Training report of ONGC Ahmedabad

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Introduction
As part of our curriculum, Training in relevant industrial establishment is held every year. This
year I, Student of M.Sc. Tech (Applied Geology), visited Forward Base, ONGC, Ahmedabad
Asset.
Ahmedabad happens to be a prolific oil producing asset of ONGC falling under western
onshore basin. All the working area of ONGC with commercial production of hydrocarbon is
broadly divided into two major components i.e. Basin and Asset. The basin is the work Centre
where all the G&G (Geological and Geophysical) activities are carried out and drillable
prospect are identified. All the necessary maps, geological sections are prepared at the basin.
All the seismic data are also interpreted at the Basin. Thus, at the basin, all the geological,
geophysical, geochemical data are generated and interpreted for the purpose of exploratory
activities.
Ahmedabad asset has many producing fields like Kalol, Limborda, Wadu Paliyad, Nandej etc.
These are almost matured fields, most of them having oldest hydrocarbon producing area. We
are assigned to forward base of Ahmedabad asset. Ahmedabad is a completely commercial
city located in central part of Gujarat. It is situated close to Sabarmati River. The nearest big
cities are Vadodara in south, Mehsana in the north. It is usually a very hot place with scanty
rain fall during the month of June and July. Ahmedabad is connected by road/railway to
Vadodara, Surat and nearby cities.
This forward base comes under the (WOB) Western Onshore Basin, Baroda and functions as
an interface between WOB and Ahmedabad asset. Its Base controls all the geological
operations pertaining to exploratory and development drilling and monitoring activities. At
forward Base Ahmedabad, we were given a comprehensive introduction to geological
operation and techniques pertaining to well site activities. The sequence of operation and the
intricacies involved therein were thoroughly explained by the geologists of forward base.

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CAMBAY BASIN: AN OVERVIEW
The Cambay rift Basin, a rich Petroleum Province of India, is a narrow, elongated rift half
graben. The total area of the basin is about 53,500 sq. km. In 1958, ONGC drilled its first
exploratory well on Lunej structure near Cambay. This turned out to be a discovery well, which
produced oil and gas.
Geographic Location of the basin:
The Cambay rift Basin extends from Surat in the south to Sanchor in the north. In the north,
the basin narrows, but tectonically continues beyond Sanchor to pass into the Barmer Basin of
Rajasthan. On the southern side, the basin merges with the Bombay Offshore Basin in the
Arabian Sea. The basin is roughly limited by Lat: 21˚00' - 25˚00'N & Long: 71˚30' - 73˚30'E.
Geologic location of the basin:
This Basin, the southern continuation of the
Barmer-Sanchor Graben is a narrow elongated
(NNW-SSE trending) intra-cratonic rift basin. It
is situated between Saurashtra craton to the
west, Aravalli swell on the northeast and
Deccan craton to the southeast. In the south, it
extends into Cambay Gulf and ultimately into
the Arabian Sea. A large part of the basin is
covered by Quaternary sediments. Cenozoic
outcrops are rare and occur only on the fringes
of the basin. The extensional architecture of the
basin is defined by three major Precambrian
trends viz., NNW-SSE trend related to
Dharwarian orogeny, NE-SW trend related to Fig: Cambay Basin
Aravalli orogeny, and ENE-WSW trend related to Satpura orogeny.

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Different Tectonic Zones with in the Basin:
Based on the cross trends the basin has been divided into five tectonic blocks.
From south to north, the blocks are:
1. Narmada block
2. Jambusar - Broach block
3. Cambay - Tarapur block
4. Ahmedabad – Mehsana block
5. Sanchor - Tharad block
Ahmedabad – Mehsana block:
>>The largest block in the basin, limited to the south by Nawagam-Wasna basement uplift
and its northern boundary (north of Mehsana horst) is arbitrary.
>>Block is segmented longitudinally into two major half grabens each with prominent down to
basin faults dipping east (most active during Palaeocene-Eocene, die upwards in the section and
absent in post-Miocene section).
>>NNW-SSE aligned structures.
>> Mehsana horst – Basement controlled topographic high(middle of the basin) formed due to
rejuvenation of along ancient faults after the deposition of cambay shale. On the horst crest cambay
shale is directly overlain by Miocene sediments and kalol and kadi formations pinch out on the
flanks of this horst.
>>Major fields include Kalol, Sanand, Ahmedabad, Wavel, Bakrol, North and South Kadi,
Indora, Jhalora, Santhal-Balol-Lanwa etc.

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STRATIGRAPHY OF CAMBAY BASIN:
Fig: Stratigraphy of Cambay Basin

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BASIN
ASSET
ONGC
PLANTS
INSTITUTES
FOR RESEARCH AND DEVELOPMENT
FOR PROCESSING
FOR PRODUCTION
FOR EXPLORATIONS
Hydrocarbon Exploration in ONGC
Oil and Natural Gas Corporation Limited (ONGC) is an Indian multinational oil and gas
company. It is a Public Sector Undertaking (PSU) of the Government of India, under the
administrative control of the Ministry of Petroleum and Natural Gas. It is India's largest oil and
gas exploration and production company. It produces around 70% of India's crude oil
(equivalent to around 30% of the country's total demand) and around 62% of its naturalgas.
ONGC is involved in exploring and exploiting hydrocarbons in 26 sedimentary basins of India.
Its international subsidiary ONGC Videsh currently has projects in 17 countries. ONGC has
discovered 6 of the 7 commercially producing Indian Basins, in the last 50 years, adding over
7.1 billion tonnes of In-place Oil & Gas volume of hydrocarbons in Indian basins. Against a
global decline of production from matured fields, ONGC has maintained production from its
brownfields like Mumbai High, with the help of aggressive investments in various IOR
(Improved Oil Recovery) and EOR (Enhanced Oil Recovery) schemes. ONGC has many
matured fields with a current recovery factor of 25–33%. On 1 November 2017, the Union
Cabinet approved ONGC for acquiring majority 51.11 % stake in HPCL (Hindustan Petroleum
Corporation Limited). On Jan 30th 2018, Oil & Natural Gas Corporation acquired the entire
51.11% stake of GOI.
Hydrocarbon Exploration and Production (E&P) operations, also referred to as upstream
operations, can be broadly grouped into three categories.

Prospect Identification by Multidisciplinary
team consisting of Geologists and Geophysicts,
Reservoir engineers etc.
Release of Exploratory
locations.
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The process of hydrocarbon exploration starts with prognostication and geo-scientific surveys
on the identified sedimentary basins. The information collected from these surveys is processed
and interpreted to construct a logical model of the basin. The model so constructed, is tested
by drilling exploratory wells. If the area proves to be hydrocarbon bearing, delineation wells
are drilled to determine the boundaries or the extent of the reservoir of the new oil or gas field.
This is followed by drilling of development wells, laying oil and gas pipe lines and installation
of facilities for regular commercial production.
Work flow of exploration activity:
Ground check
BEXB agreed locations are Further put-up in EPMB meeting
(Exploration Portfolio Management Board) chaired by Director (E).
All the recommended locations from different Basins are Put-up.
BEXB Meeting (Basin Exploratory Board) chaired by Basin
Manager, in which exploratory locations are proposed for next level
which is EPMB.
Boardmeeting

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Staking of Location i.e.
where drilling to be done.
Land acquisition for drill site
and approach road
Civil work for drill site
foundation and approach road
Rig development for drilling
Well drilling, Testing and
Completion

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What is Operation Geology?
Drilling an exploratory or production oil well involves numerous processes. The steps involved
in this process have been below. An operation geologist plays a significant role in successful
completion of oil/gas wells. They prepare the predrill geological prognosis, monitor drilling
activities, supervise wellsite geological operations, manage the overall geological data and
prepare reports for the well. Operation geologist Works with the prospect team on real time
geological interpretations, QA/QC of logs, pore-pressure detection and geosteering etc. These
steps together are known as operation geology.
The sequence of activities involved in drilling and completion of an exploratory well are as
under:
1. Preparation of proposals for exploratory locations
2. Proposals put up to Basin Exploration Board (BEXB) - Chaired by Basin Manager.
3. Agreed proposals of BEXB put up to Exploration Portfolio Management Board
(EPMB) – Chaired by Director Exploration.
4. Approved proposals of EPMB- Release of exploratory locations.
5. Staking of the location
6. LAQ- Land Acquisition for drill-site
7. Preparation of drill-site: Civil work- hardening, levelling of ground, setting up
perimeter, etc.
8. Preparation of GTO- Geotechnical order
9. Rig deployment- Transportation and rig building
10. Spudding and Drilling of well
11. Logging
12. Identification of intervals (Objects) for testing
13. Casing
14. Cementation
15. WOC- Waiting on cement
16. Cement clearing/casing scrapper
17. Casing hermaticity test
18. Production testing.
19. IF HC bearing, after reservoir study (multi-bean, BHS etc.) well can be put on
production.

