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Response of a Multidetector Pulsed Neutron Porosity ToolH. D. Scott, P. D. Wraight, J. L. Thornton, J-R. Olesen,

R. C. Hertzog, D. C. McKeon, T. DasGupta, I. J. Albertin

Schlumberger Wireline & TestingHouston, Texas

ABSTRACTThe development of a neutron porosity tool using apulsed source of 14-MeV neutrons and multiple detectorspacings provides significant improvement in thedetermination of formation hydrogen index. Optimizingthe source-to-detector spacing has substantially reducedthe unwanted effects of increased formation atom densityexhibited by clay minerals in shaly formations, and hasalso reduced the lithology effect.

The detector system has five neutron detectors: fourepithermal and one thermal. By operating the source ina pulsed mode, the detector system can measure bothneutron count rates and neutron arrival times. To reduceborehole effects, the detectors are backshielded and thetool is run eccentered in the borehole.

The epithermal neutron porosity measurement iscorrected in real time for tool standoff from the boreholewall by combining count rate ratios with the epithermalneutron slowing-down time measurement. The use ofepithermal neutron detection removes the influence ofthermal neutron absorbers commonly encountered inshaly formations. In addition, fluid salinity andtemperature effects are significantly reduced.

The improved vertical resolution of the measurementplus the lower sensitivity to clay makes it easier toidentify and evaluate thin beds.

The single thermal neutron detector is used tomeasure the capture cross section (sigma) of theformation close to the borehole. This extrameasurement has good bed resolution and is valuable forformation evaluation and gas indication.

The response of the tool to a wide range offormation and borehole parameters has been determinedusing a combination of laboratory measurements andMonte Carlo modeling. In carbonate formations, theresponses to limestone and dolomite are almostidentical.

The use of an electronically controlled pulsedneutron source eliminates the need for the conventionalradioactive AmBe neutron source for this type ofmeasurement, improving radiation safety.

This technology has been found particularly usefulfor gas exploration in shaly formations.

INTRODUCTIONNeutron porosity logging plays an important role in theevaluation of newly drilled wells when used incombination with resistivity and density logging. Formany years, this triple combination has been theindustry standard for quantifying new oil and gasreserves. Evaluations have been improved recently as aresult of better resistivity tool designs, but the basicdesign of commercial neutron tools used for openholelogging has remained unchanged for many years.Conventional neutron tools of this type typically have acontinuously emitting source of neutrons and use eitherone or two neutron detectors. The performance of suchdevices has been summarized by several authors (Algeret al., 1972; Arnold and Smith Jr, 1981; Smith, 1986;Tittman, 1986; Ellis, 1987; Galford et al., 1988;Mickael and Gilchrist, 1993).

An improved neutron porosity measurement hasbeen designed using an electronically controlledminiature (minitron) neutron source which reducesradiation hazards. The basic tool design, known as theAccelerator Porosity Sonde (APS), was described in anearlier paper (Flanagan et al., 1991). Since that time,several innovations have been implemented.

The APS measurement has the following advantagesover conventional compensated neutron porosity logs:

• The porosity response is affected primarily by thehydrogen index of the formation and is relativelyinsensitive to changes in formation atom density.

• The vertical resolution of the measurement isimproved.

• In carbonates, the response to limestone anddolomite is almost identical.

• Combining appropriately spaced measurementsallows gas detection without the use of other logs.

• Environmental effects are reduced.

• An epithermal neutron slowing-down timemeasurement is provided.

• A thermal neutron capture cross-sectionmeasurement (sigma) is also provided.

This paper gives a detailed description of the tool,results of Monte Carlo modeling of the tool response tovarious conditions, results of laboratory measurements,

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and a log example showing the present toolperformance.

APS SONDE DESIGNThe APS sonde is an important component of the newmodular IPL* Integrated Porosity Lithology nuclearmeasurement tool string. This string consists of asingle electronic cartridge to control and process all thenuclear measurements performed by three sondes: theLitho-Density* Sonde (LDS), the APS sonde, and theHostile Environment Natural Gamma Ray Sonde(HNGS). The arrangement of the tool string is shownin Figure 1. The APS sonde has a diameter of 3.625in., a temperature and pressure rating of 175°C and 20kpsi, and an eccentralizing device.

The major components of the APS sonde are shownin Figure 2. There are two fundamental differencesbetween the electronic source of the APS sonde and thechemical source of previous generation dual-detectorcompensated neutron tools:

• the neutrons have 3 times the energy (14 MeVinstead of an average of 4.5 MeV)

• eight times as many neutrons are emitted.

The lower response to porosity because of the higherenergy of the neutrons is more than offset by the abilityto pulse the electronic neutron source at differentfrequencies.

The higher source output allows epithermal neutrondetection to be used instead of thermal neutron detectionwithout increasing statistical variations. This allowsmeasurements which are free from the effects ofunknown thermal neutron absorbers in the formationand borehole regions.

Neutron DetectorsTo improve the formation-to-borehole signal ratio, thehelium-3 neutron detectors are eccentered, shielded fromthe borehole, and focused toward the formation. For theporosity measurement, the detector section uses fourepithermal neutron detectors placed at three differentspacings from the source. A single thermal neutrondetector is also used.

Below the source is the near epithermal detector(Fig. 2) which is centered in the tool and has muchlower porosity sensitivity than the other detectors. Thisdetector provides normalization of the neutron sourceoutput and partial compensation of environmentaleffects.

_______________* Mark of Schlumberger

The two array epithermal detectors are backshieldedto achieve focusing and to minimize borehole effects.The two detectors used together provide improved countrate statistics and redundancy. Measurements includethe average count rates and time distributions of theneutrons detected in each array detector. The ratio ofnear/array epithermal count rates is the principalporosity measurement provided by the APS sonde. Thetime distributions are used to measure the epithermalneutron slowing-down time.

The single thermal neutron detector provides thermalneutron decay time measurements. It is different from aconventional pulsed neutron capture (PNC)measurement since it detects neutrons rather thangamma rays and is shielded from the borehole. As aresult, this shallow device measures the formationcapture cross-section of the invaded zone. The detectorbackshielding allows the range of formation sigmameasurements to extend over a wider range thanconventional PNC devices. This is a useful shalinessindicator with a vertical resolution better than thegamma ray log.

The far detector, which is also eccentered andbackshielded, provides the longest spacingmeasurement. Its source-to-detector spacing is similarto that of a compensated neutron device. In terms ofsensitivity to formation atom density and gas effect,porosity computed from the ratio of the near-to-farcount rates shows characteristics similar to the CNL*

Compensated Neutron Log measurements.

Tool Operational CheckThe APS sonde uses a novel method to check the toolis operational. Since the neutron source is not switchedon until the tool is well below the surface, a method isneeded to confirm that the tool is working correctlybefore placement in the borehole. Previous methodsrequired an auxiliary neutron source at the wellsite.However, a major reason for developing the APS sondewas to completely eliminate the need for neutronsources.

To solve this dilemma, a special design wasdeveloped for the helium-3 neutron detectors. Eachdetector is augmented with a very weak internal alpha-particle source. The alpha sources have extremely lowactivity. All radiation is completely contained withinthe detector since the alpha particles have insignificantpenetrating power. These particles create a lowbackground count rate of ionizing pulses in eachdetector which are counted by the tool electronics. Thiscount rate is compared with that measured during toolcalibration to confirm the tool is operating correctlyprior to logging the well. This procedure eliminates theneed for an auxiliary neutron source and the complete

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logging operation can be performed with enhancedsafety and environmental protection.

