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CHAPTER 9 WELL LOGGING SUMMARY Nuclear and nonnuclear well logging tools are used in concert with each other to obtain information about the geologic media through which a borehole has been drilled. There are five main nuclear well logging tools: the density porosity tool, using cesium-137; the neutron porosity and elemental analysis tools, typically using americium-beryllium (Am-Be) radioisotope sources; and the neutron absorption and carbon/oxygen (C/O) tools, which use 14.1-MeV neutrons from deuterium-tritium (D-T) accelerators. The cesium-137 source in a density log is a vitrified Category 3 source. The Am-Be sources range from Category 3 to Category 2. It would not be difficult to replace the Am-Be source in elemental analysis logs with a D-T accelerator. Replacing the Am-Be porosity tools is more difficult, although Schlumberger does now market D-T accelerator nuclear porosity logging tools for logging while drilling (LWD) and logging drilled holes (wireline). A major reason why such logs have not been adopted widely is that well log analysis relies on a large body of data that has been accumulated for the porosity logs using Am-Be sources. These data would be less useful in analyzing the results from 14.1-MeV neutrons from D-T accelerators. Californium-252 sources might also be used to replace Am-Be sources, but they also suffer (somewhat less) from a similar lack of supporting data. In addition, californium-252 sources have a half-life of only about 2.45 years and would have to be replaced in about two half-lives. These replacement source approaches are presently being studied by Monte Carlo simulation and in-hole experiments and demonstrations. INTRODUCTION TO WELL LOGGING Well logging is the practice of measuring the properties of the geologic strata through which a well has been or is being drilled. A well log is the trace or record of the data from a down-hole sensor tool plotted versus well depth. Its most common application is by the oil and gas industries which seek out recoverable hydrocarbon zones. For oil and gas production, companies would like to have several kinds of information about a geologic layer, such as the hydrocarbon content. To measure these properties, sources and sensors loaded into housings called sondes can be lowered into an existing borehole (a technique called wireline logging) or can be mounted in a collar behind the drilling head for taking measurements while the well is being drilled (called LWD). In wireline logging, sondes and supporting electronic cartridges are strung together and lowered into an uncased borehole on a cable that has an electronic signal wire. As the string is raised, the sensors measure some or all of the following properties as functions of the depth: electrical resistivity, electron density, sound velocity, neutron moderation, thermal-neutron absorption, natural and artificial (induced) radioactivity, gamma-ray spectra, Compton scattering, borehole dimension, and occasionally nuclear magnetic resonance. The data are transmitted through the wire to computers at the surface where the data are logged. Similar measurements can be made in a cased borehole, although it is much more challenging to carry out the measurements through the steel casing. Even with that difficulty, however, there is an increasing demand for logging of previously utilized production wells 147
148 RADIATION SOURCE USE AND RADIATION because initial hydrocarbon (oil and gas) recovery fractions may have been quite low (20â30 percent of the resources in the formation), and improved logging techniques may enable drillers to double that recovery fraction. It is much less expensive to reexamine existing wells, of which the oil companies have tens of thousands, than to drill new exploratory wells, 1,000 or 2,000 of which may be drilled each year in the southern United States and Gulf of Mexico. Small, independent well logging companies find a substantial market in relogging old wells, while the major oil field services companies (Baker-Hughes, Halliburton, Schlumberger, and Weatherford) win most of the contracts for logging while drilling. All well logging was done by wireline until the 1980s when measurement-while-drilling and logging-while-drilling tools first became available. Measurement while drilling provides data on the location and direction of the drill head and logging while drilling gathers information about the features of a formation to the surface while the drill head is still in the formation. Crude signals can be used to carry some minimal information from the logs to the surface as the borehole is being drilled. When coupled with the ability to direct the drill head (geosteer) toward promising targets, these techniques offer several advantages over wireline logging. With geosteering, the pitch of a borehole can be increased to a high angle relative to a vertical line to create a so-called deviated well. Such wells can follow one of the long dimensions of an oil or gas deposit, enabling much higher recovery fractions relative to a well that just traverses the short dimension of the deposit. Because of difficulties in data transmission, only critical data are transmitted, at a very low data rate (about 10 bits per second) from logging while drilling tools to the surface by mechanically vibrating the drilling fluid, which is called mud pulsing. More detailed data are stored