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Duties and Responsibilities of Well Site Geologists
A well site geologist is in charge of the geological aspects of the well. It is his duty to
communicate effectively with the Basin or Forward Base office and the other well site
personnel to ensure smooth running of operations while adhering to the well programme.
The main responsibilities of well site geologists are:
• Collection and description of ditchsample.
• Study and analysis of Hydrocarbonshows.
• Maintaining master log.
• Deciding coring points and core description.
• Supervision during side wall coring process
• Supervising and monitoring Mud Logging Unit (MLU).
• To monitor drill breaks, torque, and mud properties.
• Monitoring sample collection procedure.
• Selecting proper casing points,
• Deciding landing depth during drilling of horizontal and multilateral wells.
• Reporting-
➢ Daily progress report (DPR)
➢ Daily geological report (DGR)
➢ Weekly/ Monthly progress report.
➢ Well completion report
• Overseeing production testing.
• Coordination and execution of programs received from base.
• Quick log interpretation.

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Geo Technical Order (GTO)
The programme of a well which covers the general, geological and other technical data is called
a “Geo Technical Order (GTO)”.
The GTO is initiated by Geology Group and subsequently prepared by Geology, Chemistry,
Drilling, Logging and Well services sections. Thus GTO becomes a guide to everyone
connected with drilling a well. The programme mentioned in GTO is only tentative and can be
altered if and when required by the concerned competent officer on site as per the actual well
behaviour during drilling.
The GTO constitutes data include mainly of general data, rig data, geological data, mud data,
drilling data, forecast of drilling days, general remarks, deviation data if any, complications
data if any and images such as location map, seismic In line and Cross line passing through the
location, Time structure map etc.
General data
It includes the under mentioned information:
WBS Element, Basin/Sub-Basin/Field, Structure/Prospect, State/District, Postal Address,
Police Station/Fire Station, Target depth, Ground level, Kelly bush, Latitude , Longitude,
Licence & Block details, Consortium members, Well type & Category, Well profile, Reference
wells, Mud logging Services, Planned well cost, Objective.
Rig data:
The rig data consists of name of the rig, type of the rig, well head set and BOP which will be
used in the well, type of mud pump and draw works, power to mud pump and draw works.
Geological data:
The Geological data consists of the expected geological parameters during drilling the well and
geological well programmes. The break-up of the geological data is as follows:
Depth, Age, Formation, Lithology, Expected hydrocarbon shows, Conventional coring, Wire
line logging, Collection of cutting samples, Angle of dip, Expected, formation temperature,
Expected Mud loss and Caving.
Mud data:
The Mud data consists of planned mud parameters and rheology based on the Geologicaldata.
The mud parameters may be changed as per the actual bottom hole condition and behaviour of
the well during drilling. The break-up of the mud data is as follows:
Mud system, Mud Weight, Funnel Viscosity, Fluid loss, Yield point, Solids (%), pH, Gel,
Salinity of mud filtrate.

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Drilling data:
The drilling data consists of the planned drilling parameters, casing policy, bit and drill string
details based on the geological data. The drilling parameters and hydraulics may be changed
as per the actual bottom hole condition and behaviour of the well during drilling. The break-up
of the drilling data is as follows:
Casing and cement rise, Drilling type (Rotary/Air Hammer etc.), Bit size & type, Number of
bits expected, Meterage per bit, Weight on bit (WOB), Rotation per minute of the rotary table
(RPM), Mud pump Discharge (LPM), Liner Size(inch), Stand pipe pressure (SPP).
Geological Data required for preparation of Geo technical Order (GTO):
The data required for generating the GTO of an exploratory well is normally collected from
the nearby well data. While selecting such wells for making the GTO, the geological objective
of the well to be drilled along with its target depth are to be kept in mind. It is better to select
such wells which are drilled in similar geological set up. In case there is a dearth of such wells
in the immediate vicinity of the well to be drilled, the nearest well data within a reasonable
geographical distance may be considered for such purpose coupled with geoscientific data
generated from interpretations at the Block level.

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Fig : Geo Technical Order

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Cutting Sample Evaluation
The drill cuttings generated during drilling are the prime source of information of a well.
Hence, collection and study of cutting sample are to be done with utmost care and sincerity,
once missed, they are missed forever. The main purpose of sample logging are –
• Evaluate the formation and associated hydrocarbon showsencountered during drilling.
• Delineation of lithological contacts and sub-surface stratigraphy.
• Determination of physical characters of different lithology encountered in the well.
Cutting sample analysis and Description
For the analysis of cutting it is necessary to take some amount of cutting from all the three
sieves as the representative sample of a particular interval. The cutting should be analysed
under microscope in wet condition as physical properties are clearer in wet condition. Some
part of dry cutting is to be used by mud logger for determination of calcimetry, shale density
and shale factor.
The sample description is generally in the following order:
• Rock type
• Colour
• Hardness
• Texture
• Cement or matrix
• Calcareousness
• Fossil and mineral accessories
• Visual porosity
• Fissility if any
• Oil and gas show
Rock Type
The most commonly used well-site method to describe rock type is based on the grain size and
induration of the fragments making of the rock. Three major sub-divisions of grain size used
to describe rock types are-
• Rudaceous: The grain size discernable to naked eye.
• Arenaceous: Grain size discernable with a microscope.
• Argillaceous: Grain indiscernible in the field.
The two major sub-divisions of induration used to describe rock types are:
• Unconsolidated: occurring in individual grains.
• Consolidated: grains held together by cement.

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Color
When determining the color of a sample, inspect the sample when wet. Dried cutting can be
viewed to allow a better discrimination of subtle hues, color shades and structures. Colour is a
useful indicator of depositional environment, especially in argillaceous rock.
Grain Size
Grain size determination from drill cuttings should follow a disciplined procedure to obtain an
accurate overall estimate of:
1. Size of individual grains
2. Mean size of grains in an individual cutting
3. Mean size of grains in all cuttings of same lithology
Grain Shape
Grain shape is a function of roundness and sphericity. Shape is of critical importance in sample
description because it gives clue to two important geological parameters:
1. Mode and distance of transport
2. Porosity and Permeability

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Evaluation of Hydrocarbon Show
Although petrophysical evaluation leads to a conclusive determination of commercial
quantities of hydrocarbon, it is the Well Site Geologist’s responsibility to report and evaluate
all hydrocarbon shows. Sometimes, in absence of electro logs of a particular interval due to
some complications the hydrocarbon shows observed and recorded by well site geologist may
help for identifying intervals interesting from hydrocarbon point of view.
Oil Show Evaluation:
The samples to be examined under ultraviolet light for the evaluation of oil show. Test and
inspection should be performed on unwashed and washed cuttings as well as individual grains.
Listed below are some of the most common methods of testing for hydrocarbon in samples and
cores that should be used by geologist during routine sample examination.
Routine Hydrocarbon detection methods:
Odor:
Odor may range from heavy for low gravity oil to light for condensate.
Oil Staining:
The amount of oil staining in ditch cuttings is primarily a function of the distribution of the
porosity and oil within the pores. The color of stain is related to the oil gravity i.e. heavy oil
stain tends to be dark brown while light oil stains tend to be colorless. The degree and color of
the stain should be noted as no visible oil stain, spotty, patchy or uniform oil stain.
Fluorescence Test:
The wet samples are to be checked under Fluoroscope for fluorescence and if positive, its
intensity, color and distribution to be noted. The color of fluorescence under UV light varies
from brown, dark green, golden, golden yellow, blue, yellow to white. The type of fluorescence
is dependent upon specific gravity of oil.
The samples showing fluorescence does not necessarily indicate the presence of oil
straightaway, as certain minerals calcite, free lime, and dolomite also give fluorescence under
ultraviolet rays. This is called mineral fluorescence. More over fluorescence can also be
caused by contaminations such as diesel in mud system.
Fluorescence test should be carried out immediately as the color tends to dull if exposed to
atmosphere for a long time due to volatilization of crude. In most cases the fluorescence will
be found around the grains in the matrix of the rock.
Among minerals calcite gives light blue fluorescence, free lime emits copper-red hue, dolomite
gives yellow to yellowish brown and limestone glows under UV light.

