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Introduction
Gamma Ray and Spontaneous Potential Logging Core
Why This Module is Important
An understanding of petrophysics and petrophysical technologyat the core knowledge level is extremely important tounderstanding well and reservoir performance.
Petrophysical data are data obtained from the wellbore duringand after drilling.
In this module, a basic introduction to Gamma Ray andSpontaneous Potential (SP) Logging is provided.
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Why This Module is Important
You will learn about Gamma Ray (GR) logging and how it provides early indications of the presence of potential reservoir rock.
GR data may be acquired either in open hole or cased hole.
Rock FormationRock Formation
Gamma Ray Logging
Why This Module is Important
The following GR logging topics are covered:• The basic physics of the Gamma Ray logging tool• Calibration standards used for GR logs and GR API Units• The three most common radioactive elements found in sedimentary
rock and applications of the Spectral GR tool• How the GR log is used to discriminate between reservoir and non-
reservoir rock • Recognize the typical GR log response in common formations
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Why This Module is Important
Spontaneous Potential (SP) logging also provides early indications of the presence of potential reservoir rock
The following SP logging topics are covered:
• The basic physics of SP logs
• The scale and units used with the SP log
• What two liquids most influence the character of the SP log
• How the SP curve can be used to estimate potential reservoir rock
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Overview of Well Logging Practices
Gamma Ray and Spontaneous Potential Logging Core
Learning Objectives
By the end of this lesson, you will be able to:
Identify three or more ways to convey openhole logging toolsinto the borehole, and the impact of hole angle
Explain why log calibrations and “repeat sections” areperformed
Recognize the key elements of an openhole log “print” and listat least five types of data on the heading
Describe the relationship between logging tool depth ofinvestigation and bed resolution
Explain what is meant by “curve blocking” and explain why it isuseful
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The Petrophysical “Detective”
Petrophysical evaluation • Identify, quantify subsurface
hydrocarbon resources • Evaluate fluid, rock
properties• Conduct single or multi-well
studies
Deliverables• Static and Dynamic
reservoir description• Fluid distribution (at and
away from wellbore)• Calibrated determination of
rock properties using multiple data types
Measurements are Both Direct, Indirect!
Direct measurements:• Direct comparison
– (e.g., ruler, weighing scales)
– Very few in petrophysics– Velocity is a direct measurement
Indirect measurements:• Via the effect on something else
– (e.g., temperature via Hg expansion)
– Porosity is an indirect measurement
Calibration is always required• Logging tool “calibration” is a verification that the logging tool is
functioning properly and reading correctly across an appropriate range
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Hydrocarbon Volume – What’s Direct?
(1 - Sw)HCVOL = A * *** h (N/G)
Traditional Logging Companies
Traditional Logging Companies for Open Hole Logging
Wireline• Schlumberger• Baker - Atlas• Halliburton• Weatherford (formerly Reeves)
Logging while drilling• Anadrill (part of Schlumberger)• Baker-Hughes Inteq (including Teleco)• Sperry (part of Halliburton)• Weatherford (Precision)
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Horizontal Well Logging
Pipe Conveyed Logging or Drill Pipe
Assisted Logging
Other options:• Coiled Tubing Unit • Use of a tractor to pull
the tool string down to desired depth
Logging While Drilling
Logging While Drilling (LWD) is the alternative to wireline logging.
LWD tools are built into drill collars and contain very sophisticated electronics designed to withstand extreme vibrations and high temperatures and pressures.
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Key Elements of THE LOG
HEADING
Key Elements of THE LOG
REMARKS
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Key Elements of THE LOG
REMARKS
Key Elements of THE LOG
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Key Elements of THE LOG
REMARKSREPEAT SECTION
Key Elements of THE LOG
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Key Elements of THE LOG
Important Information: Log Header
Depths Casing depths Permanent datum Type of mud and
properties Resistivities Rig elevation
Header Key Information:
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The “Remarks”: See Any Problems?
What occurred during logging
Services provided
Tool string makeup
Total depth remarks
Environmental corrections
The Remarks section includes:
Formation Properties
Rock type
Porosity
Permeability
Fluid type (oil, gas, water)
Fluid Volume (saturation)
Formation tops
Fractures
Open Hole Logs
Gamma Ray and SP logs
Resistivity (Laterolog, Induction)
Nuclear (Density, Neutron)
Acoustic
Nuclear Magnetic Resonance
Formation Imaging
Sampling (pressures and fluids)
Open Hole Logging Tools
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Principle of Log Interpretation
Basic Log Analysis Questions
There are “Quick-Look” and more “Advanced” Techniques
Net Pay, Porosity, Water Saturation, and Permeability
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Parameters Needed
Lithology and Porosity
Resistivity of Rocks and Fluids
Formation Factor
Formation Water
Resistivity –Rw
Water Saturation –
Sw
Hydrocarbon Indicator(s)
Permeability Relationship
End Product – “Answer Log”
Delineates Lithology, Fluids• Sandstone, limestone, dolomite, etc.
