Summer Training Report

Submitted by: SHWETA GUPTA M.SC (TECH.)GEOLOGY INSTITUTE OF SCIENCE BANARAS HINDU UNIVERSITY

Acknowledgement

I render a strong sense of indebtness to Shri Atanu Chakrborty, Director General, DGH, Noida for providing me this magnificent opportunity and facilities and to associate me with this prestigious institute.

I owe special gratitude to Mrs. Irani Bharali, HOD, HR & Administration for always being the source of every support – academic and non-academic we needed.

I also thank Department of Geology, BANARAS HINDU UNIVERSITY and our HOD, Prof. M. P SINGH for recommending and helping me in all possible ways.

I am highly grateful to K. Murli, HOD, Petrophysics; Sunaram Hembrom, HOD, Reservoir Engineering; Mr. Ahin Samajpati, Mr. Ram Kumar, learned instructors – Mr. Abhiram Deo Sharma, Mrs. Inderjeet Kaur, Mr. Promothos Barua, Mr. Himanshu Srivastava and Mr. Pradeep Kumar, for enlightening me with the indepth knowledge and providing me all the necessary help towards completion of the project.

Last but not the least, I humbly thankful to all the staffs, especially the pantry staffs of DGH for fulfilling our requirements in smooth running of the project.

- SHWETA GUPTA

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ABSTRACTAt the end of the project we were able to get concise idea of:

a) Field Geology and Geophysicsb) Petro-Physicsc) Reservoir Engineeringd) Field Development Plane) Drillingf) Productiong) Production Sharing Contract

There were many phases during the training where we felt awestruck and wonderful regarding the industry practices and applications of the classroom theories.

In the field geology and geophysics we learnt about well logging and seismic methods. The Petro-Physics training imparted us knowledge about detailed well logging and hi-tech well logging. During the reservoir engineering part we were able to get an idea of basic reservoir characteristics and production mechanisms.

We also got to know about Field Development Plan (FDP), Drilling, Production and Production Sharing Contract (PFC).

The overall training very strongly inclined me towards the industry practices and overall working of the industry.

1. INTRODUCTION

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With rising energy needs of the world, the demand for petroleum has rapidly increased the trend

that followed World War II. Initially however, the oil industry focused solely on production and

selling. Regulation of drilling and production was nonexistent; hence waste and overproduction

were widespread. This led to the development of control measures and studying the reservoir

While India has not been blessed with a bountiful of deposits, use of advanced technology has

ensured sufficient output and thus a brighter future.

1.1 Internship Overview

Earth sciences are vague without the introduction of newer field studies and technologies. For

fulfillment of this purpose, compulsory summer internship was introduced in the curriculum. By

personal interests and with persuasion of university authorities, this internship was granted at

Directorate General of Hydrocarbons (DGH) under Ministry of Petroleum and Natural Gas from

9th May to 3rd June, 2016.

Although the internship was primarily concentrated towards the field of Seismic data acquisition

and Interpretation, various aspects, such as, administrative and regulative stature of DGH and

Stages of Exploration and Production division were described in brief by lectures in their

concerned departments. The objective was mainly targeted towards learning of the Seismic

interpretation module of the well known software PETREL, developed by Schlumberger. For

understanding the outcome given by the software, seismic correlation, fault orientation

knowledge is essential.

1.2 About DGH

The Directorate General of Hydrocarbons (DGH) was established in 1993 under the

administrative control of Ministry of Petroleum & Natural Gas through Government of India

Resolution. Objectives of DGH are to promote sound management of the oil and natural gas

resources having a balanced regard for environment, safety, technological and economic aspects

of the petroleum activity.

DGH has been entrusted with several responsibilities like implementation of New Exploration

Licensing Policy(NELP), matters concerning the Production Sharing Contracts exploration and

development of non-conventional hydrocarbon energy sources like Coal Bed Methane(CBM) as

also futuristic hydrocarbon energy resources like Gas Hydrates and Shale oil for discovered

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fields and exploration blocks, promotion of investment in E&P Sector and monitoring of E&P

activities including review of reservoir performance of producing fields. In addition, DGH is also

engaged in opening up of new unexplored areas for future.

Being directly under the government of India control, it has the following governance

components-

Despite being a developing country in terms of UN definition, India is largely dependent upon

the imports to meet its energy demands in terms of Oil and gas. With the global oil price

fluctuations monopolized by the oil exporting countries (mainly OPEC, Russia, Venezuela and

Nigeria), the national growth is being critically hampered .To overcome this vicious circle, the

government of India has formulated new policies to give a push to exploration and production so

that India can be self sufficient in its oil and gas sector. Firstly, the government incorporated the

public sector Exploration and Production (E & P) giant ONGC to increase its efficiency by

making it run for a profit motive like the private sector companies, while serving the interest of

the nation.

