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1 BTC - April, 1999
Natural Formation Gamma Ray
Logging
HGNS : Highly-integrated Gamma ray Neutron Sonde
SGT-L : Scintillation Gamma ray Tool
2 BTC - April, 1999
Objectives
• Explain why a GR log can tell shale from other rock
• List the three principal elements that naturally produce radiation
• Explain pair production
• Explain Compton scattering
• Explain photoelectric effect
• Explain the types of GR detectors and their advantages/disadvantages
• Describe the applications of GR logging
• Explain the GR calibration procedure
3 BTC - April, 1999
Outline - GR Logging
• Applications
• Physics of Measurement– Radioactivity
– GR Interaction
– Detector Operation
• Hardware Description– Intro - PEx Electronic Architecture
– HGNS
– Tool Hardware
• Operations– Environmental Effects
– Parameters
– Calibrations
– Limitations
– FIT & TRIM
– Safety
• LQC
• Log Response
4 BTC - April, 1999
GR Applications
• GR measurement
– General lithology indicator
– Log correlation
– Quantitative shaliness evaluation
– Radioactive tracer logging
– Scale build-up monitoring
– Others (clay typing, density,…)
5 BTC - April, 1999
HGNS Introduction
Total Gamma Ray
One tool (cartridge) performing several functions:
Compensated Neutron
Tool Acceleration
Tool Telemetry
6 BTC - April, 1999
GR Log
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Natural Formation Gamma Ray
Logging
Physics of Measurement
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Naturally Occurring Gamma Rays
Rock formation Rock formation
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Radioactivity
Stable Atom:
One which has equal number of protons, neutrons and electrons.
Radioactivity:
A property possessed by some elements of spontaneously emitting alpha, beta particles and/or gamma rays as their atomic nuclei disintegrate.
Unstable Atom
10 BTC - April, 1999
Types of Emissions
• Alpha Particles– Positively charged particle with 2 neutrons
and 2 protons (nucleus of He atom)
– easy to stop by a thick cloth
• Beta Particles– Two kinds
• B- : electron emitted from an unstable nucleus when one of its neutrons decays to a proton
0n1 1H1 + -1e0 +
• B+: positron emitted from an unstable nucleus when one of its protons decays to a neutron
1H10n1 + 1e0 +
– Easily stopped by a thin sheet of metal
– May cause skin burn
• Gamma Rays– Massless, chargeless bundles of high-
frequency electromagnetic energy emitted when an atom passes from an excited state to a less excited/ground state
– Travel at speed of light
– Referred to as photon when it has discrete quantity of em energy.
– Penetrate rocks up to 15” (8” of concrete)
11 BTC - April, 1999
Natural Radioactivity
The 3 main radioactive series in nature:Potassium (K40) - decays to stable K39
Thorium (Th232) - decays to Pb208
Uranium (U238). - decays to Pb208
•These elements decay to their rest state through a
series of intermediate steps emitting and
particles on their way
•They are found in various proportions in crystalline
rocks
•During erosion and degradation these tend to
concentrate in shales
•Hence shales are more radioactive than sands
12 BTC - April, 1999
Gamma Ray Interactions
Three principal Gamma Ray interactions:
Pair Production (high energy)
Compton Scattering (medium energy)
Photoelectric Absorption (low energy)
Nearly all natural gamma rays in formation come from Potassium, Uranium and Thorium series
In passing through the formation, a gamma ray will experience successive Compton scattering collisions, losing energy until it is finally absorbed by an atom via the photoelectric effect.
13 BTC - April, 1999
Pair Production
– Phenomenon of conversion of neutron into an electron
and positron
– GR must have at least 1.02 MeV
– Dominates the 10MeV and up interactions
14 BTC - April, 1999
Compton Scattering
– Scattering of GR from an orbital electron
– GR loses energy & e- ejected from atom’s orbit
– 75keV < Energy range < 10MeV
– Dominates the medium energy interactions
15 BTC - April, 1999
Photoelectric Absorption
– Low-energy GR disappears as it collides with an atom
– Results in the ejection of e- from its orbit
– Gamma ray energy < 100keV
– Dominates the low energy interactions
16 BTC - April, 1999
Important Interactions
Three principal Gamma Ray interactions:
Pair Production
Compton Scattering
Photoelectric Absorption
Natural gamma rays in formation experience successive Compton scattering collisions, losing energy until they are finally absorbed by some atoms via the photoelectric effect
17 BTC - April, 1999
Gamma Ray Energies
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Recap
• There are naturally occurring radioactive elements in the formation (K, Th & U)
• K, Th & U are primarily concentrated in shales
• Gamma rays are produced by the disintegration of these elements
• These gamma rays may further interact with the formation losing energy on the way
• The detector sees these gamma rays and measures the total count rate
19 BTC - April, 1999
Natural Formation Gamma Ray
Logging
Detector Operation
20 BTC - April, 1999
GR Detectors
• Geiger-Müller
– Detects and counts
• Scintillating Crystal Detectors
– Sodium Iodide doped with Thallium
– Gadolinium Orthosilicate doped with
Cerium
– Others (BGO)
– Crystals need a photomultipler tube
to count
21 BTC - April, 1999
Geiger-Muller Detector/Counter
Geiger-Müller•Detects and counts
•“Tick-tick-tick” sound!
•Temperature insensitive
•Very inefficient (6% only!)
•Used in some downhole tools
22 BTC - April, 1999
Formation to Crystal Interaction
23 BTC - April, 1999
Environmental Effects
• Log affected by – Hole Size
– Mud weight
– Barite in Mud
– Casing
• Correction charts and software settings exist
to correct the GR reading for these effects
• However since GR is primarily used as a
correlation tool, corrections may not be
applied in some areas
• Output ECGR (from HGNS) is fully
environmentally corrected
24 BTC - April, 1999
GR Safety
• Radiation
– Treat GSR-U/Y blanket as source
• Crystal
– Deteriorates in humid air
– Very brittle
– Do not eat!
• Photo Multiplier Tube
– Avoid exposure to light
– Keep away from magnets (collar locators, CMR,
…)
• Shock hazard
– 250V cartridge power
– 2000 to 3000V PMT voltage
25 BTC - April, 1999
Natural Formation Gamma Ray
Logging
Limitations,LQC,
Typical Response
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GR Measurement Limitations ??
The biggest feature of the GR log is that it
can be run in almost any logging condition
including:
– cased wells
– open holes drilled with air
– open holes drilled with water based muds
– open holes drilled with oil based or fresh muds
27 BTC - April, 1999
LQC Checklist
Hardware LQC is within tolerance
Proper logging speed
Parameters correctly selected
Difference in before and after survey less than 7
GAPI
Log follows response (in shape) with other wells
in the region
Repeatability within specified tolerances (7%)
28 BTC - April, 1999
GR Neutron Accelerometer
HGNS Hardware LQC
29 BTC - April, 1999
Typical Response
30 BTC - April, 1999
Objectives
• Explain why a GR log can tell shale from other rock
• List the three principal elements that naturally produce radiation
• Explain pair production
• Explain Compton scattering
• Explain photoelectric effect
• Explain the types of GR detectors and their advantages/disadvantages
• Describe the applications of GR logging
• Explain the GR calibration procedure