Felder

Richard D. Felder is a research supervisor with ExxonProduction Research Co. in Houston. Since joining Exxon in1977, he has worked principally in cased-hole logging, training,and formation-evaluation research management. During 1986-87he served as Chairman of the SPE Well Logging ProgramCommittee and as an SPE Distinguished Lecturer on cased-holelogging. He is 1988-89 president of the Soc. of Professional WellLog Analysts. Felder holds a BS degree from Texas A&M U. andMA and PhD degrees from Rice u., all in physics.

sPEDistinguishedAuthor SERIES

Cased-Hole Logging for EvaluatingBypassed ReservesR.D. Felder, SPE, Exxon Production Research Co.

Summary. Cased-hole formation evaluation logs are playing a major role in efforts to increase recovery from existingreservoirs. While the logs available today are significant improvements over those of just a few years ago, care must still beexercised for successful applications. Application limitations remain, although some of these are being addressed by ongoing research.

IntroductionCurrent oilfield economics, while discouraging someexploration and drilling expenditures, has also produced agrowing interest in re-evaluating existing wells to locatebypassed intervals. In mature producing areas, the potentialreserve additions from bypassed reservoirs can be very large;moreover, the economics for recompletion can be quitefavorable because of the high chance of success in addingproduction for moderate capital expenditures. 1 Cased-hole logsare playing a major role in locating and evaluating suchbypassed hydrocarbon zones. In view of this role, a review ofthe common technology, applications, limitations, and possiblefuture developments in cased-hole logging is appropriate.Cased-hole logs for looking behind pipe are used in a broad

spectrum of applications, several of which are indicated inTable I. Although these uses range from cement evaluation tomonitoring EaR processes, for the purpose of this discussionour focus will be on recompletions, and specifically thelocation and evaluation of bypassed oil or gas zones.

TechnologyThe logs commonly used in formation evaluation behind pipeare listed in Table 2, along with the primary reservoirproperties they measure and an indication of some of the otherformation properties that influence their response. This list isnot exhaustive, but it does provide an indication of the types offormation properties that can be measured by common cased-hole tools. Further information on the operating principles ofthese tools is readily available in the literature. 2Gamma ray logs include both conventional tools that

measure total natural gamma ray count rates and newerspectral gamma ray tools, which measure the individualcontributions of potassium, uranium, and thorium to the energyspectrum of natural gamma rays. These tools arepredominantly lithology indicators and, as such, are useful indepth control, correlation, and lithology evaluation (especiallyshaliness). They also detect the buildup of radioactive scale,

Copyright 1988 Society of Petroleum Engineers

Journal of Petroleum Technology, August 1988

which in certain cases can indicate zones of prolonged watermovement, such as perforations, watered-out zones, orchannels behind pipe.Cased-hole neutron logs are also of two primary types: the

older conyentional neutron log that measures capture gammaray count rates with a single detector and the newer dual-detector, compensated neutron log that presents a log tracescaled in porosity units. These logs are good porosity, gas, andlithology indicators. With proper calibration, quantitativeestimates of porosity and semiquantitative estimates of gassaturation can be obtained, provided that shaliness effects areproperly accounted for. Because of their gas response, they arewidely used in monitoring gas/liquid contacts.Pulsed neutron capture (PNC) logs are the primary tools

used in detecting oil behind pipe. First commercially availablein the mid 1960's,3,4 they measure the macroscopic thermalneutron capture cross section, E, using a pulsed neutronsource. Present-day tools can be run through tubing as small as2 in. [5.08 cm]. Because the chlorine present in salt water hasa large thermal neutron capture cross section, these logs candistinguish high-salinity formation water from hydrocarbons,and hence can yield quantitative estimates of water saturation ifproperly calibrated. They also respond to gas saturation andporosity, but their applications can be severely limited by lowbrine salinity, low porosity, or shaliness.Pulsed neutron spectral (PNS) or induced gamma ray

spectra110gs are also commonly known as carbon/oxygen(C/O) or gamma ray spectroscopy logs. These logs, whichwere first commercially available in the mid-1970's,5-8measure the energy spectra of gamma rays from both inelasticneutron scattering and thermal neutron capture to obtainestimates of the relative concentrations of particular elementspresent in the formation. Various ratios of these elementalcomponents (such as the C/O ratio) can be used to evaluate oilsaturation, lithology, and even' brine salinity; also, count rateor component yield ratios can be used to estimate porosity.The advantage of these tools is their ability to estimate oilsaturation independent of brine salinity; however, low porosity,