Neutron Pulsing SchemeThe pulsed neutron source allows the measurement ofboth total count rates and neutron time distributions.To measure the epithermal neutron slowing-down time,short neutron bursts of 10 µsec are followed bymeasurement periods of 30 µsec. This contrasts withthe sigma measurement with neutron bursts of 100 µsecfollowed by a measurement period of 700 µsec. Afully interleaved pulsing scheme accommodates bothmeasurements simultaneously with a series of 30slowing-down time cycles followed by one sigma cycle.A neutron burst-off period is incorporated to allow themeasurement of the detector background count rates.This permits detector performance to be monitoredcontinuously, even while logging.

MONTE CARLO MODELAn MCNP model of the APS sonde has been developedfor the determination of tool response algorithms. TheMCNP-3A code (Briesmeister, 1986) is a generalpurpose Monte Carlo code for simulating the transportof neutrons and/or gamma rays in complex three-dimensional geometries. The code, developed at LosAlamos National Laboratory in the 1970s, has beenused for modeling the response of well-logging tools forover a decade.

Objectives were to have a model sufficiently accurate fordeveloping tool response algorithms while maintainingthe flexibility to handle a wide range of problems. Theuse of laboratory measurements and a large MCNPdatabase has proved to be a powerful combination. Evenwith the Environmental Effects Calibration Facility(EECF) in Houston, only a limited set of variations inwellbore conditions such as lithology, porosity,salinity, gas saturation, standoff, borehole fluid andmudcake may be practically studied. By combining thislimited range of laboratory measurements with a largerset of MCNP results, a better understanding of toolresponse can be obtained.

The APS MCNP model was designed to computethe count rates in the helium-3 detectors. Since theabsolute source strength is unknown, the model is nottypically used to calculate absolute count rates. Instead,count rate ratios are used to normalize out the sourcestrength.

The model was designed also to simulate theslowing-down time measurement of epithermalneutrons. The count rate time-profiles in the arraydetectors were computed with the code and processed tocompute the slowing-down time. The results were

benchmarked using EECF data for various lithologies,porosities and standoffs. The benchmarked model hasbeen used to predict the slowing-down time response inconditions of heavy mudcake and standoff that could notbe duplicated easily in the laboratory.

Detector PhysicsThe helium-3 detectors in the APS detect neutrons whena neutron is absorbed by a helium-3 nucleus. Thereaction causes a proton to be ejected from the excitednucleus. The recoil nucleus and the proton lose energyin the gas by stripping bound electrons from atoms.The electrons are accelerated to an anode wire by a highelectrical potential. Pulses on the anode wire correspondto neutron interactions within the detector gas.

To accurately model the active volume of thehelium-3 detectors, simulations were made of theelectrostatic potential for the detectors. Only reactionsin the active volume of the detectors were counted.

An additional reaction in a helium-3 detector iselastic scattering. Since the tool electronics eliminatespulses below 100 keV in energy, the code was modifiedto create a special elastic scattering tally to detect elasticscattering events depositing over 100 keV in the heliumgas.

BenchmarkingThe purpose of benchmarking is to prove that the modelcan calculate the tool response in environments thatcannot be easily measured. A comparison of normalizedcount rates involves determining a normalizationconstant so that the MCNP count rates (or ratios) matchthe measured values. In some cases, a normalizationfunction is required to match the MCNP values with themeasured values. The best situation is when a singlenormalization constant can be used for any combinationof wellbore conditions such as hole size, lithology,porosity and salinity. Comparisons of MCNP calculatedratios with EECF measured values have shown thatsingle normalizations for the near/array ratio and thenear/far ratio are sufficient. These normalizationconstants were determined for three lithologies (sand,lime and dolomite) and several different porosities:

Near/Array : RatioAPS = 0.937 × RatioMCNP

Near/Far : RatioAPS = 1.070 × RatioMCNP

Crossplots of the measured ratios versus the normalizedMCNP ratios are shown in Figure 3.

EPITHERMAL NEUTRON RATIOOne of the most important factors to consider whendesigning traditional neutron logging tools is thesource-to-detector spacing. Since the objective is tomeasure neutrons that have traveled through theformation rather than the borehole, the emphasis is touse as large a source-to-detector spacing as possible.

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This is balanced by the need to have good counting rateprecision, thin-bed resolution and a neutron sourcestrength within safe limits. The present dual-spaceddetector designs meet these requirements and, by usingthe ratio of detector count rates, partially compensate forborehole environmental effects.

The measurement principle of neutron porositylogging is based on the fact that hydrogen is veryefficient in the slowing down of fast neutrons. Ameasurement of the spatial distribution of neutronsresulting from the interaction of high energy neutronswith the formation can be related to its hydrogencontent. If the hydrogen, in the form of water orhydrocarbons, is contained within the pore space, themeasurement correlates with porosity. If the formationalso contains clay minerals containing bound hydrogen,the tool responds to the total hydrogen content.Hydrogen index (HI) is defined as the formationhydrogen content relative to that of water at standardconditions. Thus, at standard conditions, the HI ofwater is defined as 1.0.

The transport of neutrons through the formation iscontrolled by the various atomic nuclei present and theirrespective neutron scattering and absorption crosssections (Tittman, 1986; Ellis, 1987). Predictivemodels of compensated neutron log behavior make useof the concept of neutron slowing-down length toepithermal energy (Allen et al., 1967; Edmundson andRaymer, 1979; Scott et al., 1982). This is a measureof the size of the epithermal neutron cloud surroundinga source emitting neutrons into the formation.Although this length is dominated by the amount ofhydrogen present, it is also affected to a lesser extent byall other atoms in the formation.

PorosityThe lower half of Figure 5 shows the epithermalneutron population plotted against source-to-detectorspacing for three porosities with constant matrixdensity, such as limestone. The three curves show thedecrease of the neutron population with distance fromthe neutron source, and the relative sensitivity to HI, orformation porosity. At a fixed distance far from thesource, the detectable neutron population and, therefore,the detector count rate decreases when the formationporosity increases. Moving the detector position closerto the source decreases the sensitivity to formationporosity.

Formation Atom DensityBefore describing the effect of atom density on neutronlogging, some new terminology will be defined. Theconvenient term atom density is used in the followingdiscussion instead of the more correct term for neutronscattering, density of atomic nuclei.

For a typical well with a density log run incombination with a neutron log, two measurements areproduced: bulk density and apparent neutron porosity. Ifthe apparent neutron porosity is a correct measure of theformation HI, combining the bulk density allows thededuction of an additional property of the formation.This property has been named the dehydrated formationdensity (g/cm3) and is defined below:

Dehydrated formation density =ρb − HI( )1 − HI( )

(1)

where ρb is formation bulk density and HI is the

formation hydrogen index.

This is the formation matrix density that wouldyield the same bulk density with the formationhydrogen index entirely related to water-filled porosity.Values cover a wide range for minerals typically foundin sedimentary formations. For example, an illite clay,with a bulk density of 2.52 g/cm3 and HI of 0.35corresponds to a dehydrated formation density of 3.34g/cm3. Kaolinite and iron-chlorite, with bulk densitiesof 2.54 and 3.42 g/cm3 respectively, correspond to 3.61and 4.72 g/cm3 dehydrated formation densities. Allclays have dehydrated formation densities well abovesand or lime densities (2.65 or 2.71 g/cm3), for whichthe porosity response of conventional neutron devices iscalibrated.

It can be shown (Fig. 4) that for alumino-silicatedominated clay minerals there is an approximately linearrelationship between dehydrated formation density anddehydrated atom density (atoms/cm3), defined below:

Dehydrated atom density =Nt − 1.5NH( )

1 − HI( ) (2)

where Nt is the formation total atom density and NHis the formation hydrogen atom density. The factor 1.5is used to subtract one oxygen atom for every twohydrogen atoms since the dehydration step removesmolecules of H2O.

From Figure 4, it can be seen that the use ofdehydrated formation density is a convenient way toestimate the effect of atom density. Notable exceptionsto this approximation are anhydrite and mineralscontaining significant quantities of heavy elements,such as iron.

