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WELL LOGS
Interpreting Geophysical Well Logs
Prof. Dr. Hassan Z. Harraz
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.
Types of Logs
a) Gamma Ray
b) SP (spontaneous potential)
c) Resistivity (Induction)
d) Sonic
e) Density/Neutron
f) Caliper
a) Gamma Ray
• The gamma ray measures the natural
radioactivity of the rocks, and does not
measure any hydrocarbon or water present
within the rocks.
• Shales: radioactive potassium is a common
component, and because of their cation
exchange capacity, uranium and thorium are
often absorbed as well.
• Therefore, very often shales will display high
gamma ray responses, while sandstones and
limestone will typically show lower responses.
• 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.
b) SP (Spontaneous Potential)
• The SP log records the electric potential
between an electrode pulled up a hole and a
reference electrode at the surface.
• This potenital exists because of the
electrochemical differences between the
waters within the formation and the drilling
mud.
• The potenital is measured in millivolts on a
relative scale only since the absolute value
depends on the properties of the drilling mud.
• In shaly sections, the maximum SP response to
the right can be used to define a “shale line”.
• Deflections of the SP log from this line indicates
zones of permeable lithologies with interstitial
fluids containing salinities differing from the
drilling fluid.
• SP logs are good indicators of lithology where
sandstones are permeable and water saturated.
• However, if the lithologies are filled with fresh
water, the SP can become suppressed or even
reversed. Also, they are poor in areas where
the permeabilities are very low, sandstones are
tighly cemented or the interval is completely
bitumen saturated (ie- oil sands).
c) Resistivity (Induction)
• Resistivity logs record the resistance of
interstitial fluids to the flow of an electric
current, either transmitted directly to the rock
through an electrode, or magnetically induced
deeper into the formation from the hole.
• Therefore, the measure the ability of rocks to
conduct electrical currents and are scaled in
units of ohm-meters.
• On most modern logs, there will be three
curves, each measuring the resistance of
section to the flow of electricity.
• Porous formations filled with salt water (which is
very common) have very low resistivities (often
only ranging from 1-10 ohms-meter).
• Formations that contain oil/gas generally have
much 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”. This
is because this curve looks into the formation at a
depth of six meters (or greater), thereby
representing 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 anhydrite
or other non-porous intervals.
d) Sonic
• Sonic logs (or acoustic) measure the porosity
of the rock. Hence, they measure the travel
time of an elastic wave through a formation
(measured in ∆T- microseconds per meter).
• Intervals containing greater pore space will
result in greater travel time and vice versa for
non-porous sections.
• Must be used in combination with other logs,
particularly gamma rays and resistivity,
thereby allowing one to better understand the
reservoir petrophysics.
e) Density/Neutron
• Density logs measure the bulk electron density of the
formation, and is measured in kilograms per cubic meter
(gm/cm3 or kg/m3).
• Thus, the density tool emits gamma radiation which is
scattered back to a detector in amounts proportional to the
electron density of the formation. The higher the gamma
ray reflected, the greater the porosity of the rock.
• Electron density is directly related to the density of the
formation (except in evaporates) and amount of density of
interstitial fluids.
• Helpful in distinguishing lithologies, especially between
dolomite (2.85 kg/m3) and limestone (2.71 kg/m3).
• 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-free
sections are a reasonable estimate of the
pore spaces that could produce water.
• It is very common to see both neutron
and density logs recorded on the same
section, and are often shown as an
overlay on a common scale (calibrated
for either sandstones or limestone’s).
• This overlay allows for better
opportunity of distinguishing lithologies
and making better estimates of the true
porosity.
* When natural gas is present, there
becomes a big spread (or crossing) of
the two logs, known as the “gas effect”.
f) Caliper
• Caliper Logs record the diameter of the hole.
It is very useful in relaying information about
the quality of the hole and hence reliability of
the other logs.
• An example includes a large hole where
dissolution, caving or falling of the rock wall
occurred, leading to errors in other log
responses.
• Most caliper logs are run with GR logs and
typically will remain constant throughout.