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Cut and solvent Test:
This is the test that can be performed for confirming the presence of hydrocarbon. The most
common solvents are carbon tetrachloride, n-hexane, chloroform, ethane, acetone and
tricholoro-ethane etc. The organic solvents when added to the cuttings showing fluorescence
dissolve the hydrocarbon which comes out from the grains with the organic solvent resulting in
coloration of the solvent.
Generally, n-hexane is widely used for routine oil detection. In oil well terminology, terms
like positive or negative cut are used for reporting oil shows. If the rock is soaked in solvent
and imparts fluorescence to the solution, it is called positive cut while negative cut is
considered for the solution that does not changes its color.
The following procedure is recommended for checking fluorescence in cuttings.
• A portion of washed sample to be placed on a thoroughly cleaned watch glass and to
be observed under ultraviolet light (fluoroscope).
• A few grains or a rock chip that shows fluorescence are to be picked and placed on a
separate cleaned porcelain tray.
• A few drops of n-hexane are to be added to it.
• A cleaned test tube containing n-hexane is to be observed under UV light in order to
ensure that the solution is not contaminated. If not contaminated it will not show any
fluorescence color. If the cuttings are oil saturated, plumes will immediately arise
from the sample. These are due to solution of the oil and reduction of interfacial
tension between the oil and grains which imparts an overall fluorescence to the
solution. If a rock has low permeability the reaction will take place at a slower rate
compared to a rock with higher permeability.
• If the fluorescence of the solvent is not observed at once, the sample is to be observed
again after few minutes to be certain of absence of fluorescence.
• If the rock chips are very hard generally in case of very tight reservoirs, then they
should be crushed before adding n-hexane so that it can dissolve the
hydrocarbon easily.
• In cases where cut is not clear from the porcelain tray, the entire content to be
transferred to a test tube and observed under UV light for better result.
Table 1: Fluorescence of oil having different API gravity
API gravity Color of crude oil
<15 Brown
15-25 Orange
25-35 Yellow to cream
35-45 White
>45 Blue white to violet

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Fig : Fluoroscope Fig : Sample under UV light

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Coring
A core is a cylindrical sample taken from the formation which is a true representative of the
formation down hole. The process of cutting such a cylindrical sample of the formation is called
coring.
Objective of Coring
• To get a firsthand information about the exact lithology being drilled.
• To obtain porosity, permeability and saturation data of the reservoir.
• To obtain information regarding formation boundaries, sedimentary structures for
depiction of depositional set up, undisturbed sample for collecting insitu paleontological
information and collecting undisturbed geochemical sample.
• To obtain different formation fluid contacts like GOC, OWC etc.
• To have an idea regarding the general attitude of beds.
Types of Coring
Basically two types of coring are done at the well site:-
a) Conventional coring-at the time of drilling
b) Side wall coring-during wire line operation
a) Conventional coring
Conventional cores are also known as whole cores.
They are continuous sections of rock extracted from
the formation during drilling operations. The coring
bit is different compared to drilling bit. It is hollow
without any nozzles so that it cuts from the formation
and captures a solid cylinder of rock that can be
brought to the surface as a single piece of rock.
Generally conventional core is of 9 metre length.
Methods of conventional coring: Fig: Conventional coring process
Conventional core barrel
It consists of an inner barrel, an outer barrel, a core barrel, a core catcher and a vent or pressure relief
valve. Drilling fluid circulated between the inner and outer core barrels but cannot pass through the
inner barrel resulting in increase in core recovery. Core barrel is attached to the end of the drillpipe.
I) Diamond core assembly:
In order to increase both recovery and penetration rate diamond faced bit is used for coring in hard,
dense formations where cost of coring may be high using a normal core bit.
II) Wire line core assembly:

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In this type of assembly, instead of the core being attached to the bottom of the drill string, the wire
line assembly is attached inside the drill pipe which is smaller in diameter. In this case when the
desired length of core has been cut an overshot device is lowered into the drill pipe which grabs the
core. It is more economical in deeper wells.
b) Side wall coring (SWC)
Side wall cores are plugs of rock cut from the well
bore wall by wire line conveyed tools.
Purposes of SW
• After logging is done if any potential zone is
identified then the side wall cores are taken
from these zones for getting information
about fluid content, formation composition,
stratigraphy and palaeontology.
• When recovery of conventional core against
the interesting zone is very poor or coring
could not be carried due to some reason.
Percussion sidewall coring:
Fig : Side Wall Coring
These tools shoot hollow, retrievable, cylindrical bullets 1 inch wide by 1.75 inch long into the
borehole wall. The tool (gun) can be combined in multiples of approximately 30 bullets with
120 shots a general maximum. The gun is lowered to the desired depth then individual bullets
are electrically fired from the surface. The bullets remain connected to the gun by wires, and
movement of the gun pulls the bullets from the borehole wall.
Core description:
▪ Identification of lithology with detailed
description
▪ Description of hydrocarbon shows in detail
▪ Sedimentological observations
▪ Structural observations
▪ Visual porosity
Fig : Arrangement of core boxes
Core packing and Despatch:
The outside of the core boxes should be marked with the following information:
• Well Name
• Core Number (i.e. CC#1 or CC#2)
• The box number and total number of boxes (i.e. Box 1 of 7, Box 2 of 7)

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Drilling Technology
Drilling Rig
The arrangement used to drill a well bore is collectively called as rig. In onshore operation, rig
includes virtually everything except living quarters. Major component of the rig includes the
mud tank, mud pump, the derrick or mass, the draw work, the rotatory table or top-drive, the
drill string, the power generation equipment and auxiliary equipment. In case of offshore
drilling, the rig includes the same components as onshore, but not those of the vessel or drilling
platform itself. The rig is sometime referred to as the drilling package, particularly in offshore.
The parts of the rig can be grouped into five systems-
1. Power system
2. Rotating system
3. Hoisting system
4. Circulating system
5. Control system
POWER SYSTEM
On the basis of power system, the land rigs here are majorly divided into two types, Mechanical
and electrical.
Mechanical rig: Has a smaller block size of about 130m x 130m. It uses torque converters,
clutches powered by its engines, often diesel. The assembly can be easily transported on a truck
as the rig is comparatively of a smaller size. These rigs can drill about 1000m or soonly.
Electrical rig: Has a block size of 150m x 150m. The major items of the machinery are driven
electrical motor, which is usually driven a diesel engine. They are large rigs which can drill
deep into the formation. Their components are to the site and then assembled there.

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ROTATING SYSTEM
Rigs may either have a conventional ‘Lower Rotating System’ or a ‘Top Drive Rotating
System’.
Lower Rotating System: Operating through Kelly drive bushings,
The rotary table rotates the Kelly and through it the drill string and bit.
The Kelly drive bushings are driven by four pins which fit into openings in the
master bushing, which, in turn, fit into the rotary table.
Salient features:
• Single/joints are drilled instead of stands of drill pipe
• It holds devices called slips that support the weight of the drill string
When it is not supported by the elevators or hook andKelly.
Fig : Lower rotating system
Top Drive Rotary System: With such a system the Kelly and Kelly bushing are not required,
and the master bushing and rotary table serve only as conduits for the drill string to be raised
or lowered into the borehole. An electric motor and is located directly below and attached to
the swivel using a drive-sub, allowing normal drilling fluid circulation, uses the simple step-
type transmission to provide rotary torque to the drill stem
Salient features:
• Because the Kelly is not required, stands of drill pipe are drilled instead of single joints.
CIRCULATING SYSTEM
Circulation of the drilling fluid serve the several functions on a rig, including cooling the bit,
providing hole stability, and adding in formation evaluation. Drilling fluid is circulated by the
mud pumps. The volume of the mud being pumped is measured by the stroke counter, and the
rate of the movement is measured by the stand pipe pressure. The stand pipe connects the mud
pumps to the Kelly hose. Mud is pumped down the drilling string through the bit and up the

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annulus (the space between the drill pipe and the bore hole). Returning mud flows down the
flow-line into a surge tank (possum belly) and across the shale shakers. Shake shakers are the
vibrating screening devices that are design to shake in order to separate out the drill cutting
from the mud. The shale shakers are the first place where cuttings can be examined, and the
gas is extracted from the mud by the degasser. After going through the shale shakers, the mud
passes through a series of the tanks or pits where the fine solids are removed via de-sander, de-
silter and centrifuges, and the mud properties are adjusted. Pits are named for their function
(e.g., shale pit setting pit, volume pit, mixing pit and suction pit). The mud pumps are charged
from the suction pit. Excess mud can also be delivered from the metal mud pit into a large,
plastic lined reserve pit located to the side of the rig.
Purpose of the circulation: An essential element of drilling a well is the drilling fluid or mud.
Drilling fluid serve a number of functions:
➢ Removal of the cuttings from the bottom of thehole.
➢ Suspend cuttings and weight material.
➢ Transports cuttings and gas to the surface.
➢ Cool and lubricate the drill bit and drill string.
➢ Add bouncy to the drill string.
➢ Control sub surface pressures
Following mud and mud-filtrate properties are taken into account to properly serve the
above-mentioned functions,
MUD
➢ Density
➢ Viscosity
➢ Gel strength
➢ Filtration
➢ Sand content
MUD FILTRATE
➢ pH
➢ Alkalinity and lime content
➢ Chloride
➢ Hardness
➢ Sulphate
➢ Resistivity
➢ Salinity

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Fig: Schematic representation of mud circulatory system

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CONTROL SYSTEM
Blowout Preventer (BOP)
It’s a surface-pressure control equipment of the drilling rig. When the hydrostatic pressure
drops below the formation pressure, the formation fluid flows into the annulus, If this flow is
minimal, causing a slight decrease in the drilling fluid density (mud density), the drilling fluid
is said to be gas cut, oil cut or saltwater cut, depending on the fluid. When noticeable amounts
of formation fluids enter the bore hole, the event is known as a kick. An uncontrolled flow of
formation fluids is a blowout. By closing the BOP (usually operated remotely via hydraulic
actuators), the drilling crew usually regains control of the reservoir, and procedures can then
be initiated to increase the mud density until it is possible to open the BOP and retain pressure
control of the formation.
The closing can be done using an annular preventer, with pipe rams, or if the drill pipe is out
of the hole, using the blind rams. In addition, it will be necessary to pump drilling fluid into
the well and to allow the controlled escape of fluids. Injection of heavier drilling fluid is
possible either through the drill pipe or through a kill line. Flow from the well is controlled
using a variable orifice (choke). Choke lines will carry the fluid to a reserve pit where the
undesired fluid is discarded through a separator where the fluid is degassed and saved.

