Cased-Hole Logging for Evaluating Bypassed Reserves R.D. Felder, SPE, Exxon Production Research Co.

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1 Richard D. Felder is a research supervisor with Exxon Production Research Co. in Houston. Since joining Exxon in 1977, he has worked principally in cased-hole logging, training, and formation-evaluation research management. During he served as Chairman of the SPE Well Logging Program Committee and as an SPE Distinguished Lecturer on cased-hole logging. He is president of the Soc. of Professional Well Log Analysts. Felder holds a BS degree from Texas A&M U. and MA and PhD degrees from Rice u., all in physics. spedistinguished Author SERIES Felder Cased-Hole Logging for Evaluating Bypassed Reserves R.D. Felder, SPE, Exxon Production Research Co. Summary. Cased-hole formation evaluation logs are playing a major role in efforts to increase recovery from existing reservoirs. While the logs available today are significant improvements over those of just a few years ago, care must still be exercised for successful applications. Application limitations remain, although some of these are being addressed by ongoing research. Introduction Current oilfield economics, while discouraging some exploration and drilling expenditures, has also produced a growing interest in re-evaluating existing wells to locate bypassed intervals. In mature producing areas, the potential reserve additions from bypassed reservoirs can be very large; moreover, the economics for recompletion can be quite favorable because of the high chance of success in adding production for moderate capital expenditures. 1 Cased-hole logs are playing a major role in locating and evaluating such bypassed hydrocarbon zones. In view of this role, a review of the common technology, applications, limitations, and possible future 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 in Table I. Although these uses range from cement evaluation to monitoring EaR processes, for the purpose of this discussion our focus will be on recompletions, and specifically the location and evaluation of bypassed oil or gas zones. Technology The logs commonly used in formation evaluation behind pipe are listed in Table 2, along with the primary reservoir properties they measure and an indication of some of the other formation properties that influence their response. This list is not exhaustive, but it does provide an indication of the types of formation properties that can be measured by common casedhole tools. Further information on the operating principles of these tools is readily available in the literature. 2 Gamma ray logs include both conventional tools that measure total natural gamma ray count rates and newer spectral gamma ray tools, which measure the individual contributions of potassium, uranium, and thorium to the energy spectrum of natural gamma rays. These tools are predominantly lithology indicators and, as such, are useful in depth control, correlation, and lithology evaluation (especially shaliness). They also detect the buildup of radioactive scale, Copyright 1988 Society of Petroleum Engineers which in certain cases can indicate zones of prolonged water movement, such as perforations, watered-out zones, or channels behind pipe. Cased-hole neutron logs are also of two primary types: the older conyentional neutron log that measures capture gamma ray count rates with a single detector and the newer dualdetector, compensated neutron log that presents a log trace scaled in porosity units. These logs are good porosity, gas, and lithology indicators. With proper calibration, quantitative estimates of porosity and semiquantitative estimates of gas saturation can be obtained, provided that shaliness effects are properly accounted for. Because of their gas response, they are widely used in monitoring gas/liquid contacts. Pulsed neutron capture (PNC) logs are the primary tools used in detecting oil behind pipe. First commercially available in the mid 1960's,3,4 they measure the macroscopic thermal neutron capture cross section, E, using a pulsed neutron source. Present-day tools can be run through tubing as small as 2 in. [5.08 cm]. Because the chlorine present in salt water has a large thermal neutron capture cross section, these logs can distinguish high-salinity formation water from hydrocarbons, and hence can yield quantitative estimates of water saturation if properly calibrated. They also respond to gas saturation and porosity, but their applications can be severely limited by low brine 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, which were first commercially available in the mid-1970's,5-8 measure the energy spectra of gamma rays from both inelastic neutron scattering and thermal neutron capture to obtain estimates of the relative concentrations of particular elements present in the formation. Various ratios of these elemental components (such as the C/O ratio) can be used to evaluate oil saturation, lithology, and even' brine salinity; also, count rate or component yield ratios can be used to estimate porosity. The advantage of these tools is their ability to estimate oil saturation independent of brine salinity; however, low porosity, 969

