7 neutron

Notes

11

Neutron Porosity Measurement

Neutron Porosity

© Schlumberger 1999

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.

22


Lithology and Porosity Tools

All tools react to lithology - usually in conjunctionwith 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 porositywith a small element if lithology.

Notes

Neutrons start as “ fast Neutrons “ and rapidly loose energy passingthrough the epithermal state to reach the thermal range. The process ofslowing down is primarily caused by collision hydrogen atoms. The morehydrogen the fewer neutrons reach the detectors.The final stage is capture by an atom when a “capture” gamma ray isemitted. The oldest tools measured these gamma rays as there were nosmall reliable neutron detectors.

33


Neutrons

Notes

Older neutron tools used gamma ray detectors hence reacted to thecapture. Gamma rays emitted at the “ end “ of the thermal neutrons life.Chlorine as well as hydrogen plays a large part in this process makingthese tools very sensitive to the borehole environments.

44


Early Neutron Tools

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

They were non-directional.

The units of measurement were API units where1000 API units were calibrated to read 19% in awater-filled limestone.

The tool was badly affected by the borehole environment.

Notes

This generation used epithermal detectors a good region as it is largelyunaffected by the borehole. However the chemical neutron sources useddid not generate enough neutrons for a statistically good measurementespecially at higher porosities.The current standard tool uses a chemical source and measures thermalneutrons.The latest tool has again gone back to epithermal neutrons but uses anelectronic source to obtain the quantity of neutrons needed to make anaccurate measurement.

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 NeutronTool (CNT).

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

Notes

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

66


Hydrogen Index

Hydrogen Index is the quantity of hydrogen perunit volume.

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

The Hydrogen Index of gas is a much smallerthan that of water.

In a formation, it is generally the fluids thatcontain hydrogen.

Notes

At the end of the thermal phase the neutrons are captured by variouselements - H, Cl are the principal ones involved. A captive gamma ray isemitted.

77


Thermal Neutron Theory

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

The principal element involved in the slowingdown is Hydrogen, because it is close in size to theneutron which loses most energy in thesecollisions.The CNT measures the neutron population in thethermal region.This is why the tool measures the HydrogenIndex.

Notes

The two detectors of the CNT tool have to be placed far enough awayfrom the source to avoid local borehole effects but close enough to havegood measurement statistics.

It is also useful to have them in a region where the count rate versusporosity relationship is linear.The detectors are set in the “ Long-spacing Region “ where increasingporosity means reduced counts. The zone is also linear.

88


DetectorsTwo neutron detectors are used to produce aratio eliminating some of the borehole effectsexperienced by single detectors.The count rate for each detector is inverselyproportional to porosity with high porosity givinglow count rates.

Notes

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

99


Ratio to Porosity Transform

The count rates are first corrected for the deadtime of the detectors (when the detector is notavailable 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 atransform. (This is a combination of theoreticaland experimental work).

The current field output for the thermal neutronporosity is called TNPH.

Notes

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

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

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

1010


Borehole Effects

The logs have to be corrected for the boreholeenvironment:

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 isusually run in combination with a density tool and the latter has a calipermeasurement.

1111


Hole Size Correction

Necessary because the tools algorithm from ratioto porosity is built to "fit" a 77/8" hole.Larger holes cause the tool to see more mud(100% porosity) around the borehole, hence thetool 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 holeusing caliper measurements from the combineddensity tool.It can be made using the bit size if a caliper is notavailable.The correction can be large.

Notes

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

1212


Stand off Correction

The same explanation applies for this correction.Any space between the tool and the borehole wallis seen as 100% porosity.The value of the correction depends on the holesize:Larger holes = more correctionStand-off is rarely measured. One method is touse 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 actualhole size. Draw a line from the porosity to be corrected (34pu in theexample) 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 betweenthis value and the original value, this is the correction to be applied.

1313


Standoff Correction Chart

Notes

This correction is rarely if ever applied as it is small and the value of themud 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 themud cake and the MSFL caliper which rides on top of the mud cake (anddivide by 2).

1414


Mud Cake Correction

The mud cake absorbs neutrons before they canenter or leave the formation.mud cake = stand-off with porosity <100%.The larger the mud cake, the larger thecorrection.It is a small correction but one that is rarely everapplied because the mud cake cannot be easilymeasured.

Notes

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

1515


Mud Weight Correction

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

In this case the amount of material needed toachieve the same mud weight is less, hence thecorrection 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 apparentporosities, the effect of the Chlorine is more important, simply becausethere is more of it.The borehole salinity can be found from the mud engineer or byconverting the measured Rm into salinity.