(Reservoir)• Shale, etc. (Non-reservoir)• Oil, Gas, Water
Quantifies• Porosity• Water, Oil saturation• Net Feet of Pay
But, more information is needed• Does apparent pay flow?• Is mineralogy affecting log response?• What does water saturation (Sw)
calculation mean?• Is it a “calibrated” result?
(1585)
(1615)
(meters)
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Basic Log Interpretation
Understanding Tool Physics and Tool Response
Depth of investigation and resolution
Tools utilized and brief description of operating principles
Principles of interpretation specific to tools to determine:• Net-Gross• Porosity• Resistivity• Saturation• Permeability
Overlays, “quick-looks,” and cross-plots
Depth of Investigation and Resolution of Logging Tools
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Reading Log Responses
Logging tools have vertical resolution limits
Adjacent beds lithology contrasts affect log response
Quick look evaluation blocking averages readings over intervals
Block boundaries at same depth on all log curves for well evaluated
Curve Blocking to Highlight Zones
depth
True formation profile
Log response
Blocks used for evaluation
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Learning Objectives
Identify three or more ways to convey openhole logging tools into the borehole, and the impact of hole angle
Understand why log calibrations and “repeat sections” are performed
Recognize the key elements of an openhole log “print” and list at least five types of data on the heading
Understand the relationship between logging tool depth of investigation and bed resolution
Understand what is meant by “curve blocking” and explain why it is useful
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Gamma Ray
Gamma Ray and Spontaneous Potential
Logging Core
Learning Objectives
By the end of this lesson, you will be able to:
Describe the basic physics of the Gamma Ray (GR) logging tool
Outline the calibration standard used for GR logs and explain the GR API Units
Identify the three most common radioactive elements found insedimentary rock and explain the purpose of the Spectral GRtool
Explain how the GR log is used to discriminate between reservoir and non-reservoir rock
Recognize the typical GR log response in five or more commonformations
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Gamma Ray Logs
Gamma Ray Logging Tools:
Passive detectors respond to natural gamma radiation in the subsurface
Do not have a radioactive source
Harmless equipment
Rock FormationRock Formation
Net/Gross – Gamma Ray
To measure the natural gamma rays emitted from the formation, the Gamma Ray (GR) tool is lowered in the borehole.
The Gamma Ray tool consists of a detector and associated electronics to measure the gamma radiation originating in the volume of formation near the tool.
Gamma Ray Logging
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Net/Gross – Gamma Ray
The simplest tool is the Gamma Ray tool that measures total gamma rays
Spectral Gamma Ray (SGR)
Also known as the Natural Gamma Ray Tool (NGT)
Two Types of Gamma Ray Tools:
Radiation found in
Sedimentary Rocks
Potassium
Thorium
Uranium
This standard is made up of radioactive cement containing fixed amounts of radioactive elements
Three most commonly occurring radioactive elements in sedimentary rocks are:
• 13 ppm (116.81 mg/m3) Uranium• 24 ppm (227.73 mg/m3) Thorium• 4% Potassium
API Gamma Ray Standard
(1.219 m)
(7.315 m)
(0.1397 m)
(126.54 mg/m3)
(227.73 mg/m3)
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Gamma Ray Applications
Correlation• Well to Well correlation • Depth matching between separate trips in the well• Positioning of open-hole sampling tools• Providing the depth control needed for cased hole perforation
– Note: gamma rays do pass through steel
General lithology indicator• Discriminate between reservoir & non-reservoir • Used to determine Net to Gross ratios
Qualitative “shaliness” evaluation of the reservoir rock• Gamma Ray data can be empirically calibrated to estimate percent
shale in a shaly sand interval
GR Response to Lithology
The Gamma Ray log is a good “first-pass” indicator of lithology
The Gamma Ray log records total abundance of the radioactive isotopes of Potassium (K), Thorium (Th) and Uranium (U)
K, Th and U are usually concentrated in shales and less in sandstones and carbonates (owing to differences in mineralogy) with some notable exceptions
Common GR readings, in API units*, are:• Limestones, anhydrites,
15-20 API• Dolomites and “clean”
(quartz-rich) sandstones, 20-30 API
• Shales, average 100 API, but can vary from 75 to 300 API
• Other lithologies: coal, salt (halite, NaCl) and gypsum usually low readings (~<20 API), volcanic ash and beds of potash salts (sylvite, KCl) give high readings (>75)
* 1 API unit = 1/200th of the response generated by a calculated standard that has 2x the average radioactivity of shale with 6ppm U, 12ppm Th and 2% K
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GR Responses (Typical) in Track 1
Net/Gross – GR Interpretation in a Sand Shale Sequence
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Net/Gross, Net Reservoir
Spectral Gamma Ray Applications
The spectral Gamma Ray, also called the Natural Gamma Ray tool or NGT, has the following applications:
Assist in mineralogy identification
May improve Vshale determination
Improved clay mineral identification
Fracture detection
Aid in difficult well to well or core-to-log correlations
Highlights potential source rock
Detection of unconformities
Identification of scale build-up
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GR Spectra Can Be Useful
Energy in MeV
Net/Gross – Gamma Ray
Apparent shale caused by high uranium streak in Northern California well
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Spectral GR, Texas
Del Rio: Typical shale. High potassium content associated with illite
Buda: Limestone. Very low radioactivity (<20 API)
Eagleford: Shale and source rock. High uranium content associated with high TOC (Total Organic Carbon)
Gamma Ray Summary
Relatively simple passive measurement of naturally occurring radioactivity
Primary sand/shale discriminator because shales are more radioactive than most reservoir rocks.
