neutron log.ppt

1Neutron Porosity Logging

Neutron Porosity Logging


Introduction

• Neutron Interactions With Matter• Slowing Down & Capture• Neutron Detection• The CNL Compensated Neutron Tool• Output Channels• LQC Hints• Examples


Introduction

Neutron logging responds primarily to the amount of hydrogen in the formation. Hydrogen is contained in water, oil and gas and can be used to estimate porosity.

It was introduced commercially in 1941 by Well Surveys Inc. (eventually absorbed by Lane-Wells).

It can be recorded in open and cased holes, in any type of liquid, and combined with virtually any other log.


Neutron Interactions With Matter

Four main neutron interactions with matter:

• Elastic Scattering - moderating interaction• Inelastic Scattering - moderating interaction• Fast Absorption - neutron absorption• Thermal Absorption - result of capture or

nuclear interaction


Neutron Interactions: Elastic Scattering

Neutrons can interact elastically regardless of their kinetic energy. With each interaction, the neutron loses kinetic energy, imparting some to the bombarded nucleus, which remains in its ground state.

Neutrons lose the most energy when they interact elastically with hydrogen (H) nuclei (a proton). This is because the hydrogen nucleus has a mass equal to that of the neutron.


Neutron Interactions: Inelastic Scattering

Inelastic interactions occur when a neutron is travelling fast, almost always at kinetic energies above about 1MeV, and excites the nucleus to a higher internal energy state. Almost instantly, the nucleus falls back to its ground state, emitting gamma rays at energies unique to the target nucleus.

The energy spectrum of prompt gamma rays given off during inelastic-neutron scatterings is primarily used to measure the relative concentrations of oxygen and carbon in the formation fluids. The energy spectrum is also useful in identifying silicon (Si), calcium (Ca), iron (Fe), and sulphur (S), which can be used to determine lithology.


Neutron Interactions: Thermal Absorption

The Neutron is absorbed by the atom and a gamma ray is emitted immediately. Capture is most likely to occur with thermal neutrons although it can occur at any energy level.

Although any nucleus can capture a neutron the probability is low except for a few elements. The most common element in formations is chlorine, typically present in the formation water (salty) or in the borehole fluid. The ability for an element to capture a neutron is called Capture Cross Section. This property depends on the element and the incoming neutron energy.


Neutron Interactions: Fast Absorption

The Neutron is absorbed by the atom, the new atom decays over a relatively long period of time, emitting other particles and gamma rays.

If the neutron is fast, the nucleus that absorbs it may immediately eject a proton or other nuclear particle and become an excited, unstable nucleus of a new element.

This interaction is also known as “activation.”


Life of a Neutron

A high energy Neutron is emitted from a source. The neutron undergoes many collisions, losing energy mainly through inelastic and elastic scattering. Once it reaches equilibrium (thermal energy level) the neutron continues to make elastic collisions with nuclei, this is called the thermal diffusion phase.

Eventually the neutron will suffer a collision in which it is absorbed (captured) with the emission of a gamma ray, characteristic of the nucleus involved. It then disappears from the neutron population.


Life of a Neutron

Capture

Time (S)

Neu

tron

Ene

rgy

(eV

)

1 1001010-2

100

102

104

106

Thermal Diffusion Region

Neutron energy is reduced rapidly,through collisions, to about0.025 eV (at room temperature).

Epithermal EnergyRegion

10 eV

0.4 eV0.025 eV


Elastic CollisionsDuring an elastic head-on collision between two particles, there is maximum transfer of energy when both particles have the same mass.

Before Collision

B is lighter than R

B has the same mass as R

B is heavier than R



Before Collision After Collision

B bounces back R moves forward




B continues forwardR moves forward




B stopsR moves forward

B has transferred all its energy to R


Elastic Scattering

Since the neutron has the same mass as a proton, it will lose the most energy when it collides with hydrogen atoms, which have essentially the same mass as a proton.

Hydrogen contributes most to neutron slowing down.

Once neutrons have been slowed down to thermal level through collisions with hydrogen and other atoms, they move randomly away from the source and are eventually captured and disappear from the neutron population.


Hydrogen Index

The number of neutrons present at a given distance from the neutron source is inversely related to how efficient the formation is at slowing them down.

Since Hydrogen has the most slowing-down power, this slowing-down length is measured in terms of Hydrogen Index (HI) of the formation.

