4 - Neutron Density

11

Neutron Porosity Measurement

Neutron Porosity

© Schlumberger 1999

A

22


Lithology and Porosity

The next major step in the procedure is lithology identification. Lithology data gives information on porosity and other parameters.

Lithology of a formation can be:

Simple

Dirty

Complex

33


Lithology and Porosity Tools

All tools react to lithology - usually in conjunction with the porosity.

Major lithology tools are:

Neutron - reacts to fluid and matrix.

Density - reacts to matrix and fluid.

Sonic - reacts to a mixture of matrix and fluid, complicated by seeing only primary porosity.

NGT - identifies shale types and special minerals.

CMR - magnetic resonance reacts to the porosity with a small element if lithology.

44


Early Neutron Tools

The first neutron tools used a chemical neutron source and employed a single detector which measured the Gamma Rays of capture

They were non-directional.

The units of measurement were API units where 1000 API units were calibrated to read 19% in a water-filled limestone.

The tool was badly affected by the borehole environment.

55


Neutron Tools

The second generation tool was the Sidewall Neutron Porosity (SNP).This was an epithermal device mounted on a pad.

The current tool is the Compensated Neutron Tool (CNT).

The latest tool is the Accelerator Porosity Sonde (APS), using an electronic source for the neutrons and measuring in the epithermal region.

66


Hydrogen Index

Hydrogen Index is the quantity of hydrogen per unit volume.

Fresh water is defined as having a Hydrogen Index of 1.Hence oil has a Hydrogen Index which is slightly less than that of water.

The Hydrogen Index of gas is a much smaller than that of water.

In a formation, it is generally the fluids that contain hydrogen.

77


DetectorsTwo neutron detectors are used to produce a ratio eliminating some of the borehole effects experienced by single detectors.The count rate for each detector is inversely proportional to porosity with high porosity giving low count rates.

88


Ratio to Porosity Transform

The count rates are first corrected for the dead time of the detectors (when the detector is not available to receive counts).

The count rates are calibrated with the master calibration.

A ratio of these is then taken.

The ratio is translated into porosity using a transform. (This is a combination of theoretical and experimental work).

The current field output for the thermal neutron porosity is called TNPH.

99


Borehole Effects

The logs have to be corrected for the borehole environment:

Borehole size.

Mud cake.

Borehole salinity.

Mud weight.

Temperature.

Pressure.

Formation salinity.

Stand-off.

1010


Hole Size Correction

Necessary because the tools algorithm from ratio to porosity is built to "fit" a 77/8" hole.Larger holes cause the tool to see more mud (100% porosity) around the borehole, hence the tool reads too high in larger hole sizes.

The chart is entered with the porosity;

Go down to hole size.Follow trend lines to 7 7/8".Read of .

A correction is made automatically in open hole using caliper measurements from the combined density tool.It can be made using the bit size if a caliper is not available.The correction can be large.

1111


Mud Cake Correction

The mud cake absorbs neutrons before they can enter or leave the formation.mud cake = stand-off with porosity <100%.The larger the mud cake, the larger the correction.It is a small correction but one that is rarely ever applied because the mud cake cannot be easily measured.

1212


Borehole Salinity Correction

This arises due to Chlorine.The more Chlorine present, the more neutrons absorbed in the borehole. ==> decrease count rate.The largest effect is seen in salt-saturated muds.

Go down to the borehole salinity.Follow trend lines to zero.Read .

1313


Mud Weight Correction

The extra material in heavier muds means there is less hydrogen, hence more neutrons reach the formation.It also changes if the mud is full of barite.

In this case the amount of material needed to achieve the same mud weight is less, hence the correction is less.

Select normal or barite mud.Enter with porosity.Go down to mud weight.Follow lines to 8 lb/gal.Read .

The correction is quite small.

1414


Formation Temperature Correction

The correction is large and depends on the porosity.This is a dual effect:

The expansion of the water reduces the quantity of Hydrogen seen by the tool.Change in the borehole fluid capture cross-section.

Enter with porosity at the top.Go down to hole temperature.Follow trend lines to 75ÞF.Read .

1515


Pressure Correction

The effect is caused by the compression of the fluids downhole.In standard water-based muds the effect is small.

Select oil-based or water-based mud.Enter with porosity at the top.Go down to hole pressure.Follow trend lines to zero.Read .

In oil-based muds the correction is large.

1616


Formation/Salinity Correction

There are two factors affecting the neutron measurement in the formation:

The chlorine in the formation water.The rock matrix capture cross-section.

The simplest method is to assume that the matrix is clean and that the matrix '' known.This leaves salinity (mud filtrate) as the only "variable".

The complete solution is to measure the total formation '' and use this to compute the correction.The correction can be large but is not applied in the field because the lithology is unknown, hence the '' unknown.It is taken into account in the interpretation phase.

1717


Stand off Correction

Any space between the tool and the borehole wall is seen as 100% porosity.The value of the correction depends on the hole size:Larger holes = more correctionStand-off is rarely measured. One method is to use the SA curve recorded with a PCD.

The chart is entered with the porosity at the top;

Go to the nearest hole size.Go down to the stand-off value, e.g. 0.5".Follow the lines to zero.Read the (always negative).

1818


Standoff Correction Chart

1919


Thermal Neutron Parameters

Vertical resolution:Standard (TNPH) 24"Enhanced 12"

Depth of investigation 9"-12"

Readings in zero porosity:

Limestone (0%) 0Sandstone (0%) -2.00Dolomite (0%) 1.00Anhydrite -2.00Salt -3.00

Typical Readings

Shale 30-45Coal 50+

2020


Thermal Neutron Interpretation/Uses

The tool measures hydrogen index.

Its prime use is to measure porosity.

Combined with the bulk density, it gives the best possible answer for lithology and porosity interpretation.

