petrophyisct 11 Neutron

Notes

11

Neutron Porosity Measurement

Neutron Porosity

© Schlumberger 1999

Notes

The classification into simple, dirty or complex is arbitrary. In reality the formations lithology is always complex. Odd minerals can also be present, such as micas in sandstones or anhydrites in carbonates which complicate the problem further.Zoning is the process by which the classification is made, the objective is to simplify the problem and make it easy to interpret.

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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

Notes

A more complex method, geochemical logging, identifies 10 elements;K, U, Th, Al, Si, Ca, S, Fe, Gd, TiFrom these the exact mineralogy can be computed.

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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.

Spectroscopy - identifies shale types and special minerals.

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

Notes

Neutrons start as “ fast Neutrons “ and rapidly loose energy passing through the epithermal state to reach the thermal range. The process of slowing down is primarily caused by collision hydrogen atoms. The more hydrogen the fewer neutrons reach the detectors.The final stage is capture by an atom when a “capture” gamma ray is emitted. The oldest tools measured these gamma rays as there were no small reliable neutron detectors.

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Neutrons

Notes

This generation used epithermal detectors a good region as it is largely unaffected by the borehole. However the chemical neutron sources used did not generate enough neutrons for a statistically good measurement especially at higher porosities.The current standard tool uses a chemical source and measures thermal neutrons.The latest tool has again gone back to epithermal neutrons but uses an electronic source to obtain the quantity of neutrons needed to make an accurate measurement.

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Neutron Tools

The best region to measure neutrons is the epithermal as there are fewer borehole effects.However there are also fewer neutrons.

The standard tool is a thermal neutron device using a chemical source.

The Accelerator Porosity Sonde (APS), uses an electronic source to generate a large neutron fluxand measures in the epithermal region. It produces a borehole corrected log.

Notes

At the end of the thermal phase the neutrons are captured by various elements - H, Cl are the principal ones involved. A captive gamma ray is emitted.

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Thermal Neutron Theory

Neutrons are slowed down from their initial "fast" state by collisions with the formation nuclei. At each collision there is some energy lost by the neutron.

The principal element involved in the slowing down is Hydrogen, because it is close in size to the neutron which loses most energy in these collisions.

This is why the tool measures the Hydrogen Index.

Notes

The tools read a hydrogen Index. Fresh water has a value of one while salt is less. (chlorine replaces some of the hydrogen). Gas has a very low value hence the change seen by the neutron tool in a gas zone. Porosity reads too low.

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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.

Notes

The ratio to porosity transform has undergone a number of changes over the years with the earlier versions superceded by more precise relationships. The latest transform is the result of theoretical, experimental and practical work, including extensive Monte Carlo modelling.

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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.

Notes

The thermal neutron tools have to be corrected for several effects of the borehole environment. The effects of the borehole are numerous but well known and characterised. The basic reading can be corrected using charts or field and/or office computers.

The major effects are the mud which is seen as 100% by the tool. This is corrected by the hole size, stand off and mud weight corrections. The chlorine in the mud is corrected by the borehole salinity correction. The effect of temperature and pressure are also important, especially the former.

Traditionally the hole size correction is applied at the time of logging. Modern surface acquisition systems allow the other corrections to be applied as well. The mudcake correction is very small and rarely applied due also to the problem measuring mudcake thickness. The formation salinity correction is not applied as it is taken into account in the crossplot.

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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.

Notes

This correction has always been made in real time as the neutron tool is usually run in combination with a density tool and the latter has a caliper measurement.

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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 ∆φ.

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.

Notes

Stand off is a major correction especially in larger hole sizes. Even in small (8 1/2 ‘) holes the value is around 0.5’, rarely zero. Unfortunately this cannot easily be measured. A fixed number is usually input to the correction.

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Stand off Correction

The same explanation applies for this 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 ∆φ (always negative).

Notes

The chart has a selection of hole sizes. Select the one closest to the actual hole size. Draw a line from the porosity to be corrected (34pu in the example) down to the relevant hole size chart.Enter the stand off on the y-axis to intersect the porosity line.Follow the lines down to the zero. Read the difference in porosity between this value and the original value, this is the correction to be applied.