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Fig : Drilling rig
Index
1. Mud Tank
2. Shale Shaker
3. Suction Line (Mud Pump)
4. Mud Pump
5. Power Source
6. Vibrating Hose
7. Draw –Works’
8. Stand Pipe
9. Kelly Hose
10. Goose-Neck
11. Travelling Block
12. Drill Line
13. Crown Block
14. Derrick
15. Monkey Board
16. Stands (Of Drill Pipe)
17. Set Back (Floor)
18. Top Drive
19. Kelly Drive
20. Rotary Table
21. Drill Floor
22. Bell Nipple
23. Blowout Preventer (Bop)
(Annular Type)
24. Blowout Preventer (Bop) (Pipe
Ram & Blind Ram)
25. Drill String
26. Drill Bit
27. Casing Head
28. Flow Line

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DIFFERENT PARTS OF DRILLING RIG
DRILL STRING:
A drill string transmits the drilling fluid (via the mud pump) and torque (via the top drive or
Kelly drive) to the drill bit. The term is loosely applied to assembled collection of Drill pipes,
drill collars, tools and drill bit. The drill string is hollow so that the drilling fluid can be pumped
down through it to the drill bit and back to the top through the annulus (the void/ gap between
drill string the casing/ open hole).
Drill string components
1. Bottom Hole assembly (BHA)
2. Transition Pipe, which is often heavy weight drill pipe (HWDP)
3. Drill Pipe
BOTTOM HOLE ASSEMBLY (BHA)
Bottom Hole Assembly consists of the drill bit, drill collars, which are heavy thick walled tubes
which are used to provide the weight upon the drill bit and the drilling stabilizers, which keeps
the assembly centered in the hole. It may contain other component such as downhole motor
and rotary steerable system, measurement while drilling (MWD) and logging while drilling
(LWD) tools. The component are joined using rugged threaded connections.
TRANSITION PIPE (HWDP)
This may be used in transition from drill collars to drill pipe. The function of HWDP is to make
a flexible transition between the drill collars and the drill pipe. This helps to reduce the fatigue
failures seen directly above BHA. The secondary use is to add weight to the drill bit.

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DRILL PIPE
It makes the majority of the drill string. Each drill pipe comprises of a long tubular section with
a specified outside diameter (for instance, 3 ½ inch, 4 inch, 5 inch, 5 ½ inch, 5 7
/8 inch, 6 5
/8
inch). At each end of the drill pipe tubular, larger diameter portions called the tool joints are
located. One end of the pipe has the male connection while the other has the female connection.
DRILL BITS
They are the cutting tools used to drill hole which is generally circular in cross section. There
are two types of drill bits, fixed cutter and roller cone. A fixed cutter has no moving parts and
the drilling occurs due to shearing, abrasion of the rock and scraping.
A major factor in drill bit selection is the type of the formation to be drilled. The effectiveness
of the drill bit varies by the formation type. Information from the adjacent well is used in proper
selection of the drill bits. Drill bits are pinched with metallic tungsten carbide and industrial
diamond.

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Drilling Fluids
Drilling fluid is a multi-functional system to provide conducive environment to carry out
effective and efficient drilling operations and to impart bore hole stability for performing
various operations necessary for exploration and exploitation of hydrocarbon.
Drilling fluid have a number of alternative names, most commonlyused name is mud or drilling
mud. Other acronyms are water based mud (WBM), oil based mud (OBM) etc.
Functions of Drilling Fluid
The important functions of drilling fluid are summarized below:
• Removal of cutting: Removal of cutting is one of the most important function of drilling
fluid. Fluid flowing from the bit nozzle exerts jetting action that keeps the face of the
hole and the edge of the bit clear of cuttings. This ensures longer bit life and greater
efficient drilling.
• Control of subsurface pressure: The formation pressure of a reservoir rock is kept in
check by the drilling fluids.
• Well building: A good drilling fluid should deposit good filter cake on the borehole
wall to consolidate the borehole wall and retard the passage of formation fluid intothe
borehole.
• Cooling and lubrication: considerable heat is generated due to the friction in the bit and
the formation. Drilling fluid helps in heat dissipation and lubrication to some extent for
better efficiency and longer bit life.
• A good drilling fluid helps in holding the cuttings in suspension when the circulation is
stopped.
• Transmitting hydraulic power to the bit.

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Drilling Complications
Complication is a problem in the well bore that prevents safe drilling, logging, casing lowering,
well testing etc. Common types of drilling complications encountered are:
Heaving Shale Problem: Shale sections containing bentonite and hydrous clays, swell and fall
in to the hole. In drilling such clays or shales is called as heaving shales. Heaving shales may
cause stuck up, excessive solid build up in the mud. While drilling such section high calcium
content and gypsum mud is generally used to reduce hydration.
String Stuck Up: A string is said to be stuck when no free movement is possible. String stuck
up occurs due to following reasons:
• Differential Stuck up: During drilling through a porous and permeable portion thick
mud cake is formed on the side of the well bore. When the drilling string is stationary,
the portion lying on one side of the well bore against permeable and porous formation
is isolated in such a way that mud cake restricts pressure communication due to the seal.
The pressure acting on the side in contact with the well bore is equal to the formation
pressure whereas on the remaining side is equal to hydrostatic head of mud. The
differential pressure so generated results in the string being pressed against the well
bore and subsequently getting differentially stuck although circulation maycontinue.
• Mechanical Stuck up: Mechanical stuck up can occur due to improper hole cleaning,
formation instability, and well bore geometry. Factors affecting hole cleaning are –
Mud Weight, Annular Velocity, Hole Inclination, Flow Rates, ROP.
Mud Loss: Mud loss or circulation mud loss represents the loss of mud in bore hole. Mud
flowing or entering to the formation implies less mud return through the flow line. The
reduction of annular velocity above the loss zone reduces the carrying capacity of the mud.
Hence cuttings may accumulate any may fall into the bottom of the hole resulting in stuck up.
Mud loss can be controlled byplacements of different types of pills like lost circulation material
(LCM pill), in extreme cases cement plugs may be placed against the zone.
Well Activity: A kick is defined as influx of formation fluid or gases into the well bore. It is
an uncontrolled from the formation into the bore hole. It generally occurs when formation
pressure exceeds than the hydrostatic pressure exerted by the mud. Blowout is a result of
uncontrolled kick. Conditions can lead to kick are-
• Abnormal formation pressure,
• insufficient mud weight than what is required,
• swabbing
• Lost circulation.
String failure and Casing failure causes drilling complications.

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Objectives of well planning:
Well Profile
The objectives of well planning is to formulate from many variables a program for drilling a
well that has the following characteristics:
• Safe
• Minimum cost
• Usable
Unfortunately, it is not always possible to accomplish these objectives on each well because of
constraints based on:
• Geology
• Drilling equipment
• Temperature
• Casing limitations
• Budget
It is always preferred to drill vertical wells to minimize complications. However, due to
unavoidable circumstances arising out of LAQ, geographical setting, environment constraints
etc. inclined wells are drilled with the help of directional drilling, from suitable surface position
to achieve predetermined subsurface objective.
Directional drilling
Directional drilling is defined as an art and science involving deflection of wellbore in a
specified direction in order to reach a predetermined objective below the surface of the earth.
The need of directional drilling is to drill the wells in inaccessible location to overcome the
geographical and geological problems like faults, salt domes etc.
Directional drilling terminologies
A directional well consist of build, hold, drop and hanging section.
• Kick off point (KOP): The KOP is the depth from where wellbore is intentionally
deviated from the vertical.
• Build up: It is the start of the inclination or angle in the hole. In the build section,
inclination or angle of the well is gradually increased up to planned value with the
preplanned build rate.
• Hold: The inclination that is planned to hold for any length of the well.
• Drop: It is the decrease of inclination in a planned curve of the well. It includes start
of drop and end of drop.
• True vertical depth (TVD): The vertical depth of the well measured from areference
point.
• Measured depth (MD): The actual distance travelled along the well bore.