969

TABLE 1-MAJOR APPLICATION AREAS OF LOGSFOR LOOKING BEHIND PIPE TABLE 3-IMPROVEMENTS IN CASED·HOLE DATA

TABLE 2-PRIMARY LOGS USED IN CASED-HOLEFORMATION EVALUATION

CompletionsPerforation depth controlCement evaluation

RecompletionsLocating bypassed hydrocarbonsIdentifying depleted zones

Reservoir ManagementMonitoring gas/liquid contacts and OWC'sLocating water or gas breakthrough

EaR ProjectsResidual oil saturationMonitoring pilot performance

LogsGamma rayNeutronPNCPNS

PrimaryProperties Investigated

Lithology¢, Sg' lithologySw' Sg, ¢

So, lithology, ¢

OtherInfluencing ConditionsRadioactive zones

ShalinessShaliness, brine salinityComplex lithology

Wellsite computer recording and processingBetter tool diagnostics and quality-control informationGreater raw (count-rate) data availabilityImproved statistical precisionBetter borehole corrections

TABLE 4-BOREHOLE ENVIRONMENT FACTORSINFLUENCING CASED-HOLE LOG RESPONSE

Borehole size, shape, rugosityBorehole fluid invasion (mud type, formation damage,backflow through open perforations, etc.)

Downhole plumbingNumber, size, and weight of liners, tubing, andcasing strings

Packers, collars, and shoesCentering of pipe in borehole

Cement thickness and compositionType of fluid in tubing and annulus, and location offluid contacts

Completion characteristics (e.g., gravel packing, acidizing,fracturing)

Well productionGas or water coningFlowing vs. shut-in loggingBuildup of radioactive scale

complex lithology, and measurement statistics can limit thisapplication. Furthermore, the large diameter of these tools

in. cm]) prevents their use in wells havingstandard tubing.Significant advances have been made in cased-hole logging

instrumentation over the last 10 years, resulting in manyimprovements in tool designs and measurement capabilities.These improvements are reflected in the logging data providedto the user, as summarized in Table 3 and discussed below.Dramatic changes have resulted from the introduction of

wellsite computers for data acquisition and processing. As inopenhole logging, the impact of these computers has beensubstantial, providing faster logging operations, simultaneousacquisition of larger data volumes, improved real-time qualitycontrol, and quality field tapes for the customer. Wellsiteprocessing and interpretation of certain logs are also available,although the user is advised to be wary of quick wellsiteinterpretations, which may be incomplete.Having computers at the wellsite (and microprocessors

downhole) has helped make possible most of the remainingimprovements listed in Table 3. Several of the tool designsintroduced in the last few years provide supplemental datacurves for the purpose of monitoring tool operations andproviding information to evaluate the quality of the datameasured. As an example, some modern PNC logs have entirebackup presentations devoted to showing just quality-controlcurves. 8-1O Among other data, these curves provideinformation concerning log repeatability, boreholecontributions, background count-rate levels, and even checks ofthe algorithms used to determine log values from the rawcount-rate data.In addition to quality-control information, a greater number

and variety of data curves are being made available forpresentation uphole. These include raw count-rate and spectraldata as two of the options offered by some of the newer pulsedneutron logs. Access to these types of data can be very usefulin certain applications, where the user may wish to do his ownprocessing to maximize sensitivity to a particular formationproperty of interest.Along with the other enhancements has come a noticeable

improvement in the statistical precision, or repeatability, ofmeasurements. This has been the product of improved nuclearsources and detectors as well as enhanced filtering techniques.

970

Correction for borehole effects is a major need for manycased-hole tools, and the last decade has seen significantimprovements in this area. 9-19 Some measurements have beendesigned to determine and to remove the borehole portion ofthe total measured signal, leaving only the formationcontribution, which is of primary interest. In other cases, newmodels of the borehole and formation response of tools havebeen developed from hundreds of laboratory test-formationmeasurements. These approaches represent majorimprovements over the old correction charts that applied onlyto a limited set of borehole and formation conditions.

