Cased-Hole Logging For Evaluating Bypassed Reserves

sPEDistinguishedRichard 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.AuthorSERIESFelderCased-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 broadspectrum 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 casedhole tools. Further information on the operating principles ofthese tools is readily available in the literature. 2Gamma ray logs include both conventional tools thatmeasure 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 EngineersJournal of Petroleum Technology, August 1988which 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: theolder conyentional neutron log that measures capture gammaray count rates with a single detector and the newer dualdetector, 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 toolsused 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 rayspectra110gs 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 PIPECompletionsPerforation depth controlCement evaluationRecompletionsLocating bypassed hydrocarbonsIdentifying depleted zonesReservoir ManagementMonitoring gas/liquid contacts and OWC'sLocating water or gas breakthroughEaR ProjectsResidual oil saturationMonitoring pilot performanceTABLE 2-PRIMARY LOGS USED IN CASED-HOLEFORMATION EVALUATIONOtherPrimaryLogsProperties Investigated Influencing ConditionsRadioactive zonesLithologyGamma rayShalinessNeutron , S g' lithologyShaliness, brine salinityPNCSw' Sg, Complex lithologyPNSSo, lithology, complex lithology, and measurement statistics can limit thisapplication. Furthermore, the large diameter of these toolsin.cm]) prevents their use in wells havingstandard tubing.Significant advances have been made in cased-hole logginginstrumentation 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 ofwellsite 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 microprocessorsdownhole) 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 numberand 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 noticeableimprovement in the statistical precision, or repeatability, ofmeasurements. This has been the product of improved nuclearsources and detectors as well as enhanced filtering techniques.970TABLE 3-IMPROVEMENTS IN CASED·HOLE DATAWellsite computer recording and processingBetter tool diagnostics and quality-control informationGreater raw (count-rate) data availabilityImproved statistical precisionBetter borehole correctionsTABLE 4-BOREHOLE ENVIRONMENT FACTORSINFLUENCING CASED-HOLE LOG RESPONSEBorehole size, shape, rugosityBorehole fluid invasion (mud type, formation damage,backflow through open perforations, etc.)Downhole plumbingNumber, size, and weight of liners, tubing, andcasing stringsPackers, collars, and shoesCentering of pipe in boreholeCement thickness and compositionType of fluid in tubing and annulus, and location offluid contactsCompletion characteristics (e.g., gravel packing, acidizing,fracturing)Well productionGas or water coningFlowing vs. shut-in loggingBuildup of radioactive scaleCorrection 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 logresponse 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 logapplications 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 beenJournal of Petroleum Technology, August 1988