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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.

Gamma Ray and Spontaneous Potential Logging Core ═════════════════════════════════════════════════════════════════════════

© PetroSkills, LLC. All rights reserved._____________________________________________________________________________________________

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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


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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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


REMARKS



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REMARKS




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REMARKSREPEAT SECTION




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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


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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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


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


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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