Primary well-to-well and logging run-to-logging run correlation tool
Vshale indicator via transform
Spectral version helps with fracture detection mineral identification, clay typing and infiltration of fluids carrying dissolved radioactive minerals (production scale)
Gamma ray, like all radioactive measurements, are statistical and log quality is affected by logging speed
High pressure, high temperature (HPHT) versions of tools available
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Learning Objectives
Describe the basic physics of the Gamma Ray logging tool
Outline the calibration standard used for GR logs and explainthe GR API Units
Identify the three most common radioactive elements found insedimentary rock and explain the purpose of the Spectral GR tool
Explain how the GR log is used to discriminate betweenreservoir and non-reservoir rock
Recognize the typical GR log response in five or more commonformations
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Spontaneous Potential (SP)
Gamma Ray and Spontaneous Potential
Logging Core
Learning Objectives
By the end of this lesson, you will be able to:
Describe the basic physics of Spontaneous Potential (SP)logs
Recognize the scale and units used with the SP log
Identify what two liquids most influence direction and magnitudeof the SP log
Recognize how the SP curve can be used to estimate potentialreservoir rock
Outline petrophysical capabilities and limitations inidentifying producing versus non-producing intervals
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Electrode
V
Isolated cable
Ground on surface
Borehole
Net/Gross – Spontaneous Potential
The SP log is recorded by placing a movable electrode in the borehole and measuring the difference between the electrical potential of this movable electrode and the electrical potential of a fixed surface electrode. The recorded curve is scaled in millivolts (abbreviated as mV).
SP log requires conductive (water-based) mud and cannot be recorded in oil base mud.
SP Presented in Track 1 of API grid
Key characteristics:• SP Scale has no zero• Calibrated for sensitivity• SP reference is shale
baseline• Looking for maximum
deflection
Affected by:• Salinity contrast (Rmf & Rw)• Bed thickness• Tight, resistive formation
Shallow
Deep
SP is recorded not generated!
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Direction ions move is dependent upon the relative salinities of the fluids involved:
– “Negative” SP deflection when Rmf>Rw
– “Positive” SP deflection when Rmf<Rw
– If Rmf ~= Rw no SP will develop
Important to properly establish shale line to distinguish permeable and non-permeable zones
– Relevant to Rw
calculations (later)
Factors Affecting SP: Salinity Contrast
negative positive
Interpretation Problems with SP Log
SP
When Rmf Rw(May indicate highor low permeability)
SP (“Normal”)
When Rmf>Rw(May indicate highpermeability)
SP (“Reversed”)
When Rmf<Rw(May indicate highpermeability)
v
Permeable beds defined well in relatively thick, porous sand, shale sequences
Thinly bedded, low permeability formations are poorly resolved
SP log measures differences in ionic activities (relative salinity) between drilling mud and formation waters
In salt muds, SP often useless because spontaneous potential at depths of interest are small (Rmf Rw)
Because boundary definition with low resistivity mud and high resistivity formations is very poor
SP curve can reverse under certain circumstances
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SP Lithology Indication
Shale LineSand Line
Net/Gross – SP as Sand Shale Indicator
The Net to Gross ratio is calculated by dividing the Net by the Gross interval.
Caution: The gross interval should be selected in consultation with the Geologist and/or Geophysicist!
Watch mud filtrate and formation water resistivities!
Shale LineSand Line
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SP Shape for Depositional Environment
SP Summary
Indicator of porous, permeable intervals
Available in most wells with water-based mud
Useful for a number of things:• Field-wide correlation• Net/Gross estimation• Vshale estimation• Rw calculation (discussed later)
Number of factors can affect the response and interpretation• Hydrocarbon saturation• Salinity contrast between mud filtrate and formation water resistivity
“Last resort” method of shale volume estimation, and Rwcalculation
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Learning Objectives
Describe the basic physics of Spontaneous Potential (SP) logs
Recognize the scale and units used with the SP log
Identify what two liquids most influence direction and magnitude of the SP log
Recognize how the SP curve can be used to estimate potential reservoir rock
Outline petrophysical capabilities and limitations in identifying producing versus non-producing intervals
PetroAcademyTM Foundations of Petrophysics
Petrophysical Data and Open Hole Logging Operations Core
Mud Logging, Coring and Cased Hole Logging Operations Core
Gamma Ray and SP Logging Core
Porosity Logging (Density, Neutron and Sonic) Core
Formation Testing Core
Resistivity Logging Tools and Interpretation Core
Petrophysical Evaluation Core
Core Analysis Core Knowledge
Special Petrophysical Tools: NMR and Image Logs Core
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