1998 Behind Casing With a Modern Pulsed Neutron Tool SPE39872

Society of Petroleum Engineers

SPE 39872

Evaluating and Monitoring Reservoirs Behind Casing With a Modern Pulsed NeutronTool

G. Simpson, SPE and L. Jacobson, SPE, Halliburton Energy Services, F. Salazar, SPE, Halliburton de Mexico, S.A.

Copyright 1998, Society of Petroleum Engineers, Inc.

This paper was prepared for presentation at the 1998 SPE International Petroleum Conferenceheld in Villahermosa, Tabasco, Mexico, 3�5 March 1998.

This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of this paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paperfor commercial purposes without the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restricted to an abstract of not more than 300words. Illustrations may not be copied. The abstract should contain conspicuous acknowledg-ment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836,Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

AbstractPulsed neutron capture logs have long provided a means ofdetermining water saturation and estimating porosity of forma-tions behind casing. Recent technological advances in pulsedneutron logging have resulted not only in improvements insaturation and porosity data, but also in the capability to provideinformation regarding water entry and flow, lithology, and im-proved differentiation of gas-filled zones from tight formations.These advances provide greater accuracy in reservoir evaluationand monitoring as well as improved completion and productiondiagnostics.

Inelastic, capture, borehole, and background spectra are nowutilized to enhance porosity estimates, identify water flow, andhelp determine lithology. Modular tool design allows the pulsedneutron tool to be combined with production logging tools formore reliable production diagnostics. Simple tool modificationspermit quantitative measurements of water velocity and bore-hole oil holdup. The modular design also allows additional de-tectors to be placed in the toolstring for lithology identificationand gravel pack evaluation.

This paper discusses this new pulsed neutron capture tech-nology and gives an overview of the expanded range of pulsedneutron applications. Field examples of waterflow detection,gravel pack evaluation, oil/gas discrimination, and conventionalanalysis are presented from offshore wells in the Gulf of Mexicoand from onshore wells in Mexico and the U.S.

IntroductionThe Thermal Multigate Decay-Lithology (TMD-L* ) tool repre-sents the latest generation of Halliburton�s pulsed neutron capture(PNC) tools. This tool provides all of the capabilities of itspredecessor, plus new measurements and enhanced capabilitiesfor high-quality water-flow logging, improved gas and porositymeasurement, and lithology evaluation, as well as conventionalcased hole formation evaluation.

Technical advancements include: larger diameter detectorsfor higher count rates, increased time gates to record the entireburst cycle from buildup through decay, improved backgroundmeasurements for better environmental corrections, gamma rayspectra recorded over the entire span of the neutron pulse anddecay cycle, and an improved telemetry package.

The following sections describe the tool�s capabilities andtheir application to reservoir evaluation and monitoring.

Tool CapabilitiesThe new tool is shorter and more modular in construction than itspredecessor, and has higher count rates, finer decay time gating,and spectral lithology measurements from the far-spaced detec-tor. The tool is 25.2 ft long (including its telemetry and gamma raysections), with a diameter of 1.69-in. It is rated to 15,000 psi and350° F. The modular construction allows easy reconfigurationfor water-flow measurements. The tool may also be run incombination with other production-logging sensors placed be-neath it. New processing software complements the higher countrates to improve statistical precision in the determination offormation capture cross section.

These features enable the tool to be used for standard cased-hole formation evaluation, qualitative and quantitative water-flowevaluation, gravel-pack evaluation, and lithology evaluation. Italso has the capability to discriminate between gas, oil, and wa-ter; as well as discriminate between tight formations and gas.

* A mark of Halliburton Energy Services, Inc.

2 EVALUATING AND MONITORING RESERVOIRS BEHIND CASING WITH A MODERN PULSED NEUTRON TOOL SPE 39872

Time GatesThe new tool distinguishes itself from its predecessors throughgreatly expanded measurement, processing, and telemetry capa-bilities. Improvements include a 10-fold increase in gates tomeasure gamma ray counts, dynamic gating to optimize the decaymeasurements, and spectral processing of the neutron burst andthe ensuing decay. A detailed description of these features isprovided in Jacobson, et al.1

Spectral Data. A key feature of the new tool is the spectralrecording of the gamma ray interactions in the long-spaceddetector. Fig. 1 shows the timing sequence for the new tool. Fig.2 displays the decay spectra for capture measurement. Four 256-channel spectra are recorded during each burst cycle, as seen inFig. 3:1. The first spectral gate begins 20 microseconds after the start

of the neutron burst and is 60 microseconds wide. It accumu-lates primarily inelastic data.

