Well Logs Neutron Log

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Neutron Logging used for:

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- gas in subsurface
- lithology (shales and general)
- evaporites
- hydrated minerals
- igneous rocks

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CNL

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Compensated Neutron Log

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DNL

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Dual Neutron Log

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SNP

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Sidewall Neutron Porosity

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CN

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Compensated Neutron

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SEN

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Sidewall Epithermal Neutron

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CNS

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Compensated Neutron Sonde

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ENS

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Epithermal Neutron Sonde

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Two sources for neutron logging:

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1. chemical
2. pulsed

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Sources for Chemical (2 combos)

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- Plutonium/Beryllium
- Americium/Beryllium

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Pulsed Sources use: (2)

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- neutron accelerator
- target

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Fast Neutrons

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5 MeV or 28,000 km/s

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Epithermal Neutrons

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0.2-1 eV or ~5 km/s

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Thermal Neutrons

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same energy as surrounding matter

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Capture Gamma Ray

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When neutron reaches thermal state and captured by nucleus that emits gamma ray

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Conventional neutron tools operate using __ or __

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- spatial distribution of neutrons
- capture gamma ray

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Pulsed neutron tools assess distribution as a function of __.

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time

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GNT (describe)

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- no longer in use
- quantitative indicator of porosity
- affected by hole size and salinity
- single detector

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SNP (describe)

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- limited use
- single detector of epithermal neutrons
- mudcake affected reading

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CNL (describe)

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- currently in use
- two detectors for thermal neutrons

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Two elements that dominate behavior of all neutron tools:

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- Hydrogen
- Chlorine

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Neutron needs to collide an avg of __ times to reach thermal energy

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18

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__ best to slow down neutrons

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Hydrogen

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__ absorbs thermal neutrons 100 times faster than other elements

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Chlorine

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CNL can be run in __ or __ holes

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- open
- cased

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CNL measures:

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neutron porosity index which relates to porosity if lithology and formation fluid are known

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Thermal neutron flux

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- defined as the number of thermal neutrons crossing unit area in unit time
- controlled by hydrogen content in formation

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CNL normalized to correct for:

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environmental effects

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Shale porosity relationship

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N > D

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Gas porosity relationship

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D > N

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Gas Porosity Equation

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write it down

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Water/Brine Porosity Equation

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write it down

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Sand/Shale sequence standard

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sandstone matrix setting with porosity of 60% to 0% left to right

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Carbonate/Evaporites /Unknown Lithology

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- limestone matrix setting with porosity of 45% to -15% left to right
- if density log recorded, scale is 1.95 to 2.95 g/cc

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Advantages of sandstone scales are (2)

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1. gas can easily be spotted
2. shales distinguished from sands

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Depth of Investigation

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90% for 10 inches

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Two natural benchmarks for CNL are: (2)

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- salt
- anhydrite

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API neutron unit =

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- 1/1000 of the difference between instrument zero
- log deflection opposite a 6 ft zone of Indiana limestone (p=19%) in neutron calibration pit

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CNT-G (describe)

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two thermal neutron detectors and two epithermal detectors such as boron and gadolinium

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TDT

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Thermal Decay Time

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NLL

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Neutron Lifetime Log

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Pulsed neutron tools (TDT and NLL) measure and environment:

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- time required for high energy neutrons to be absorbed by formation
- run in cased holes

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Pulsed neutron tool uses: (2)

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- qualitatively differentiate between water, oil, and gas sands
- quantitatively estimate water saturation