WELL LOG
(The Bore Hole Image)
Interpreting Geophysical Well Logs
Prof. Dr. Hassan Z. Harraz
What is well Logging
Well 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.
HISTORY
 1912 Conrad Schlumberger give the idea of using electrical measurements to map subsurface
rock bodies.
 in 1919 Conrad Schlumberger 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 photographic – film 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
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
Work Flow Chart
Well logging
From Warrior Energy Services Website, www.warriorenergyservices.com
TYPICAL WIRELINE TRUCK
From Welaco
TYPICAL WIRELINE SKID UNIT
Welaco Unit at Ormat’s Puna Geothermal Venture in Hawaii
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
 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
Depth Of Investigation Of Logging Tools
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)
POROSITY LOGS
• Neutron tool
– Neutron source
– High energy neutrons are slowed down by hydrogen atoms in
water (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
GAMMA RAY LOG
• Gamma ray detector measures natural
radioactivity 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 that
measure gamma ray energy
• Run with other tools to correlate logs
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 cased hole
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).
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 kept
in 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.
The Density Log
• The formation density log is a porosity log that measures electron
density of a formation
• Dense formations absorb many gamma rays, while low-density
formations absorb fewer. Thus, high-count rates at the detectors indicate
low-density formations, whereas low count rates at the detectors indicate
high-density formations.
• Therefore, scattered gamma rays reaching the detector is an indication
of 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.
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 is
known
• Oil or gas saturation can be calculated if porosity
and water salinity are known
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,
• The ability to conduct electric current depends upon:
• The Volume of water,
• The Temperature of the formation,
• The Salinity of the formation
The Resistivity Log:
Resistivity logs measure 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 used
to:
Determine Hydrocarbon versus Water-bearing zones,
Indicate Permeable zones,
Determine Resisitivity Porosity.
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
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, EMI – Halliburton
• Pads with many buttons map small scale resistivity
– Ultrasonic imaging tools
• USIT – Schlumberger, CAST – Halliburton
• 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
OTHER GEOPHYSICAL
LOGS
• Mineral identification
– Pulsed neutron source stimulates gamma ray
emissions
– Tool measures energy spectrum of returning
gamma rays
– Percentage of elements (silica, calcium, etc.)
• Magnetic resonance
– Detects free water
– Determine permeability
GEOTHERMAL APPLICATIONS
• Geophysical tools designed for sedimentary
formations
– 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 quantify
stress regime
• Neutron and density tools can identify lithology,
– if samples are available to create correlations
– if there is variation in rock type
Well logging
Well logging
Schlumberger Litho-Density Log
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 are
more useful for well testing and
production
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
Well logging
TEMPERATURE PROFILE
TEMPERATURE
DEPTH
SURFACE
CONDUCTIVE GRADIENT
HYDROTHERMAL SYSTEM
UPFLOW
CONDUCTIVE HEAT SOU
OUTFLOW ZONE
TEMPERATURE
REVERSAL
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
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
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 DEPTH
FLASH DEPTH
SPINNER LOG
• Propeller measures flow in wellbore
• Identifies production (or injection) zones
• Calculate fluid velocity from series of up
and down runs at different cable speeds
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
TYPICAL SHALLOW WELL LOGGING UNIT
From USGS website, nc.water.usgs.gov
TYPICAL SLICK LINE WINCH
From BOP Controls Inc. website, bopcontrols.net
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
PRESSURE CONTROL
Should 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 well
Blow out preventor
– Valve below lubricator that closes around
wireline
– Useful if pack-off fails or wireline gets stuck in
pack-off
Grease tubes for high pressure
– Placed below pack-off
– For thousands of psi
– Grease pumped in high pressure end flows to
low pressure
Grease in
High pressure
Grease out
Low pressure
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.