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Fig : well Profile
Types of Well Profiles:
“L” Type: This is the most common and simplest profile for
directional well. The well is drilled down vertically upto KOP,
where the well is deviated to required inclination and further
maintained to target. This profile can be applied where large
displacements are required at relatively shallow target depth. This
profile includes only build and hold part.
Fig : “L” Profile
“S” Type: This profile is similar to “L” type profile up to hold
section. After that profile enters in a drop of section, there
inclination is reduced and in some cases becomes vertical as it
reaches the target. It is also applicable when target is deep and
horizontal displacement is small. It includes build, hold and drop.
Fig : “S” Profile

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“J” Type: It is used in particular situation like salt dome drilling,
fault drilling etc. In these profile vertical section will be more as
compare to the above profiles and KOP point will be at greater
depth. Formation may be harder and less responsive to deflection.
It includes Deep KOP and Build.
Fig : “J” Profile
Horizontal well profiles
Horizontal wells are categorized by the radius of curvature
adopted to make the well horizontal. They are also classified by
build up rates which is inversely proportional to radius of
curvature. They are long radius (radius length 1000-5000ft),
medium radius (286-716ft), short radius (16-57ft) and ultra-short
radius (less than 10ft). The main advantage is, it increases the
drainage area and increase the penetration of producing
formation. It helps in increasing the efficiency of enhanced oil
recovery (EOR).
Fig : Horizontal Profile

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Introduction
Mud Logging Unit
Mud logging is the technique used in well site for overall monitoring of drilling Parameters,
mud parameters and gas parameters along with geological parameters. The primary objective
of mud logging is to forecast drilling hazards by monitoring the drilling, mud and gas
parameters. That is the reason why Mud logging technique is termed as the technique of
“Looking ahead the bit”.
Different sensors are used to calibrate the data in MLU. There are 4 parameters on which basis
the analysis is done:-
1. Drilling Parameter
2. Mud parameter
3. Gas indication
4. HC show
Drilling Parameters
• Hook Load: Weight of drill string hung from hook. There are several things which can
be known from hook load i.e. Drill string loss, WOB etc.
• Weight on Bit (WOB): Amount of downward force exerted on the drill bit. In ONGC
it is measured in pounds. As drilling depth increases drilling rate reduces so to make it
faster we increase the WOB.
• Rate of Penetration (ROP): The time taken for drilling 1m of formation. It is measured
in metre per hour or feet per minute. Generally ROP increases with fast drilling
formation such as sandstone and decreases in slow drilling formation such as shale. It
can indicate about Drill break (A sudden increase or decrease in ROP during drilling
known as positive drilling break or reverse drilling break simultaneously) also.
• Rotation per Minute (RPM): The number of times the rotary table makes one revolution
in one minute. It is measured by proximity sensor.
• Stand Pipe Pressure (SPP):
• Strokes per Minute (SPM): The number of strokes the mud pump completes in one
minute. This determines the rate at which a liquid is pumped. If the no. of strokes per
minute is increased the pump rate also increases. A proximity sensor is used to calibrate
SPM. It generally varies between 35-60.
Pump discharge=
stroke length×(liner size)2
25.9
• Torque: The moment required to rotate the entire drill string and the bit on the bottom
of the hole. It is measured in PSI unit.

Mud Parameters
• Mud Flow: A Mud flow meter is used to detect mud flow for which a potentiometer
sensor is used. In flow meter a paddle is placed in the flow channel so that flow ofmud
moves the paddle downward or upward. This paddle is attached to the potentiometer
and any movement of the paddle rotates the
potentiometer.
• Mud Temperature: IN/OUT
• Mud Resistivity: IN/OUT
• Mud Viscosity: IN/OUT
• Mud Salinity: IN/OUT
• Mud Density: IN/OUT
• Mud Weight: IN/OUT
GAS INDICATION
• Gas Detector: For detection of any gas
coming from the formation within mud fluid.
Fig : Gas Chromatograph & HC Analyser
• Gas Chromatograph: It is used for gas analysis .It is based on the principle of Flame
ionisation detection (FID). An FID typically uses a hydrogen/air flame into which the
sample is passed to oxidise organic molecules.Then electrically charged particles(ions) are
produced which are collected to generate electrical signals which are measured.FID
measurements are often labelled total hydrocarbon content (THC).
THC= C1+2C2+3C3+4(iC4+ nC4)+5(iC5+nC5)
Fig : MLU gas component
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Where:
C1: Methane
C2: Ethane
C3: Propane
iC4: Iso Butane
nC4: Neo Pentane
iC5: Iso Pentane
nC5: Neo Pentane
Sensors:
The classification of Sensors used in mud logging based on the place where the sensor is
engaged is as follows:
Rig Floor Sensors Pit Room Sensors Shaker sensors
Hook load Sensor Pit level recorders Flow Out or Return Sensor
Standpipe pressure Sensor SPM Counter (Proximity
Switch)
Mud density Out
RPM Counter (Proximity
Switch)
Mud density In Mud Temperature Out
Rotary Encoder (Draw
works)
Mud temperature In Mud Conductivity Out
Torque Mud Conductivity In
Role of Well site Geologist in Mud logging
The well site geologist must ensure and be certain that the equipment necessary to monitorthe
well activities is working properly and is used properly. The role of Well site geologist begins
as early as the mud logging crew commission their unit in Well site. Once the unit is
commissioned, well site Geologist being the client representative validates the functioning of
the mud logging unit and mark the approval for the unit to stand operational on site. So, it is
important that a well site Geologist must understand the basic working principles of all the
sensors and equipment used in mud logging.
Following is a check list to be adhered to:
• Ensuring the unit has got all the equipment, inventories and specifications as per the
clause of contract.
• Ensuring that the unit and equipment are installed properly as per standard Operating
procedures and maintained as and when required.

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• Regular Sensor calibrations should be performed and witnessed. A record should be
kept of these calibrations.
• Regular Gas calibrations (TG, Chromatograph, CO2 and H2S) should be performed
and witnessed, record of calibration to be maintained.
• Gas detectors are to be correctly zeroed on injecting pure air and calibrated with gases
of known composition. A record should be kept of these calibrations.
• Ensuring the sensors installed in the correct place and position, sensors properly
connected to data acquisition system, proper working of Data Acquisition System and
channel mapped correctly to display the output in the relevant field of the software.
• Calculation of Lag time:-
Lag time is the time taken by the mud to travel from the specified depth to the surface.
Lag Time=
𝐴𝑁𝑁𝑈𝐿𝐴𝑅 𝑉𝑂𝐿𝑈𝑀𝐸(𝑙𝑖𝑡𝑟𝑒𝑠)
𝑀𝑈𝐷 𝐹𝐿𝑂𝑊 𝑅𝐴𝑇𝐸(𝐿𝑃𝑀)
Where, Annular Volume is the total volume of the annulus between the running drill string
and the casing/formation. This check is important as the accuracy of the entire sampling
depends on the lagtime and lag depth.

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Casing
It is the lowering of a long section of pipe that is assembled and inserted into the recently drilled
section of a borehole and typically held into place with cement. Casings are heavy walled, high
tensile steel pipes which are periodically lowered into the well after the completion of the
respective drilling phase.
Why casing is required
Casings provide the following functions:
• Prevents collapse of the bore hole.
• Prevents caving tendencies of unconsolidated
formation, especially in surface hole.
• Prevents escape of the formation fluids through
the well from one stratum to another.
• Prevents contamination of ground water.
• Provides a high strength conduit (passage) for
produced fluids.
• Allows safe control of formation fluid pressures
with the help of BOP.
Fig : Types of casing
Casing design and policy:
The casing lowered should effectively serve above mentioned purpose and accordingly it
should be designed. It must be of sufficient strength to withstand the stress exerted to it. It
should be water tight, particularly if it is to be used in sealing off water and should be made of
material that resists corrosion particularly when it comes in contact with saline ground water.
Also the material should be hard enough to resist abrasion and distortion by contact with the
rock or drilling tools.
Casing policy is prepared considering the depth of the well and the formation to be encountered
during the course of drilling. . There are two major casing policy, which is selected depending
on the conditions encountered. These are 4CP and 3CP. 4CP involves four casing strings
whereas 3CP uses 3 casing string. 2CP casing policy is also used, in case of a shallow depth
well or a barefoot well.
The inside diameter of the final casing string must accommodate the production tubing and
associated hardware such as packers, gas lift mandrels and sub-surface safety valves.
Typically, a well contains multiple intervals of casing successively placed within the previous
casing run

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CASING TYPES:
Casings are classified into different types according to the relative positions they are installed
inside the well.
1. Conductor Casing
2. Surface Casing
3. Intermediate Casing
4. Production Casing
Conductor casing: serves as a support during drilling operations, to flow back returns during
drilling and cementing of the surface casing, and to prevent collapse of the loose soil near the
surface. It can vary from 18 inch to 30 inch in diameter and is typically 20 inch in diameter.
Surface casing: The purpose is to isolate the fresh water zones and prevent it from getting
contaminated. The typical size of surface casing is 13 3/8 inch.
Intermediate casing: It is used on longer drilling intervals where the necessary drilling mud
weight to prevent blowouts maycause a hydrostatic pressure that can fracture shallow or deeper
formations. Casing placement is selected so as to ensure that the hydrostatic pressure of the
drilling fluid remains at a pressure level that is between formation pore pressures and fracture
pressures. It is typically of 9 5/8 inch.
Production casing: The final casing which passes through the prospect zone. It is typically of
outside diameter of 7 inch or 5 ½
Fig : 3CP/3000m Casing and Cementing