2 TABLE 1-MAJOR APPLICATION AREAS OF LOGS FOR LOOKING BEHIND PIPE Completions Perforation depth control Cement evaluation Recompletions Locating bypassed hydrocarbons Identifying depleted zones Reservoir Management Monitoring gas/liquid contacts and OWC's Locating water or gas breakthrough EaR Projects Residual oil saturation Monitoring pilot performance TABLE 2-PRIMARY LOGS USED IN CASED-HOLE FORMATION EVALUATION Logs Gamma ray Neutron PNC PNS Primary Properties Investigated Lithology, S g' lithology Sw' Sg, So, lithology, Other Influencing Conditions Radioactive zones Shaliness Shaliness, brine salinity Complex lithology TABLE 3-IMPROVEMENTS IN CASED HOLE DATA Wellsite computer recording and processing Better tool diagnostics and quality-control information Greater raw (count-rate) data availability Improved statistical precision Better borehole corrections TABLE 4-BOREHOLE ENVIRONMENT FACTORS INFLUENCING CASED-HOLE LOG RESPONSE Borehole size, shape, rugosity Borehole fluid invasion (mud type, formation damage, backflow through open perforations, etc.) Downhole plumbing Number, size, and weight of liners, tubing, and casing strings Packers, collars, and shoes Centering of pipe in borehole Cement thickness and composition Type of fluid in tubing and annulus, and location of fluid contacts Completion characteristics (e.g., gravel packing, acidizing, fracturing) Well production Gas or water coning Flowing vs. shut-in logging Buildup of radioactive scale complex lithology, and measurement statistics can limit this application. Furthermore, the large diameter of these tools in. cm]) prevents their use in wells having standard tubing. Significant advances have been made in cased-hole logging instrumentation over the last 10 years, resulting in many improvements in tool designs and measurement capabilities. These improvements are reflected in the logging data provided to 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 in openhole logging, the impact of these computers has been substantial, providing faster logging operations, simultaneous acquisition of larger data volumes, improved real-time quality control, and quality field tapes for the customer. Wellsite processing and interpretation of certain logs are also available, although the user is advised to be wary of quick wellsite interpretations, which may be incomplete. Having computers at the wellsite (and microprocessors downhole) has helped make possible most of the remaining improvements listed in Table 3. Several of the tool designs introduced in the last few years provide supplemental data curves for the purpose of monitoring tool operations and providing information to evaluate the quality of the data measured. As an example, some modern PNC logs have entire backup presentations devoted to showing just quality-control curves. 8-1O Among other data, these curves provide information concerning log repeatability, borehole contributions, background count-rate levels, and even checks of the algorithms used to determine log values from the raw count-rate data. In addition to quality-control information, a greater number and variety of data curves are being made available for presentation uphole. These include raw count-rate and spectral data as two of the options offered by some of the newer pulsed neutron logs. Access to these types of data can be very useful in certain applications, where the user may wish to do his own processing to maximize sensitivity to a particular formation property of interest. Along with the other enhancements has come a noticeable improvement in the statistical precision, or repeatability, of measurements. This has been the product of improved nuclear sources and detectors as well as enhanced filtering techniques. 970 Correction for borehole effects is a major need for many cased-hole tools, and the last decade has seen significant improvements in this area Some measurements have been designed to determine and to remove the borehole portion of the total measured signal, leaving only the formation contribution, which is of primary interest. In other cases, new models of the borehole and formation response of tools have been developed from hundreds of laboratory test-formation measurements. These approaches represent major improvements over the old correction charts that applied only to a limited set of borehole and formation conditions. Accuracy and Limitations Before leaving the subject of borehole corrections, it is important to emphasize that the borehole environment imposes basic limitations on the ability of cased-hole logs to measure formation properties with accuracy. This is because, for the most part, cased-hole logs are nuclear measurements with very shallow depths of investigation-i.e., with most of the signal originating from within a foot of the tool. Because the formation of interest may be separated from the tool by several inches of intervening pipe strings, cement, and annuli filled with oil, gas, or water, it is not surprising that borehole environment strongly influences these measurements and thus limits quantitative interpretation. Some of the major borehole influences on cased-hole log response are listed in Table 4. Because it is fairly obvious that these conditions can affect the logs, the specific influences on response will not be elaborated. Rather, it is sufficient to note that each of these factors can have significant contributions; moreover, the influence of some of them can vary with such formation properties as porosity, saturation, or water salinity. Because borehole environment effects can be so important, it is essential that these conditions be evaluated as thoroughly as possible when cased-hole logs are run. For this purpose, useful information includes openhole log data and complete descriptions of the completion, production, and workover histories. Other limitations on the accuracy of cased-hole log applications result from the difficulty in calibrating certain tools and the lack of suitable formation parameters and models required for log interpretation. 20 Issues regarding the calibration of neutron and pulsed neutron logs have been

