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WELL LOGS

Interpreting Geophysical Well Logs

Prof. Dr. Hassan Z. Harraz


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Historical Aspect-Schlumberger brothers, Conrad and Marcel, are credited with

inventing electrical well-logs.

- On September 5, 1927, the first well-log was created in a

small village named Pechelbroon in France.

- In 1931, the first SP (spontaneous potential) log was

recorded. Discovered when the galvanometer began wiggling

even though no current was being applied.

-The SP effect was produced naturally by the borehole mud at

the boundaries of permeable beds. By simultaneously

recording SP and resistivity, loggers could distinguish between

permeable oil-bearing beds and impermeable nonproducing

beds.


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Types of Logs

a) Gamma Ray

b) SP (spontaneous potential)

c) Resistivity (Induction)

d) Sonic

e) Density/Neutronf) Caliper


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a) Gamma Ray

The gamma ray measures the naturalradioactivity of the rocks, and does notmeasure any hydrocarbon or water presentwithin the rocks.

Shales: radioactive potassium is a commoncomponent, and because of their cationexchange capacity, uranium and thorium areoften absorbed as well.

Therefore, very often shales will display highgamma ray responses, while sandstones andlimestone will typically show lower responses.


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The scale for GR is in API (American

Petroleum Institute) and runs from 0-125

units. There are often 10 divisions in a GR

log, so each division represents 12.5 units.

Typical distinction between between a

sandstone/limestone and shale occurs

between 50-60 units. Often, very clean sandstones or carbonates

will display values within the 20 units range.


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b) SP (Spontaneous Potential)

The SP log records the electric potentialbetween an electrode pulled up a hole and areference electrode at the surface.

This potenital exists because of theelectrochemical differences between thewaters within the formation and the drillingmud.

The potenital is measured in millivolts on arelative scale only since the absolute valuedepends on the properties of the drilling mud.


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In shaly sections, the maximum SP response tothe right can be used to define a shale line.

Deflections of the SP log from this line indicateszones of permeable lithologies with interstitialfluids containing salinities differing from thedrilling fluid.

SP logs are good indicators of lithology wheresandstones are permeable and water saturated.

However, if the lithologies are filled with freshwater, the SP can become suppressed or even

reversed. Also, they are poor in areas wherethe permeabilities are very low, sandstones aretighly cemented or the interval is completelybitumen saturated (ie- oil sands).


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c) Resistivity (Induction)

Resistivity logs record the resistance ofinterstitial fluids to the flow of an electriccurrent, either transmitted directly to the rockthrough an electrode, or magnetically induceddeeper into the formation from the hole.

Therefore, the measure the ability of rocks toconduct electrical currents and are scaled inunits of ohm-meters.

On most modern logs, there will be threecurves, each measuring the resistance ofsection to the flow of electricity.


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Porous formations filled with salt water (which isvery common) have very low resistivities (oftenonly ranging from 1-10 ohms-meter).

Formations that contain oil/gas generally havemuch higher resisitivities (often ranging from 10-500 ohms-meter).

With regards to the three lines, the one we are

most interested in is the one marked deep. Thisis because this curve looks into the formation at adepth of six meters (or greater), therebyrepresenting the portion of the formation most

unlikely undisturbed by the drilling process. One must be careful of extremely high values, as

they will often represent zones of either anhydriteor other non-porous intervals.


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d) Sonic

Sonic logs (or acoustic) measure the porosityof the rock. Hence, they measure the traveltime of an elastic wave through a formation(measured in T- microseconds per meter).

Intervals containing greater pore space willresult in greater travel time and vice versa fornon-porous sections.

Must be used in combination with other logs,particularly gamma rays and resistivity,thereby allowing one to better understand thereservoir petrophysics.


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e) Density/Neutron

Density logs measure the bulk electron density of theformation, and is measured in kilograms per cubic meter(gm/cm3 or kg/m3).

Thus, the density tool emits gamma radiation which isscattered back to a detector in amounts proportional to theelectron density of the formation. The higher the gammaray reflected, the greater the porosity of the rock.

Electron density is directly related to the density of theformation (except in evaporates) and amount of density ofinterstitial fluids.