The Hydrogen Index of fresh water is defined as 1.


Elastic Scattering Efficiency

Element Atomic Number ( Z ) Number of Collisions

Hydrogen (H) 1 18

Carbon (C) 6 115

Oxygen (O) 8 150

Sodium (Na) 11 215

Aluminum (Al) 13 250

Silicon (Si) 14 261

Chlorine (Cl) 17 329

Calcium (Ca) 20 371

Iron (Fe) 26 514

This table shows the average number of collisions with a given element, that is necessary to lower the neutron energy from 2 MeV down to the 0.025 eV thermal level.


Basic Design of a Neutron Porosity Tool

Neutron Source

Neutron Detector

FormationBorehole

Neutron count rate at a fixeddistance from the source

Fo

rmat

ion

Hyd

rog

en In

dex


Neutron Sources

Two types of neutron sources are used in the well logging industry:

• Chemical Sources• Electronic Sources


Chemical Neutron Source (AmBe)

The chemical neutron source used for neutron logging is a mixture of americium (Am241) and beryllium (Be).

Am241 is an alpha emitter. When the alpha particles interact with beryllium, neutrons are produced. The energy emission level from this source ranges from 1 to 10 MeV, with a peak between 4 and 5 MeV.

The americium-beryllium (AmBe) neutron source has a half-life of 433 years.


Chemical Neutron Source (AmBe)


Electronic Neutron Source (Minitron)

The minitron is an electronic neutron accelerator. Neutrons are produced by "pulsing" the neutron accelerator.

It consists of a ceramic tube containing tritium and deuterium at low pressure. The deuterium ions are accelerated by a high voltage and hit a tritium target. This creates neutrons at an energy of 14 MeV.

A minitron typically emits eight times as many neutrons with three times as much energy as a conventional chemical logging source.


Electronic Neutron Source


Neutron Sources Comparison

Chemical (AmBe)Simple and reliableAlways onLow neutron outputLow energy (4-5 MeV)

Electronic (Minitron)Complex designCan be pulsed (turned on and off)High neutron output (eight times more)High energy (14 MeV)


Compensated Neutron Tool (CNT)

AmBe chemical neutron source.

Two He3 Neutron Detectors.

Mandrel design, with radial symmetry, i.e. there is no back-shielding and no preferential orientation.

Tool is run eccentralized, applied to the formation by a bow spring.

Ratio of count rates at the near and far detectors is converted to hydrogen index.


Neutron Porosity Logs (Schlumberger)

NPHI Derived from the instantaneous ratio of near and far count rates. The ratio is calibrated and converted to porosity. Hole size correction is usually applied at the well site.

TNPH Near and far count rates are corrected for dead time and calibrated. They are depth-matched and resolution matched before the ratio is taken. Ratio is converted to porosity using a new transform. All borehole corrections can be applied at the well site, but TFE request that only hole size be applied

NPOR As TNPH, with the addition of deconvolution based on the near-detector count rates. This improves the vertical resolution.

HTNP As TNPH, with higher sampling rate (2-inch instead of 6-inch).

HNPO As NPOR, with higher sampling rate (2-inch instead of 6-inch).


Neutron Porosity Logs (Halliburton)

NPHI Neutron Porosity.

ENPH Enhanced Vertical Resolution Neutron Porosity.


Neutron Borehole Corrections Options

• HSCO Hole size (casing size) correction option

• SOCO Standoff correction option

• MCCO Mudcake correction option

• BSCO Borehole salinity correction option

• FSCO Formation salinity corrections options

• MWCO Mud weight correction option

• PTCO Pressure and temperature correction option

• CCCO Casing and cement thickness correction option


Neutron Tool Calibration

Because neutron detector sensitivities and mechanical tolerances may vary from one tool to another, the measurement must be calibrated. This is performed in a water-filled tank with the logging source in place. The basic procedure consists of:

1. Measurement of housing diameter and water temperature are critical for an accurate calibration.

2. Background measurement. The value of the background is determined by the alpha sources in the detectors.

3. Plus measurement in the water tank with logging source.

4. Computation of gains and offsets. The offset is due to the ALPHA CHECK sources. 50Bq U-234 painted on each detector

5. Plateau checks. While not actually required for the calibration, the plateau checks ensure functioning of the tool instrumentation.