2. The second spectral gate begins 130 microseconds after thestart of the burst and is 60 microseconds wide. It accumulatesdata associated with the borehole decay.

3. The third spectral gate begins 250 microseconds after timezero and is 1000 microseconds wide. It accumulates datafrom the formation component of the decay process.

4. The fourth spectral gate begins at the start of the backgroundperiod and is 5 milliseconds wide. It measures activationgamma rays as well as background gamma ray activity.

Logging speeds of 5 to 10 ft/min are required to obtain op-timal spectral measurements, rather than the 20 to 30 ft/min re-quired for sigma data.

Lithology IndicatorThe formation spectrum is corrected for the presence of back-ground activation and is used to estimate the silicon and calciumpresent in the formation. These estimates provide a mechanismfor distinguishing gas sands from tight calcareous zones, andeliminate errors caused by activation and radioactive scale.

Lab and field data show that the silicon yield is especiallyuseful in distinguishing sandstone from limestone. It is output tothe log as a lithology indicator. However, when a sand reservoiris saturated with saltwater, a reduction in the silicon responsecan be expected (as compared to the freshwater- or oil-saturatedcase) due to increased neutron absorption by the chlorine in thesaltwater.

The combinations of silicon and calcium curves with thestandard ratio of the near and far capture counts (RTMD) pro-vide the necessary information to distinguish gas from low-po-rosity formations. Fig. 4 shows the difference between sandstoneand limestone detected by the capture spectra.

The log in Fig. 5 demonstrates this capability. This log wasrun in a well in Oklahoma. The tool was run in brine-filled 5 ½-in. casing, set in a 7 7/8-in. borehole. This particular example high-lights the capability of the tool to distinguish between sand and

lime intervals. The silicon-index curve (SI) serves as the lithol-ogy indicator, and clearly distinguishes the sand zones A, B, andE from the lime zones C and D. Note also the shale laminationswithin zone A. Zone F is seen to be a limy-sand.

Distinguishing gas from low porosityGas sands and tight calcareous zones generally contain very littlesaltwater, making differentiation of these zones possible.

A low silicon value will indicate either a porous saltwater-saturated sand or a calcareous zone. A high silicon value indi-cates an oil- or gas-saturated sand.

The ΣFM curve will distinguish a saltwater sand from a cal-careous zone. The RTMD will distinguish oil from gas. Collec-tively, these log curves differentiate tight zones from gas sands.

The example in Fig. 6 is from a well in South Texas. Theoperator encountered drilling problems due to extreme over pres-sures, and was not able to obtain openhole logs below Z350 ft.The well was drilled to a total depth of Z650 ft, and 2 7/8-in.casing was set in a 6 1/8-in. hole. The area was known to be gasproductive, but it was known to have tight intervals as well. Be-cause openhole logs were not obtained, it was decided to run thenew tool one week after drilling, in order to distinguish possiblegas zones from tight zones.

The log was run over the section of the well that had notbeen previously logged. The log was continued over a section ofthe well that had been logged by open-hole tools, so that the datagathered with the new tool could be compared with the previousopenhole data. The log was run at 5 ft/min, to obtain a combina-tion of inelastic data and capture data for analysis. The analysisused the inelastic counts recorded during the neutron burst ingates 3 through 9 from the near and far detectors.

The ratio of the near to far inelastic counts is denoted RIN.Gamma rays produced during the neutron burst associated withinelastic scattering resemble the Compton scattering effect re-lated to gamma-gamma logging for openhole density logs. TheRIN measurement can therefore be plotted with RTMD to dis-tinguish gas from tight- or low-porosity zones in a qualitativeinterpretation.

In Fig. 6, the openhole Density and Neutron porosities areplotted in Track 2, and are shaded red when the two curves crossover each other to indicate gas. Track 3 shows the RTMD curveplotted in blue, with the RIN curve plotted in red. When thecurves cross over, the area between the curves is shaded red.The intrinsic formation sigma (SGIN) is plotted in black acrossTracks 2 and 3.