Secondly, Government decided to give up the exploration fields by inviting the participation of

the various private companies. Hence the New Exploration Licensing Policy (NELP) was

formulated in 1997-98 to provide a level playing field in which all parties could compete on

equal terms for the award of exploration acreage. Different blocks were put on the market under

successive rounds of NELP for the various companies to bid for them. Moreover, various

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incentives were provided to encourage the private companies to invest in exploration and

production.

1.3 Vision and Objectives of DGH

To be an upstream advisory & technical regulatory body of international repute, creating value

for society through proliferation & dissemination of E&P knowledge optimal hydrocarbon

resources management & environment friendly practices by

1. Promotion of sound management of Indian petroleum and natural gas resources having a

balanced regard to environment, safety, technological and economic aspects of the

petroleum activity.

2. Recommendation to Government of India, the correct and best mix of blocks to be

offered for exploration and exploitation keeping in mind the national geological

objectives of the country.

1.4 Functions and Responsibilities

Advise Government on offering and award of acreages under NELP & CBM rounds for

exploration as well as, matters relating to relinquishment of acreages.

Technical advice to MOP&NG on issues relevant to exploration and optimal exploitation

of oil and gas.

Preservation, upkeep and storage of data / samples pertaining to petroleum exploration,

production, etc. and the preparation of data packages for acreage on offer.

Field Surveillance of producing fields and blocks to monitor their performance

Compliance of Ministry of Defence (MOD) guidelines.

All other matters incidental there to and such other functions as may be assigned by

Government.

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2. OVERVIEW

2.1 Seismic Data Acquisition and Seismic Data Processing

There are mainly three types of seismic survey:

1. 2D Seismic survey

2. 3D seismic survey

3. 4Dseismic survey

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2D seismic survey:

In 2D Seismic survey, receivers are placed in a straight line and the source is placed in same line,

thereby generating a 2D profile (distance, time).The source can be placed either at the end of the

line or in the middle also. Placement of the source is decided depending upon the geological

objective of subsurface. When the source is kept at the end of the receivers, the configuration is

known as inline shooting. When the source is placed in the middle of the receivers, such

configuration is called split-spread.

3D Seismic Survey:

In 3D Seismic, data is recorded with more than one no. of parallel receiver lines. Data is

recorded from all the planted receivers lying inline. And the the seismic sections in crosslines are

generated by using softwares like Petrel, Geoframe so that the final processed output is sorted to

bin-size(x and y direction), while time is third dimension. Inlines are generally acquired in a

direction perpendicular to the dip direction of the beds in an area. Thus, a final processed output

is a data cube instead of single lines as described in 2D.

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4D seismic survey

Apart from 2D and 3D, 4D surveys have also being carried out. 4D Survey is done for reservoir

monitoring/ management. It is time lapse 3D survey in which fourth dimension is time.

Variations in the seismic characteristics are monitored within the reservoir at periodic intervals

for planning best exploitation strategy to get maximum recovery of hydrocarbons from the

reservoir under study.

Seismic exploration method consists of three main stages:

Data Acquisition

Data Processing

Data Interpretation

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2.2 Seismic Data Acquisition

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Subsurface geologic structures containing hydrocarbons are found either land or sea. Different

data acquisition methods are adopted for onshore, offshore and transition zone. All the methods

have a common-goal, imaging the sub-surface. But, because the environments (on-land & off-

shore) differ, each acquisition methodology requires unique technology and terminology.

Seismic survey is a program for acquiring field data with an objective of mapping sub-surface

geological features/ structure by recording the reflected seismic waves, generated with artificial

sources. The recorded arrival time of the waves are reflected from various sub-surface layers,

having acoustic- impedance contrasts or refracted waves as per the design of survey.

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Land Data Acquisition:

In land acquisition, a shot is fired (i.e., energy is transmitted) and reflections from the boundaries

of various litho-logical units within the sub- surface are recorded at a number of fixed receiver

stations on the surface. These geophone stations are usually in-line although the shot source may

not be. When the source is in-line with the receivers – at either end of the receiver line or

positioned in the middle of the receiver line – a two-dimensional (2D) profile through the earth is

generated. In majority of land surveys, efforts are made in moving the line/ survey equipment

along and / or across farm fields or through populated area.

Marine Seismic Data Acquisition

In a marine operation, a ship tows one or more energy sources fastened parallel with one or more

towed seismic receiver lines. In this case, the receiver lines take the form of cable called Steamer

containing a number of hydrophones. The vessel moves along and fires a shot, with reflections

recorded by the streamers. If a single streamer and a single source are used, a single seismic

profile may be recorded in like manner to the land operation. If a number of parallel sources and/

or streamers are towed at the same time, the result is a number of parallel lines recorded at the

same time. If many closely spaced parallel lines are recorded, a 3D data volume is recorded.

Transition–Zone Seismic Data Acquisition

Because ships are limited by the water depth in which they safely can conduct operations, and

because land operations must terminate when the source approaches the water edge, or shore

lines, transition-zone recording techniques have been developed to provide a continuous seismic

coverage required over the land and then into the sea.