2121


Thermal Neutron in Cased Hole

The CNT can be run in cased hole for the porosity.

In addition to the standard corrections some others are needed to take into account the extra elements of casing and cement.

The standard conditions are:

83/4" borehole diameter.Casing thickness 0.304".Cement thickness 1.62".Fresh water in the borehole / formation.No stand-off.75ÞF.Atmospheric pressure.Tool eccentred in the hole.

2222


Corrections in Cased Hole

2323


Bulk Density Measurement

© Schlumberger 1999

A

2424


Gamma Ray Physics -density -1

The Density Tools use a chemical gamma ray source and two or three gamma ray detectors.

The number of gamma rays returning to the detector depends on the number of electrons present, the electron density, e.The electron density can be related to the bulk density of the minerals by a simple equation.

e = ( 2Z/A )

Where Z is the number of electrons per atomand A is the atomic weight.

2525


Gamma Ray Physics -density 2

The assumption made in the interpretation is that:

Z/A = 0.5

This is very close for most elements commonly encountered, except hydrogen which has little effect on the measurement. Therefore e =

Element Z/AH 0.9921C 0.4996O 0.5Na 0.4785Mg 0.4934Al 0.4819Si 0.4984S 0.4989Cl 0.4794K 0.4860Ca 0.499

2626


Calibration

The tool measured density, b, has been experimentally related to the electron density;

b = 1.0704 e - 0.1883

The tool needs to be calibrated in a known condition.This condition is fresh water and limestone, densities, 1.00 and 2.71 respectively.

The bulk density versus the electron density equation fits for all the common minerals with a few exceptions:Salt - true density 2.165

density tool value 2.03

Sylvite - true density 1.984density tool value 1.862

2727


Spine and Ribs

The spine represents the line of increasing formation density on the plot of the long spacing count rate versus short spacing count rate.

The presence of mud cake causes a deviation from the line in a predictable manner. Thus a correction can be made to obtain the true density.

.

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

Mud cakewith barite

Mud cakewithoutbarite

IncreasingMud cakeThickness


A

B

C

Long

Spa

cing

Cou

nt R

ate

Short spacing Count Rate

2828


Spine and Ribs

Example:The correct reading is at point A.An increasing mud cake thickness moves the point to B or C depending on whether there is heavy material (barite) in the mud or not.

.

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

Mud cakewith barite

Mud cakewithoutbarite



A

B

C

Long

Spa

cing

Cou

nt R

ate

Short spacing Count Rate

2929


Density Outputs

The outputs are:RHOZ/RHOB (b), the corrected bulk density.

DRHO (), the correction that has beenapplied to b (LDT only).

RHOZ/RHOB is the main output;

DRHO is a quality control curve (LDT only).

3030


Borehole Effects

The LDT is a pad tool with collimated source and detectors. It experiences little or no environmental effect.

In large holes, the curvature of the pad versus that of the hole causes a minor error that needs to be corrected.

3131


Borehole Effects

Hole rugosity may affect the measurement.

The source and detectors "see" different formations/borehole.

The effect is an erratic and incorrect log.

3232


Alpha Processing

As the density tool also uses two detectors it can be Alpha processed in exactly the same way as the CNT.

The resulting log shows a great improvement over the standard output.

3333


Density Parameters

Vertical resolution:

Standard 18"Enhanced 6"

Depth of investigation 6"-9"

Readings in:

Limestone (0pu) 2.71Sandstone (0pu) 2.65Dolomite (0pu) 2.85Anhydrite 2.98Salt 2.03Shale 2.2-2.7Coal 1.5

3434


Interpretation/Uses

The density tool is extremely useful as it has high accuracy and exhibits small borehole effects.

Major uses include:Porosity.

Lithology (in combination with the neutron tool).

Mechanical properties (in combination with the sonic tool).

Acoustic properties (in combination with the sonic tool).

Gas identification (in combination with the neutron tool).

3535


Density Porosity

There are two inputs into the porosity equation: the matrix density and the fluid density.

The fluid density is that of the mud filtrate.

1mafb

fma

bma

3636


Scaling/PorosityThe density tool is usually run with the neutron.To aid quicklook interpretation they are run on "compatible scales".This means that the scales are set such that for a given lithology the curves overlay.The standard scale is the "limestone compatible" where the neutron porosity scale is:

To fit this the density log has to have its zero limestone point (2.7 g/cc) on the same position as the neutron porosity zero and the range of the scale has to fit the neutrons 60 porosity units hence the scale is:

Changing to a sandstone compatible scale would put the zero sandstone density, 2.65, over the neutron porosity zero to give:

3737


Pef Physics

The Photoelectric effect occurs when the incident gamma ray is completely absorbed by the

electron.

It is a low energy effect hence the Photoelectric Absorption index, Pe, is measured using the lowest energy window of the tool.

Pe is related directly to Z, the number of electrons per atom, hence fixed for each element.

Pe = ( Z/A )3.6

Its units are barns/electron.

3838


Pef Theory

Pe can be easily computed for any lithology by summing the elemental contributions.

Measurement is virtually porosity and fluid independent.

Major use is Lithology identification.Another way of using it is express it in volumetric terms as:

U = Pee

This is called the Volumetric photoelectric absorption index.This parameter can then be used in a formula for computing the components of the reservoir.

U = Uf + (1 - ) Uma

3939


Pef Parameters

Vertical resolution:

Standard 4"

Readings in:

Limestone 5.08Sandstone 1.81Dolomite 3.14Shale 1.8-6Anhydrite 5.05Salt 4.65

4040


4141


Crossplot Porosity Calculations

4242


Interpretation Challenge

4343


Gas Zone Calculations

4444


N/D Response on LS Matrix

Documents

4 - Neutron Density