Returning now to Figure 5, the upper half showshow the neutron population depends on the distancefrom the neutron source and the value of the atomdensity when the HI is held constant. At the farposition from the source, the detectable neutronpopulation decreases when the atom density increases.This increase in the number of atoms (nuclei) per unit

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volume causes increased neutron scattering andabsorption, limiting the number of neutrons arriving atthe far detector. Increasing or decreasing the atomdensity has an effect similar to increasing or decreasingthe formation porosity.

Moving the detector closer to the source decreasesthe sensitivity to atom density. By moving the detectorstill closer to the source, the backscattering of theneutrons toward the detector becomes the dominantfactor influencing the count rate. In this region,increasing the atom density causes the detector countrate to increase since the additional scattering maintainsthe neutrons closer to the source. At the crossover zone(at an intermediate source-to-detector spacing), thedetector has negligible count rate sensitivity to changesin the atom density with HI held constant. In otherwords, for formations containing most clay minerals,there is no sensitivity to the dehydrated formationdensity.

The lower part of Figure 5 shows the approximateposition of the APS near-, medium- and far-spaceddetectors. The medium-spaced detector consists of anarray of two small detectors labeled 'array'. The near-spaced detector exhibits little sensitivity to porosity andprovides the means to normalize the neutron sourceyield and partially compensate for environmental effects.The optimized spacing of the medium-spaced detectorminimizes the effect from dehydrated formation densitywhile exhibiting reasonable sensitivity to formationporosity. The far-spaced detector is highly sensitive toporosity with a sensitivity to atom density similar tothat of conventional compensated neutron devices. Thisincludes the excavation effect response to gas since thisis equivalent to a low dehydrated formation densityeffect.

Recognition of the importance of atom density onthe far-spaced detector measurement and inclusion of acloser-spaced detector that minimizes this effect whileretaining HI sensitivity is a step forward in neutronporosity logging tool design.

Modeling ResultsThe Monte Carlo code was used to study in detail theresponse of the APS detectors to changes in porosityand dehydrated formation density. Figure 6 shows theresults of some of these computations. The APSsketch on the left indicates relative detector spacings.The three plots on the right indicate near-, array- and far-detector count rate sensitivities to formation hydrogenindex and to dehydrated formation density for a watersand (solid line), a gas-bearing sand (dotted line) and adense shale (dashed line). Note that these plots usereciprocal count rates on the vertical axis.

Sensitivities to formation HI and to dehydratedformation density behave as illustrated in Figure 5. The

sensitivity to HI increases with detector spacing;smallest in the near detector and greatest in the fardetector. The far detector is most sensitive to dehydratedformation density. With HI constant, count ratedecreases with dehydrated formation density. The neardetector is less sensitive to dehydrated formation densityand shows an increase in count rate with an increase indehydrated formation density. The array detector isinsensitive to dehydrated formation density whileretaining a reasonable sensitivity to formation hydrogenindex.

It should be noted that the far detector, which issimilar to the far detector of conventional compensatedneutron tools, is sensitive to both formation HI anddehydrated formation density. Therefore, using thenear/far ratio retains the traditional sensitivity toporosity and also the excavation effect in gas zones.

The above modeling results were obtained for clasticformation mixtures of quartz sand and clays. Resultsfor carbonates show a small systematic difference fromclastics which appears to be related to the nuclear crosssections for the dominant elements in carbonates, ratherthan to a difference in atom density. This difference isdiscussed later as part of the basic tool response.

Laboratory ResultsMeasurements in laboratory formations were used tocompare the APS near/array and near/far porosityresponses with the CNL response. The results aresummarized in Table 1.

The tool calibrations were confirmed in a 35-p.u.freshwater sandstone formation. An extremely denseshale formation was simulated using aluminum oxidegrains with a grain density of 3.97 g/cm3. Thisformation had a true porosity of 47 ±2 p.u. and a bulkdensity of 2.57 g/cm3 with fresh water in the porespace.

A gas-bearing sand was simulated using a newapproach employing grains of fused quartz with a graindensity of 2.18 g/cm3. A formation porosity of 20 ±1p.u. was obtained using particles of different sizes. Theadvantage of using fused quartz is that it containsexactly the same atoms - silicon and oxygen - as anormal quartz formation but has lower bulk density,similar to a gas sand. When saturated with fresh water,the fused quartz formation had a bulk density of 1.94g/cm3, exactly equivalent, in terms of atom density, to aquartz sand formation with a porosity of 34 p.u. with awater saturation of 58%.

Results shown in Table 1 use the sandstone porosityscale. In the simulated dense shale, the APS near/arrayporosity gave a reading of 51 p.u., only slightly higherthan the true porosity value of 47 ±2 p.u. despite thevery high grain density of 3.97 g/cc. This small

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deviation is caused by a small atom density effectintroduced by the near detector.

In contrast, the APS near/far porosity was 85 p.u.and the CNL porosity was 81 p.u. The high APSnear/far porosity reading shows a strong sensitivity tothe dehydrated formation density effect. This is true forthe CNL reading also, but included in the CNL shift isabout 4 p.u. induced by the presence of neutronabsorbers in the aluminum oxide formation, such asboron. The APS sigma value of 25 c.u. for thisformation indicates an equivalent boron content of 84ppm.

When the CNL tool was first introduced, its thermalneutron response did not reflect the true formation HI inshales. The increase in apparent porosity was attributedto the possible presence of thermal neutron absorbers inclays. Log results with the epithermal CNL tool showthat only about one third of the apparent porosityincrease in shales is due to such absorbers. Sensitivityto the dehydrated formation density is responsible forthe remaining effect.

In the simulated gas-bearing formation, thenear/array response was 19 p.u., very close to thecorrect porosity (HI) of 20 ±1 p.u. The APS near/farand CNL porosities exhibited the classic excavationeffect, both reading the significantly lower value of 14p.u.

These experiments confirm that the near/arrayporosity measurement is practically independent ofdehydrated formation density in alumino-silicateformations, measuring close to the true HI of theformation.

RESPONSE TO POROSITY ANDSIGMAThe basic responses of the APS tool fall into threecategories:

• Integrated count rates from the four epithermaldetectors for porosity determination from countrate ratios

• Neutron counts as a function of time for the twoepithermal array detectors to determine slowing-down time, also for porosity

• Neutron counts as a function of time for thethermal array detector to determine formationcapture cross section (sigma)

Count Rate RatiosFigure 7 shows the results of near/array count rate ratiomeasurements made to establish the response of the toolin freshwater limestone formations. Thesemeasurements were made with the tool eccentered infreshwater boreholes with diameters in the range 7.875

to 8.5 in. The data include measurements in the APIlimestones at the University of Houston (Belknap et al.,1959; API, 1974), the EUROPA formations inAberdeen (Locke and Butler, 1993) and the EECFfacility. For the EUROPA data set, the HI of eachformation was used. This was computed from coreporosity and core analysis of trace bound hydrogen(Locke and Butler, 1993).

EECF and EUROPA measurements in sandstone anddolomite formations show that the limestone anddolomite responses are almost identical, differing byless than 1 p.u., and the difference between sandstoneand limestone is significantly less than that for theCNL measurement (Galford et al., 1988). These resultshave been confirmed with Monte Carlo modeling.Limestone data are shown in Figure 8 for the near/farratio. This ratio measurement has larger sandstone anddolomite effects, relative to limestone, similar inmagnitude to the CNL measurement but different indetail.

A comparison of these lithology effects is shown indetail in Figures 9a and 9b. Here, the magnitude of theporosity correction to obtain the true porosity insandstone or dolomite is shown. Monte Carlo modelingindicates that in terms of total hydrogen content (HI),the near/array ratio response for quartz sand, and quartzsand containing typical clay minerals (shale) fall on thesame line.

