Radioactive Properties of Rocks

7/27/2019 Radioactive Properties of Rocks

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Summary of Lecture 3 Chapter 5

Simplest resistivity measurement and calculationof geometric factor to convert resistance intoresistivity

Normal device

Lateral device

Boundary and thin bed behavior

Focused devices Induction devices

Corrections to estimate true resistivity


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Radioactive Properties of Rocks


3/25

Reading Assignment

Bassiouni Ch. 2 Radioactive Properties of Rocks


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Where are we heading?

What reservoir properties can be inferred by

radioactive measurements?


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Atomic StructureCartoon only

Electrons occupy orbitals, many ofwhich are not spherical

Size of nucleus is much smaller Size of atom ~ 10-10 = angstrom

Size of proton ~ 1 femtometer ~ 10-15 m~ .00001 size of atom, i.e., tiny


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Atomic Structure

Cartoon only

Electrons occupy orbitals,many of which are notspherical

Size of nucleus is muchsmaller Size of atom ~ 10-10 = angstrom

Size of proton ~ 1 femtometer~ 10-5 angstrom

Courtesy: C.C. Lin,

University of

Wisconsin

Conventional units

Na = Avogadro's number

= 6.023 x 1023

Mass:

1 amu = (1 g/Na)

me = .5 MeV/c2

Energy = eV

Emitted light is visible

to ultraviolet


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Nuclear Structure

The nucleus also has energy levels

Conventional units

mp

~ mn

~ 1 amu ~938 MeV/c2

Note: masses are often expressed as

energy/(speed of light)2 to make it

easier to calculate the energy of

emitted photons since E = h = hc/Energy = MeV

Emitted light is gamma radiation


8/25

Types of Natural Radioactive Decay Alpha particle emission

two protons + two neutrons = He nucleus

Beta particle emissionone electron from the nucleus not the atomicorbitals

Proton conversion to neutron Positron emission

Electron capture (K shell capture)

16 12 4

8 6 2 (which is alpha particle)O C He +

14 14

6 7 (which is beta particle)

n p e

C N e

+

+ +

+


9/25

Decay Rates

0 0

0

ln 2

0

0

ln 2(constant)

ln 2

ln 2

ln 2ln

When then2

hl

hl

hl

N t

hlN

hl

t

hl

dN Ndt Ndt

dNdt

N

dN

dtN

N t

N

N N e

Nt N

= =

=

=

=

=

= =


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Natural Gamma Radiation in Rocks

Clean sandstones emit very little radiation

Clays attract radioactive minerals

So for real rocks

Shales are about 3% K, of which about 0.01% isradioactive K40 (or about 300 ppm) + 6 ppm U + 20ppm Th

Sandstones contain about 100 ppm K40 (about 1/3 of

shale concentration) + 6 ppm U + 20 ppm ThSummary: shale emits about 3X the gamma rays of

sandstone


11/25

Significance of Natural Gamma Rays

For sandstone-shale sequences, simply

recording the nature gamma ray emissions from

the formation provides a shale indicator


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Absorption of Gamma Rays

Photoelectric effect low energy photons

Depends on atomic number Z of atoms

Compton scattering medium energy photons

Independent of Z, just saps energy from

Pair production high energy photons

Independent of Z, just saps energy from


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Photoelectric Effect


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Compton Scattering


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Gamma Ray Absorption Logging

Litho-density tool

Photoelectric effect absorption is a function of Z

so absorption can be calibrated to estimate

lithology (types of rocks and minerals)

Compton scattered just depends on density of

electrons which in turn depends on density of

rock so absorption due to Compton scattering can

be used to estimate density


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Neutron Scattering

Fast neutrons are produced by bombarding

targets with accelerated positive ions

Neutrons are neutral bullets


17/25

Inelastic scatteringNeutron hits nucleus, excites it, the nucleus emits a thatis detected. The has a particular frequency that isassociated with a particular element.


18/25

Elastic Scattering

Hydrogen is the most effective moderator

because the mass of a hydrogen nucleus is

about the same as the mass of a neutron

Where is most of the hydrogen in a reservoir?


19/25

Why is H the most effective

moderator?

An effective neutron moderator is a nucleus to which a neutron will

transfer a lot of energy in each elastic collision. What should the mass of

an effective neutron moderator be?

You have all the tools to derive this result. Dont get lost in the weeds.


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1. Physics


21/25

2. Algebra to Obtain Velocities


22/25

3. Algebra to Obtain Energy Transfer


23/25

4. Calculus to Obtain

Maximum Energy Transfer


24/25

4. Calculus to Obtain

Maximum Energy Transfer (continued)


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Summary of Lecture 4 Chapter 2

Types of natural radioactive decay

Units

Decay rates and half-life Natural gamma radiation as shale indicator

Gamma ray absorption

Preview of gamma ray absorption logging

Elastic and inelastic neutron scattering

Documents

Radioactive Properties of Rocks