Helpful in distinguishing lithologies, especially betweendolomite (2.85 kg/m3) and limestone (2.71 kg/m3).


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Neutron Logs measure the amounts of

hydrogen present in the water atoms of a

rock, and can be used to measure porosity.

This is done by bombarding the the formation

with neutrons, and determing how many

become captured by the hydrogen nuclei.

Because shales have high amounts of water,the neutron log will read quite high porosities-

thus it must be used in conjunction with GR

logs.

However, porosities recorded in shale-freesections are a reasonable estimate of the

pore spaces that could produce water.


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It is very common to see both neutronand density logs recorded on the samesection, and are often shown as anoverlay on a common scale (calibratedfor either sandstones or limestones).

This overlay allows for better

opportunity of distinguishing lithologiesand making better estimates of the trueporosity.

* When natural gas is present, therebecomes a big spread (or crossing) ofthe two logs, known as the gas effect.


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f) Caliper

Caliper Logs record the diameter of the hole.It is very useful in relaying information aboutthe quality of the hole and hence reliability ofthe other logs.

An example includes a large hole wheredissolution, caving or falling of the rock walloccurred, leading to errors in other logresponses.

Most caliper logs are run with GR logs andtypically will remain constant throughout.


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WELL LOG

(The Bore Hole Image)Interpreting Geophysical Well Logs

Prof. Dr. Hassan Z. Harraz


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What is well LoggingWell log is a continuous record of measurement made in bore hole respond to

variation in some physical properties of rocks through which the bore hole is drilled.

Traditionally Logs are display on girded papers shown in figure.

Now a days the log may be taken as films, images, and in digital format.


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HISTORY

1912 Conrad Schlumberger give the idea of using electrical measurements to map subsurface

rock bodies.

in 1919 ConradSchlumberger and his brother Marcel begin work on well logs.

The first electrical resistivity well log was taken in France, in 1927.

The instrument which was use for this purpose is called SONDE, the sond was stopped at

periodic intervals in bore hole and the and resistivity was plotted on graph paper.

In 1929 the electrical resistivity logs are introduce on commercial scale in Venezuela, USA and

Russia For correlation and identification of Hydrocarbon bearing strata.

The photographicfilm recorder was developed in 1936 the curves were SN,LN AND LAT

The dip meter log were developed in 1930

The Gamma Ray and Neutron Log were begin in 1941
http://www.oilfield.slb.com/content/services/evaluation/other/lwf.asp?


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LOGGING UNITS

Logging service companies utilize a variety of

logging units, depending on the location

(onshore or offshore) and requirements of the

logging run. Each unit will contain the

following components:

logging cable

winch to raise and lower the cable in the well

self-contained 120-volt AC generator set of surface control panels

set of downhole tools (sondes and cartridges)

digital recording system


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Work Flow Chart


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From Warrior Energy Services Website, www.warriorenergyservices.com


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TYPICAL WIRELINE TRUCK

From Welaco


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TYPICAL WIRELINE SKID UNIT

Welaco Unit at OrmatsPuna Geothermal Venture in Hawaii


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TYPES OF LOGS

Geophysical Logs

Resistivity

Porosity

Gamma Ray

Dip Meter

Borehole Imaging

Other

Production Logging Pressure

Temperature

Spinner Fluid Density

Well Inspection Sonic

Caliper

Electro-magnetic

Ultrasonic

RA Tracer

Video


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depth to lithological boundaries

lithology identification

minerals grade/quality

inter-borehole correlation

structure mapping

dip determination

rock strength

in-situ stress orientation

fracture frequency

porosity

fluid salinity


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Depth Of Investigation Of Logging Tools


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LOG INTERPRETATION OBJECTIVES