Even though the near/array ratio has a smallerlithology effect than the near/far or CNL measurement,it is still possible to obtain good lithologyidentification using the neutron-density cross plot, asshown in Figure 10, since the density log providesexcellent lithology separation.

Slowing-Down TimeThe epithermal neutron slowing-down time (SDT) isdetermined from an analysis of the decay rate of theepithermal neutron population (Mills et al., 1988,1989; Flanagan et al., 1991). Hydrogen is the mostimportant element in this slowing-down process.Figure 11 shows the epithermal neutron timedistributions acquired during 5-min periods in laboratoryformations of zero-, medium-, and high-porosityfreshwater limestone with the tool fully eccentered in an8-in. freshwater borehole. Also shown is the result foran infinite water tank representing 100 p.u.

The total epithermal neutron population decreaseswith increasing porosity. To facilitate comparison ofthe decay rates, the measurements shown in Figure 11were normalized to the count rate immediately after the10-µsec neutron burst in the zero-porosity formation.Sensitivity of the decay rate to formation porosity isexcellent at low to medium porosities. At higherporosities, the sensitivity is reduced and counting rates

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become lower. The slowing-down time processing atpresent is a simple exponential fit to the data in thetime window 2 to 13 µsec after the neutron burst.

The response to porosity (HI) in the API, EUROPAand EECF freshwater limestone formations is shown inFigure 12.

SigmaThe APS sigma measurement is a direct measurementof the decay rate of the thermal neutron population.This is different from other thermal neutron decay timetools that measure capture gamma rays produced in theformation by capture of thermal neutrons. The APSsigma measurement has a shallower depth ofinvestigation since the neutrons must return to the toolto be detected.

Typical thermal neutron decay curves have beenpresented previously (Flanagan et al., 1991). At present,sigma is computed by fitting the data to a simplesingle-exponential function over the time period 160 to700 µsec after the 100 µsec neutron burst. The resultsobtained in the well-characterized EUROPA formationsare shown in Figure 13. Two of these limestoneformations are saturated with salt water having asalinity of 200 kppm NaCl, with the same salinity saltwater also present in the borehole. Two similarlimestone formations have a salinity of 100 kppm.These four formations provide a range of sigma valuesfrom 15 to 30 c.u. The measured values in Figure 13are mostly within 1 c.u. of the formation sigma valuescomputed from core and known fluid salinity data.

BED RESOLUTION AND DEPTH OFINVESTIGATIONThe bed resolution of the neutron measurements hasbeen determined empirically by an analysis of the toolporosity and sigma responses in a specially designedtest pit. The formations were made from stackeduniform formation blocks of porosities alternating fromnearly 0 p.u. to 18 to 24 p.u. and thicknesses of 1 ft or3 ft. The high resolution near/array (APLC) andnear/far (FPLC) porosities and sigma (SIGF) responsewere measured along with the LDS density porosity(DPO). The working definition used for thin-bedresponse was the vertical distance required for theneutron measurement to go from 10% to 90% of thetotal change from one equilibrium value to the next asthe tool moved past a step change of the formationporosity. The results are shown in Table 2. Dependingon the actual porosities across the step, the results willvary to some extent. Since the vertical resolution ofthe near/array neutron and the density logs are now morecompatible than previously, and since on the IPL stringboth measurements are focused in the same direction,

formation analysis of thin beds is dramatically improved(Olesen et al., 1994).

The APS neutron radial spatial responses anddepths of investigation were investigated by MCNPmodeling studies (Couet and Watson, 1993). Theworking definition of depth of investigation (DOI) wasdefined as the radial distance that contributes the first90% of the cumulative (or saturated) tool response intothe formation. As with the vertical resolution, the DOIis dependent on the formation porosity. The results ofthe integrated radial porosity sensitivity study for aporosity of 15 p.u. are shown in Table 3. All theneutron measurements have increasing DOI in lowerporosities.

ENVIRONMENTAL CORRECTIONSIN OPENHOLEThe standard reference conditions for the APS laboratorymeasurements are as follows:

• 8-in. borehole diameter

• Freshwater in borehole and formation

• No standoff or mudcake

• 75°F temperature

• Atmospheric pressure

• Tool eccentered in hole

Corrections to the uncorrected log data are required toaccount for the difference between actual loggingconditions and the standard conditions. To the degreethat the inputs to these corrections may be poorlyknown, an important design criterion for any newneutron porosity tool is to minimize the need for suchcorrections. In part this is accomplished by using theratio of count rates to derive the porosity since boreholeconditions that cause each detector count rate to changeby the same factor cancel in the ratio and hence result inno net effect. However, to the extent that one detectorcount rate changes more than the other, a correctionbecomes necessary.

The environmental corrections for the APSnear/array and near/far porosities along with those forthe CNL are compared in Table 4 for a 15-p.u.limestone formation with a typical selection of boreholeenvironmental conditions. A mid-porosity example waschosen both as representative of typical loggingconditions and because mid-porosities tend to be thehardest cases. Since the APS detects epithermalneutrons, corrections for temperature and salinity effectsare reduced significantly compared with CNLcorrections. The APS corrections for borehole size arealso smaller. The APS mud weight corrections arelarger than for CNL corrections in the 8-in. boreholeexample chosen, but in a 16-in. borehole the correctionsfor the near/array and near/far porosities reduce to 0.6

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and 1.0 p.u., respectively. The effect for tool standoffis somewhat larger for the APS than for the CNL tool.However, for APS a technique has been developed thatboth measures the effective tool standoff during loggingand makes the necessary corrections at each depth.Thus, the standoff-corrected APS logs are more accuratethan CNL logs since for the CNL logs the engineer canonly estimate the average tool standoff over the logginginterval.

To determine the APS environmental corrections thetool response has been measured in roughly 1000different combinations of lithology, porosity, mudweight, borehole size, standoff, and formation andborehole salinity. These measurements andcomputations of the pressure and temperature effect wereused to derive the individual environmental correctionsas well as their principle interdependencies, such as theborehole size and standoff effect dependence on mudweight and salinity. All these corrections are availablein the standard wellsite product. Each of these effects isdiscussed below.

Mud WeightIncreasing the mud weight with additives such as baritedecreases the water volume fraction of the mud.Therefore, the mud hydrogen density (HI) is decreased.This allows more neutron transmission through theborehole fluid, resulting in an increase in count rate atthe detectors. When changing from fresh water to 12lbm/gal barite mud, the increase is less than 7% for theAPS detectors in an 8-in. borehole with no toolstandoff. Each detector behaves slightly differentlydepending on the amount of backshielding from theborehole fluid. Using detector count rate ratios tocompute apparent porosity reduces the overall effect ofmud weight.

Measurements have been made in laboratoryformations using 12- and 16-lbm/gal freshwater baritemuds and in a 15 lbm/gal oil-base mud. The HI of thesemuds is 0.87, 0.73 and 0.69, respectively. Thecorrections required for both the near/array and near/farratio porosities in an 8-in. borehole are shown in Figure14 for the freshwater barite mud case. The results foroil-base muds are similar, although reduced somewhatfrom those for a freshwater barite mud with the sameHI.

The sign of the APS mud weight correction isopposite that for the CNL correction due to thebackshielding of APS array and far detectors. Since theAPS near detector is centered and only partiallybackshielded, its count rate increases most as the mudweight is increased. This results in a net positive mudweight effect. For CNL measurements, the far detectorcount rate increases most, resulting in a mud weightcorrection of the opposite sign.

Borehole SizeThe borehole size effect has been determined withboreholes varying from 6 to 16 in. diameter and withborehole fluids of water and each of the three mudsdescribed above. The effect for the near/array porosityin water and in a 16 lbm/gal freshwater barite mud areshown in Figure 15. Figure 16 shows the same effectfor the near/far porosity. Since the array and fardetectors are backshielded and focused toward theformation, the borehole size effect is controlled by thecrescent of borehole fluid between the tool face and theborehole wall. Therefore, bit size rather than the caliperlog is used to characterize the curvature of the boreholewall opposite the tool and to compute the boreholecorrections. As expected, the uncorrected porosityincreases with borehole size as the crescent gets largerand the tool sees more borehole fluid. As can be seenfrom the figures, the borehole size effect decreases withmud weight as heavier muds contain less hydrogen.