1616


Borehole Salinity Correction

This arises due to Chlorine.The more Chlorine present, the more neutrons absorbed in the borehole. ==> decrease countrate.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 chartsto make this correction. However again it is rarely applied as it is socomplex. The major unknown is the matrix capture cross-section, which isknown if the matrix is clean but can be very different if there are someminerals present.

1717


Formation/Salinity Correction

There are two factors affecting the neutronmeasurement in the formation:

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

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

The complete solution is to measure the totalformation 'Σ' and use this to compute thecorrection.The correction can be large but is not applied inthe field because the lithology is unknown, hencethe 'Σ' unknown.It is taken into account in the interpretationphase.

Notes

This is the major correction in most reservoir cases. It is large even atmodest temperatures. It is in the opposite direction to the standoff, anotherlarge correction. However, the former is larger in the deeper part of thewell where the small hole size will minimise the stand off correction. Thestandoff correction will be largest in the larger surface holes where thelower temperature will minimise the temperature correction.

1818


Formation TemperatureCorrection

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

The expansion of the water reduces the quantityof 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 largercompressibility of the oil. However this is, in most cases, a relativelysmall effect as the pressure at the bottom of the well is quite low. Forexample, a 10000 foot well, with a mud weight of 1.2g/cc, will have abottom hole pressure of around 5000psi.

1919


Pressure Correction

The effect is caused by the compression of thefluids 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 thepossibility of using a higher resolution measurement to enhance thestandard log. In the case of the neutron porosity and density tools thathigher resolution is available on the tool itself with the nearer spacingdetector.

The process follows three major steps. The first is to put the twomeasurements at the same depth so that they read the same bit offormation.

2020


Alpha Processing

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

It utilizes the higher resolution of the neardetector to increase the resolution of the moreaccurate far detector.

Notes

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

2121


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 measurementfrom the original reading made by the same detector. The residualinformation is the High resolution data that is required. This can then beadded to the standard reading to give the final high resolutionmeasurement.

2222


Alpha Processing

The difference between the two readings nowgives the "high frequency" information - whichhighlights thin beds missed by the far detector.

Notes

The whole system works if the two detectors are reading the sameformation. If the hole is in bad condition the method will not work. Rapidchanges in the parameter being measured will also cause problems for thistechnique.The final output curve uses the same algorithm as TNPH and hence is agood neutron porosity.

2323


Alpha processing

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

Notes

The depth of investigation of this tool depends on the porosity of thesystem.The tool will only read zero porosity truly in limestone as it is calibratedto this mineral. Other minerals will show a deviation from this value dueto 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.

2424


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 thecalibration is made in this mineral. It is normally combined with thedensity tool when the combination will handle the different minerals.

2525


Thermal NeutronInterpretation/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 porosityinterpretation.

Notes

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

2626


Thermal Neutron in Cased Hole

The CNT can be run in cased hole for theporosity.

In addition to the standard corrections someothers are needed to take into account the extraelements 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.

Notes

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

2727


Corrections in Cased Hole

Notes

This is a complex problem as a trace of these elements suffice to affect the log. The capture cross section of Gadolinium is ~30000 comparedto ~10 for other common borehole minerals. Hence a small percentage has a large effect.for example, 0.1% Gd ahs a capture cross section of 30.

2828


Thermal neutron absorbers

Some elements such as Gadolimium and Boronhave very high Neutron Capture Cross sections

This means that they capture neutronsefficiently.

If they are present in the formation (matrix orfluids) thermal neutron tools will read aporosity that is too high

Notes

The Accelaeator Porosity Sonde is a new type of device, employing an electronic source instead of the older chemical type. This produces moreneutrons an allows the main measurement to be made in the epithermal range. The advantage of this is the vastly reduced environmentalcorrections. The major remaining correction to the measurement is stand-off. One of the secondary measurements, the epithermal slowing downtime is used to compute the stand-off in real time and hence make this correction.The other measurements on the tool are the near-far ratio which allows a lithology determination similar to the thermal neutron measurement. Inaddition the capture cross section of the invaded zone is measured which can help if there are neutron absorbers present.

2929


Accelerator Porosity Sonde

Notes

This log shows an experiment to test the standoff correction. The log was run both centred and eccentred. The stand off measured is in track 1for both passes. The resultant of the stand off correction is in track 2, blue curves. The correction in both case has worked well with both thecurves reading virtually together.

3030


Stand off measurement

7 neutron

Documents

sidewall neutron porosity

porosity toolsall tools

epithermal neutrons

porosity readstoo low

hydrogen index of gas

accelerator porosity

generation tool

current tool