In this well, the mud filtrate was expected to affect the sigmareadings, because only one week had passed since drilling; there-fore, no quantitative water saturation figures were calculated.Notice that the SGIN curve reads almost the same in each of thesands having similar gamma ray response, except in the zonefrom Y840 to Y980 ft, which shows slightly lower SGIN. Overthis interval, both the openhole Density/Neutron and the RTMD/RIN indicate gas, as seen by the red shading. Notice that the

SPE 39872 G. SIMPSON, L. JACOBSON, AND F. SALAZAR 3

amount of crossover deflection on the RTMD/RIN is greater thanthat of the Density/Neutron crossover.

It was determined that the excessive overpressure causedthe gas to be flushed away from the near- wellbore region duringthe openhole logging operation. It was also determined that thiszone was a higher permeability zone. At Z240 ft, both the Den-sity/Neutron and the RTMD/RIN show a tight zone. Below theopen-hole data, the log showed two tight zones at Z420 and Z520ft; however, there were no noticeable gas zones identified by theRTMD/RIN crossover. Looking at the curves closely, notice thatthe curves move toward each other as a density/neutron log would.After considering the mud weights used to drill this section, itwas determined that these zones were of lower permeability thanthe zones at Y840 ft, and most likely contained gas.

Two zones were tested. The first zone from Z608 to Z628 fttested gas at 350 Mcf/D, and was not considered to be commer-cial. The second zone from Z510 to Z560 ft tested gas at 7.8MMcf/D.

Oil/Gas DiscriminationThis example shows a log from an offshore well from the Gulf ofMexico. The well was drilled in January 1997, and logged withLWD tools. In July 1997, the operator elected to run a pulsedneutron tool to obtain a log for reservoir monitoring. The primarypurpose of the log was to monitor production from a deeperperforated interval.

Fig. 7 shows a gas zone from X120 to X247 ft, as indicatedby the LWD Density/Neutron log in Track 1 of the example. Theoperator was later surprised to discover that the pulsed neutronlog now indicated a fluid contact in a zone previously thought tocontain only gas. The fluid contact is shown in Track 2 of Fig. 7at a depth of X180 ft. This zone was currently being produced inan offset well at a rate of 30 MMcf/D.

Geological maps indicated that, if anything, an oil leg shouldhave moved up into this interval. The response of the formationsigma on the pulsed neutron log over the liquid interval could beattributed to either oil or a mixture of gas and water.

The operator was facing a multi-million dollar dilemma. Ifthe interval contained oil, a workover would be needed to pro-duce it. If the zone actually contained water, the operator wouldleave things as they were, and continue to produce the zone fromthe offset well.

The original log was run at 15 ft/min, which was too fast toobtain suitable spectral data for a detailed analysis of the zone.However, the zone showed two distinct intervals: one containinggas from X120 to X180 ft, and another containing liquid fromX180 to X247 ft. Since these intervals appeared homogeneous,the inelastic and capture spectra were summed across each inter-val to yield spectra showing energy peaks that could be identi-fied for interpretation purposes.

If the liquid interval contained oil, the inelastic spectra shouldshow an increase in carbon over the carbon peak in the gas inter-val around channel 100. If the zone actually contained salt water

and gas, then the capture spectra would indicate an increase inthe chlorine peaks caused by the high salinity water.

The spectra shown in Fig. 7 show increases of chlorine onthe capture spectra, while the inelastic spectra show no hint ofincreased carbon response. The spectral information confirmedthat the liquid interval contained a mixture of gas and water. Thewater-saturation analysis in Track 3 of the figure indicates thatabout 40% gas was left in the liquid interval, as compared to thewater saturation provided by openhole LWD data.

Activation LoggingThough the TMD-L tool typically measures gamma rays emittedduring the neutron burst and subsequent decay, it can alsomeasure activation gamma rays. These activation gamma rays areemitted by long-lived isotopes created by neutron interactionwith the elements during or following the generator�s neutronburst. In activation decay, both the energy distribution and thedecay rate serve as distinct characteristics of the particularactivated element.

Oxygen Activation. Oxygen activation is used to detect waterflow. Oxygen activation occurs when oxygen is irradiated byneutrons having energies in excess of 10 MeV. The followingreaction occurs:

16O + n → 16N* + p ............................................................ (1)

The nitrogen is left in an excited state, and beta-decays backto oxygen with a 7.35-second half-life. The oxygen emits 6.13-MeV gamma rays, which are counted by the tool�s detector.