CEMENTING
Well cementing consists of two principal operations
• Primary cementing
• Secondary cementing
Primary Cementing: It is the process of placing a sheath of cement in the annulus between
the casing and the formation. After the casing string is lowered to the bottom. The bottom end
of the casing string is protected by a casing shoe. The casing shoe is tapered and has a check
valve to prevent the back flow of the cement from annulus into the casing. Further, a float collar
above the casing shoe also acts as an additional check valve and contains contaminated cement,
thereby ensuring that no contaminated cement goes into the annulus region. Centralizers are
placed along critical casing sections to help ensure placement of a uniform cementing. Cement
slurries and drilling fluid are usually chemically incompatible.
Therefore, chemical washes and spacer fluids may be pumped after the drilling fluid and before
the cement slurry. These fluids have the added benefit of cleaning the casing and formation
surfaces, which helps achieve good cement bonding. In order to precisely place the cement
slurry at the required interval on the outside of the casing, the amount of cement required is
properly estimated by the cementing engineer using the calliper log.
After the cementing operation, wait for the cement to cure, set and develop strength - known
as waiting on cement (WOC).
Nearly all cementing operations use Portland cement, which consists mainly of anhydrous
calcium silicate compounds that hydrate when water is added. In addition, many additives are
added to allow proper cementing operation and achieve better cementing result.
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Secondary Cementing
It occurs after primary cementing when cement is
injected to strategic well location for various
purposes, including well repair and well
abandonment. When logging operations indicatethat
the cement job is defective, either in form of poor
cement bonding or communication between zones, a
remedial cementing technique known as squeeze
cementing maybe performed to establish zonal
isolation. The casing is perforated in the defective
interval and cement slurry is forced through the
perforation and into the annulus to fill the voids. In
addition, squeeze cementing may be an effective
technique for repairing casing leaks caused by a
corroded or split casing. At the end of a well’s
productive life, the well is abandoned by performing
plug cementing. The casing interior is filled with
cement at various depths, thereby preventing inter-
zonal communication and fluid migration into
underground freshwater sources.
Fig : Secondary Cementation
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Well Completion and Well Testing
Well Completion
Well completion refers to the process of making a well ready for the production after the
completion of drilling. Well completion incorporates all the steps taken to transform a drilled
well into a producing well. The steps include operations like casing, cementing, perforating,
gravel packing, installing X-mas tree etc.
Types of completion:
• Open hole completion: In this type casing is set only to the top or slightly into the
completion interval before drilling the zone. In competent formation the zone might be
left entirely bare, but some sort of sand control measures (Gravel packing) are usually
incorporated. It is not suitable for weak or unstable formation.
• Cased and perforated Completion: This involves the setting of casing through a
production zone and cementing it in place. The casing is then perforated to provide
communication between the well bore and formation. The main advantage of this type
of completion is that the well can be drilled and logged to total depth prior to running
and cementing production casing.
Well Testing
Testing refers to the execution of a set of planned activities in which subsurface data is acquired
for determining the various characteristics of reservoir and hydrocarbon properties within it. In
other words testing provide a measure of the production potential of thereservoir.
Perforation Techniques
The object of perforation is to achieve communication between well bore and formation.
Perforations must penetrate the casing beyond cement, into the hydrocarbon bearing formation.
The perforation should be clean and of uniform size and depth. It uses shaped charge explosive
which create a jet of high pressure, high velocity jet perforation. The different types of
perforation are:
• Conventional method: The method of conventional perforation is very simple. It avoids
the cumbersome pressure control equipment and the diameter of the gun is restricted to
the internal diameter of the casing. In these method, perforation is made prior to
insertion open end tube.
• Through tubing perforation (TTP): When through tubing perforation (TTP) is carried
out, the tubing are lowered with a bellbottom to the required depth and positioned.
Subsequently a suitable perforating system by wire line is lowered and perforated
against the zone of interest. Pressure control system is a requirement here to perform the
job.
• Tubing Conveyed perforation (TCP): The use of tubing, drill pipe or coiled tubing to
convey perforating guns to the required depth. Initially, the technique was developed as

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a means for conveying the gun string on the production tubing, with the guns remaining
in the well until they are removed during the first work over. The subsequent popularity
of highly deviated and horizontal wells increased the requirement for tubing-conveyed
perforating as the only means of gaining access to the perforating depth.
Perforating operations carried out in two ways:
• Overbalanced perforation: Under these conditions the well bore pressure is greater
than formation pressure so there is a tendency of the well bore fluid to enter into the
formation.
• Underbalanced perforation: Under these conditions the well bore pressure is lesser
than formation pressure so there is a tendency of formation fluid to enter the well
bore.
Well Activation
Underbalance condition does not require well activation techniques but in case of overbalance
condition well activation techniques are required. The activation techniques used are:
• Displacement: Objective of this process is to reduce the hydrostatic head so as to
create drawdown at the formation, there by inducing it to flow. The process involves
the displacement of the drilling fluid or well fluid in the well, with lighter fluids in
several displacement cycles.
• Compressor application: In this method, compressed air is applied into the annular
space. Subsequently the water in the annular space is pushed into the tube well from
where an equivalent quantity of water is displaced at the surface. After the compressed
air is released liquid level in the well falls due to this displacement. The air from the
annulus is released under controlled conditions through a bean. If the decreased
pressure at the bottom hole is less than the formation pressure, than the formation fluid
will start to move from the formation to the well bore.
• Liquid nitrogen application: this is the most common and extensively used activation
method. This method involves pumping of liquid nitrogen to a vaporizer where gaseous
nitrogen is released through a manifold into the well. Liquid nitrogen helps to displace
fluids from deeper levels.
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WELL LOGGING
Logging: Systematic and chronologic recording of data.
Wireline logging is a conventional form of logging that employs a measurement tool suspended
on a cable or wire that suspends the tool and carries the data back to the surface. These logs are
taken between drilling episodes and at the end of drilling. Recent developments also allow
some measurements to be made during drilling. The tools required to make these measurements
are attached to the drill string behind the bit, and do not use a wire relying instead on low band-
width radio communication of data through the conductive drilling mud. Such data is called
MWD (measurement while drilling) for simple drilling data, and LWD (logging while drilling)
for measurements analogous to conventional wireline measurements.
There are several types of log:
Calliper 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.
Log Representation-
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.
Application:
Fig : Presentation of calliper log
• Contributory information for lithological assessment.
• Indicator of good permeability and porosity zones (reservoir rock) due todevelopment
of mudcake in association with gamma ray log.
• Measurement of bore hole volume.
• Estimation of required cement for cementation job.
Resisitivity Logging
Resistivity is sensitive to rock properties such as porosity, shaliness, compaction or degree of
sedimentation, pore distribution and pore fluids. Determination of the true formation resistivity,

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(resistivity of the undisturbed formation) is required in order to quantitatively evaluate
formation of interest, which is used in
determination of hydrocarbon saturation.
Sedimentary minerals normally encountered in
oil wells are generally poor conductors, having
resistivity in the range of 0.2-2000 ohm-m.
Resistivity tool has 3 component of measurement.
• Shallow-which measure resistivity of
flushed zone.
• Intermediate-which measure intermediate
zone
• Deep- which measure virgin zone
Applications:
• Determine hydrocarbon versus water bearing zones.
• Indicate permeable zones.
Self-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.
Applications:
• Determination of resistivity of water.
• Indication of shaliness of a formation.
• Correlation.
Sonic Log
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.
The main use is to support and calibrate seismic data and to form Synthetic Seismogram along
with other logs (Density log and Neutron log).
Applications:
• Provision of a record of “seismic” velocity and travel time throughout a borehole.This
Fig : Depth of Invasion

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Information can be used to calibrate a seismic data set (i.e., tie it in to measured values of
seismic velocity).
• Provision of “seismic” data for the use in creating synthetic seismograms.
• Determination of porosity.
• Stratigraphic correlation and identification of lithology.
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. 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. 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.
Application:
• The main use neutron log is to estimate porosity.
• Idea about the lithology when compared with another logs.
• Better lithological identification using neutron-density logs.
Density Log
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’s 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.
Applications:
• In association with neutron log provides clear idea about formation and its quality.
• Density from the formation density log is often combined with acoustic velocity from
the sonic log to calculate acoustic impedance down a well.
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