Seismic Field Equipment

There are basically 3 types of seismic equipment that are used in seismic data acquisition, they

are as follows:-

1. Navigation or positioning

2. Sources or charge

3. Receivers or seismogram

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1. Navigation or positioning

The equipments that are used in positioning are called navigators. Some of them are explained

briefly below:

i. Transit Satellite Positioning

This one of the method used for positioning the sources (ships) in the area that is under vision.

Satellites are 1075 Km from the earth and take about 107min. to circle the earth waves of

frequencies 150 & 400 MHz are continuously emitted from it. The frequencies measured by a

receiver are Doppler shifted. The satellite transmits information that gives its locations every 2

min.

ii. Global Positioning System (GPS)

It permits determination of latitude, longitude and elevation by trilateration. The system is

extensively used for geophysical positioning in the marine environment and is also used to set

base stations on land.

iii. Locating the streamer

A seismic ship usually tows a long streamer behind the ship. Even though the location of the ship

is known, the streamer can drift by appreciable amounts. However, it is often impossible in

rough seas, to distinguish the tail buoy reflection from the water wave back scatter, particularly

when the tail buoy is in the wave trough. A radio or GPS receiver can be mounted on the tail

buoy so that its location is known in the radio positioning or GPS system being used to locate the

seismic ship.

2. Seismic energy sources

i. Impulsive energy sources

These are the sources that induce energy by means of some forces like dynamite and some

explosives. They can be categorized under following headings.

a. Dynamite

Dynamite produces more energy and a broader bandwidth than any other sources. Most often

explosive land sources are detonated in drilled holes called Shot holes.

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b. Thumper/weight dropper

The thumper is a popular source in desert areas. It generates high level of horizontally travelling

surface waves and relative to other impulsive sources, is of low energy level. The weight is a 2

ton steel block, dropped from a height of 3m. The records from the multiple drops are summed.

In thumper survey the geophone groups are designed to reduce ground rolls.

c. Air gun

Air gun consists of a steel bell filled with water and sealed, at the bottom by a diaphragm that

rests on a base plate. When the air gun is fired, the air bursts into the water, causing expansion of

the diaphragm which transmits the pressure wave to the base plate.

d. Water gun

The water gun is a relatively new marine seismic sound source that produces an acoustic signal

by an impulsive rather than explosive mechanism. A comparison of the source characteristics of

two different-sized water guns with those of conventional air guns shows the water gun signature

is cleaner and much shorter than that of a comparable-sized air gun.

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ii. Non-impulsive energy sources

a. Vibroseis

Non impulsive sources differ from impulsive sources in that they transmit energy in the ground

for an extended period of time. The most common type of non impulsive source is a vibrator or

vibroseis. In the vibroseis system energy is produced by a pad pressed firmly to the ground

which vibrates in a carefully controlled way. The pressure fluctuation driving the pad is carefully

monitored by an electronic oscillator. It produces oscillations of continuously varying frequency

for a specified interval of time. One sequence of vibration is called sweep. This sweep is input

signal by a energy source.

b. Sparker

A seismic sparker generates an acoustic pulse by discharging an electrical pulse between

electrodes located on the tips and a ground point on the sparker body, in the conducting medium

of seawater.

c. Detonating Cord

Detonating cord is a thin, flexible tube with an explosive core. It is a high-speed fuse which

explodes, rather than burns, and is suitable for detonating high explosives, usually pentaerythritol

tetra nitrate (PETN, Pentrite).

d. Gas Gun

The principle is based on the rapid combustion of a mixture of propane and oxygen in a

cylindrical rubber sleeve.

e. Boomer

Boomer sound sources are used for shallow water seismic surveys, mostly for engineering survey

applications. Boomers are towed in a floating sled behind a survey vessel.

3. Seismic data receivers

There are generally two types of receivers that are used in seismic surveying that are Geophones

and Hydrophone.

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i. Geophone

Conventional geophones are based on Faraday’s law of electromagnetic induction. This law

states that relative motion of a conductor through a magnetic field induces an electromagnetic

force (EMF) which causes a current to flow through the conductor, if, if the conductor is an

element of an electrical circuit.

ii. Hydrophones

The hydrophone is an electro acoustic transducer that converts a pressure pulse into an electrical

signal by means of the piezoelectric effect. If mechanical stress is applied on tow opposite faces

of a piezoelectric crystal, then electrical charges appear on some other pair of faces. If such a

crystal is placed in an environment experiencing changes in pressure, it will produce a voltage

proportional to those variations in pressure.

SHOT GATHER:

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2.3 Seismic data processing

Objectives of Data Processing

The objectives of data processing may be summarized as follows:

To enhance the signal to noise ratio (S/N).

To produce seismic cross section representative of geology.

To meet the exploration objectives of the client.