GEOPHYSICAL LOGGING TOOLS
AND WIRELINE WINCHES FOR
SALE
• Companies that used to make tools and
sell wireline systems went out of
business in the 1990’s
• Companies that sell systems now are
on the internet
Well logging
The Big Three
• SCHLUMBERGER
• HALLIBURTON
• BAKER ATLAS
Worldwide Geophysical, Production
& Inspection Logging
Video
• DOWNHOLE VIDEO – Oxnard
CA
• many other companies
Geothermal Production Logging
• WELACO – Bakersfield CA
• PACIFIC PROCESS SYSTEMS –
Bakersfield CA
• SCIENTIFIC PRODUCTION
SERVICES – Houston TX
• INSTRUMENT SERVICES INC. –
Ventura CA
Pressure-Temperature-Spinner
& some other services
Sell and service equipment
Many other companies in Japan, New
Zealand, Philippines, Iceland,
Kenya (KenGen), etc.
COMMERCIAL BOREHOLE
LOGGING COMPANIES
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 Problems
Reservoir Description
Well logging
Well logging
Some Well Mechanical Problems
Well logging
Important Questions
Is the Well Producing at Its Potential?
If It Is Not , Why Isn’t It?
What is the Well Production Potential?
Is It: the Well Production on Well Test
OR
Is It: What Well Is Capable to Produce
Causes of Low / Production Disturbance
A- Non- Treatable Problems
1- Low Formation KH
2- Poor Relative permeability
3- High GOR or WOR
4- High Viscosity
B- Treatable Problems
1- Formation Problems
( Organic & Inorganic Precipitates, Stimulation
Fluids, Clay Swelling, Mud Effects)
2- Production Equipments Problems
( Cement & casing, Tubing, Artificial Lifts)
It is fine to Understand Types of
Problems and Their Causes.
But It Is More Important To Determine
That A Problem Does Exist.
Diagnosis of Causes
A- Surface Data Analysis
B- Drilling Report
C- Workover, Completion and
Stimulation Data
Well Log Classification
Overview
Well Log Classification and
Cataloging
Industry Data
Company Data
Well Log Catalog
Well Log Data Repository PWLS Class Repository
Activities Enabled by PWLS
Meta Data
• Classify well logs
• Classify well log channels
• Query for well logs
• Query for well log channels
Classify a Well Log / Channel
/ Parameter
• well log  well_log_service_class
– by interpretation of well log header
• channel  company_channel_class
– validate against dictionary
• parameter  company parameter spec.
– validate against dictionary
Genericity of classification
• original acquired data  primarily co.
data
– company channel class
– well log service class
• computed data  primarily industry
data
– well log curve class
– well log tool class
• processed data  combined approach
Query by technology
• goal: logs of a given technology
• industry classification:well log tool class
• company classification: well log service
class
• catalog: classification by well log
service class
• result: well log data
Query by channel attributes
• goal: channels of a given object,
property, function, ...
• industry classification:well log curve
class
• company classification: company
channel class class
• catalog: classification by company
channel class
• result: well log data
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 company
channel class
• result: well log data
Parameter-Augmented Query
• goal: well logs, subject to parameteric
constraints e. g. total_depth > 33000 ft
• industry classification: param spec (property
type) e. g. Bottom_Depth
• company classification: company parm spec
e. g. BOTTOM_DEPTH
• catalog: parametric classification e. g.
BOTTOM_DEPTH=44000(m)
• result: well log data
Existing Data
Well Log Catalog
Well Log Data Repository
15:MDL : xxxxxxxxx
150:CDL : xxxxxxxxx
280:SLD : xxxxxxxxx
440:LDS : xxxxxxxxx
Dictionary
query
engine
Queries
Well Log Catalog
Well Log Data Repository
15:MDL : xxxxxxxxx
150:CDL : xxxxxxxxx
280:SLD : xxxxxxxxx
440:LDS : xxxxxxxxx
Dictionary
Where are my density logs?
Existing Data
Well Log Catalog
Well Log Data Repository
15:MDL : xxxxxxxxx
150:CDL : xxxxxxxxx
280:SLD : xxxxxxxxx
440:LDS : xxxxxxxxx
Dictionary
query
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 : xxxxxxxxx
Neutron : xxxxxxxxx
... density ...
Prof. Dr. H. Z. Harraz
Textbook & References
Textbook:
1- Hill, A.D., 1990," Production Logging- Theoretical and
Interpretive Elements", SPE Series, vol.14.