Silicon Activation. Silicon activation is used in lithologyand gravel-pack evaluation. Silicon in sand may be activated byfast neutrons through the following reaction:

28Si + n → 28Al* + p ........................................................... (2)

The 28Al* activity dies away with a half-life of 2.24 min-utes, with a 1.78-MeV gamma ray emitted by each decay.

Activation via fast neutrons (such as oxygen and silicon ac-tivation) is fairly shallow, because the neutrons quickly lose en-ergy as they penetrate the surrounding formation. This charac-teristic can be usefully exploited for gravel pack logging, be-cause changes in the relatively distant formation lithology willnot significantly affect the measurement of gravel-pack efficiency.

Thermal Neutron Capture. Likewise, the thermal neutroncapture by aluminum in aluminum oxide (Al2O3) involves a re-action that yields the same end product:

nTh + 27Al → 28Al* + p ...................................................... (3)

Thus, gravel packs composed of either SiO2 or Al2O3 canbe evaluated through this technique.

Optimal Velocity. At very slow logging speeds, the sourcewill activate a region completely, but this activity may decayaway before it can be sensed by the detector. At the other ex-


treme, at very fast logging speeds, a region may be sensed by thedetector before significant decay can occur; however, that re-gion may not have been in the source field long enough to be-come sufficiently activated. Fig. 8 shows a chart of this relation-ship. In this figure, Vopt is the velocity at which optimum re-sponse occurs, and is a function of activation half-life and detec-tor spacing. At Vopt, the response will only be weakly affected bysmall velocity variations. However, significant velocity changes(either fast or slow) will result in lower measured activity. Opti-mal velocity is discussed fully in reference 2.

The following subsections discuss applications for activa-tion logging.

Water-Flow Detection. Movement of water behind casing or inthe cement annulus can often be detected by oxygen activationlogging. Oxygen-activation from flowing water is commonlyobserved in the background spectrum of TMD-L logs. A strongoxygen-activation gamma ray peak usually indicates that water isflowing past the neutron generator, toward the detector.

The ratio of counts in a spectral window spanning the gammaray peak to counts in a spectral window spanning just below thepeak can be used to indicate the distance between the tool andthe water flow, thereby discriminating between inside and out-side flow. Water flowing near the tool will generate spectra withless downscattering, compared to spectra of water flowing out-side of casing.

Oxygen-activation measurements obtained from multiplepasses can often be used to estimate flow velocity. In addition,an accurate identification of the fluid entry point can be made byusing the logging speed to calculate the oxygen-activation de-lay.2

TMD-L Waterflow. The example in Fig. 9 comes from anoffshore well in the Gulf of Mexico. In this application, the newtool was run to monitor production from two producing intervals.The bottom zone was perforated from X722 to X778 ft, and wascompleted with a gravel pack using a 3 ½-in. screen. The upperzone was perforated from X520 to X525 ft, and completed witha gravel pack using a 4 ½-in. screen. Production was assisted withgas lift valves.

During the monitor log operation, the oxygen activationcurve (OAI) indicated water entry into the bottom zone. In Track1 of this example, the Gamma Ray curve (shown in green) de-flects to the right. On logs from older tools, this deflection mighthave been considered an indicator of a water entry point. How-ever, the OAI curve (plotted in blue and shaded in Track 2) showsthat the actual water entry point is 6 ft higher than the gamma raydeflection. The high gamma activity in the bottom 6 ft is causedby radioactive salt deposits on the casing or in the perforations.The ΣBH curve (shown in red in Track 1) decreases at X750 ft toindicate Gas production, and corresponds with a reduction ofSigma Formation (SGFM) in Track 3 to indicate a Gas/Watercontact.

The changes in shape of the OAI curve are due to changesin flow rate of the produced fluids. In this case, these changeswere attributed to water speeding up, and then possibly fallingback due to the gas production.

