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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
result: well log data
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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
result: well log data
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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)
result: well log data
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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
Well Log Data Repository
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
Well Log Data Repository
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.