The full well had been logged at one run and the surface temperature is 250C. Geothermal gradient of the area is varying from 16-220C/km but the most common value is 180C/km .So for purpose of petrophysical analysis 180C/km has been taken. Zone Surface Tenperature (in 0C ) Surface Temprature(in 0F )

1 25 77

2 25 77

3 25 77

4 25 77

5 25 77

Formation temperature: Zones Formation Depth(m) Formation temperature (in 0F )

1 3685 196.3

2 3692 196.6

3 3748 198.4

4 3796 199.9

5 3807 200.3

Zonal Parameters: Zonal parameters used for petrophysical analysis are listed below, Zone Rmf

(in Ωm)

GRmax (inAPI)

GRmin (inAPI)

GRlog (inAPI)

ρf (gm/cc)

a

m

n

1 1.289 120.0 22.5 52.5 1.12 0.62 2.15 2 2 1.287 121.0 21.0 45.0 1.12 0.62 2.15 2 3 1.275 142.5 19.5 37.5 1.12 0.62 2.15 2 4 1.266 120.0 19.5 34.5 1.12 0.62 2.15 2 5 1.266 112.5 22.5 40.0 1.12 0.62 2.15 2 Formation water resistivity (Rw) calculation: Zone Tf(0F) Rmf@Ts SSP Rmf@Tf Rmfe Rwe Rw

1 196.3 3.125 -10.32 1.289 0.358 0.2718 0.3062 2 196.6 3.125 -19.81 1.287 0.358 0.2472 0.2722

3 198.4 3.125 --------- 1.275 ---------- ---------- ---------

4 199.9 3.125 -13.75 1.266 0.357 0.2476 0.2727

5 200.3 3.125 -16.19 1.266 0.357 0.2334 0.2537

Shale volume calculation: Zone IGR Clavier Steibel Vshale(accepted)1 .307 0.162 0.128 0.128 2 0.240 0.120 0.095 0.095 3 0.146 0.068 0.054 0.054 4 0.149 0.098 0.055 0.055 5 0.194 0.094 0.074 0.074 Porosity Calculation: Zone Vshale ρb ФD ФN ФDC ФNC ФEffective. 1 0.128 2.22 0.308 0.19 0.282 0.124 0.2178 2 0.062 2.12 0.352 0.15 0.332 0.101 0.2459 3 0.054 2.20 0.320 0.09 0.316 0.071 0.2292 4 0.055 2.25 0.289 0.096 0.285 0.089 0.2111 5 0.074 2.25 0.289 0.15 0.284 0.141 0.2243

Saturation Calculation: Zone Depth ФEffective

. F Rw Rt Sw(in%)

Shc(in%)

1 3685-3691 0.2178 16.427 0.3062 150 18.31 81.62

2 3692-3705 0.2459 12.654 0.2722 760 6.73 93.27

3 3748-3768 0.2292 14.720 -------- 850 15.08 84.92

4 3796-3804 0.2111 17.568 0 .2727 180 16.31 83.69

5 3807-3811 0.2243 15.420 0 .2537 80 22.11 77.89

Zone Sw Shc Rmf@Tf RXO SXO

1 0.1831 0.8162 1.289 25 0.9203

2 0.0673 0.9327 1.287 40 0.6380

3 0.1508 0.8492 1.275 40 0.6850

4 0.1631 0.8369 1.266 30 0.8610

5 0.2211 0.7789 1.266 40 0.6986

Movable-immovable Hydrocarbon out of total pore fluids: Zone Sw Shc SXO Shcim

(1- SXO) Shcm

1 0.1831 0.8162 0.9203 0.0797 0.7365

2 0.0673 0.9327 0.6380 0.3620 0.5707

3 0.1508 0.8492 0.6850 0.3150 0.5342

4 0.1631 0.8369 0.8610 0.1390 0.6979

5 0.2211 0.7789 0.6986 0.3014 0.4775

Fig 6.1(Pie chart representing the percentage of Sw , Shcm and Shcim for different zones)

Chapter:7 Discussion and Conclusion We have analyzed the five zone of interest for hydrocarbon accumulation and found good results. These zones are showing prospective hydrocarbon accumulation and having good porosity. Detail discussion about these zones is given below. Discussion of Zones: Zone:1 Depth (3685-3691)m

1. This zone is having relatively high Gamma value(52.5API).Shale volume is 12.8%.

2. Effective pososity of the zone is good. 3. Low Rxo and high Rt value against the formation which indicate good

mount of movable hydrocarbon. 4. Hydrocarbon saturation is 81.6% which is quite good.

Zone:2 Depth (3692-3705)m

1. This zone having 45 API gamma ray value which is relatively low. Shale volume is 9.5%.

2. Effective porosity is 24.5% which is quite good. 3. True resistivity is 760 ohm-m which is very high. 4. Neutron –density crossover is present which is indicating that it is a gas

zone. 5. Density porosity is very high because of the presence of gas. 6. Hydrocarbon saturation is 92.7% which is very good.

Zone:3 Depth (3748-3768)m

1. This zone having 37.5 API gamma ray value which is low. Shale volume is 5.4%.

2. Effective porosity is 22.9% which is good. 3. True resistivity is 850 ohm-m which is highest resistivity in the five

zones. 4. In this zone there are two type of SP deflection in the same formation,

which indicates a sudden change in formation water resistivity.Here empirical relation Sxo=(Sw)1/5 has been used to calculate water saturation.

5. Density porosity is very high because of the presence of gas. 6. Neutron –density crossover is present which is indicating that it is a gas

zone. 7. Hydrocarbon saturation is 84.9% which is very good.

Zone:4 Depth (3796-3804)m

1. This zone having 34 API gamma ray value which is low. Shale volume is 5.5%.

2. Effective porosity is 21.1% which is good enough. 3. True resistivity is 180 ohm-m which is high. 4. Neutron –density crossover is present which is indicating that it is a

gas zone. 5. Hydrocarbon saturation is 83.6% which is quite good.

Zone:5 Depth (3807-3691))m

1. This zone having 40 API gamma ray value which is relatively high. Shale volume is 7.4%.

2. Effective porosity is 22.43% which is good enough. 3. True resistivity is 80 ohm-m which is relatively low. 4. Neutron –density crossover is present which is indicating that it is a

gas zone. 5. Hydrocarbon saturation is 77.9% which is good.

Conclusion:

1. There are five hydrocarbon bearing zones having thickness varying from 4 m to 20 m.

2. Shale volume is varying from 5.4% to 12.8%. 3. Effective permeability of the zones is varying from 22.1% to 24.6%. 4. Water saturation is varying from 18% to 22%. 5. Movable hydrocarbon saturation is varying from 47.7% to 73.6%. 6. These zones are gas zone which is indicated by low bulk density

and neutron-density cross-over.

References

1. Brock,James G.,Applied open hole log analysis,1986

2. Rider Malcolm,The Geological Interpretation of Well Logs,1996 3. Glover Poul,Petrophysical M.Sc. Course notes 4. Schlumburger, Introduction to open-hole logging

5. HLS Asia Limited, Basic Log Interpretation Tutorial

6. Dakota-petrophysics, Kansas Geological Survey

7. John H. Doveton, Kansas Geological Survey,Basic Oil and Gas Log

Analysis