3 .. O!!.. CCl IGUl.. GO INDUCTION LOG (18781 JII IOUAL 'GO """ '\ XIl1 x t T Fig. 1-Suitable formation conditions for PNC log application defined by the region to the right of the q,(i;w -I;h)=3 c.u. line. GRAYIl."'K Fig. 2-Appllcatlon example of a modern PNC log (adapted from Ref. 33). studied both in the past and more recently21,22; these studies point to the continuing need for better calibration standards and procedures for these tools. As an example of the contributions of uncertainties in interpretation, consider the use of the common PNC log to determine water saturation quantitatively. In the simplest interpretation model,23,24 the tool-measured E value, Er, is composed of matrix, water, and hydrocarbon components: E r =(1-fjJ)E ma +fjjswew+fjj(l-sw)eh (1) \--.v--j \..-v-...j matrix water hydrocarbon Solving for water saturation, Sw, yields where fjj is porosity and E refers to the thermal neutron capture cross sections for the matrix, rna, water, w, and hydrocarbon, h, respectively. Factors that influence the accuracy of the calculated saturation are the correctness of the interpretation model and the magnitudes of and uncertainties in each of the model parameters (Le., fjj and E values). Discussion of anyone of these factors could be a story in itself; rather than elaborating on these factors, a few general observations are offered. 1. Given typical uncertainties in the parameters of Eq. 2, PNC logs are usually not able to distinguish hydrocarbon from water zones when fjj(ew-e h ) < 3 C.U., with fjj given in fractional units. 25 This limitation on normal applications is illustrated in Fig. 1 as a function of E w and fjj for a E h value representative of oil (20 c.u.). Also shown is the equivalent water salinity corresponding to various E w values. Application accuracy improves for larger values of fjj(ew-e h ) (i.e., more favorable formation conditions) as porosity increases, brine salinity (and hence E w ) increases, or E h decreases (lighter hydrocarbons). This range of applicability has improved slightly over the years with enhancements in PNC log technology that have improved the accuracy of measured Er values. 2. Independent calibration measurements or special logging procedures (e.g., repeat runs or time-lapse logging) can improve the applicability and accuracy of PNC and other cased-hole logs,20,26 whereas large uncertainties in interpretation parameters can significantly decrease accuracy. For example, the variability in E ma values can introduce large uncertainties in interpreting PNC logs, especially in formations with large E ma 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 defining proper interpretation equation parameters. 4. For favorable formation and borehole conditions, the uncertainty in calculated saturation values from PNC logs can approach ±10 saturation units (% PV); for PNS logs the uncertainty is larger, and this usually limits these logs to qualitative interpretation. Applications Despite the uncertainties associated with quantitative evaluation of fluid saturations from cased-hole logs, these logs are very effective in qualitatively evaluating bypassed zones in many situations. 27 Both neutron porosity and PNC logs have been used effectively in measuring gas saturations, especially when comparing logs with earlier base logs run in the same well For the more challenging application of identifying bypassed oil zones, PNC logs have been used very widely and effectively.10, 11,31,32 As an example of a common application, Fig. 2 illustrates the use of a modem PNC log to evaluate oil zones in a high-salinity, high-porosity formation. 33 The primary logging curve presentation is shown, along with a borehole sketch and an operthole induction log run several years earlier. Not shown are the additional quality-control curves that are part of this PNC log and that aid in interpretation. As seen on the borehole sketch, a portion of the logged interval covers a gravel-packed zone, and several of the available quality-control curves provided useful diagnostic information 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 from X629 ft [X192 m] downward (indicated by the high resistivities on the operthole induction log). Smaller E values «24 c.u. in this example) on the formation capture cross section, E f, curve also indicate hydrocarbons; however, the E curve indicates that the oil/water contact (OWC) in this zone is now at X659 ft [X201 m], suggesting considerable depletion after several years of production. Moreover, a gas cap, identified by the small E values, small near/far detector countrate ratio, R NF, and separation of the short- and long-spaced detector count rate (G3-6) curves, now extends from the top of the reservoir down to X651 ft [X198 m], leaving a remaining 8-ft [2.4-m] oil column (low E, high R NF ) from X651 to X659 ft [X198 to X201 m]. 971

4 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 resistivity values, are also apparent on the PNC log, which identifies both as oil zones from the small E values, large R NF values, and lack of separation in the count rate (G3-6) curves. Neither of these zones had been completed in this well, and indeed the upper ZOne shows no depletion since the well was drilled (the OWC remained at X326 ft [X99 m] on the E curve). The middle zone, however, shows that the OWC has moved up to X448 ft [X137 m] on the PNC log, suggesting that this reservoir 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 in the literature These logs see major use where PNC logs can no longer be applied effectively because of low formation brine salinities or brine salinities that are unknown or changing. By nature, effective use of PNS logs is often much more difficult, especially in mixed-lithology and lower-porosity zones. Yet, in certain fields where considerable application experience has been obtained, impressive records of successful recompletions based on PNS log evaluations have been compiled. The most successful applications of cased-hole logs often occur when a plan for reservoir surveillance is developed and implemented early in the life of a field. As