The objective of log interpretation depends very much on the user. Quantitative analysis of well

logs provides the analyst with values for a variety of primary parameters, such as:

porosity water saturation, fluid type (oil/gas/water)

lithology

permeability

From these, many corollary parameters can be derived by integration (and other means) to arrive

at values for:

hydrocarbons-in-place

reserves (the recoverable fraction of hydrocarbons in-place)

mapping reservoir parameters

But not all users of wireline logs have quantitative analysis as their objective. Many of them are

more concerned with the geological and geophysical aspects. These users are interested in

interpretation for:

well-to-well correlation

facies analysis regional structural and sedimentary history

In quantitative log analysis, the objective is to define

the type of reservoir (lithology)

its storage capacity (porosity)

its hydrocarbon type and content (saturation)

its producibility (permeability)


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POROSITY LOGS Neutron tool

Neutron source

High energy neutrons are slowed down by hydrogen atoms inwater (or oil) and detected by tool

Porosity is function rock type and slow neutron count

Density tool Gamma ray source Electrons reflect gamma rays back to detector in tool

Electrons in formation proportional to density

Porosity is function of rock type and density

Sonic tool Measures speed of sound in formation

Porosity slows sound

Porosity is function of rock type and measured speed of sound


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GAMMA RAY LOG

Gamma ray detector measures naturalradioactivity of formation

Mostly due to Potassium in Shale

Shale has porosity but no permeability Uranium and Thorium

Less common sources natural radioactivity

Detected by more sophisticated tools thatmeasure gamma ray energy

Run with other tools to correlate logs


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GAMMA RAY LOG

Gamma Rays are high-energy electromagnetic waves which are emitted by atomic nuclei as a form

of radiation Gamma ray log is measurement of natural radioactivity in formation verses depth.

It measures the radiation emitting from naturally occurring U, Th, and K.

It is also known as shale log.

GR log reflects shale or clay content.

Clean formations have low radioactivity level.

Correlation between wells,

Determination of bed boundaries,

Evaluation of shale content within a formation,

Mineral analysis,

Depth control for log tie-ins, side-wall coring, or perforating.

Particularly useful for defining shale beds when the sp is featureless

GR log can be run in both open and casedhole


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Spontaneous Potential Log (SP)

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).


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Neutron Logging

The Neutron Log is primarily used to evaluate

formation porosity, but the fact that it is really

just a hydrogen detector should always be keptin mind

It is used to detect gas in certain situations,

exploiting the lower hydrogen density, or

hydrogen index

The Neutron Log can be summarized as the

continuous measurement of the induced

radiation produced by the bombardment of that

formation with a neutron source contained in

the logging tool which sources emit fast

neutrons that are eventually slowed by

collisions with hydrogen atoms until they are

captured (think of a billiard ball metaphor where

the similar size of the particles is a factor). The

capture results in the emission of a secondary

gamma ray; some tools, especially older ones,

detect the capture gamma ray (neutron-gamma

log). Other tools detect intermediate

(epithermal) neutrons or slow (thermal)

neutrons (both referred to as neutron-neutron

logs). Modern neutron tools most commonly

count thermal neutrons with an He-3 type

detector.


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The Density Log

The formation density log is a porosity log that measures electrondensityof a formation

Dense formations absorb many gamma rays, while low-densityformations absorb fewer. Thus, high-count rates at the detectors indicatelow-density formations, whereas low count rates at the detectors indicatehigh-density formations.

Therefore, scattered gamma rays reaching the detector is an indicationof formation Density.

Scale and units:

The most frequently used scales are a range of 2.0 to 3.0 gm/cc or 1.95

to 2.95 gm/cc across two tracks.

A density derived porosity curve is sometimes present in tracks #2 and

#3 along with the bulk density (rb) and correction (Dr) curves. Track #1

contains a gamma ray log and caliper.


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RESISTVITY LOGS Measure bulk resistivity of formation

Laterlog The original well log

Electrodes direct current into formation to ground

electrodes on surface Induction

Magnetic field induces current in formation

Used with low conductivity well fluids

Porosity can be calculated if water salinity isknown

Oil or gas saturation can be calculated if porosityand water salinity are known


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Resistivity Log

Basics about the Resistivity:

Resistivity measures the electric properties of the formation,

Resistivity is measured as, R in W per m,

Resistivity is the inverse of conductivity,

Theability to conduct electric current depends upon:

The Volumeof water,

The Temperatureof the formation,

The Salinity of the formation

The Resistivity Log:

Resistivity logsmeasure the ability of rocks to

conduct electrical current and are scaled in units of

ohm-

meters.