At X530 ft, the OAI values decrease because of slower wa-ter velocity caused by larger diameter tubing. OAI values alsoshow a small reduction just above the upper set of perforations,possibly resulting from additional water flow from the upperperforations. The OAI values increase again when the fluid fromthe upper perforations exits the top of the screen at X430 ft. TheOAI values decline once more, as a result of a significant veloc-ity increase, when the tool enters a restriction coming into theproduction tubing at X392 ft. The OAI values further decline atX317 ft, where gas entering at a gas lift valve further speeds upthe flow relative to the tool. In this instance, an understanding ofoptimal velocity is needed to reconcile the fact that both fasterand slower velocities lead to a reduction in recorded activity (re-fer to Fig.8). The initial flow was measured near the optimalvelocity, and was then followed by a significant velocity varia-tion (either faster or slower), which resulted in reduced mea-sured activity.

Track 4 shows a plot of the summed background spectraover 30-ft intervals. The displayed spectra clearly show the 6.13-MeV gamma ray peaks from oxygen activation. The bottom spec-tra plotted in the track shows no oxygen-activation peaks, andthus provides a zero reading on the OAI curve.

Quantitative Water Flow Logging. The modular design of thenew tool allows it to be easily reconfigured at the wellsite. Whenwater flow is detected, the tool can be modified to providequantitative measurements of the flow velocity. This new serviceis known as Spectra Flow Logging (SpFL*). The Spectra Flowconfiguration uses a detector package that increases the spacingbetween the source and the detector by 2 ft, and provides spectralrecording capabilities for both detectors. This configurationsimplifies calibration and reduces the effects of stationary oxygenactivation at the detectors. A unique neutron generator pulsingsequence is also employed to optimize the oxygen activationmeasurement.

The water-flow log can be run in different modes, depend-ing upon well conditions:

• Stationary � for very low flow velocities in eitherdirection

• Continuous � for upward water flow• Continuous � for downward water flow• Continuous � against the flow for low velocity flowA forthcoming paper will discuss Spectra Flow logging in

more detail.The example shown in Fig. 10 is taken from an early field

test of the new water-flow logging tool. Fig. 10 shows a simpli-fied wellbore schematic, three of the spectral measurements madewith the water-flow logging tool, and a table of results.

The operator had previously conducted pressure tests thatindicated the presence of either a hole in the casing or a leak in


the packer. Because the well was located offshore, in the Gulf ofMexico, environmental concerns prohibited the use of radioac-tive tracers to locate the problem. It was therefore decided to trythe new water-flow tool, and to use oxygen activation to locatethe problem. The problem was solved by pumping water downthe annulus between the casing and tubing at a rate of ¼ bbl/min,while making stationary measurements with the water-flow toolin the tubing.

The first measurement is at X500 ft, and indicates flow, asseen from the oxygen activation gamma rays at 6.13 MeV on thegamma ray energy spectra in Fig. 10. The next measurement wasat Z500 ft, and indicates no flow, as shown by the absence ofoxygen activation gamma rays on the spectra. These measure-ments showed that the flow was located between the two mea-sure points. By making a series of stationary measures betweenthese points, a hole was finally located at Z396 ft. Measurementstaken just 4 ft lower indicated no flow.

Lithology and Gravel-Pack Evaluation. Sand, which is typi-cally composed of SiO2, can be activated by the TMD-L tool toaid in the determination of lithology or evaluation of gravel packs.

A gamma ray detector below the neutron generator in thetool measures the silicon activation when the tool logs upwardthrough a sand or gravel pack. Because fast neutron reactionsoccur in close proximity to the tool, this process is sensitive tothe SiO2 density near the tool, and provides good sensitivity togravel-pack efficiency.

The thermal-neutron capture by aluminum oxide providesinformation used in the assessment of Al2O3 gravel packs.

Gravel packs are used in wells to control the production offormation sand. A high-quality gravel pack is essential to sus-taining production in wells that would normally tend to produceformation sand.

Conventional gravel-pack evaluations use logging tools thatemploy radioactive sources to measure the density of the pack.These density measurements can become quite inaccurate whenhigh-density completion fluids are used with light gravel packmaterial.

Silicon activation has been used successfully to determinegravel-pack quality without the problems associated with con-ventional density-type tools. The modular design of the TMD-Ltool allows a second gamma ray detector to be placed 9 ft belowthe neutron generator. At a logging speed of 5 to 10 ft/min, thesilicon-activation gamma rays can be measured to provide anaccurate evaluation of gravel pack quality.