2- Instructor Notes: Production Logging & Cased–Hole 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", SPE
Series vol.40.

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Well logging

  • 1. WELL LOGS Interpreting Geophysical Well Logs Prof. Dr. Hassan Z. Harraz
  • 2. 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.
  • 3. Types of Logs a) Gamma Ray b) SP (spontaneous potential) c) Resistivity (Induction) d) Sonic e) Density/Neutron f) Caliper
  • 4. a) Gamma Ray • The gamma ray measures the natural radioactivity of the rocks, and does not measure any hydrocarbon or water present within the rocks. • Shales: radioactive potassium is a common component, and because of their cation exchange capacity, uranium and thorium are often absorbed as well. • Therefore, very often shales will display high gamma ray responses, while sandstones and limestone will typically show lower responses.
  • 5. • 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.
  • 6. b) SP (Spontaneous Potential) • The SP log records the electric potential between an electrode pulled up a hole and a reference electrode at the surface. • This potenital exists because of the electrochemical differences between the waters within the formation and the drilling mud. • The potenital is measured in millivolts on a relative scale only since the absolute value depends on the properties of the drilling mud.
  • 7. • In shaly sections, the maximum SP response to the right can be used to define a “shale line”. • Deflections of the SP log from this line indicates zones of permeable lithologies with interstitial fluids containing salinities differing from the drilling fluid. • SP logs are good indicators of lithology where sandstones are permeable and water saturated. • However, if the lithologies are filled with fresh water, the SP can become suppressed or even reversed. Also, they are poor in areas where the permeabilities are very low, sandstones are tighly cemented or the interval is completely bitumen saturated (ie- oil sands).
  • 8. c) Resistivity (Induction) • Resistivity logs record the resistance of interstitial fluids to the flow of an electric current, either transmitted directly to the rock through an electrode, or magnetically induced deeper into the formation from the hole. • Therefore, the measure the ability of rocks to conduct electrical currents and are scaled in units of ohm-meters. • On most modern logs, there will be three curves, each measuring the resistance of section to the flow of electricity.
  • 9. • Porous formations filled with salt water (which is very common) have very low resistivities (often only ranging from 1-10 ohms-meter). • Formations that contain oil/gas generally have much 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”. This is because this curve looks into the formation at a depth of six meters (or greater), thereby representing 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 anhydrite or other non-porous intervals.
  • 10. d) Sonic • Sonic logs (or acoustic) measure the porosity of the rock. Hence, they measure the travel time of an elastic wave through a formation (measured in ∆T- microseconds per meter). • Intervals containing greater pore space will result in greater travel time and vice versa for non-porous sections. • Must be used in combination with other logs, particularly gamma rays and resistivity, thereby allowing one to better understand the reservoir petrophysics.
  • 11. e) Density/Neutron • Density logs measure the bulk electron density of the formation, and is measured in kilograms per cubic meter (gm/cm3 or kg/m3). • Thus, the density tool emits gamma radiation which is scattered back to a detector in amounts proportional to the electron density of the formation. The higher the gamma ray reflected, the greater the porosity of the rock. • Electron density is directly related to the density of the formation (except in evaporates) and amount of density of interstitial fluids. • Helpful in distinguishing lithologies, especially between dolomite (2.85 kg/m3) and limestone (2.71 kg/m3).
  • 12. • 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-free sections are a reasonable estimate of the pore spaces that could produce water.
  • 13. • It is very common to see both neutron and density logs recorded on the same section, and are often shown as an overlay on a common scale (calibrated for either sandstones or limestone’s). • This overlay allows for better opportunity of distinguishing lithologies and making better estimates of the true porosity. * When natural gas is present, there becomes a big spread (or crossing) of the two logs, known as the “gas effect”.
  • 14. f) Caliper • Caliper Logs record the diameter of the hole. It is very useful in relaying information about the quality of the hole and hence reliability of the other logs. • An example includes a large hole where dissolution, caving or falling of the rock wall occurred, leading to errors in other log responses. • Most caliper logs are run with GR logs and typically will remain constant throughout.
