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slb log interpretation

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  • 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.

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    Neutron Porosity Measurement

    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.

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    Neutron Porosity Measurement

    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

    Neutron Porosity Measurement

    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 Porosity Measurement

    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

    Neutron Porosity Measurement

    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

    Neutron Porosity Measurement

    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

    Neutron Porosity Measurement

    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

    Neutron Porosity Measurement

    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.

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    Neutron Porosity Measurement

    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

    Neutron Porosity Measurement

    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 .

    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.

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    Neutron Porosity Measurement

    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 (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.

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    Neutron Porosity Measurement

    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

    Neutron Porosity Measurement

    Mud Cake Correction

    The mud cake absorbs neutrons before they canenter or leave the formation.mud cake = stand-off with porosity

  • Notes

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

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    Neutron Porosity Measurement

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