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Standoff Correction Chart

Notes

This correction is rarely if ever applied as it is small and the value of the mud cake thickness is difficult to obtain as it is of the order of magnitude (0.25”) as the caliper accuracy. Some methods to compute the thickness are :Take the difference between the bit size and the caliper (and divide by 2).Take the difference between the density caliper which cuts through the mud cake and the MSFL caliper which rides on top of the mud cake (and divide by 2).

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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.

Notes

This is one of the few cases where barite mud has less effect on the logging measurement than standard mud.

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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 ∆φ.

The correction is quite small.

Notes

The effect has a notable porosity dependence. At higher apparent porosities, the effect of the Chlorine is more important, simply because there is more of it.The borehole salinity can be found from the mud engineer or by converting the measured Rm into salinity.

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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 ∆φ.

Notes

This can be a very large correction. There are a series of correction charts to make this correction. However again it is rarely applied as it is so complex. The major unknown is the matrix capture cross-section, which is known if the matrix is clean but can be very different if there are some minerals present.

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Formation and SalinityCorrection

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

The chlorine in the water seen by the tool, typically the filtrate.The rock matrix capture cross-section.

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

The complete solution is to measure the total formation 'Σ' 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 'Σ' unknown.It is taken into account in the interpretation phase.

Notes

This is the major correction in most reservoir cases. It is large even at modest temperatures. It is in the opposite direction to the standoff, another large correction. However, the former is larger in the deeper part of the well where the small hole size will minimise the stand off correction. The standoff correction will be largest in the larger surface holes where the lower temperature will minimise the temperature correction.

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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 ∆φ.

Notes

Note the large oil-based mud correction because of the larger compressibility of the oil. However this is, in most cases, a relatively small effect as the pressure at the bottom of the well is quite low. For example, a 10000 foot well, with a mud weight of 1.2g/cc, will have a bottom hole pressure of around 5000psi.

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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 ∆φ.

In oil-based muds the correction is large.

Notes

The idea behind the technique of alpha processing is that there is the possibility of using a higher resolution measurement to enhance the standard log. In the case of the neutron porosity and density tools that higher resolution is available on the tool itself with the nearer spacing detector.The process follows three major steps. The first is to put the two measurements at the same depth so that they read the same bit of formation.

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Alpha Processing

Alpha Processing is a method that enhances the resolution of the standard measurement.

It utilizes the higher resolution of the near detector to increase the resolution of the more accurate far detector.

Notes

The next step is to match the resolution. This is effectively stretching the measurement so that it matches the standard one. This gives the same porosity (in this case) but shifted because of environmental effects.

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Alpha Processing

The first step is to depth-match the two detectors' responses.

The next step is to match the resolution of both detectors.

Notes

The final step is to take the resolution matched near detector measurement from the original reading made by the same detector. The residual information is the High resolution data that is required. This can then be added to the standard reading to give the final high resolution measurement.

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Alpha Processing

The difference between the two readings now gives the "high frequency" information - which highlights thin beds missed by the far detector.

Notes

The whole system works if the two detectors are reading the same formation. If the hole is in bad condition the method will not work. Rapid changes in the parameter being measured will also cause problems for this technique.The final output curve uses the same algorithm as TNPH and hence is a good neutron porosity.

2222


Alpha processing

The "high frequency" information is added to the far detector signal to give the final enhanced log.

Notes

The depth of investigation of this tool depends on the porosity of the system. The tool will only read zero porosity truly in limestone as it is calibrated to this mineral. Other minerals will show a deviation from this value due to the formation salinity effect and the calibration. Shales have a high apparent porosity because of the water (i.i hydrogen) trapped by them. The actual value depends on the clay type.

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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+

Notes

The neutron tool is recorded on a scale of “apparent neutron porosity”. This is equal to the actual porosity only in a clean limestone because the calibration is made in this mineral. It is normally combined with the density tool when the combination will handle the different minerals.

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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.

Notes

The tool is a standard size hence will not run through a normal tubing string.

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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:

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

Notes

These charts are used in the same manner as the open hole set. In addition to these corrections the borehole salinity, mud weight and hole size corrections have to be made.

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Corrections in Cased Hole

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