Neutron Tool Calibration

Primary Artificial Porous Formation at the University of Houston

Master NCT-B Tank (Shop) The proportions of water, air and Al at 75 degf give an apparent porosity = 18 %, simulating limestone matrix and hole size 7-7/8”.

Field CNB or Alpha detectors (Check)


CNL Curve Outputs

• Standard Resolution– Sampling rate: 6 inches– NPHI, TNPH, NPOR, NPHI– This log should be recorded at 1800 ft/hr.

• High Sampling Rate– Sampling rate: 2 inches – HTNP, HNPO, ENPH– This log should be recorded at 900 ft/hr– Curves are over-sampled and noisy


Neutron Log Display and Scales


Neutron Porosity Borehole Corrections

Standard Conditions:

Hole Diameter = 8 inches

No mud cake

Borehole Fluid = fresh water

Temperature = ambient

Pressure = atmospheric

Tool is eccentered


Neutron Porosity Hole Size Correction

Actual Conditions:


No mud cake




Tool is eccentered


Neutron Porosity Hole Size Correction

Chart Por-14c


Neutron Porosity Mud Cake Correction

Actual Conditions:


Mud cake is present




Tool is eccentered


Neutron Porosity Mud Cake Correction

Chart Por-14c


Neutron Porosity Mud Salinity Correction

Actual Conditions:


No mud cake

Borehole Fluid = salt water



Tool is eccentered


Neutron Porosity Mud Salinity Correction

Chart Por-14c


Neutron Porosity Mud Weight Correction

Actual Conditions:


No mud cake

Borehole Fluid = mud



Tool is eccentered


Neutron Porosity Mud Weight Correction

Chart Por-14c


Neutron Porosity Temperature Correction

Actual Conditions:


No mud cake


Temperature = hot


Tool is eccentered


Neutron Porosity Temperature Correction

Chart Por-14c


Neutron Porosity Pressure Correction

Actual Conditions:


No mud cake



Pressure = high

Tool is eccentered


Neutron Porosity Pressure Correction

Chart Por-14c


Neutron Porosity Standoff Correction

Actual Conditions:


No mud cake




Tool standoff


Neutron Porosity Standoff Correction

Chart Por-14d


Neutron Borehole Corrections (Halliburton)

Chart Por-4a


Porosity from Neutron Log

The Compensated Neutron logging tool does NOT measure porosity. It measures Hydrogen Index. This Hydrogen Index is function of porosity, but also of lithology and fluid type.

For convenience (?) the measurement is displayed as an apparent limestone porosity.

This means that the measurement, after all borehole corrections, only represents true formation porosity in a clean limestone formation filled with fresh water.

This is seldom the case in the Kutai basin.



Chart Por-13bApparent Limestone Porosity (p.u.)

Tru

e P

oros

ity fo

r In

dica

ted

Mat

rix

20 p.u.

25 p.u.

17 p.u.

20 p.u.



Measured porosity (in Limestone units) is 25 p.u.Determine true porosity, using Chart Por-13b.

1. Assuming matrix is quartz sandstone2. Assuming matrix is calcite3. Assuming matrix is dolomite



Chart Por-13bApparent Limestone Porosity (p.u.)

Tru

e P

oros

ity fo

r In

dica

ted

Mat

rix

25 p.u.


Light Hydrocarbon Effect

The CNL neutron porosity is calibrated assuming that the fluid in the pore space is fresh water (hydrogen index equal to 1).

Gas has a very low hydrogen index: the amount of hydrogen per unit volume is much less than in water.

In a gas-bearing reservoir, the CNL “sees” much less hydrogen than it would if the same reservoir were filled with water.

What effect does this have on inferred porosity?


Light Hydrocarbon Effect

In a gas-bearing formation, neutron “porosity” is much lower than the actual porosity of the rock.

This is because the CNL actually measures the hydrogen index of the formation, not its porosity.


Gas Effect on Neutron Log

Gas/Oil Contact


Neutron Response

TNPH45 -15

RHOB 2.951.95

GR0 150

CALI6 16

Shale

Halite

Borehole Washout

20 p.u. Limestone (water)

20 p.u. Dolomite (water)


20 p.u. Sandstone (water)


20 p.u. Limestone (oil)

20 p.u. Limestone (gas)

Pore Fluid

Lithology

Porosity

neutron log.ppt

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