Seismic Data Processing:

There are various steps involved in seismic data processing, they are mentioned below:-

1. Loading of data/conversion

2. Demultiplexing

3. Geometry

4. Editing

5. Amplitude corrections

6. Static correction

7. Frequency filter

8. Deconvolution

9. CMP – sorting

10. Velocity analysis

11. NMO/DMO – correction

12. Stacking

13. Migration

14. Post processing

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2.4 Seismic Data Interpretation:

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TIME STRUCTURAL MAPS1. Time Structural Map for Basement

Fig 16: Time structural map of Basement

The white boxes in the map represent the major faulted regions in the basement. The major trend of faults seems to be in the NW-SE direction. The faults in the map are indicative by the contour shift wherever a fault occurs. The black portions represent the throw of the fault. Since the black portions visible are less in area, it can be concluded that most of the faults in this area have a steep slope and some of them are vertical.

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2. Time Structural Map for HZ1

Fig 17: Time structural map of HZ1

The effect of the faulting in the basement is not much prominent in the horizon just above the basement. The horizon seems to be unaffected by the faults by which it can be concluded that the fault is older in age than the horizon.

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Well drilling and logging:For exploration activities, satellite surveys, gravity- magnetic surveys, aerial surveys etc. are done to know the local geology of the area under consideration and ascertain highs and lows along with the basement depth and other details and thickness of sediments. These help in understanding how prospective the area is. Structural and stratigraphic traps are obtained from seismic surveys in which bright amplitude indicate the presence of hydrocarbons. Drilling starts at the most prospective area and is continuously monitored in the mud-logging system. A drill break is applied at the point at which the velocity of drilling increases drastically. This happens only when a soft formation is encountered after a hard formation. Drilling stops and coring starts at that point as hydrocarbons are expected from that formation.

A mud logging system is an eminent part of the drilling unit as it monitors pH, sand quantity, viscosity etc of the formation. Mud is circulated in the drill hole to bring up drill cuttings, cool the drill bit, and maintain back pressure on the formation to prevent blow-out. Mud is mostly bentonite, but barite is mixed at times, to increase back pressure on the formation. The mud flows through a flow line, after encountering the drill bit, and reaches a shell shaker where the cuttings are separated from it. Then it moves to a desander, followed by a desilter, and a centrifuge to separate sand, silt and clay sized particles respectively. After the centrifuge, the mud is clear and hence, it goes back to the mud pit, followed by the pump and finally to the well, thus getting recirculated.

A Geo-Technical Order (GTO) is the plan made before drilling a well and consists of three parts:

I. Geological

II. Chemical

III. Drilling

The geological section determines the rock type, structures, pressure in the formations (hydrostatic/over-hydrostatic) and hence the local geology of the area under consideration. The chemical section decides the mud type to be used and the drilling section determines the bit type and casing to be used.

Wells can be of four types:

Vertical well.

Deviated well- J band.

Deviated well- S band.

Horizontal well.

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Horizontal wells have higher (2-2.5 times) production rates than vertical wells at 1.5 times the cost. They are more efficient than vertical wells as the water coning effect is much less. Deviated wells are drilled at places where the site is not suitable for drilling vertical wells directly above the reservoir.

All oil wells are drilled telescopically and cased so that the wells do not collapse or pollute the ground water in the vadose zone. Casing policy is decided based on the target, geology of the area etc. On land, there are three to four casing levels whereas offshore has greater than four casing levels. The first casing is called conductor casing or surface casing and is up to 100-250 meters from the surface. It isolates fresh water zones. The bit diameter is 171/2’’ and casing diameter is 133/8’’.The intermediate hole has a diameter of 121/4’’ with a casing diameter of 95/8’’ and extends up to 1300 meters. The production hole has a diameter of 81/2’’ with a casing diameter of 51/2’’ and extends up to 2000 meters. Each casing lands at a casing shoe which is to be implanted in a hard formation only. The length of one casing pipe is the length between two float collars, and is normally 9.3 meters. Casing is the costliest affair in the well as it is imported, hence, it is retrieved after all the work is over in the well by cutting at a particular depth and reused.

Pilot hole is drilled at first to identify the oil-water contact (OWC) and then the working well is drilled. Good cementing of casing against the interested zone is needed for effective hydrocarbon production. A perforation gun perforates the cement in the hydrocarbon-bearing horizon in a vertical drill hole for proper interaction with the hydrocarbon-bearing horizon.

Borehole environment:The area beside the bore hole can be divided into three zones:

Flushed zone: The zone nearest to well bore which is flushed with mud.

Invaded zone: The zone next to flushed zone containing both formation fluid and mud filtrate.

Virgin zone: It is the uninvaded zone next to the invaded zone and contains mostly hydrocarbons.

Well logging: “Well Logging” refers to the process of recording or data acquisition of some property of a well ( borehole) as a function of well depth. The property/ measurement may include:

– One or more physical properties of the earth formation

– Some geometric measurements of the borehole

– Wellbore flow properties

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– Well completion job

The data recorded are called the Well Logs.