Accuracy and LimitationsBefore leaving the subject of borehole corrections, it isimportant to emphasize that the borehole environment imposesbasic limitations on the ability of cased-hole logs to measureformation properties with accuracy. This is because, for themost part, cased-hole logs are nuclear measurements with veryshallow depths of investigation-i.e., with most of the signaloriginating from within a foot of the tool. Because theformation of interest may be separated from the tool by severalinches of intervening pipe strings, cement, and annuli filledwith oil, gas, or water, it is not surprising that boreholeenvironment strongly influences these measurements and thuslimits quantitative interpretation.Some of the major borehole influences on cased-hole log

response are listed in Table 4. Because it is fairly obvious thatthese conditions can affect the logs, the specific influences onresponse will not be elaborated. Rather, it is sufficient to notethat each of these factors can have significant contributions;moreover, the influence of some of them can vary with suchformation properties as porosity, saturation, or water salinity.Because borehole environment effects can be so important, it isessential that these conditions be evaluated as thoroughly aspossible when cased-hole logs are run. For this purpose, usefulinformation includes openhole log data and completedescriptions of the completion, production, and workoverhistories.Other limitations on the accuracy of cased-hole log

applications result from the difficulty in calibrating certaintools and the lack of suitable formation parameters and modelsrequired for log interpretation. 20 Issues regarding thecalibration of neutron and pulsed neutron logs have been

Journal of Petroleum Technology, August 1988

..O __ !!JII IOUAL •

..CCl ...1.. IGUl ..

GO

INDUCTION LOG (18781

'GO

""" '\

x••XIl1 t

TGRAYIl."'K

Fig. 1-Suitable formation conditions for PNC log applicationdefined by the region to the right of the q,(I;w -I;h)=3 c.u.line.

studied both in the past and more recently21,22; these studiespoint to the continuing need for better calibration standards andprocedures for these tools.As an example of the contributions of uncertainties in

interpretation, consider the use of the common PNC log todetermine water saturation quantitatively. In the simplestinterpretation model,23,24 the tool-measured E value, Er , iscomposed of matrix, water, and hydrocarbon components:

Er =(1-fjJ)Ema +fjJSwEw+fjJ(l-Sw)Eh (1)\--.v--J \..-v-...Jmatrix water hydrocarbon

Solving for water saturation, Sw, yields

where fjJ is porosity and E refers to the thermal neutroncapture cross sections for the matrix, rna, water, w, andhydrocarbon, h, respectively. Factors that influence theaccuracy of the calculated saturation are the correctness of theinterpretation model and the magnitudes of and uncertainties ineach of the model parameters (Le., fjJ and E values).Discussion of anyone of these factors could be a story initself; rather than elaborating on these factors, a few generalobservations are offered.1. Given typical uncertainties in the parameters of Eq. 2,

PNC logs are usually not able to distinguish hydrocarbon fromwater zones when fjJ(Ew-Eh)< 3 C.U., with fjJ given infractional units. 25 This limitation on normal applications isillustrated in Fig. 1 as a function of Ew and fjJ for a Eh valuerepresentative of oil (20 c.u.). Also shown is the equivalentwater salinity corresponding to various Ew values. Applicationaccuracy improves for larger values of fjJ(Ew-Eh) (i.e., morefavorable formation conditions) as porosity increases, brinesalinity (and hence Ew) increases, or Eh decreases (lighterhydrocarbons). This range of applicability has improvedslightly over the years with enhancements in PNC logtechnology that have improved the accuracy of measured Ervalues.2. Independent calibration measurements or special logging

procedures (e.g., repeat runs or time-lapse logging) canimprove the applicability and accuracy of PNC and othercased-hole logs,20,26 whereas large uncertainties ininterpretation parameters can significantly decrease accuracy.For example, the variability in Ema values can introduce large

Journal of Petroleum Technology, August 1988

Fig. 2-Appllcatlon example of a modern PNC log (adaptedfrom Ref. 33).

uncertainties in interpreting PNC logs, especially in formationswith large Ema values (e.g., shaly sands).3. Openhole log data (especially porosity logs) and other

well information (e.g., completion diagram, production history,fluid analysis, and core data) playa major role in definingproper interpretation equation parameters.4. For favorable formation and borehole conditions, the

uncertainty in calculated saturation values from PNC logs canapproach ±10 saturation units (% PV); for PNS logs theuncertainty is larger, and this usually limits these logs toqualitative interpretation.