46 | P a g e
occurs until the gamma ray is of such a low energy that it is completely absorbed by the
formation.
Applications:
• Calculation of shale volume.
• Identification of lithology (mainly sand and shale).
• It has very high vertical resolution and hence sometime used for depth matching.
• GR log indicate presence of radioactive minerals.
Fig : Composite Log
Cased Hole Log
The logging carried out in the cased wells is called Cased Hole Logging. Main applications of
Cased-hole logging are in Completion Services, Reservoir monitoring, and in production
logging. In completion service, we check cement quality and control log for perforation by
cased-hole log. In Reservoir monitoring, we used cased-hole log to detect water flooding of
any zone, to detect rise in fluid contacts and to locate bypassed hydrocarbon (left over HC).
And in Production logging we do case-hole logging to detect channeling behind casing, to find
contribution from different zones and to detect type of fluid produced.
Cement bond log (CBL) - Variable density log:

47 | P a g e
• CBL log is based on casing ringing. When the casing pipe is free, the ringing effect will
be more hence larger will be amplitude. When casing pipe is cemented on the outside,
ringing effect will be lessvand sonic signal will be attenuated more and hence lesser
will be amplitude.
• VDL record the arrival of the sonic log from the casing, cement and the formation.
Generally represented by series of bars. If the bond between cement and formation is
good VDL representation will be fade and viceversa.
Log Representation:
Good Cement:
CBL- lesser will be amplitude, VDL- Fade appearance.
Fig : CBL-VDL Log
No Cement:
CBL- Higher will be amplitude, VDL- Prominent Series of bars.
Fig : CBL-VDL Log

48 | P a g e
Pore pressure estimation from well logs
Pore Pressure: - The pressure exerted by the pore fluid contained in a formation is called pore
pressure. The term pore pressure is synonymous with formation pressure and reservoir
pressure.
Prediction, detection, evaluation and estimation of pore pressure are of uttermost importance
during drilling of exploratory wells, particularly in areas where abnormal pore pressure has
been observed. Evaluation of pore pressure help in designing the mud policy and drilling plan
including casing, setting depth. There are many methods to estimate pore pressure before
drilling, during drilling and after drilling. Well logs can be used to detect abnormal pore
pressure before or after drilling. However, now with introduction of LWD (Logging While
Drilling) this method of pore pressure detection and estimation can be applied during drilling
itself. Eaton’s method of pore pressure estimation from well log is widely used in the industry
and the same is discussed below.
Estimation of pore pressure- Eaton’s method
Estimation of pore pressure from well logs is a tested method and has been is use in the industry
for the last 5 decades. (Hottman and Johnson, 1965 Eaton 1975-1982, Jincai Zhang 2012 and
references therein) apart from logs most of the techniques used to predict and evaluate pore
pressure are based on concept of Normal Compaction Trend (NCT). In geological
environment where deposition is rapid pore fluid cannot be squeezed out from underlying
sediments due to lack of time. These sediments remain under compacted with pore fluid inside.
These under compacted sediments share part of the overburden stress and thus generate
abnormal pore pressure in sediments.
Under normal compaction shale tend to get denser and denser with depth. A sudden deviation
towards lesser value indicates under compaction and high pressure within the trapped fluid in
shale.

49 | P a g e
The techniques involved in Estimation of pore pressure from well logs envisage this normal
compaction trend as mentioned above and magnitude of abnormal pore pressure is a function
of difference between the deviated value at the depth of study and value from the normal trend
line at the same depth.
Pore Pressure estimation from Resistivity Log
Fig : Resistivity vs depth plot showing decreasing values in under compacted zone and the
Normal Compaction Trend (NCT)
As mentioned earlier, in a shale section, resistivity readings should show an increasing trend
with depth. A sudden fall in resistivity value at depth indicate under-compaction and hence the
possibility of higher pressure in the trapped fluid

50 | P a g e
Fig : Plot of Resistivity vs depth and decreasing values of resistivity inrelation to NCT line in
the undercompacted zone indicatng high pore pressure
Estimation of pore pressure from Resistivity Log by applying Eaton's Equation is shown below
in Fig 37.
Given that overburden Gradient is 0.95 psi / ft and Normal Pore Pressure Gradient as 0.465 psi
/ ft, pore pressure at the depth of interest is estimated to be 0.73 psi / ft indicating overpressure
to the tune of 60 % above hydrostatic pressure, the equivalent mud weight being 1.56 gm/cc.

51 | P a g e
Fig : Estimation of pore pressure from Resistivity Log by applying Eaton's Equation.
Pore Pressure estimation from Sonic Log
In case of Sonic Log, the del-t values should decrease with depth and in an under compacted
zone, the sonic del-t value should increase, just the reverse of resistivity and density logs.
Thus in Eaton's equation for pore pressure estimation from sonic log, numerator and
denominator are just opposite of Eaton's equation for density and resistivity logs. The
exponential value in Eaton's equation for Resistivity and Density logs are 1.2 whereas the same
is 3.0 for sonic log.
The process of estimation of pore pressure from sonic log by applying Eaton's Equation.is
depicted behind (Fig-38)

52 | P a g e
Fig : Plot of Sonic ∆t values vs depth. Note deviation of the data towards incresing side in
the under compacted over-pressured zone.
1000Sonic (usec/m)100
3500
3000
2500
2000
1500
1000
response
in casing
Cycle
Skipping
Top of
Overpressure
NCT
TVD(m)

Application of Eaton's Equation
Example :
sGiven that Overburden Gradient is 1 psi / ft, Normal Hydrostatic Gradient is 0.433 psi / ft,
actual del-t value 70 usec/ft and normal del-t value as 55 usec/ft, pore pressure gradient is
estimated from sonic log by Eaton's Equation is 0.725 psi / ft.
53| Page

Hydrocarbon Exploitation
After casing and perforation due to the natural forces the well flows by its own and in course
of time the forces are decreases and the rate of production also decreases. The following
methods are used for further production.
SELF FLOW WELL:
During first phase the well flow on its own, due to the forces that exist in the reservoir.
These self flow wells are of only gas wells or oil-gas wells. The production rate of these
well are generally decreases with time due to declining of reservoir forces. The production is
so adjusted that the time period of production is maintained to certain period after that the
well needs to be produced by an Artificial lift.
ARTIFICIAL LIFT:
After the first phase of production i.e. self flow stage the pressure of the reservoir decreases
so you have to exert certain pressure to produce oil.
SUCKER ROD PUMP (SRP):- Sucker rod pump is similar to the
domestic ground water pump. In this method, a pump is connected to a rigid rod is lowered
down the tubing to the bottom.
COLLECTION OF OIL AND GAS FROM THE WELL
Oil and gas are produced from the well and are transported through the pipe lines and road
tankers. Oil and gas collected at well itself or collected at those places where number of
wells are gathered at centrally located unit called Group Gathering Station. The essential
features of production installation are mentioned below.
(A) WELL HEAD INSTALLATION A well head installation handle about 30-40
tonnes/day.produced from one well. The installation consists of an oil and gas separator and
water both. Oil is separated and collected in horizontal tank and transported by road tankers.
(B) GROUP GATHERING STATION This is centrally located unit where flow line from
various productions well is connected to the manifold platform. In this unit oil, water and
gas are separated individually and different separators separate the gas from crude oil. This
crude oil is further pumped down to central tank form.
(C) CENTRAL TANK FORM This is the storage unit for crude oil received from different
GGS. The capacity of storage tank in the unit is over 1000 cubic meter. The oil and gas are
totally separated from each other. The oil is then stored at central tank form and pumped
through pipe line to the refinery and gas is sent gas collecting stations.
(D) GAS COLLECTION STATION Gas collection station called LPG plant. It is situated
near the central tank. Gas is soluble in oil and separated from pressure. Some reservoirs have
dry gas or free gas with high pressure. The high pressure gas obtained from gas well
collected at gas collecting stations. The gas is passed through separators to separate gas and
condensate, the later is piped to storage tank where as gas at high pressure (40- 60/cm2 ) is
sent down pipe line to consumers. The gas consists of 80-90% of methane which is highly
inflammable.
ENHANCE OIL RECOVERY When natural driving forces are declined, these leaves a
considerable amount of oil in reservoir, to recover this amount of oil secondary methods are
adopted which is called enhance oil recovery. Water injection and gas injection is most
important enhanced oil recovery methods. The EOR methods are of following types:-
(a) GAS INJECTION Gas injection is the commonly used technique for secondary
recovery of oil. In this case carbon dioxide and nitrogen etc. gases are injected into the well
with high pressure. This gas tends to come out along with oil.
54| Page