  • 15. WELL LOG (The Bore Hole Image) Interpreting Geophysical Well Logs Prof. Dr. Hassan Z. Harraz
  • 16. What is well Logging Well 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.
  • 17. HISTORY  1912 Conrad Schlumberger give the idea of using electrical measurements to map subsurface rock bodies.  in 1919 Conrad Schlumberger 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 photographic – film 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
  • 18. 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
  • 21. From Warrior Energy Services Website, www.warriorenergyservices.com
  • 23. TYPICAL WIRELINE SKID UNIT Welaco Unit at Ormat’s Puna Geothermal Venture in Hawaii
  • 24. 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
  • 25.  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
  • 26. Depth Of Investigation Of Logging Tools
  • 27. 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)
  • 28. POROSITY LOGS • Neutron tool – Neutron source – High energy neutrons are slowed down by hydrogen atoms in water (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
  • 29. GAMMA RAY LOG • Gamma ray detector measures natural radioactivity 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 that measure gamma ray energy • Run with other tools to correlate logs
  • 30. 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 cased hole
  • 31. 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).
  • 32. 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 kept in 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.
  • 33. The Density Log • The formation density log is a porosity log that measures electron density of a formation • Dense formations absorb many gamma rays, while low-density formations absorb fewer. Thus, high-count rates at the detectors indicate low-density formations, whereas low count rates at the detectors indicate high-density formations. • Therefore, scattered gamma rays reaching the detector is an indication of 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.
  • 34. 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 is known • Oil or gas saturation can be calculated if porosity and water salinity are known
  • 35. 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, • The ability to conduct electric current depends upon: • The Volume of water, • The Temperature of the formation, • The Salinity of the formation The Resistivity Log: Resistivity logs measure 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 used to: Determine Hydrocarbon versus Water-bearing zones, Indicate Permeable zones, Determine Resisitivity Porosity.
  • 36. 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
  • 37. 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, EMI – Halliburton • Pads with many buttons map small scale resistivity – Ultrasonic imaging tools • USIT – Schlumberger, CAST – Halliburton • 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
  • 38. OTHER GEOPHYSICAL LOGS • Mineral identification – Pulsed neutron source stimulates gamma ray emissions – Tool measures energy spectrum of returning gamma rays – Percentage of elements (silica, calcium, etc.) • Magnetic resonance – Detects free water – Determine permeability
  • 39. GEOTHERMAL APPLICATIONS • Geophysical tools designed for sedimentary formations – 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 quantify stress regime • Neutron and density tools can identify lithology, – if samples are available to create correlations – if there is variation in rock type
  • 43. 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 are more useful for well testing and production
  • 44. 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
  • 46. TEMPERATURE PROFILE TEMPERATURE DEPTH SURFACE CONDUCTIVE GRADIENT HYDROTHERMAL SYSTEM UPFLOW CONDUCTIVE HEAT SOU OUTFLOW ZONE TEMPERATURE REVERSAL
  • 47. 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
  • 48. 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
  • 49. 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 DEPTH FLASH DEPTH
  • 50. SPINNER LOG • Propeller measures flow in wellbore • Identifies production (or injection) zones • Calculate fluid velocity from series of up and down runs at different cable speeds
  • 51. 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
  • 52. TYPICAL SHALLOW WELL LOGGING UNIT From USGS website, nc.water.usgs.gov
  • 53. TYPICAL SLICK LINE WINCH From BOP Controls Inc. website, bopcontrols.net
  • 54. 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
  • 55. PRESSURE CONTROL Should 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 well Blow out preventor – Valve below lubricator that closes around wireline – Useful if pack-off fails or wireline gets stuck in pack-off Grease tubes for high pressure – Placed below pack-off – For thousands of psi – Grease pumped in high pressure end flows to low pressure Grease in High pressure Grease out Low pressure
  • 56. 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.