Well Logs reveal a picture of (variations of) physical properties of earth formations as a function of depth.

Well Logs are the most universal documents describing oil and gas wells, hence most valuable Geological data source.

– Geologists use them for locating prospective hydrocarbon bearing zones

– Geophysicists use them for seismic correlation and interpretation

– Reservoir Engineers can locate reservoirs and estimate their hydrocarbon content

– Production Engineers use them to produce the wells in the best possible way

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Basic Well Logging Equipments are:

• A surface computer system mounted on a specialized truck (called the Logging Unit)

• An electro-mechanical cable (called the logging cable or the wireline) with drum mounted on the truck and controlled by a hydraulically operated winch

• A sonde or an electronic instrument (called the Logging Tool) containing appropriate sensors for the type of measurement.

Well logging is done to:

• To find Hydrocarbons

• To identify the Reservoir

• Determine the reservoir characteristics : lithology, porosity and hydrocarbon saturation

• Determine hydrocarbon type

• Determine the depth and thickness of pay

• Determine what type of fluid will flow and at what rate

Formation Matrix Definitions:

Well logging helps to determine the following attributes:

• Saturation

• Density, Porosity

• Lithology

• Permeability

• Formation Pressure

• Mobility & Samples

• Reservoir Size

• Accurate Reserves

• Basic Water Saturation Equation (Archie’s equation)

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Where

Sw – water saturation;

a- Tortuosity factor;

Rw – water resistivity;

Φ- porosity;

m- cementation exponent;

Rt – resistivity of uninvaded zone;

F- formation factor.

This equation is applicable to a sandstone reservoir only.

Well Logging Operations Two Broad Types:

1. Open Hole Logging Operations:

This type of logging operation is carried out in an open or uncased hole, usually immediately after the drill-string has been withdrawn. All basic petro-physical properties of the formation like resistivity, density, porosity, travel time etc. are measured here. Also, formation core samples and fluid sampling, pressure measurements are also done under open hole logging. The data so gathered are interpreted for detection of possible pay zones and overall formation evaluation.

2. Cased Hole Logging Operations:

This type of logging operation is carried out in a cased hole. It includes some nuclear and acoustic measurements, all well completion operations (like perforation, plug/packer setting etc.) and production logging operations.

SOME BASIC MEASUREMENTS CARRIED OUT IN OPEN HOLE ARE:

• Spontaneous Potential

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• Formation Natural Radioactivity

• Formation Resistivity

• Formation Density

• Formation porosity

• Formation Travel Time

• Formation Pressure/Fluid Sampling

SP Log – Principle of Measurement

• Existence of SP was discovered accidentally in 1931 by a field engineer.

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• SP currents originate principally through the electrochemical effects of the salinity differences between the borehole fluid and the formation water.

• The spontaneous currents flow when the fluids come into contact through a porous medium (the diffusion potential) or else, when they come into contact through a shale which acts as a semi-permeable membrane (the shale potential).

• The actual SP currents which are measured in the borehole are formed as a result of the combination of the two electrochemical effects.

• SP is analogous to a naturally – occurring battery downhole.

• Uses of SP are- calculate Rw, lithology identification and identifying tight formations.

• The spontaneous potential (SP) curve records the naturally occurring electrical potential (voltage) produced by the interaction of formation connate water, conductive drilling fluid, and shale

• The SP curve reflects a difference in the electrical potential between a movable electrode in the borehole and a fixed reference electrode at the surface

• Though the SP is used primarily as a lithology indicator and as a correlation tool, it has other uses as well:

– permeability indicator,

– shale volume indicator

– porosity indicator, and

– measurement of Rw (hence formation water salinity).

MEASUREMENT OF FORMATION RESISTIVITYFormation Resistivity Measurement has the following applications:

Distinguish between salt-water bearing and hydrocarbon-bearing formations

Determine true formation resistivity for calculating water saturation

Estimate invasion diameter

Measure flushed zone resistivity and calculate flushed zone water saturation

Indicate movable hydrocarbons

Correlate formations

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Delineate thin beds

• Resistivity is a property of the earth formation and refers to the degree by which the formation impedes the flow of electric current .

• All earth formations are somewhat conducting due to presence of saline water.

• Hydrocarbons are non-conducting fluids, hence porous beds containing hydrcarbons exhibit high resistivities.

• Since drilling fluid invades the porous / permeable beds, resistivity measurement is carried out at several depths of investigation to estimate the invasion profile.

• Thus we have several resistivity measurements:

– DEEP resistivity, refers to the uninvaded or virgin zone

– SHLALLOW resistivity, refers to the invaded zone

– FLUSHED ZONE resistivity, refers to the flushed zone

• Two types of resistivity tools are available- induction type and laterolog type.

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MEASUREMENT OF NATURAL GAMMA RAY• Natural formation gamma radiation comes from radioactive isotopes of uranium, thorium

and potassium.