StandoffThe tool standoff effect for the APS is larger than thatfor the CNL tool since the APS is a focused device.The unfocused CNL tool, even though it is usually runeccentered, does receive some signal from the formationbehind the tool when the tool stands off from theborehole wall. However, an estimate of the actual toolstandoff during logging is not easy to determine.

A series of APS measurements have been made withstandoffs from 0 to 2 in. in laboratory boreholesranging from 6 to 16 in. filled with water or any of thethree muds described above. Since the effect is largerthan that for CNL measurements, an independentmeasurement of the effective tool standoff has beendeveloped using the epithermal neutron slowing-downtime measurement.

The response of the slowing-down timemeasurement to formation porosity was shownpreviously in Figure 11. Since this measurement has ashallow depth of investigation, it is also sensitive totool standoff. Figure 17 illustrates the effect of standoffon the neutron time distributions with the tool in a 0-p.u. limestone formation and an 8-in. borehole as thestandoff is progressively increased. Each timedistribution consists of a slow formation decaycomponent, more visible at later decay times, and anearly borehole component whose amplitude increaseswith increasing standoff. The single exponential fit tothe data in the time window shown, discussedpreviously, provides a good formation porositymeasurement when there is no tool standoff. With toolstandoff, the result is a good standoff indicator. For thisapplication the slowing-down time apparent porosityhas been named standoff porosity.

-9-

In perfect borehole conditions, there is very littledifference between the porosity values derived fromslowing-down time and the near/array ratio. Bothmeasurements respond primarily to the formation HI,and both are insensitive to thermal neutron absorbersand the dehydrated formation density. They differslightly in the sensitivity to lithology but mostly inthe sensitivity to tool standoff.

Figure 18 is a crossplot of the two porosities, withthe uncorrected near/array ratio porosity on thehorizontal axis and the slowing-down time apparentporosity on the vertical axis. Data shown are for threelaboratory formations of 0, 17 and 44 p.u. Without toolstandoff, both porosity measurements agree. Withincreased tool standoff, both apparent porosities increasebut the increase is several times faster for the standoffporosity than for the ratio porosity. Crossplottingthese two independent measurements provides formationporosity and tool standoff.

To accomplish the above standoff correctionprocedure during logging, an algorithm has beendeveloped to correct for standoff using the differencebetween the uncorrected near/array porosity and theslowing-down time porosity, the bit size and the mudweight. The effect in water was shown in Figure 18.As the mud weight increases, both the near/array and theslowing-down time porosities increase less withstandoff since the mud HI is less and the overall effect isas shown in Figure 19 for a 16-lbm/gal mud. A similarstandoff correction is made for the near/far ratio porosityusing the standoff information derived from the arraydetector.

Sal inityWith previous generation dual-spaced thermal neutronporosity tools, corrections for salinity are a complexcombination of hydrogen displacement by NaCl andthermal neutron absorption by the chlorine. For theepithermal APS measurement, the correction issimplified to that for hydrogen displacement only. Thecorrection for saltwater in the borehole is computed inthe same fashion as the mud weight correction for alight mud. The correction for formation saltwater is asimple HI correction computed knowing the salinity,temperature, pressure and water saturation.

A series of APS measurements for a range ofsaltwater salinities in the borehole and formation hasbeen acquired to characterize the effect on apparentneutron porosity and sigma. The results for formationsaltwater in Figure 20 clearly show the effect ofdecreasing hydrogen index with increasing watersalinity. Relative to fresh water with a HI of 1.0,saltwater with a salinity of 200-kppm NaCl has ahydrogen index of 0.92.

Temperature and PressureThe choice of epithermal neutron detection for the APSsimplifies the needed corrections for temperature andpressure to a single correction for borehole HI. A majorbenefit is the much smaller temperature correction,about a factor of seven less than for thermal neutronsystems. Dual-spaced thermal neutron porosity toolshave a very large temperature correction resultingmainly from the choice of using thermal neutrondetection. As shown in Table 4, corrections of the orderof 8 p.u. are often required (Galford et al, 1988; Mickaeland Gilchrist, 1993). The offsetting correction forincreased pressure with depth is relatively small.

MudcakeThe effect of mudcake thickness is the combined effectof reduced borehole size and tool standoff. The changesto the individual detector count rates and the slowing-down time caused by the presence of barite mudcakehave been computed using the MCNP model. For atypical 1-in. thick mudcake with a HI of 0.42, the toolbehaves in low- and medium- porosity formations as ifit stood off from the formation at a distance of about0.35 in., approximately the product of true thicknessand HI. Thus, the standoff correction processing,described previously, is also effective for correcting forthe presence of mudcake.

RESPONSE TO MINERALSAs shown previously from both modeling andexperimental data, the APS near/array ratiomeasurement provides a result which is very close tothe true HI for typical reservoir formations. To developa formation evaluation procedure for complexlithologies, it is necessary to know the specificresponse to individual hydrogenous minerals in theformation. The Monte Carlo model has been used forthis purpose.

Minerals Containing HydrogenSince minerals containing hydrogen, such as clays,commonly exist as a component of the formationmixture, modeling was done for a volumetric mixture of50% of the mineral with 50% of quartz sand having aporosity of 30 p.u. The results obtained for several clayminerals are shown in Table 5, together with results formuscovite and iron chlorite. For this study, clayscontaining clay bound and interlayer water were used tosimulate typical formation conditions. Similar resultsusing dry clay formation mixtures have been reportedseparately (La Vigne et al., 1994).

Listed in the table for each mineral mixture are theHI, bulk density and dehydrated formation density fromEq. (1). It can be seen that for all these minerals, thedehydrated formation density is substantially greaterthan that for quartz sand (2.65 g/cm3). The modeling

-10-

results are the apparent neutron porosity on thesandstone scale for near/array ratio (APSC) and near/farratio (FPSC).

Results show that for the five clay minerals listed,APSC is very close to the true HI of the mixture. Thisis also true for muscovite. The agreement for ironchlorite is not as good but is acceptable considering thevery high dehydrated formation density and the fact thatin nature a 50% volume of iron chlorite is unlikely.Typical formation volumes of iron chlorite are close to5%. These results indicate that quantitative evaluationof the APS near/array apparent neutron porosity log(APSC) should be possible using the mineral HI for theinterpretation.

The results for the near/far ratio porosity showsignificantly increased values caused by the high valuesof dehydrated formation density.

HaliteThe APS near/array ratio response has been optimizedfor the typical formation conditions found in oil and gasreservoirs: sandstones, limestones and dolomites withvarying amounts of clays and some fraction of heavyminerals. One particular formation type that does notfit into this framework is halite (NaCl). Halite oftenexists as a bed of 100% pure mineral with a density of2.165 g/cm3. Modeling results for the apparent neutronporosity on a limestone scale (APLC) gave a value of21 ±2 p.u., in agreement with well log values of 21 to24 p.u.. The correctness of the model was confirmed,also, using a laboratory dry halite formation. Thereason for the high apparent porosity in halite is thevery low atom density and the large neutron slowing-down length (43.5 cm.) that exceeds the source to arraydetector spacing.

Modeling of the near/far ratio apparent porositygave a value of 0.5 ±0.1 p.u., close to the correct HIvalue of zero. This was in good agreement with a valueof 0.3 p.u. measured in the laboratory formation andwell log values close to zero.