ApplicationsDespite the uncertainties associated with quantitative evaluationof fluid saturations from cased-hole logs, these logs are veryeffective in qualitatively evaluating bypassed zones in manysituations. 27 Both neutron porosity and PNC logs have beenused effectively in measuring gas saturations, especially whencomparing logs with earlier base logs run in the samewell. 28-30 For the more challenging application of identifyingbypassed oil zones, PNC logs have been used very widely andeffectively .10, 11 ,31 ,32 As an example of a common application,Fig. 2 illustrates the use of a modem PNC log to evaluate oilzones in a high-salinity, high-porosity formation. 33 Theprimary logging curve presentation is shown, along with aborehole sketch and an operthole induction log run severalyears earlier. Not shown are the additional quality-controlcurves that are part of this PNC log and that aid ininterpretation. As seen on the borehole sketch, a portion of thelogged interval covers a gravel-packed zone, and several of theavailable quality-control curves provided useful diagnosticinformation concerning this zone (as discussed in Ref. 33).This well originally was perforated just below the base of

the logged interval in the oil-saturated zone that extends fromX629 ft [X192 m] downward (indicated by the highresistivities on the operthole induction log). Smaller E values« 24 c.u. in this example) on the formation capture crosssection, Ef , curve also indicate hydrocarbons; however, the Ecurve indicates that the oil/water contact (OWC) in this zone isnow at X659 ft [X201 m], suggesting considerable depletionafter several years of production. Moreover, a gas cap,identified by the small E values, small near/far detector count-rate ratio, RNF, and separation of the short- and long-spaceddetector count rate (G3-6) curves, now extends from the top ofthe reservoir down to X651 ft [X198 m], leaving a remaining8-ft [2.4-m] oil column (low E, high RNF) from X651 to X659ft [X198 to X201 m].

971

Further uphole, two hydrocarbon zones (X285 to X326 ft[X87 to X99 m] and X436 to X456 ft [X133 to X139 m]),originally indicated on the induction log by higher resistivityvalues, are also apparent on the PNC log, which identifiesboth as oil zones from the small E values, large RNF values,and lack of separation in the count rate (G3-6) curves. Neitherof these zones had been completed in this well, and indeed theupper ZOne shows no depletion since the well was drilled (theOWC remained at X326 ft [X99 m] on the E curve). Themiddle zone, however, shows that the OWC has moved up toX448 ft [X137 m] on the PNC log, suggesting that thisreservoir is being drained by offset production.PNS logs have also been widely used to evaluate bypassed

oil reservoirs, as affirmed by the many application examples inthe literature.34-39 These logs see major use where PNC logscan no longer be applied effectively because of low formationbrine salinities or brine salinities that are unknown orchanging. By nature, effective use of PNS logs is often muchmore difficult, especially in mixed-lithology and lower-porosityzones. Yet, in certain fields where considerable applicationexperience has been obtained, impressive records of successfulrecompletions based on PNS log evaluations have beencompiled.The most successful applications of cased-hole logs often

occur when a plan for reservoir surveillance is developed andimplemented early in the life of a field. As part of such aplan, it is fairly common in new, large fields to run cased-holebase logs in wells shortly after completion; these logs can becalibrated with the results of openhole log analysis.Comparison of subsequent cased-hole logs with the early baselogs greatly improves interpretation accuracy by eliminatingmany of the variables that introduce uncertainty. Implementingsuch a surveillance program during early field development canreap significant benefits in later field life.

Future Developmentsdevelopments in cased-hole logging methods will consist

of both improvements to existing tools and methods andintroduction of new technology. Enhancements in existingneutron porosity, PNC, and PNS logging technology willcontinue, especially in the areas of environmental-effectcorrections and interpretational models. As examples of newdevelopments, two techniques that show promise in futureapplications are worth noting.The first of these is the use of new digital, multireceiver,

full-waveform sonic waveform logs in cased holes. 40 Improvedhardware and processing methods have made possible theextraction of formation acoustic data from these logs in cased-hole environments. These data are beginning to be used inseveral cased-hole applications, including identifying gas,estimating porosity, evaluating lithology, and designinghydraulic fracturing treatments. 41 ,42 Each of these uses can beimportant in the recompletion of old wells; e.g., comparison ofthe sonic porosity with the compensated neutron porosity canbe an effective means of identifying gas zones,41 providinginformation similar to that obtained by using the commonoverlay of openhole density and neutron porosity curves. Theneed to supplement the cased-hole neutron porosity log inidentifying gas in many cases has even resulted in theoccasional application of density logs in cased holes for thispurpose. 43A second developing technology is the application of high-

resolution gamma ray spectroscopy to evaluate saturations inlow-salinity reservoirs. 44-46 Such logs use high-resolutiongermanium detectors to measure the chlorine content in thegamma ray spectra from thermal neutron capture; from themeasured chlorine component, water saturation can beestimated if porosity and salinity are known. While chlorinelogging is not a new concept,47,48 high-resolution spectroscopyhas the potential of significantly improving the measurement ofchlorine content. This, in turn, may provide useful saturationestimates in numerous low-salinity reservoirs where PNC logscannot be used and conventional interpretation of PNS logs has