The Usage:

Resistivity logs are electric logs which are usedto:

Determine Hydrocarbon versus Water-bearing zones,

Indicate Permeable zones,

Determine Resisitivity Porosity.


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Prof. Dr. H. Z. Harraz

Acoustic tools measure the speed of sound waves in

subsurface formations. While the acoustic log can be

used to determine porosity in consolidated formations, it

is also valuable in other applications, such as:

Indicating lithology (using the ratio of compressional

velocity over shear velocity),

Determining integrated travel time (an important tool for

seismic/wellbore correlation),

Correlation with other wells

Detecting fractures and evaluating secondary porosity,

Evaluating cement bonds between casing, and formation,

Detecting over-pressure,

Determining mechanical properties (in combination with

the density log), and

Determining acoustic impedance (in combination with

the density log).

Acoustic Log


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DIP METER AND BOREHOLE IMAGING

Dip Meter Four or six arms with few buttons measure small scale resistivity Wellbore inclination and orientation

Map bedding planes of sedimentary formations

Imaging Tools

Resistivity imaging tools FMI - Schlumberger, EMIHalliburton Pads with many buttons map small scale resistivity

Ultrasonic imaging tools USITSchlumberger, CASTHalliburton

Spinning ultrasonic transducer measures I.D. and sonic impedance

Borehole image Dip and orientation of fractures

Structure and stress of formation Borehole breakout

Drilling induced fractures


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OTHER GEOPHYSICAL

LOGS Mineral identification Pulsed neutron source stimulates gamma rayemissions

Tool measures energy spectrum of returning

gamma rays

Percentage of elements (silica, calcium, etc.)

Magnetic resonance

Detects free water

Determine permeability


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GEOTHERMAL APPLICATIONS

Geophysical tools designed for sedimentaryformations Algorithms for sandstone, shale, limestone, dolomite

Special algorithms required for crystalline rock Resistivity tool is sufficient to quantify porosity when

water salinity is known

Sonic tool puts seismic surveys on depth

Density tool calibrates gravity surveys

Formation imaging tools map fractures and quantifystress regime

Neutron and density tools can identify lithology, if samples are available to create correlations

if there is variation in rock type


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Schlumberger Litho-Density Log


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PRODUCTION LOGS

Very useful in geothermal wells

Can be run with simple or sophisticated

equipment

Temperature surveys are essential for

exploration work

Pressure & Temperature surveys aremore useful for well testing and

production


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TEMPERATURE LOGS Most important parameter in geothermal wells

Thermocouple wire easiest for shallow holes

RTD most accurate

Mechanical tool Only option for deep hot wells 10 years ago

Electronic surface readout tool in thermal flask Requires high temperature wireline

Electronic memory tool in thermal flask State of the art

Slick line or braided cable

Fiber Optics

Instantaneous temperature profile of entire wellbore Good for measuring transients

High temperature electronics Not yet commercial


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TEMPERATURE PROFILE

TEMPERATURE

DE

PTH

SURFACE

CONDUCTIVE GRADIENT

HYDROTHERMAL SYSTE

UPFLOW

CONDUCTIVE HEAT SO

OUTFLOW ZONE

TEMPERATURE

REVERSAL


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PRESSURE LOG

Second most important reservoir parameter pressure drives flow

producing drawdown indicates reservoir productivity (or injection buildup)

drawdown curves analyzed to determine reservoir permeability

Water level, easily measured used in hydrology but less useful in geothermal systems

dependant on wellbore temperature and gas or steam pressure above water

Mechanical pressure tool common ten years ago

Capillary tubing filled with nitrogen or helium reservoir pressure is measured at surface