  • 57. GEOPHYSICAL LOGGING TOOLS AND WIRELINE WINCHES FOR SALE • Companies that used to make tools and sell wireline systems went out of business in the 1990’s • Companies that sell systems now are on the internet
  • 59. The Big Three • SCHLUMBERGER • HALLIBURTON • BAKER ATLAS Worldwide Geophysical, Production & Inspection Logging Video • DOWNHOLE VIDEO – Oxnard CA • many other companies Geothermal Production Logging • WELACO – Bakersfield CA • PACIFIC PROCESS SYSTEMS – Bakersfield CA • SCIENTIFIC PRODUCTION SERVICES – Houston TX • INSTRUMENT SERVICES INC. – Ventura CA Pressure-Temperature-Spinner & some other services Sell and service equipment Many other companies in Japan, New Zealand, Philippines, Iceland, Kenya (KenGen), etc. COMMERCIAL BOREHOLE LOGGING COMPANIES
  • 60. 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 Problems Reservoir Description
  • 65. Important Questions Is the Well Producing at Its Potential? If It Is Not , Why Isn’t It? What is the Well Production Potential? Is It: the Well Production on Well Test OR Is It: What Well Is Capable to Produce
  • 66. Causes of Low / Production Disturbance A- Non- Treatable Problems 1- Low Formation KH 2- Poor Relative permeability 3- High GOR or WOR 4- High Viscosity B- Treatable Problems 1- Formation Problems ( Organic & Inorganic Precipitates, Stimulation Fluids, Clay Swelling, Mud Effects) 2- Production Equipments Problems ( Cement & casing, Tubing, Artificial Lifts)
  • 67. It is fine to Understand Types of Problems and Their Causes. But It Is More Important To Determine That A Problem Does Exist. Diagnosis of Causes A- Surface Data Analysis B- Drilling Report C- Workover, Completion and Stimulation Data
  • 69. Well Log Classification and Cataloging Industry Data Company Data Well Log Catalog Well Log Data Repository PWLS Class Repository
  • 70. Activities Enabled by PWLS Meta Data • Classify well logs • Classify well log channels • Query for well logs • Query for well log channels
  • 71. Classify a Well Log / Channel / Parameter • well log  well_log_service_class – by interpretation of well log header • channel  company_channel_class – validate against dictionary • parameter  company parameter spec. – validate against dictionary
  • 72. Genericity of classification • original acquired data  primarily co. data – company channel class – well log service class • computed data  primarily industry data – well log curve class – well log tool class • processed data  combined approach
  • 73. Query by technology • goal: logs of a given technology • industry classification:well log tool class • company classification: well log service class • catalog: classification by well log service class • result: well log data
  • 74. Query by channel attributes • goal: channels of a given object, property, function, ... • industry classification:well log curve class • company classification: company channel class class • catalog: classification by company channel class • result: well log data
  • 75. 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 company channel class • result: well log data
  • 76. Parameter-Augmented Query • goal: well logs, subject to parameteric constraints e. g. total_depth > 33000 ft • industry classification: param spec (property type) e. g. Bottom_Depth • company classification: company parm spec e. g. BOTTOM_DEPTH • catalog: parametric classification e. g. BOTTOM_DEPTH=44000(m) • result: well log data
  • 77. Existing Data Well Log Catalog Well Log Data Repository 15:MDL : xxxxxxxxx 150:CDL : xxxxxxxxx 280:SLD : xxxxxxxxx 440:LDS : xxxxxxxxx Dictionary query engine
  • 78. Queries Well Log Catalog Well Log Data Repository 15:MDL : xxxxxxxxx 150:CDL : xxxxxxxxx 280:SLD : xxxxxxxxx 440:LDS : xxxxxxxxx Dictionary Where are my density logs?
  • 79. Existing Data Well Log Catalog Well Log Data Repository 15:MDL : xxxxxxxxx 150:CDL : xxxxxxxxx 280:SLD : xxxxxxxxx 440:LDS : xxxxxxxxx Dictionary query 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 : xxxxxxxxx Neutron : xxxxxxxxx ... density ... Prof. Dr. H. Z. Harraz
  • 80. Textbook & References Textbook: 1- Hill, A.D., 1990," Production Logging- Theoretical and Interpretive Elements", SPE Series, vol.14. 2- Instructor Notes: Production Logging & Cased–Hole 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", SPE Series vol.40.