• To record this radiation, there are presently two types of gamma ray tools in use:

– The gross (simple) gamma tool which is usually referred to as a natural

gamma tool.

– The spectral gamma ray tool.

• The gross gamma tool records the total gamma activity in the wellbore without regard to the source, while the spectral gamma tool is a spectral analyzer that identifies the source and gives the contribution (concentration) of each of the elements (potassium, uranium and thorium) to the overall spectrum.

• Natural Gamma Ray Tools34

This simple gamma tool consists of a detector and a counter. The detector is usually a scintillation type that outputs a discrete electrical pulse for each gamma ray detected. The height of the pulses are proportional to incident gamma energy, and the tool records the total count rate (counts/second) per depth sample. In order to output a standard result log, independent of tool systems, a unit of measurement called the API is used.

• Uses Gamma Ray log

– Lithology identification & indicate permeable zones

– Estimate shale volume

– Correlate well depths & formations

– Delineate bed boundaries

MEASUREMENT OF FORMATION DENSITY• Density tool principle is based on the interaction of Gamma Rays with formation atoms.

• The tool uses an artificial GR source (cs137) to emit strong gamma rays into the formation.

• These gamma rays collides with the electrons in the formation atoms, thereby knocking out the electrons and losing some of its original energy.

• This process is repeated over and over as the GR passes through the formation.

• Finally, the GR loses enough energy, and absorbed photoelectrically by the atom.

• For high density materials, collisions per unit length will be more, hence a GR detector placed at a distance from the source will record less GR counts.

• The detector reading is a function of the number of electrons that the formation contains - its electron density (electrons/cm³) – which in turn is very closely related to bulk density (gm/cm³).

MEASUREMENT OF FORMATION POROSITY• Porosity is measured using Neutrons tools (CNT or DSNT)

• Measurement is based on the principles of Neutron-Nuclei interactions in the borehole environment

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• Tool consists of an instrument section containing the electronics and two Neutron detectors

• A source sub, placed at a distance from the detectors, houses an intense Neutron source bombarding fast neutrons at an initial energy of 4.6 Mev

• Neutrons collide with the nuclei of the surrounding formation and loses energy (goes to thermal state)

• Of all atomic nuclei, Hydrogen atom causes the maximum loss of energy to the neutrons during diffusion because both are of equal mass.

• The detector is designed to detect the thermalized neutrons and the total count rate is sent to the surface computer via the logging cable

• Appropriate calibrations factors are then applied to output formation porosity from the count rates

• High formation porosity means high neutron moderation (by Hydrogen), hence less counts at the detector, and vise-versa.

CASED HOLE LOGGING

• As the name suggests, these type of jobs are carried out in a cased hole i.e. in a wellbore where casing has been run and cemented

• Cased Hole logging operations include:

– Cement Bond evaluation logging

– Cased hole depth correlation logging (GR, casing collars etc.)

– Completion operations (perforation, plugs, packers etc.)

– Production Logging operations

– Other specialized services

CEMENT BOND EVALUATION

• The Cement Bond Log is run in cased wells to determine cement to pipe and cement to formation bond which then aids in determination of effective zone isolation.

• A bond log records compressional wave travel time and the magnitude (the amplitude) of the acoustic energy at the tool's receivers. The first arrival peak (called E1) is assumed to be from the wave traveling through the bonded casing, because it has the shortest travel path and steel has a high sound velocity compared to cement and formation rocks.

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• Three separate measurements are simultaneously recorded on the log: travel time, casing signal amplitude, and the total acoustic waveform or Variable Density Log (VDL).

• The gamma ray, neutron and CCL are used for correlation purposes.

• The casing amplitude curve is derived from the first arrival at the receiver and is maximum in pipe not in contact with the cement sheath or formation (free pipe) allowing the pipe to vibrate freely, while minimum amplitude occurs in pipe completely surrounded by the cement sheath and unable to vibrate.

• Travel Time (TT) is the amount of time required for the acoustic signal to travel from the transmitter to the receiver.

• The first arrival amplitude curve with travel time gives the casing to cement bonding. The VDL gives cement to formation bonding.

PERFORATION

• Perforation means shooting holes in steel casing and cement to establish communication with oil and gas zones to the borehole

• Perforation creates conductive tunnels that link oil and gas reservoirs to steel-cased wellbores which finally lead to surface

• Different perforating systems (called perforating Guns) alongwith high-power perforating explosives are used to carry out perforation.

• Over-balance perforations are carried out thru’ casing and commonly known as casing gun perforation

• Perforations may be carried out:

• Over-balance condition: mud column pressure is more than the expected reservoir pressure. Involves the risk of formation damage

• Under-balance condition: mud column pressure is less than the expected formation pressure. Requires surface production set-up ready. Can be produced immediately.

• Under-balance perforations are carried out thru’ production tubing, and are commonly called Through Tubing Perforation (TTP).

• Casing Collar Log (CCL) or Gamma Ray (GR) log is recorded during perforation for depth correlation.