972

not been effective. 45 Because work to date has only proved thefeasibility of this measurement, much development work isrequired before useful logs will be available.At a more exploratory stage is the use of high-resolution

spectroscopy to obtain improved elemental abundances formeasuring carbon-to-oxygen ratio and for determiningformation lithology and mineralogy in cased hole. 49 ,50 As inmany cased-hole measurements, correction for the influence ofborehole fluid, casing, and cement will pose a major challengeto successful applications. A different approach to obtaindetailed elemental concentrations has been proposed that uses ageochemical model along with measurements of natural,activation, and neutron capture gamma rays with conventional(sodium iodide) detectors. 51 While this technique showspromise in openhole environments, its meaningful applicationin cased hole will be extremely difficult because of theinterference from casing and cement.

ConclusionsCurrent economic conditions are providing an opportunity andchallenge to increase recovery from existing reservoirs, anactivity in which cased-hole logs are playing a major role. Thecased-hole logs available today are a significant improvementover the tools available just a few years ago, withenhancements in processing, measurement precision, andborehole corrections. Knowledge of tool limitations andintegrated use of all available well data, however, remainessential elements of any cased-hole log application. Cased-hole logs are used effectively in a wide variety of conditions,although situations still remain in which current logs are notsufficiently diagnostic. Cased-hole logging technologycontinues to advance, with favorable prospects for improvinginterpretation and extending the range of application for these tools.

NomenclatureRNF = near/far detector count-rate ratioS = saturationE = macroscopic thermal neutron capture cross section

EBH-SS = borehole capture cross section measured in short-spaced detector

Ef = formation capture cross sectionEqual = calculated formation counts divided by measured

total counts in Gate 6cP = porosity

Subscriptsg = gash = hydrocarbon

rna = matrix0= oilT = tool-measuredw = water

References1. "Stimulation Activity ofOld Wells Increases in New Economic Setting,"IPT (Dec. 1987) 1552-56.

2. Ellis, D.V.: Well Logging for Earth Scientists, Elsevier SciencePublishing Co., New York City (1987) 227-304.

3. Youmans, A.H. et al.: "Neutron Lifetime, a New Nuclear Log," IPT(March 1964) 319-29; Trans., AIME, 231.

4. Wahl, J.S. et al.: "The Thennal Neutron Decay Time Log," SPEJ(Dec. 1970) 365-80.

5. Lock, G.A. and Hoyer, W.A.: "Carbon-Oxygen (C/O) Log: Use andInterpretation," IPT (Sept. 1974) 1044-54.

6. Schultz, W.E. and Smith, H.D.: "Laboratory and Field Evaluation ofa Carbon/Oxygen (C/O) Well Logging System," IPT (Oct. 1974)1103-10.

7. Culver, R.B., Hopkinson, E.C., and Youmans, A.H.: "Carbon/Oxygen(C/O) Logging Instrumentation," SPEJ (Oct. 1974) 463-70.

8. Hertzog, R.C.: "Laboratory and Field Evaluation of an Inelastic-Neutron-Scattering and Capture Gamma Ray Spectroscopy Tool," SPEJ(Oct. 1980) 327-40.

Journal of Petroleum Technology, August 1988

9. Schultz, W.E. et al.: "Experimental Basis for a New Borehole CorrectedPulsed Neutron Capture Logging System (TMD)," Trans., SPWLA24th Annual Logging Symposium (June 1983) paper CC.

10. Smith, H.D. Jr., Arnold, D.M., and Perlman, H.E.: "Applicationsof a Borehole Corrected Pulsed Neutron Capture Logging System(TMD)," Trans., SPWLA 24th Annual Logging Symposium (June1983) paper DD.

II. Randall, R.R. et al.: "PDK-IOO Log Examples in the Gulf Coast,"Trans., SPWLA 26th Annual Logging Symposium (June 1985) paperXX.