The example in Fig. 11 is taken from a deep-water field inthe Gulf of Mexico. The silicon activation log was run to detectmajor voids in the gravel pack. Track 3 of the log shows a plotof the silicon-activation gamma rays, minus a background passto subtract gamma ray counts from iron activation in the casing.The top of the screen is shown at Y180 ft, and the top of the packis clearly shown at Y025 ft. The Depth Track shows a plot of thecomputed gravel pack percent, scaled 0 to 100%. Three majorvoids were detected at Y280 ft, Y300 ft, and Y325 ft. It was

determined later that these voids were associated with blank sec-tions in the tubing-conveyed perforating guns. Minor voids werealso detected in the interval above the top of the screen to nearthe top of the pack. In the absence of a gravel pack, the siliconactivation technique can also be used to measure silicon contentfor lithology determination. Carbonate zones exhibit lower sili-con-activated gamma ray counts relative to sandstone formations.

ConclusionsThis paper has shown how a modern pulsed neutron tool

can use spectral data and activation analysis to provide improve-ments in lithology determination, tight-vs-gas evaluation, gravel-pack evaluation, and formation fluid analysis. This enhancedcapability greatly expands the range of applications availablefor cased-hole reservoir monitoring and evaluation, and extendsthe utility of pulsed neutron tools beyond that of earlier pulsedneutron tools.

Personal AcknowledgmentsThe authors wish to thank Charles R. Conley for graphics

work and Matt Varhaug for editorial assistance. The authors grate-fully acknowledge the management of Halliburton Energy Ser-vices for allowing this paper to be published, as well as the oper-ating companies that granted permission to publish their log ex-amples.

NomenclatureAl = aluminium

FSIN = far spaced inelastic count ratesFTMD = far spaced formation count rates

GI = gas indexGR = gamma ray

NPHI = formation neutron porosityNTMD = near spaced capture count rates

nTh = thermal neutronOAI = oxygen activation indicator

p = protonRHOB = formation bulk density (gm/cc)RTMD = ratio of near to far detector capture count rates

RIN = ratio of near to far detector inelastic count ratesSGIN = intrinsic formation sigma

SGFM = formation sigmaSi = silicon

Sw = water saturationΣBH = borehole sigmaΣFM = formation sigma

References1. Jacobson, L.A., Ethridge, R., and Wyatt, D.F.: �A New Thermal

Multigate Decay � Lithology Tool,� paper M presented at the 1994Annual SPWLA Symposium, Tulsa, OK, U.S.A.

2. Jacobson, L.A., Smith, H.D., and Wyatt, D.F.: �Water Flow De-tection using Modern PNC and PNS Logs,� paper presented at theSeptember, 1996 SPWLA meeting in Beijing, China.


Fig. 1�TMD-L Timing Gates: 61 decay-curve time gates, spanning 1250 ms, are shown in the upper half of the figure.The lower half shows the arrangement of the tool�s three background gates. These background gates are 1-ms, 1-ms,and 3-ms wide, respectively, spanning a total of 5-ms. The time spans for the spectral windows are shown above thetime gates: IN = inelastic spectrum, BH = borehole spectrum, and the formation spectrum are all shown at the upperhalf of the figure, while the background spectrum is shown over the lower half of the figure. NB indicates the time frameof the neutron burst.

Fig. 2�Composite Decay Curve for a high-porosity, oil-saturated sand

CC

H00

010

IN BH

Gates 1-32each 10 µs wide Gates 33-48

each 20 µs wide1250 µs

Gates 49-60each 50 µs wide Gate 61

10 µs wide

NBNB

0.0 0.2 0.4 0.6

Formation Spectrum

Decay Gates

Background Spectrum

Background Gates

0.8 1.0 1.2 1.4time (ms)

time (ms)20 25

SS DECAY SPECTRUM FS DECAY SPECTRUM

SIGB = 94.3ABT = 34,493.0SIGF = 15.6AFT = 12,875.9

Start Depth = 7,710.0Stop Depth = 7,670.0

1000

1000

100

100

10

10

0 200 400 600 800 1000 1200Time (µs)

Cou

nts

Cou

nts

SIGB = 86.5ABT = 7,051.6SIGF = 12.2AFT = 5,652.3

0 200 400 600 800 1000 1200Time (µs)

CC

H00

062


Inelastic SpectrumEnergy (MeV)

20 40 60 80 100 120 140 160 180 200 220 240Channel

Nor

mal

ized

Cou

nt

0.00

0.01

0.02

0.03

0.041 2 43 5 6 7 8

0

CC

H00

065

Borehole SpectrumEnergy (MeV)