AnhydriteModeling results for anhydrite with a bulk density of2.96 g/cm3 gave a value of 1.8 ±0.5 p.u. for thenear/array ratio porosity (APLC). Field tests indicate anaverage value of 1.5 p.u. For the near/far ratio,modeling gave a value of 0 ±0.2 p.u., also in agreementwith field tests.

EXAMPLEA typical APS log run in the Amoco Catoosa field,Rogers County, Oklahoma, is shown in Figure 21.The section shown is a series of shales and shaly sandsover the bottom interval leading to an almost clean sandat the top of the sequence. The thin zone from 617 ft to

609 ft is an isolated limestone bed indicated by the highvalue of PEF and the lowest GR and sigma readings inthis section.

In this relatively shallow well, the APS was run at aspeed of 800 ft/hr to accommodate other tools on thestring. The standard logging speed is 1800 ft/hr, and900 ft/hr if the high-resolution sampling mode isrequired. Shown with the APS log are companion logsfrom the IPL tool string: the LDS density andphotoelectric factor logs and the HNGS uranium-freeGR. Also shown is the CNL log from a separate passrun at 900 ft/hr. The APS logs shown are thefollowing:

• APS epithermal array ratio porosity corrected forborehole conditions and tool standoff presented ona sandstone scale (APSC)

• APS epithermal far ratio porosity, also on asandstone scale (FPSC)

• Sigma formation (SIGF)

• APS computed standoff (STOF)

The primary porosity measurement is APSC.SIGF usually correlates with GR as a shalinessindicator but has added value in terms of thinner bedresolution and lithology evaluation.

The figure shows two interesting shale sectionsfrom 730 to 685 ft and 668 to 655 ft. Both sectionshave a relatively constant bulk density and PEF butshow a trend of decreasing apparent neutron porosity(HI) with decreasing depth. This trend is mirrored in theGR log which also decreases with decreasing depth. Themeasurements are consistent with a gradual decrease inthe clay content across the shale section with decreasingdepth.

The FPSC curve for this well reads 4 to 8 p.u.higher than APSC in the shales, as expected due to theincreased dehydrated formation density. Comparison ofthe two curves across the above two shale sectionsshows a gradual reduction in separation consistent witha reduction in clay content with decreasing depth. In thesand from 600 to 550 ft FPSC exceeds APSC by 3p.u., indicating that the sand has some clay present.This is confirmed by the value of PEF, which reads 2.2units.

The CNL apparent thermal neutron porosity , alsopresented on a sandstone scale, is significantly higherthan the epithermal array porosity (APSC) across thewhole section for two reasons: increased dehydratedformation density effect and thermal neutron absorbers.Increased dehydrated formation density effect is similarto that for FPSC. The thermal neutron absorber effectcan be deduced from the APS formation sigmameasurement, SIGF. Values of SIGF in the shale

-11-

sections range from 26 to 40 c.u. Published correctioncharts for sandstones (Ellis et al., 1987) indicate that inthis sigma range the necessary correction is about -6p.u., consistent with the separation between CNLneutron and FPSC.

The SIGF sigma curve is very similar in overallstructure to the uranium-free GR curve in this well.The improved bed resolution of this curve over GR isvisible at bed boundaries. The computed tool standoff(STOF) reads close to zero over the section indicatinggood eccentralization of the tool string in a good-quality borehole.

Other ExamplesThe above example illustrates the use of the APS forimproved formation evaluation in shaly sand reservoirs.Several other examples, including improved evaluationfor gas, are available and the subject of anotherpublication (Olesen et al., 1994).

CONCLUSIONSThe development of a new neutron porosity tool using apulsed source of 14-MeV neutrons and multiple neutrondetector spacings provides a significant improvement toformation HI measurements. By optimizing the source-to-detector spacing, the unwanted effects of formationatom density have been substantially reduced,particularly for shaly formations.

The use of epithermal neutron detection removes theinfluence of thermal neutron absorbers commonlyencountered in shaly formations and in formations withsalty brines.

The dynamic standoff measurement and porositycorrection obtained from the slowing-down time dataimprove the neutron porosity behavior in moderatelyrugose boreholes.

The improved vertical resolution of the APSmeasurement plus the lower sensitivity to clay makes iteasier to identify and evaluate thin beds.

The inclusion of a thermal neutron detector tomeasure formation capture cross section (sigma) hasprovided benefits of improved bed resolution and anindependent indicator of gas in the region close to theborehole.

Laboratory measurements and Monte Carlomodeling show that the APS carbonate response is onlyslightly influenced by the degree of dolomitization inthe formation. In carbonates with unknown dolomitecontent the APS provides a more accurate porositydetermination.

Finally, the use of an electronically controlledpulsed neutron source eliminates the need for theconventional radioactive AmBe neutron source for thistype of measurement, improving radiation safety.

ACKNOWLEDGMENTSWe thank Amoco Production Company for permissionto publish the example from the Catoosa field. Thanksare due to John Locke and Robert Jorro, AEATechnology UK, for assistance in gathering data at theEUROPA facility. We also acknowledge many helpfuldiscussions with our colleagues P. Albats, B. Couet,D. Ellis, C. Flaum, T. Loomis, R. Namjoshi and K.Stephenson.

REFERENCESAlger, R.P., Locke, S., Nagel, W.A., and Sherman, H.,

1972, The dual spacing neutron log-CNL, Journalof Petroleum Technology, v. 24, p. 1073-1083.

Allen, L.S., Tittle, C.W., Mills, W.R., and Caldwell,R.L., 1967, Dual-spaced neutron logging forporosity, Geophysics, v. 32, p. 60-68.

Arnold, D.M., and Smith Jr, H.D., 1981, Experimentaldetermination of environmental corrections for adual-spaced neutron porosity log, paper VV, in22nd Annual Logging Symposium Transactions ofthe Society of Professional Well Log Analysts.

API RP 33 Third Edition, 1974, Recommended practicefor standard calibration and format for nuclear logs,American Petroleum Institute, Dallas, Texas.

Belknap, W.B., Dewan, J.T., Kirkpatrick, C.V., Mott,W.E., Pearson, A.J., and Rabson, W.R., 1959,API calibration facility for nuclear logs, Drillingand Production Practices, API.

Briesmeister, J.F. (ed.), MCNP - A General MonteCarlo Code for Neutron and PhotonTransport,Version 3A, Los Alamos NationalLaboratory report LA-7396-M Rev. 2, 1986.

Couet, B., and Watson, C., 1993, Applications ofMonte Carlo differential neutron sensitivitycalculations to geophysical measurements, NuclearGeophysics, v. 7, no. 2, 215-229.

Edmundson, H., and Raymer, L.L., 1979, Radioactivelogging parameters for common minerals, paperO, in 20th Annual Logging SymposiumTransactions of the Society of Professional WellLog Analysts.

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Ellis, D.V., 1987, Well Logging for Earth Scientists,Elsevier, New York.

Ellis, D.V., Flaum, C., Galford., and Scott, H.D.,1987, The effect of formation absorption on thethermal neutron porosity measurement, SPE16814: Society of Petroleum Engineers, in 62ndAnnual Technical Conference and Exhibition of theSPE, Dallas, Texas.

Ellis, D., Howard, J., Flaum, C., McKeon, D., Scott,H., Serra, O., and Simmons, G., 1988, Minerallogging parameters: Nuclear and acoustic, TheTechnical Review, v. 36, no. 1, 38-53.

Flanagan, W.D., Bramblett, R.L., Galford, J.E.,Hertzog, R.C., Plasek, R.E., and Olesen, J.R.,1991, A new generation nuclear logging service,paper Y, in 32nd Annual Logging SymposiumTransactions of the Society of Professional WellLog Analysts, Midland, Texas.

Galford, J.E., Flaum, C., Gilchrist Jr, W.A., Soran,P.D., and Gardner, J.S., 1988, Improvedenvironmental corrections for compensated neutronlogs, SPE Formation Evaluation, 371-376.