FISHING OPERATION

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• Sometimes, a logging tool lowered in the hole (open or cased), can not be pulled out. This happens when either the tool or the logging cable gets stuck in the hole.

• Fishing operations are carried out to free the stuck tool (or cable) and to safely take it out of the hole.

• Fishing is a normal happening in well logging operations.

• Specially designed fishing grabs or overshots are run at the end of the drill-string or production string to catch the “Fish” by neck.

• Once engaged, it is carefully pulled out of the hole.

• For cable stuck situations in open holes, a special fishing operation called “Cut & Thread” method is carried out.

• Fishing operation is a team work between logging and drilling personnel.

ACOUSTIC MEASUREMENTS

• Conventional acoustic logging is still based almost entirely upon the detection of the first compressional wave arrival and the production of “Delta-T” logs.

• Sonic logging tools were used primarily to determine the formation compressional velocity ( Vp ) and its inverse (1/ Vp). This value is referred to as slowness OR delta-t (Dtp), the “interval transit time” or simply “transit time.”

• The basic sonic tool has a Transmitter to send high frequency sound waves to formation

• Waves reflected by the formation are received by two receivers placed at different distances

• Recording the travel times of both receivers allows to calculate the transit time of the formation

• Significant achievements have been made in understanding the effect of formation properties on seismic wave propagation.

• As a result, acoustic velocity and attenuation can be related more quantitatively to formation porosity, permeability, fluid saturation, and lithology.

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2.4 RESERVOIR and PRODUCTION

Reservoir rock

Petroleum reservoir is a body of porous and permeable rock containing oil and gas through

which fluids move towards recovery openings under the existing pressure or that may be applied.

Porosity

Porosity is expressed as void ratio, which is the ratio of the volumes voids to total rock volume.

ф = pore volume/ bulk volume

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Where ф = porosity.

Types of Porosity on Basis of Origin

(a) Primary porosity

Primary porosity (or intergranular porosity) is originally generated during formation of the rock

units. Clays may have even up to 90% primary porosity; but it has no effective permeability for

oil.

(b) Secondary porosity

Secondary porosity is imposed on rocks after its formation by deformational (fracturing,

jointing) or chemical solution processes involved in diagenesis (e.g. dolomitisation of limestone)

or later events.

Porosity can also be classified as:

Absolute Porosity

The absolute porosity is defined as the ratio of the total pore space in the rock to that of the bulk

volume. The absolute porosity is generally expressed mathematically by the following

relationships:

Effective Porosity

The effective porosity is the percentage of interconnected pore space with respect to the bulk

volume.

PERMEABILITY

Permeability is a property of the porous medium that measures the ability of the formation to

transmit fluids. The rock permeability, k, is a very important rock property because it controls

the directional movement and the flow rate of the reservoir fluids in the formation.

SATURATION

Saturation is defined as that fraction, or percent, of the pore volume occupied by a particular

fluid (oil, gas, or water). Thus, all saturation values are based on pore volume and not on the

gross reservoir volume. This property expressed mathematically by the following relationship:

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Fluid saturation = total volume of the fluid / pore volume

WETTABILITY

Wettability is defined as the tendency of one fluid to spread on or adhere to a solid surface in the

presence of other immiscible fluids. This spreading tendency can be expressed more

conveniently by measuring the angle of contact at the liquid-solid surface. This angle, which is

always measured through the liquid to the solid, is called the contact angle q. The contact angle q

has achieved significance as a measure of wet ability. As the contact angle decreases the wetting

characteristics of the liquid increases.

CLASSIFICATION OF RESERVOIR AND RESERVOIR FLUIDS

Petroleum reservoirs are broadly classified as oil or gas reservoirs:

The composition of the reservoir hydrocarbon mixture.

Initial reservoir pressure and temperature.

Pressure and temperature of the surface production

Fig. 1. P-T DIAGRAM FOR A MULTICOMPONENT SYSTEM

Some Terms used in Phase Diagram

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Bubble Point Pressure: The pressure at which first bubble of gas comes out from liquid while

lowering the pressure.

Dew Point Pressure: The pressure at which first drop of liquid condenses out of gas on

increasing the pressure.

Bubble Point Curve: The line joining all the Bubble Point Pressures (the pressure at which first

gas bubble comes out from liquid on lowering the pressure) at different temperatures is called

‘Bubble Point Curve.” Fluid above this line, therefore, is in single liquid phase at all the

temperatures.

Dew Point Curve: The line joining all the Dew Point Pressures (the pressure at which first drop

of liquid forms out of gas on increasing the pressure) at different temperatures is called “Dew

Point Curve”. Fluid below this line, therefore, is in single gaseous phase at all the temperatures.

Critical Point: The point where Bubble Point Curve & Dew Point Curve meets is called as

Critical Point. At this unique point, for a particular temperature and pressure, there is no

difference between Bubble Point & Dew Point i.e. between liquid and gas phase.

