1btc - april, 1999 natural formation gamma ray logging hgns : highly-integrated gamma ray neutron...

1 BTC - April, 1999

Natural Formation Gamma Ray

Logging

HGNS : Highly-integrated Gamma ray Neutron Sonde

SGT-L : Scintillation Gamma ray Tool

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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

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GR Applications

• GR measurement

– General lithology indicator

– Log correlation

– Quantitative shaliness evaluation

– Radioactive tracer logging

– Scale build-up monitoring

– Others (clay typing, density,…)

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HGNS Introduction

Total Gamma Ray

One tool (cartridge) performing several functions:

Compensated Neutron

Tool Acceleration

Tool Telemetry

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GR Log

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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

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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)


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


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.


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


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


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


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


Gamma Ray Energies


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



Logging

Detector Operation


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


Geiger-Muller Detector/Counter

Geiger-Müller•Detects and counts

•“Tick-tick-tick” sound!

•Temperature insensitive

•Very inefficient (6% only!)

•Used in some downhole tools


Formation to Crystal Interaction


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


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



Logging

Limitations,LQC,

Typical Response


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


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%)


GR Neutron Accelerometer

HGNS Hardware LQC


Typical Response


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

1btc - april, 1999 natural formation gamma ray logging hgns : highly-integrated gamma ray neutron...

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