20 40 60 80 100 120 140 160 180 200 220 240Channel

Nor

mal

ized

Cou

nt

0.00

0.01

0.02

0.03

0.041 2 43 5 6 7 8

0

Background SpectrumEnergy (MeV)

20 40 60 80 100 120 140 160 180 200 220 240Channel

Nor

mal

ized

Cou

nt

0.00

0.01

0.02

0.03

0.041 2 43 5 6 7 8

0

Formation SpectrumEnergy (MeV)

20 40 60 80 100 120 140 160 180 200 220 240Channel

Nor

mal

ized

Cou

nt

0.00

0.01

0.02

0.03

0.041 2 43 5 6 7 8

0

Fig. 3�Typical spectra obtained with the new water-flow tool in a 12% limestone test formation. Three measurementsof 300 seconds each are shown in each diagram. The upper curve is magnified by a factor of 5 to emphasize thespectral structure at high energy levels.

Fig. 4�Formation spectra for sandstone and limestone formations

X5

LimestoneSandstone

Formation Spectrum Energy (MeV)

0 20 40 60 80 100 120 140 160 180 200 220 240 260Channel

Nor

mal

ized

Cou

nt

0.00

0.01

0.02

0.03

0.040 1 2 43 5 6 7 8

CC

H00

063


Fig. 5�Lithology identification using silicon-yield curve


Fig. 6�South Texas tight versus gas example


0.012

Nor

mal

ized

Cou

nts

240

0.010

0.008

0.006

0.004

0.002

70 80 100 120 140 160 180 200 220Channel

Liquid ZoneGas Zone

Capture Spectra

CC

H00

071

Increase due toChlorine

0

0.012

Nor

mal

ized

Cou

nts

240

0.010

0.008

0.006

0.004

0.002

60 80 100 120 140 160 180 200 220Channel

Inelastic Spectra

CC

H00

072

No Difference inCarbon Peak

Liquid ZoneGas Zone

0.014

0

GasInterval

LiquidInterval

Fig. 7�Oil and gas discrimination example


0.2

Nor

mal

ized

Act

ivity

0.1

0.0

0 1 3

Vopt

a

bc

d

0 10 20 30 40

CC

H00

069

Velocity/Spacing (1/min)Oxygen Activation

Silicon ActivationVelocity/Spacing (1/min)

Velocity = Velocity/Spacing*Spacing

2

Fig. 8�This graph shows the relationship betweenmeasured activity and logging velocity. The variableVopt represents the tool, or flow, velocity at which themaximum activity will be measured. A direct relation-ship between the measured activity and velocity can-not be expressed because the curve is double-valued,as shown by the two dashed curves. To determine v,multiply the upper scale (for silicon activation) or thebottom scale (for oxygen activation) by the source-detector spacing, measured in feet.

Fig. 9�Qualitative water-flow identification using the new tool


CC

H00

066

Spectra Flow Test Results

Depth Result Depth Result

Z520 No flowZ440 No flowZ410 No flowZ392 DownflowZ401 No flowZ396 Downflow

X,500

Z,500

Z,396C

CH

0007

8

W500X500Z500Y500Z000Z250Z375

DownflowDownflowNo flowDownflowDownflowDownflowDownflow

0

0

1

2

3

4

NO

RM

ALI

ZE

D C

OU

NT

S

5

100 20020 120 22060CHANNEL

16040 140 240 26080 180

OBI OAI

NEAR OAIFAR OAI

0

0

1

2

3

4

NO

RM

ALI

ZE

D C

OU

NT

S

5

100 20020 120 22060CHANNEL

16040 140 240 26080 180

OBI OAI

NEAR OAIFAR OAI

0

0

1

2

3

4

NO

RM

ALI

ZE

D C

OU

NT

S

5

100 20020 120 22060CHANNEL

16040 140 240 26080 180

OBI OAI

NEAR OAIFAR OAI

X,500

Z,396(hole)

Z,500

1/4

bbl/m

in in

ject

ion

rate

Fig. 10�Quantitative water-flow example usinga Spectra Flow Log


Fig. 11�Gravel-pack evaluation via silicon-activation logging

Documents

1998 Behind Casing With a Modern Pulsed Neutron Tool SPE39872