12. Randall, R.R. et al.: "A New Digital Multiscale Pulsed Neutron LoggingSystem," SPEFE (Dec. 1987) 395-400; Trans., AIME, 290.

13. Preeg, W.E. and Scott, H.D.: "Computing Thermal Neutron DecayTime Environmental Effects Using Monte Carlo Techniques," SPEFE(Feb. 1986) 35-42; Trans., AIME, 284.

14. Steinman, D.K. et al.: "Dual-Burst Thermal Decay Time LoggingPrinciples," SPEFE (June 1988) 377-85.

15. Schweitzer, J.S., Manente, R.A., and Hertzog, R.C.: "Gamma RaySpectroscopy Tool Environmental Effects," lPT(Sept. 1984) 1527-34.

16. Grau, J.A., Roscoe, B.A., and Tobanou, J.R.: "A Borehole CorrectionModel for Capture Gamma Ray Spectroscopy Logging Tools," SPEFE(March 1988) 62-88; Trans., AIME, 285.

17. Galford, J.E. et al.: "Improved Environmental Corrections forCompensated Neutron Logs," SPEFE (June 1988) 371-76.

18. Smith, M.P.: "Neutron Absorption Effects on Dual-Spaced ThermalNeutron Logging Tools," Trans., SPWLA 28th Annual LoggingSymposium (June 1987) paper R.

19. Smith, H.D. Jr., Wyatt, D.F., and Arnold, D.M.: "Obtaining IntrinsicFormation Capture Cross Sections with Pulsed Neutron Capture LoggingTools," Trans., SPWLA 29th Annual Logging Symposium (June 1988)paper SS.

20. Thomas, E.C. et al.: "The Scope and Perspective ofROS Measurementand Flood Monitoring," lPT(Nov. 1987) 1398-1406; Trans., AIME,287.

21. Mills, W.R. et al.: "A Proposed Calibration Facility for Pulsed NeutronLogging Tools," The Log Analyst (Jan.-Feb. 1977) 3-5.

22. Wiley, R. and Allen, L.S.: "Recommendations for Neutron Loggingfrom the SPWLA Subcommittee for Log Calibration Guidelines," TheLog Analyst (May-June 1988) 204-15.

23. Clavier, C., Hoyle, W., and Meunier, D.: "Quantitative Interpretationof Thermal Neutron Decay Time Logs: Part I. Fundamentals andTechniques," lPT (June 1971) 743-55; Trans., AIME, 251.

24. Clavier, C., Hoyle, W., and Meunier, D.: "Quantitative Interpretationof Thermal Neutron Decay Time Logs: Part II. Interpretation Example,Interpretation Accuracy, and Time-Lapse Technique," lPT (June 1971)756-63; Trans., AIME, 251.

25. Log Interpretation, Volume II-Applications, Schlumberger Ltd., NewYork City (1974) 57-58.

26. Felder, R.D. and Hoyer, W.A.: "The Use of Well Logs to Monitora Surfactant Flood Pilot Test," lPT (Aug. 1984) 1379-92.

27. Fertl, W.H.: "Well Logging and Its Applications in Cased Holes,"lPT (Feb. 1984) 249-66.

28. Vacca, H.L. and Walker, C.M.: "The Cased-Hole CompensatedNeutron Log Enhances Shaly Gas Identification, " Trans., SPWLA 26thAnnual Logging Symposium (June 1985) paper BB.

29. Smith, M.P. and Wyatt, D.F. Jr.: "Quantitative Flood MonitoringUtilizing a New Pulsed Neutron Tool Modeling Concept," paper SPE16815 presented at the 1987 SPE Annual Technical Conference andExhibition, Dallas, Sept. 27-30.

30. Dupree, J.H.: "Cased-Hole Nuclear Logging Interpretation, PrudhoeBay, Alaska," Trans., SPWLA 29th Annual Logging Symposium (June1988) paper TT.

31. PulsedNeutron Logging, W.A. Hoyer (ed.), Reprint Series, SPWLA,Houston (1979) 139-273.

32. Lisbon, T.E., Vacca, H.L., and Meehan, D.N.: "Stratton Field, TexasGulf Coast: A Successful Cased-Hole Re-Evaluation of an Old FieldTo Determine Remaining Reserves and To Increase Production Level,"lPT (Jan. 1985) 105-23.

Journal of Petroleum Technology, August 1988

33. Buchanan, J.C. et al.: "Applications of TMD* Pulsed Neutron Logsin Unusual Downhole Logging Environments," Trans., SPWLA 25thAnnual Logging Symposium (June 1984) paper KKK.