good for long term reservoir pressure monitoring of hot wells

Electronic surface readout tool in thermal flask

requires high temperature wireline Electronic memory tool in thermal flask

state of the art

slick line or braided cable

STATIC PRESSURE AND TEMPERATURE PROFILES


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STATIC PRESSURE AND TEMPERATURE PROFILES

0

200

400

600

800

1000

1200

0 50 100 150 200 250 300 350

PRESSURE TEMPERATURE

DEPTH

STATIC PRESSURE STATIC TEMPERATURE

WATER LEVEL

STATIC AND FLOWING PRESSURE AND TEMPERATURE PROFILES


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STATIC AND FLOWING PRESSURE AND TEMPERATURE PROFILES

0

200

400

600

800

1000

1200

0 50 100 150 200 250 300 350

PRESSURE TEMPERATURE

DEPTH

STATIC PRESSURE

FLOWING PRESSURE

STATIC

TEMPERATURE

FLOWING

TEMPERATURE

PRESSURE DRAWDOWN

FLASH DEPTHFLASH DEPTH


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SPINNER LOG

Propeller measures flow in wellbore

Identifies production (or injection) zones

Calculate fluid velocity from series of upand down runs at different cable speeds

FLOWING SPINNER SURVEY


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FLOWING SPINNER SURVEY

0

200

400

600

800

1000

1200

-10 0 10 20 30 40 50

SPINNER COUNTS

DEPTH

Log down 100 fpm Log up 100 fpm

MAIN PRODUCTION ZONE

FLASH DEPTH


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TYPICAL SHALLOW WELL LOGGING UNIT

From USGS website, nc.water.usgs.gov


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TYPICAL SLICK LINE WINCH

From BOP Controls Inc. website, bopcontrols.net


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WELL INSPECTION LOGS

Sonic Cement Bond Log (Same tool as sonic porosity log) Measures quality of cement on outside of casing

Difficulty with large geothermal well casing

Difficulty with micro-annulus caused by temperature and pressure changes

Caliper

Measures I.D. of casing Detects corrosion, scale, washouts, parted casing

Electro-magnetic Measures metal loss

Detects corrosion, holes and parted casing

Ultrasonic (same as imaging tool) Measures I.D. and thickness of casing, and impedance of material behind casing

Detects corrosion, holes and cement

RA Tracer Injects slug of iodine 131 into wellbore

Gamma ray detector measures radioactive slug

Detects leaks in casing and flow behind pipe

Video Identify well problems

Requires very clear water


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PRESSURE CONTROLShould be used there is any possibility of well flowing

Pack-off Rubber cylinder tightens around wireline

Few hundred psi

Lubricator Length of pipe below pack-off

Necessary to run tool in pressurized wellBlow out preventor Valve below lubricator that closes around

wireline

Useful if pack-off fails or wireline gets stuck inpack-off

Grease tubes for high pressure

Placed below pack-off For thousands of psi

Grease pumped in high pressure end flows tolow pressure

Grease in

High pressure

Grease out

Low pressure


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PRESSURE-TEMPERATURE-

SPINNER TOOLS FOR SALE MADDEN SYSTEMS (Odessa, TX)

Flasked surface readout and memory tools

KUSTER COMPANY (Long Beach, CA) Mechanical tools

Flasked surface readout and memory tools

Anyone with a slickline or braided cable

winch can run memory tools.

G O S C OGG G OO S


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GEOPHYSICAL LOGGING TOOLS

AND WIRELINE WINCHES FOR

SALE

Companies that used to make tools and

sell wireline systems went out ofbusiness in the 1990s

Companies that sell systems now areon the internet


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COMMERCIAL BOREHOLE


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The Big Three

SCHLUMBERGER

HALLIBURTON

BAKER ATLAS

Worldwide Geophysical, Production& Inspection Logging

Video

DOWNHOLE VIDEOOxnardCA

many other companies

Geothermal Production Logging

WELACOBakersfield CA

PACIFIC PROCESS SYSTEMSBakersfield CA

SCIENTIFIC PRODUCTION

SERVICESHouston TX INSTRUMENT SERVICES INC.

Ventura CA

Pressure-Temperature-Spinner

& some other services

Sell and service equipment

Many other companies in Japan, NewZealand, Philippines, Iceland,Kenya (KenGen), etc.