(b) WATER INJECTION Water Injection is the most important method for injection of
well. In this case, water is injected below the oil water contact. The injectivity of well
depends upon the permeability of the formation. The injecting water sweeps the remaining
oil through the reservoir to the producing well.
(c)POLYMER FLOODING Conventional water flooding can often be improved by the
addition of polymers to injection water to improve (decrease) the mobility ratio between the
injected and in place fluids. The polymer solution affects the relative flow rates of oil and
water sweeps a large fraction of the reservoir than water alone, thus containing more of the
oil and moving it to production wells. Polymers currently in use, are produced both
synthetically (poly cryl-amides) and biologically (poly-saccharides).
(d)SURFACTANT FLOODING Surfactant flooding (Fig. 5.24) is a multiple slug process
involving the addition of surface active chemicals to water. These chemicals reduce the
capillary forces that trap the oil in the pores of the rock. The surfactant slug displaces the
majority of the oil from the reservoir volume contacted, forming a flowing oil/ water bank
that is propagated ahead of the surfactant slug. The principle factors that influence the
surfactant slug design are interfacial properties, slug mobility in relation to the mobility of
the oil / water bank, the persistence of acceptable slug properties and integrity in the
reservoir and cost.
(e)MICROBIAL ENHANCE OIL RECOVERY (MEOR) The microbes are cultured
artificially. Crude oil has different contents of materials. These materials are settled in the
reservoir zone and chock the reservoir rock so the migration of oil is slow. To clean the
reservoir rocks, microbes are injected in the reservoir rocks. These microbes eat the
unwanted material and clean the reservoir rocks and the flow of oil takes place freely. This
is microbial enhance oil recovery. The microbes are alive at 60° C, but by some technique it
is increased up to 90° C. it decreases the interfacial tension, viscosity of oil and improves
permeability of rocks.
(f)IN-SITU COMBUSTION In-situ combustion is normally applied to reservoir containing
low gravity oil. Heat is generated within reservoir by injecting and burning part of the crude
oil. This reduces the oil viscosity and partial vaporizes the oil in place. The oil is driven
forward by a combination of steam, hot water and gas drive. The relatively small portions of
the oil that remains after these displacement mechanisms have acted become the fuel for the
in-situ combustion process. Production is obtained from well offsetting the injection
locations. In some applications, the efficiency of the total in-situ combustion operation can
be improved by alternating water and air injection. The injected water tends to improve the
utilization of heat by transferring heat from the rock behind the combustion zone to the rock
immediately ahead of combustion zone.
55| Page

56 | P a g e
DISCUSSION AND CONCLUSION
The operational geological work starts after a location for exploratory drilling is released and
approved. It begins with staking the location followed by land acquisition (LAQ). Civil work
at the site is initiated at this stage. The main objective of civil work is to level and harden the
ground, construct motorable road for the movement of heavy vehicles carrying equipment and
to construct a solid basement for the installation of the rig. Once the rig is deployed at the site
the whole thing is reassembled and erected at the specified spot on the soil. The job of well site
geologist starts with spudding the well and commencement of drilling. Drilling is carried out
in phases whereby the hole size becomes successibly smaller in stages with corresponding
reduction in respective casing sizes. After completion of each phase of drilling the well is
logged, cased and cemented.
Before lowering a casing, the well is logged. Different suites of logs are recorded to know the
formation type, its density, porosity, resistivity etc. In porous formation some special suite of
logs like MDT can also indicate the type of fluid occurring in the formation. The boundary
between different formations and their thickness can be accurately marked on logs. These
combinations of logs also identify the zones to be tested for their hydrocarbon production
potential.
The next phase of drilling starts with drilling the excess cement inside the casing and the float
collar, cement and the casing shoe is drilled along with 2-3 meter of fresh formation. At this
stage a pressure test is carried out in mud to check for the maximum pressure the formation
can withstand. This is called LOT (Leak off Test). The equivalent mud weight of LOT pressure
is taken as the upper limit of mud weight that can be used to control the well in case of any
activity during the new phase of drilling. During drilling monitoring of ROP is very important.
A sudden faster drilling rate indicates a drilling break; meaning a change of formation with
good amount of porosity. In such cases particularly in exploratory wells drilling is stopped and
a complete cycle of mud circulation of mud is made to check the bottom sample. If the bottom
sample shows the presence of hydrocarbon then a core is cut at this depth.

57 | P a g e
After the casing is lowered the gap between casing and the well wall called annulus is cemented
to secure the casing in the borehole and also to prevent influx of formation fluid into borehole.
The quantity of cement slurry to be pumped depends on the requirement of rise of cement in
the annulus which is to be decided by the geologist.
For cementation job initially a liquid, usually water is pumped which is known as preflush and
this is followed by pumping of cement slurry. The cement slurry is displaced from casing by
pumping mud so that the rise of slurry in the annulus and its placement are appropriate. The
cement is let to be settled for a prefixed time period known as “Waiting on Cement” (WOC).
After this the well is scrapped and the cement is cleared up to top of float collar and the casing
is pressure tested for its integrity. To know the quality of cementation job and bonding of the
cement with casing and formation wall, a cased hole log, CBL-VDL is recorded. When the
boning of cement is found to be poor, a cement repair job is undertaken which is known as
Cement Squeeze job. It is to be noted that the last casing or the final casing is the production
testing. Production casing is lowered in such a manner that the float collar depth is sufficiently
below the bottom most object to be tested, because in production casing the hole is cleared only
up to top of float collar. The final/production casing is tested hermetically in water for its
integrity and successful completion of this test marks the end of drilling phase of the well. After
this the well enters the production testing phase.
During the course of drilling the operations are closely monitored by the site geologist. All the
geological data and few drilling and mud parameters are monitored and noted theoretically and
are presented in a single platform known as masterlog. Masterlog contains a lithocoloumn
accompanied by rate of penetration (ROP), hydrocarbon shows, and few drilling parameters
like RPM, WOB (weight on bit). Similarly the data generated in a mudlogging unit are used
to prepare geo-pressure log. The masterlog and geo-pressure log containing all the important
and relevant information are studied in detail to select intervals of interest which can be tested
as objects.
The geologist at well site has to be alert all the time during drilling and closely monitor all the
proceedings at well site. Any indication of approaching high pressure can be detected and
evaluated in the mudlogging unit.
Apart from mudlog unit, pore pressure can also be estimated from well logs, like resistivity,
density and sonic log using Eaton’s equation.

58 | P a g e
BIBLIOGRAPHY
Atlantic Richfield Indonesia, 1978: Geologist's Well Site Manual. ARCO, Jakarta, August,
1978.
Biswas, S.K. et al , 1993: Classification of Indian Sedimentary Basins in the framework of
plate tectonics; Proc; second seminar on Petroliferous Basins of India, 1, 1-46
Biswas, S.K., Rangaragu, M.K., Thomas, J., and Bhattacharya, S.K., 1994, Cambay-Hazad(!)
petroleum system in the South Cambay Basin, India, in Magoon, L.B., and Dow, W.G., eds.,
The petroleum system – from source to trap: AAPG Memoir, no. 60, p. 615-624.
Blackbourn, G.A.; 1990 : Core and Core Logging for Geologists, Whittles Publishing Services,
London, 1990.
Boatman, W.A; 1967 : Measuring and Using Shale Density to Aid in Drilling Wells in High-
pressure Areas. American Petroleum Institute, Drilling & Production Practices, New York,
1967
Bowers, G. L., 1995, Pore pressure estimation from velocity data: Accounting for overpressure
mechanisms besides under compaction: SPE Drilling and Completions, June, 1– 19.
Eaton, B. A., 1968, Fracture gradient-prediction and its application in oil field operations: J.
Pet. Tech., 10, October, 1353–1360.
Eaton, B.A., 1972. The effect of overburden stress on geopressures prediction from well logs.
Paper 746 SPE3719. JPT, Aug., 1972:929-934.
Eaton, B. A., 1975. The equation for geopressure prediction from well logs. SPE 50th Annual
Fall Meeting Proceedings, paper SPE 5544.
Exlog, 1979 : Field Geologist's Training Guide, Expl. Logging Inc, Sacramento, USA.
Fertl, WH; 1978: Abnormal formation pressure; Lelsivebier, Amsterdam

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Fertl, Walter H; 1973: Significance of Shale Gas as an indicator of Abnormal Pressure; Sixth
conference on Drilling and Rock Mechanics, SPE, Austin, Texas, SPE Paper 4230 (Preprint)
Hottman, C. E., and Johnson, R. K., 1965, Estimation of formation pressures from log-derived
shale properties: J. Petr. Tech., 17, 717–722.
Geoservices, 1983 : Well site operational manual. Geoservices,
Glenn L. B. 2001. Determining an Appropriate Pore-Pressure Estimation Strategy, Offshore
Technology Conference, 30 April-3 May. 13042-MS.
McPhater, D, MacTiernan B; Well-site Geologist's Handbook, McKinley-Smith International
Ltd, Penn well Publishing Company, Tulsa, Oklahoma.
Madanfc Mohan; 1995: Cambay Basin - A Promise of Oil and Gas Potential; Journal of the
Palaeontological Society of India,40, 41-47
Mishra, Somen and Patel, B. K.; 2011: Gas Shale Potential of Cambay Formation, Cambay
Basin, India, Search and Discovery Article #10317 (2011), Adapted from extended abstract
presented at geo-India, Greater Noida, New Delhi, India, January 12-14, 2011
Schlumberger, 1987: Log Interpretations Principles/Applications, Schlumberger Educational
Services, Houston, USA.
Whittaker, R; 1982 : Well site geological operations. Exlog Publication, New York, USA.

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