part of such a plan, it is fairly common in new, large fields to run cased-hole base logs in wells shortly after completion; these logs can be calibrated with the results of openhole log analysis. Comparison of subsequent cased-hole logs with the early base logs greatly improves interpretation accuracy by eliminating many of the variables that introduce uncertainty. Implementing such a surveillance program during early field development can reap significant benefits in later field life. Future Developments developments in cased-hole logging methods will consist of both improvements to existing tools and methods and introduction of new technology. Enhancements in existing neutron porosity, PNC, and PNS logging technology will continue, especially in the areas of environmental-effect corrections and interpretational models. As examples of new developments, two techniques that show promise in future applications are worth noting. The first of these is the use of new digital, multireceiver, full-waveform sonic waveform logs in cased holes. 40 Improved hardware and processing methods have made possible the extraction of formation acoustic data from these logs in casedhole environments. These data are beginning to be used in several cased-hole applications, including identifying gas, estimating porosity, evaluating lithology, and designing hydraulic fracturing treatments. 41,42 Each of these uses can be important in the recompletion of old wells; e.g., comparison of the sonic porosity with the compensated neutron porosity can be an effective means of identifying gas zones,41 providing information similar to that obtained by using the common overlay of openhole density and neutron porosity curves. The need to supplement the cased-hole neutron porosity log in identifying gas in many cases has even resulted in the occasional application of density logs in cased holes for this purpose. 43 A second developing technology is the application of highresolution gamma ray spectroscopy to evaluate saturations in low-salinity reservoirs Such logs use high-resolution germanium detectors to measure the chlorine content in the gamma ray spectra from thermal neutron capture; from the measured chlorine component, water saturation can be estimated if porosity and salinity are known. While chlorine logging is not a new concept,47,48 high-resolution spectroscopy has the potential of significantly improving the measurement of chlorine content. This, in turn, may provide useful saturation estimates in numerous low-salinity reservoirs where PNC logs cannot be used and conventional interpretation of PNS logs has 972 not been effective. 45 Because work to date has only proved the feasibility of this measurement, much development work is required before useful logs will be available. At a more exploratory stage is the use of high-resolution spectroscopy to obtain improved elemental abundances for measuring carbon-to-oxygen ratio and for determining formation lithology and mineralogy in cased hole. 49,50 As in many cased-hole measurements, correction for the influence of borehole fluid, casing, and cement will pose a major challenge to successful applications. A different approach to obtain detailed elemental concentrations has been proposed that uses a geochemical model along with measurements of natural, activation, and neutron capture gamma rays with conventional (sodium iodide) detectors.51 While this technique shows promise in openhole environments, its meaningful application in cased hole will be extremely difficult because of the interference from casing and cement. Conclusions Current economic conditions are providing an opportunity and challenge to increase recovery from existing reservoirs, an activity in which cased-hole logs are playing a major role. The cased-hole logs available today are a significant improvement over the tools available just a few years ago, with enhancements in processing, measurement precision, and borehole corrections. Knowledge of tool limitations and integrated use of all available well data, however, remain essential elements of any cased-hole log application. Casedhole logs are used effectively in a wide variety of conditions, although situations still remain in which current logs are not sufficiently diagnostic. Cased-hole logging technology continues to advance, with favorable prospects for improving interpretation and extending the range ofapplication for these tools. Nomenclature R NF = near/far detector count-rate ratio S = saturation E = macroscopic thermal neutron capture cross section E BH - SS = borehole capture cross section measured in shortspaced detector E f = formation capture cross section Equal = calculated formation counts divided by measured total counts in Gate 6 cp = porosity Subscripts g = gas h = hydrocarbon rna = matrix 0= oil T = tool-measured w = water References 1. 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Dewan, J.T., Stone, O.L., and Morris, R.L.: "Chlorine Logging in Cased Holes," lpt(june 1961) Tanner, H.L.: "Evaluation oflow-resistivity CItsed-OffReserves with the Shale Compensated Chlorine Log," SPEFE (Sept. 1987) ; Trans., AIME, Mellor, D.W. and Underwood, M.C.: "Formation Properties from High-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 of Rock Formations," lpt(july 1986) Hertzog, R.C. et al.: "GeochemicalLogging with Spectrometry Tools, " paper SPE presented at the 1987 SPE Annual Technical Conference and Exhibition, Dallas, Sept JPT This paper is SPE Distinguished Author Series articles are general, descriptive presentations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: To inform the general readership of recent advances in various areas of petroleum engineering. A softbound anthology, SPE Dis6nguished Author Series: Dec Dec. 1983, is available from SPE's Book Order Dept. 973