34. Oliver, D.W., Frost, E., and Fertl, W.H.: "Continuous Carbon/Oxygen(C/O) Logging-Instrumentation, Interpretive Concepts, and FieldApplications, " Trans., SPWLA 22nd Annual Logging Symposium (June1981) paper TT.

35. Gilchrist, W.A. Jr. et al.: "Application of Gamma Ray Spectroscopyto Formation Evaluation," Trans., SPWLA 23rd Annual LoggingSymposium (June 1982) paper B.

36. Neuman, C.H. and Oden, A.L.: "Cased-Hole Measurement ofOH inPlace, San Joaquin Valley, California," lPT (June 1982) 1295-1301.

37. Fertl, W.H. etal.: "Evaluation and Monitoring of Enhanced RecoveryProjects in California and Cased-Hole Exploration and Recompletionsin West Texas Based on the Continuous Carbon/Oxygen Log," lPT(Jan. 1983) 143-57.

38. Quirein, J., La Vigne, J., and Chapman, S.: "Enhancements to thePulsed Neutron Gamma Ray Interpretation Process, " Trans., SPWLA28th Annual Logging Symposium (June 1987) paper Y.

39. Hull, R.L. and Johnson, D.E.: "Muldoon Field: An Evaluation ofBehind Casing Pay Zones in a Freshwater Environment," SPEFE(March 1988) 92-96.

40. Tubman, K.M., Cheng, C.H., and Toksoz, M.N.: "Determination ofFormation Properties in Cased Boreholes Using Full Waveform Logs,"Trans., SPWLA 25th Annual Logging Symposium (June 1984) paperCC.

41. Bettis; F. et al.: "Sonic Logging in Cased Wells: Opportunities forEnhancing Oil Fields," Schlumberger Technical Review (Dec. 1987)14-18.

42. Cannon, D.E. and La Vigne, J.A.: "Through-Casing ReservoirEvaluation," SPEFE (June 1987) 201-08; Trans., AIME, 290.

43. Cigni, M. and Magrassi, M.: "Gas Detection from Formation Densityand Compensated Neutron Logs in Cased Hole," Trans., SPWLA 28thAnnual Logging Symposium (June 1987) paper W.

44. Baicker, J.A. et al.: "Carbon/Oxygen Logging Using a Pulsed NeutronGenerator and a Germanium Cryosonde, " Trans., SPWLA 26th AnnualLogging Symposium (June 1985) paper BBB.

45. Neuman, C.H.: "Test of a High-Resolution Spectroscopy Logging Toolto Measure Chlorine in a Low-Salinity Reservoir," SPEFE (March 1988)40-46.

46. Myers, G.D.: "Practical Pulsed Neutron Spectroscopy Logging witha High-Resolution Gamma Ray Detector," Trans., SPWLA 29th AnnualLogging Symposium (June 1988) paper RR.

47. Dewan, J.T., Stone, O.L., and Morris, R.L.: "Chlorine Logging inCased Holes," lPT(June 1961) 531-37.

48. Tanner, H.L.: "Evaluation of Low-Resistivity CItsed-OffReserves withthe Shale Compensated Chlorine Log," SPEFE (Sept. 1987) 284-88;Trans., AIME, 263.

49. Mellor, D.W. and Underwood, M.C.: "Formation Properties fromHigh-Resolution Neutron Activation Gamma Ray Spectra," Trans.,SPWLA 26th Annual Logging Symposium (June 1985) paper FFF.

50. Scott, H.D.: "New Developments in Remote Elemental Analysis ofRock Formations," lPT(July 1986) 711-13.

51. Hertzog, R.C. et al.: "Geochemical Logging with Spectrometry Tools, "paper SPE 16792 presented at the 1987 SPE Annual TechnicalConference and Exhibition, Dallas, Sept. 27-30.

JPT

This paper is SPE 18507. Distinguished Author Series articles are general, descriptivepresentations that summarize the state of the art in an area of technology by describingrecent developments for readers who are not specialists in the topics discussed. Writtenby individuals recognized as experts in the area, these articles provide key references tomore definitive work and present specific details only to illustrate the technology. Purpose:To inform the general readership of recent advances in various areas of petroleumengineering. A softbound anthology, SPEDis6nguishedAuthorSeries: Dec. 1981-Dec. 1983,is available from SPE's Book Order Dept.

973