La Vigne, J., Herron, M., and Hertzog, R., 1994,Density-neutron interpretation in shaly sands, in35th Annual Logging Symposium Transactions ofthe Society of Professional Well Log Analysts,Tulsa, Oklahoma.

Locke, J., and Butler, J., 1993, Characterization of rockformations for the improved calibration of nuclearlogging tools, paper R, in 15th EuropeanFormation Evaluation Symposium Transactions ofthe Society of Professional Well Log Analysts,Stavager, Norway.

Mickael, M.W., and Gilchrist Jr, W.A., 1993,Evaluation of environmental corrections ofcompensated neutron instruments using MonteCarlo modelling, paper DDD, in 34th AnnualLogging Symposium Transactions of the Societyof Professional Well Log Analysts, Calgary,Canada.

Mills, W.R., Allen, L.S., and Stromswold, D.C.,1988, Pulsed neutron porosity logging based onepithermal neutron die-away, Nuclear Geophysics,v. 2, no. 2, 81-93.

Mills, W.R., Stromswold, D.C., and Allen, L.S.,1989, Pulsed neutron porosity logging usingepithermal neutron lifetime, The Log Analyst, v.30, no. 3. 119-128.

Olesen, J-R., Flaum, C., and Jacobsen, S., 1994,Wellsite detection of gas reservoirs with advancedwireline logging technology, in 35th AnnualLogging Symposium Transactions of the Societyof Professional Well Log Analysts, Tulsa,Oklahoma.

Scott, H.D., Flaum, C., and Sherman, H., 1982, Dualporosity CNL count rate processing, SPE 11149:Society of Petroleum Engineers, in 57th AnnualTechnical Conference and Exhibition of the SPE,New Orleans, Louisiana.

Smith, M.P., 1986, Calibration, checking, and physicalcorrections for a dual-spaced neutron porosity tool,paper YY, in 27th Annual Logging SymposiumTransactions of the Society of Professional WellLog Analysts, Houston, Texas.

Tittman, J., 1986, Geophysical Well Logging,excerpted from Methods in Experimental Physics,Volume 24: Geophysics, (Academic Press, Inc.,Orlando, Florida).

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ABOUT THE AUTHORS

Hugh Scott is a Scientific Advisor in the Nuclear Department atthe Schlumberger Houston Product Center (HPC) facilities. Hismain interest is the development of new applications for nuclearwell logging measurements. Recently, he has been involved withthe development of the Reservoir Saturation Tool and the WaterFlow Log. He joined Schlumberger-Doll Research, CT., in 1978,and moved to HPC in 1982. He holds a Ph.D. degree in nuclearphysics from the University of Liverpool. He is a member ofSPWLA and SPE and former Associate Editor of The LogAnalyst.

Peter Wraight is currently Manager of the Nuclear Product Line atthe Schlumberger Houston Product Center (HPC) facilities. Hejoined Schlumberger as Field Engineer in 1974 after graduatingfrom the University of Birmingham, U.K., with a Ph.D. inPhysics. He joined Schlumberger's engineering center near Parisin 1976 and was involved in the development of several wirelinetools. In 1982 he transferred to HPC to work on gamma rayspectroscopy tools. In 1986 he transferred to the newly formedlogging while drilling group in Sugar Land, Texas and led thedevelopment of the density-neutron tool. In 1989 he returned toHPC as Manager of the Nuclear Tools Department.

James Thornton is the Product Manager for the Integrated PorosityLithology system. Since joining Schlumberger in 1984 he haswritten software, worked on tool physics, managed the Electricaland Nuclear Tool Software groups in Houston, and served as atechnical advisor for the worldwide MAXIS* software developmenteffort. After starting out as a starving musician in his hometownof Seattle, James eventually awakened to the need to do somethingpractical, and so decided to study elementary particle physics. Heholds a B.S. in Physics from the University of Washington and aPh.D. in Physics from Stanford University.

Jean-Remy Olesen is Marketing Coordinator for the IntegratedPorosity Lithology system. He is based at the SchlumbergerHouston Product Center facilities. Jean-Remy graduated from theFederal Institute of Technology, Lausanne, Switzerland in 1971with a master's degree in electrical engineering. He joinedSchlumberger in 1974. After numerous overseas field assignmentsas a Field Engineer, Staff Engineer and InterpretationDevelopment Engineer, he joined the Houston Engineering groupin 1985. Before his present assignment, he was involved with theCNL and TDT* Thermal Decay Time tool projects. He holdsseveral patents in the field of nuclear logging and is a member ofSPWLA and SPE.

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Russel Hertzog, Technical Consultant at the SchlumbergerHouston Product Center facilities, works on tool design,characterization and new applications for well loggingmeasurements. Before joining Schlumberger, he did research ingamma ray spectroscopy at the University of Liverpool andneutron induced direct reaction spectroscopy at Oxford University.He received a Ph.D. degree in nuclear physics from BrownUniversity.

Donald C. McKeon is a Project Manager in the NuclearDepartment at the Schlumberger Houston Product Centerfacilities. Since joining Schlumberger in 1986, he has worked onMonte Carlo modeling, tool response physics, softwaredevelopment, and project management. Recently, he has beeninvolved with the development of the Water Flow Log, theReservoir Saturation Tool, and the Geochemical ReservoirAnalyzer Tool. He holds a B.S. and M.S. in Nuclear Engineeringfrom the University of Michigan, a Ph.D. in Nuclear Engineeringfrom the University of New Mexico, and a M.B.A. from theUniversity of Houston. He is a member of SPE and ANS.

Toni DasGupta received her Ph.D. degree in particle physics in1990 from the University of Minnesota where she studied protondecay and cosmic ray showers. Following her degree, she workedon modeling the near-earth environment and analyzing solar flaresfor NASA funded satellite experiments. Toni joined Schlumbergerin 1992 as Project Engineer and has been mainly involved inMonte-Carlo modeling of nuclear tools. Her other interests includeacquisition software for downhole logging tools.

Ivanna Albertin is a Senior Development Engineer in the NuclearDepartment at the Schlumberger Houston Product Centerfacilities. She received B.S. and Ph.D. degrees in physics from theUniversity of California at Berkeley and worked two years atColumbia University and Brookhaven National Laboratory, NewYork, as a Postdoctoral Research Associate in particle physics.Since joining Schlumberger in 1990 she has worked in toolphysics and software development.

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Table 1. Apparent Neutron Porosities* for APS and CNL Tools in LaboratoryFormations

Formation APS Sonde CNL Sonde

Near/Array(p.u.)

Near/Far(p.u.)

Sigma(c.u.)

ThermalTNPH(p.u.)

ThermalTNPH

with Sigmacorrection

(p.u.)35 ± 1 p.u. sandGrain density=2.65 g/cm3 35 35 12 34 34

47 ± 2 p.u. aluminum oxideGrain density=3.97 g/cm3 51 85 25** 81 77

20 ± 1p.u. fused quartzGrain density=2.18 g/cm3 19 14 8.5 14 14

* Sandstone scale.** Equivalent boron content of 84 ppm.

Table 2. 10 to 90% Bed Resolution in Inches

Response Typical Resolution (in.)

φNear/Array (APLC) 14

φNear/Far (FPLC) 17

Σform (SIGF) 12

LDS density porosity (DPO) 11

Table 3. Depth of Investigation

Response DOI at 15 p.u. (in.)