COMMERCIAL BOREHOLE

LOGGING COMPANIES


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1- Formation Evaluation

A- Virgin Reservoir

(Mainly Open Hole Logs)

B-Developed & Depleted Reservoirs(Mainly Cased Hole Logs)

2- Monitoring Reservoir Performance

Reservoir Performance Problems

Well Performance ProblemsReservoir Description


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Some Well Mechanical Problems


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Important Questions

Is the Well Producing at Its Potential?

If It Is Not , Why Isnt It?

What is the Well Production Potential?

Is It: the Well Production on Well Test

OR

Is It: What Well Is Capable to Produce


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Causes of Low / Production Disturbance

A- Non- Treatable Problems1- Low Formation KH

2- Poor Relative permeability

3- High GOR or WOR4- High Viscosity

B- Treatable Problems1- Formation Problems

( Organic & Inorganic Precipitates, Stimulation

Fluids, Clay Swelling, Mud Effects)2- Production Equipments Problems

( Cement & casing, Tubing, Artificial Lifts)


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It is fine to Understand Types ofProblems and Their Causes

But It Is More Important To DetermineThat A Problem Does Exist.

Diagnosis of Causes

A- Surface Data Analysis

B- Drilling Report

C- Workover, Completion and

Stimulation Data


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Well Log Classification

Overview


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Well Log Classification and

Cataloging

Industry Data

Company Data

Well Log Catalog

Well Log Data Repository PWLS Class Repository


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Activities Enabled by PWLS

Meta Data Classify well logs

Classify well log channels

Query for well logs

Query for well log channels


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Classify a Well Log / Channel

/ Parameter well logwell_log_service_class

by interpretation of well log header

channelcompany_channel_class validate against dictionary

parametercompany parameter spec.

validate against dictionary


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Genericity of classification

original acquired dataprimarily co.

data

company channel class well log service class

computed dataprimarily industry

data well log curve class

well log tool class

processed datacombined approach


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Query by technology

goal: logs of a given technology

industry classification:well log tool class

company classification: well log serviceclass

catalog: classification by well log

service class

result: well log data


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Query by channel attributes

goal: channels of a given object,property, function, ...

industry classification:well log curveclass

company classification: companychannel class class

catalog: classification by companychannel class



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Query by propery type

goal: channels of a given property type

industry classification: well log curve

class company classification: company

channel class class

catalog: classification by companychannel class



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Parameter-Augmented Query

goal: well logs, subject to parametericconstraints e. g. total_depth > 33000 ft

industry classification: param spec (property

type) e. g. Bottom_Depth

company classification: company parm spece. g. BOTTOM_DEPTH

catalog: parametric classificatione. g.BOTTOM_DEPTH=44000(m)



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Existing Data

Well Log Catalog

Well Log Data Repository

15:MDL : xxxxxxxxx

150:CDL : xxxxxxxxx

280:SLD : xxxxxxxxx

440:LDS : xxxxxxxxx

Dictionary

queryengine


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Queries

Well Log Catalog


15:MDL : xxxxxxxxx

150:CDL : xxxxxxxxx

280:SLD : xxxxxxxxx

440:LDS : xxxxxxxxx

DictionaryWhere are my densitylogs?


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Existing Data

Well Log Catalog


15:MDL : xxxxxxxxx

150:CDL : xxxxxxxxx

280:SLD : xxxxxxxxx

440:LDS : xxxxxxxxx

Dictionaryquery

engine

Industry Data

PWLS

Company Data

15:MDL : xxxxxxxxx : Density

150:CDL : xxxxxxxxx : Density

280:SLD : xxxxxxxxx : Density

440:LDS : xxxxxxxxx : Density

Density : xxxxxxxxx

Acoustic : xxxxxxxxxNeutron : xxxxxxxxx

... density ...

Prof Dr H Z Harraz

T tb k & R f


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Textbook & References

Textbook:1- Hill, A.D., 1990," Production Logging- Theoretical and

Interpretive Elements", SPE Series, vol.14.

2- Instructor Notes: Production Logging & CasedHole Logging

in Vertical and Horizontal Wells).

References:1- Schlumberger, 1987," Cased- Hole Log Interpretation:

Principles / Applications", Schlumberger Ltd., Houston.

2- Rollins, D.R., et al, 1995," Measurement While Drilling", SPESeries vol.40.

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