Two-Phase Region: The area falling between the Bubble Point Curve & Dew Point Curve is

called as Two-Phase region. In this region, both liquid and gas phase co-exist. This area cab be

divided into a number of segments indicating reducing percentage of liquid phase and hence

increasing percentage of gas phase.

CLASSIFICATION OF RESERVOIRS

In general, reservoirs are conveniently classified on the basis of the location of the point

representing the initial reservoir pressure Pi and temperature T with respect to the pressure-

temperature diagram of the reservoir fluid. Accordingly, reservoirs can be classified into two

types. These are:

(a) Oil reservoirs-If the reservoir temperature T is less than the critical temperature Tc of the

reservoir fluid, the reservoir is classified as an oil reservoir.

(b) Gas reservoirs-If the reservoir temperature is greater than the critical temperature of the

hydrocarbon fluid, the reservoir is considered a gas reservoir.

Reservoir Management

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During the life of the reservoir we use to monitor its performance and well condition. it is useful

to monitor changes in average reservoir pressure. So that we could refine forecast of reservoir

performance. In special circumstances it may be possible to track the movement of fluid front

within the reservoir ex. Water flooding or in-situ combustion. Knowledge of front location will

allow us to evaluate the effectiveness of the displacement process and to forecast subsequent

performance.

Reservoir Description

Reservoirs are complex and contain different type of rocks stratigraphic interface fault barrier

and fluid fronts. Some of this feature may influence the pressure transition behavior to a

measurable extent and most of this also effect reservoir performance. This reservoir description

aids forecasting of reservoir performance and field development plan.

2.5 PRODUCTION

Production is the operation that deals with bringing hydrocarbons to the surface and preparing

them for their trip to the refinery or processing plant.

Production begins after the well is drilled.

The mixture of oil, gas and water from the well is separated on the surface.

The water is separated and the oil and gas are treated, measured, and tested.

Production operations include bringing the oil and gas to the surface, maintaining production,

purifying, measuring and testing. After a well is drilled, the operating company considers all the

data obtained from the various tests run on the formation of interest and a decision is made on

whether to plug and abandon the well or to set production casing and complete the well If the

decision is to abandon it, the hole is considered to be “Dry” not capable of producing oil or gas

in commercial quantities, cannot justify the expense of completing the well. Therefore, several

cement plugs will be set in the well to seal it off more or less permanently. If the operating

company decides to set production casing then well Completion is done.

Well completion

Well completion allows the flow of petroleum or natural gas out of the formation and up to the

surface. It includes strengthening the well hole with casing, evaluating the pressure and

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temperature of the formation, installing the proper equipment to ensure an efficient flow of oil

and natural gas out of the well. Installing casing in the well is an important part of both the

drilling and completion process. Cement is then forced into the annulus between the casing and

the borehole wall to prevent fluid movement between formations.

The production casing or oil string is the final casing for most wells. It provides a conduit from

the surface of the well to the petroleum producing formation.

Perforating

Casing must be perforated to allow liquids to flow into the well. This is a perforated completion.

Perforations are simply holes through the casing and cement, extending into the formation. The

most common method of perforating is using shaped-charged explosives. A perforating gun is

lowered into the well opposite the producing zone on a wire line& fired by electronic means

from the surface. After perforations are made, the tool is retrieved.

Well Completion after Perforating

The well is not produced through the casing. A small diameter pipe, called tubing, is used to

transmit oil or gas to the surface. A device called packer that fits around the tubing is lowered

just above the producing zone. It expands and seals off the space between the tubing and the

casing, forcing the produced fluids to enter the tubing to the surface.

Artıfıcıal Lıft

When pressures in the oil reservoir have fallen to the point where a well will not be produced by

natural energy, some method of artificial lift must be used.Artificial lift uses oil well pumps and

high pressure gas to lift the oil from the reservoir. The most common method of pumping oil in

land-based wells is beam pumping. The beam pumping creates an up-and-down motion to a

string of rods called sucker rods. 

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Water Flood Definition: - The practice of injecting water into a reservoir in order to increase oil recovery and maintain pressure.

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FIELD DEVELOPMENT PLANThe workflow of the FDP is as expressed under:

Format A

Format B (Potential Commercial Interest)

Appraisal

Commerciality (Basic Data Regarding Reservoir)

Distribution of Data among Departments (Analysis and Evaluation of Obtained Data)

FDP

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REFERENCES1. Yilmaz O. (2001), Seismic data Analysis, vol- 1& 22. Keary, P., and M, Brooks, An introduction to Geophysical Exploration, Blackwell

Scientific publication, Oxford, England (1998) 3. Robinson E.S & Coruch C. (1998), Basic exploration Geophysics, John Wiley & sons

inc., USA4. Telford W.M., Geldart L.P., Sheriff R. E. & Keys D. A. (1976), Applied Geophysics,

Cambridge University Press, New York 5. INFORMATION DOCKET- CAUVERY BASIN

6. INTERNET

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