φArray-Detector 7

φSDT 5

φNear/Far 9

-16-

Table 4. Environmental Effects Comparison

EnvironmentalEffect

A P Snear/arrayporosity(APLU)(p.u. )

APS near/farporosity(FPLU)(p.u. )

CNL thermalporosity(TNPH)(p.u. )

Borehole Size(4 inch increase)

1.3 1.1 2.4

Mud Weight(16 lb/gal mud)

1.1 1.2 -0.8

Detector Standoff(0.5 inch)

2.8 2.4 1.6

Borehole Salinity(0 to 200 ppk)

0.6 0.5 -1.4

Formation Salinity(0 to 200 ppk)

-0.9 -1.4 4.1

Temperature(250 F increase)

-1.2 -1.2 -8.4

Pressure(15 kpsi increase)

0.7 0.7 1.0

Table 5. APS Response to Minerals Containing Hydrogen

Formation Mixture: Equal Volumes of Mineral and 30-p.u. Fresh Water Sand

MineralBulk

Density ofMixture(g/cm3)

DehydratedFormation

Density(g/cm3)

HydrogenIndex

(x 100)

Near/ArrayPorosity(APSC)(p.u. )

Near/FarPorosity(FPSC)(p.u. )

Muscovite1

2.84 g/cm3, HI = 0.13 2.50 2.92 22 22 25

Fe-Chlorite1

3.42 g/cm3, HI = 0.35 2.79 3.63 32 37 53

Kaolinite2

2.54 g/cm3, HI = 0.41 2.35 3.08 35 38 45

Illite2

2.52 g/cm3, HI = 0.35 2.34 2.97 32 35 41

Montmorillonite.4H2O1

2.22 g/cm3, HI = 0.462.19 2.92 38 38 45

Smectite2

2.02 g/cm3, HI = 0.67 2.09 3.10 48 49 58

Glauconite1

2.85 g/cm3, HI = 0.13 2.50 2.90 21 23 25

1 Ellis et al., 1988.2 Includes clay bound and interlayer water.

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HNGS sonde• Thorium, uranium

and potassiumconcentrations

APS sonde• Epithermal neutron

porosities• Epithermal neutron

slowing-down time• Invaded formation

sigma• Detector standoff

IPLC cartridge• Raw spectral and

time distributiondata transmittedto surface

LDS sonde• Compensated bulk

density• Photoelectric factor• Borehole diameter

Sensors

Electronicneutronsource

Measurements

Near-array ratio• Hydrogen ind• Reduced litho• No thermal n

effects• Reduced env• Improved ver

Epithermal slow• Standoff dete

Thermal neutr• Formation c

of invaded z

Near-far ratio• Lithology ind• Stand-alone

(in clean for

Nearepithermal

detector

Arrayepithermal

Array thermal

Far epithermaldetector

Figure 1. IPL tool string. Figure 2. APS sensor layout.

Sensors

Electronicneutronsource

Nearepithermal

detector

Arrayepithermal

detectors

Array thermaldetector

Far epithermaldetector

-18-

Figure 4. Comparison between dehydrated formation density (Equation 1) and dehy-drated atom density (Equation 2) for alumino-silicate dominated clay minerals. Dataare for the formations listed in Table 1 and the mineral mixtures in Table 5 with the

omission of iron chlorite.

Figure 3. Comparison of APS measurements with normalized Monte Carlo modelingresults for freshwater formations with 8-in. diameter freshwater borehole.

0.2

0.4

0.6

0.8

1

1.2

0.2 0.4 0.6 0.8 1 1.2

Limestone

Sandstone

Dolomite

Water

Norm

aliz

ed M

CN

P N

ear/

Arr

ay

Ratio

Measured Near/Array Ratio

0 pu

15 pu

42 pu

100 pu

0

2

4

6

8

0 2 4 6 8

Limestone

Sandstone

Dolomite

Water

Norm

aliz

ed M

CN

P N

ear/

Far

Ratio

Measured Near/Far Ratio

0 pu

15 pu

42 pu

100 pu

0.0

1.0

2.0

3.0

4.0

0 2 4 6 8 10 12 14

EECF FormationsClay, sand, water mixture

Deh

ydra

ted

Form

atio

n D

ensi

ty (g

/cc)

Dehydrated Atom Density (atoms/cc)X 1022

20 pu fw fused quartz (gas sand)

35 pu fw sand

47 pu aluminum oxide (dense shale)

-19-

Sensors

Electronicneutronsource

Measurements and features

Near-array ratio porosity• Hydrogen index measurement• Reduced lithology effect• No thermal neutron absorber

effects• Reduced environmental effects• Improved vertical resolution

Epithermal slowing-down time• Standoff determination

Thermal neutron decay rate• Formation capture cross section

of invaded zone

Near-far ratio• Lithology indicator• Stand-alone gas indicator

(in clean formations)

Nearepithermal

detector

Arrayepithermal

Array thermal

Far epithermaldetector

Figure 6. APS detector sensitivity to formation hydrogen index and dehydrated forma-tion density. Reciprocal count rates are used.

Figure 5. Epithermal neutron population sensitivity to formation hydrogen index andatom density.

-20-

Figure 7. APS near/array ratio response in limestonecalibration formations.

Figure 8. APS near/far ratio response in limestonecalibration formations.

Figure 9a. APS near/array porosity lithology correc-tion determined in sandstone and dolomite calibrationformations.

Figure 9b. APS near/far porosity lithology correctiondetermined in sandstone and dolomite calibration for-mations.

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50

API

EUROPA

EECF

Nea

r/A

rray

Rat

io

Hydrogen Index (pu)

0

2

4

6

0 10 20 30 40 50

API

EUROPA

EECF

Nea

r/F

ar R

atio

Hydrogen Index (pu)

-21-

Figure 10. APS near/array neutron-density crossplot.

-22-

0

10

20

30

40

0 10 20 30 40

Fresh water boreholeSalt water borehole

Measu

red S

igm

a (

c.u.)

Formation Sigma (c.u.)

Low fSand

Dolo

Water

Figure 12. APS epithermal neutron slowing-downtimes measured in limestone calibration formations.

Figure 13. APS sigma response for the EUROPA for-mation.

Figure 12. APS epithermal neutron slowing-downtimes measured in limestone calibration formations.

2

4

6

8

10

12

0 10 20 30 40 50

API

EUROPA

EECF

Epi

ther

mal

Slo

win

g-D

own

Tim

e (m

icro

sec)

Hydrogen Index (pu)

Figure 11. Effect of formation porosity on epithermalneutron time distributions measured by the array

epithermal detectors.

-23-

Figure 16. APS near/far borehole size effect in waterand 16 lbm/gal barite mud.

Figure 15. APS near/array borehole size effect inwater and 16 lbm/gal barite mud.

Figure 14. APS corrections for freshwater baritemud.

-24-

Figure 17. Effect of tool standoff on epithermal neutron time distributions measured bythe array epithermal detectors.

Figure 18. Method for determining tool standoff andporosity correction by combining near/array and

slowing-down time (SDT) porosities in freshwater.

Figure 19. Method for determining tool standoff andporosity correction by combining near/array andslowing-down time (SDT) porosities in 16 lbm/gal

barite mud.

Figure 20. Response of the APS near/array porosity measurement to changes in forma-tion salinity.

0

50

100

150

200

250

0 10 20 30 40 50

Form

ation

Sali

nity (

kppm

NaC

l)

Apparent Limestone Porosity (pu)

17 pu 44 pu

-25-

2

Figure 21. Comparison of APS, LDS and CNL logs run in Amoco Production Companywell CTF-DM#21, Catoosa field, Rogers County, Oklahoma

Sigma Formation (SIGF)(CU)10 40

APS Computed Standoff (STOF)(IN)-1 4

APS Epithermal Array Porosity Sandstone Corrected (APSC)(PU)60 0

Uranium free GR (HCGR)(GAPI)0 150

LDS Caliper (LCAL)(IN)6 16

LDS Long Spaced Photoelectric Effect(PEFL)

(----)0 10

LDS Bulk Density (RHOM)(G/C3)1.65 2.65

APS Epithermal Far Porosity Sandstone Corrected (FPSC)(PU)60 0

Neutron Porosity (TNPH)(V/V)0.6 0

Time Mark Every 60 S

700

600