in electronic data chips which are extracted from the logging collar when it is removed from the borehole. Another advantage of logging while drilling is in offshore drilling where the well is cased as the hole is drilled to prevent fluid intrusion into the well. In both deviated wells, where strings of instruments would get stuck, and offshore wells, where wireline logging is only possible through the borehole casing, logging while drilling is the most attractive option. In both wireline and logging while drilling, time is a critical factor. The cost of running operations on an offshore drilling rig is very high: drilling a well might cost $1â2 million per day of operations. In such operations, downtime and logging-equipment failures are expensive. Well logging equipment costs are only a small part of the cost of drilling operations and generally a very small fraction of the hydrocarbon production costs. Modifications that improve the accuracy of logging without compromising reliability of the data are welcome in the industry even if they raise the cost. As a result, many techniques have been used for well logging. Several techniques are discussed below. Well loggers use combinations of both radiation-based and non-radiation-based tools (called nuclear and nonnuclear tools in this field) to examine the earth formations surrounding the well and sensors to detect the mediaâs response to interrogation tools. An analyst examines detector logs to look for some or all of the following parameters of the formation: formation water saturation, porosity, rock characteristics, carbon/oxygen ratio, and permeability. Because of the complexity of earth formations, only a combination of all the logs allows the log analyst to draw accurate conclusions for the formation parameters. For example, combining resistivity and nuclear logs, the log analyst can determine porosity, water content, and density. Figure 9-1 illustrates a typical set of well logs. Even with multiple logs, well log analysis is an interpretive science in that it relies on data that do not uniquely determine the solution. Different well log analysts may, and often do, interpret the same logs differently. As a result, there are differences of opinion on which tools are the most important ones and which ones are valuable in what media and in combination with what other tools (see, e.g., differing views in Jacobsen et al., 2006; Badruzzaman, 2002; Ayan et al., 1999; Chang et al., 1993; Ellis, 1987).
WELL LOGGING 149 FIGURE 9-1 Typical logs and evaluation results from a string of well logging tools, including a natural gamma-ray log, a neutron log, an array induction log, and resistivity logs. The hydrocarbon volume result is also shown at the right. SOURCE: van Popta et al. (2004), courtesy of the Society of Petrophysicists and Well Log Analysts. For these reasons, the committee has not attempted to prioritize among the different tools, and no prioritization should be inferred from the order of presentation. The major tools for well logging are listed in Table 9-1. The tools are described in the sections that follow, first the nonnuclear well logging tools and then the nuclear well logging tools. NONNUCLEAR WELL LOGGING METHODS There are many nonnuclear well logging tools, including acoustic arrays, various resistivity tools, nuclear magnetic resonance tools, and formation pressure logs. Electric logs, which measure the resistivity of formations, were the first well logging technique to be used. In its simplest form, electric logging simply measures the resistivity between electrodes or transmitter/receiver coils on the sonde, which enables a well logger to identify the kind of rock within a layer and, to some extent, the likely porosity and fluid content of the pores. Most resistivity tools use induction coils through which a 20-kHz to 2-MHz signal is transmitted into the formation and induced signals from the formation are received to obtain formation resistivity.
150 RADIATION SOURCE USE AND RADIATION TABLE 9-1 Well Logging Tools Typical Method of Geologic Tool Logging Source of Signal Parameter Sought Nonnuclear Tools Resistivity LWD or wireline Electrodes Formation water saturation Spontaneous potential Wireline Electrodes Formation composition and water content Induction LWD or wireline Coils Formation composition and water content Radiofrequency (rf) Wireline rf antenna Formation dielectric composition and water content Formation pressure Wireline Pressure in formation Fluid content Acoustic Wireline Sonic transceiver Fluid content and porosity Nuclear magnetic Wireline Media in magnetic field Fluid content and resonance porosity Nuclear Tools Natural gamma LWD or Wireline K-40, Th, and U in Formation formation composition Gamma-gamma density LWD or wireline Cs-137 Formation density and shale content Neutron cross section LWD or wireline D-T accelerator Rock density and porosity Elemental composition LWD or wireline Am-Be or D-T accelerator Formation composition Neutron moderation LWD or wireline Am-Be or D-T accelerator Hydrogen content C/O ratio Wireline D-T accelerator Hydrocarbon and water content NOTE: Am-Be = americium-241âberyllium; C/O ratio = carbon-to-oxygen ratio; Cs-137 = cesium-137; D-T = deuterium-tritium; K-40 = potassium-40; LWD = logging while drilling; Th = thorium; U = uranium. SOURCE: Provided by committee. Occasionally, a dielectric constant tool, which measures the electrical permeability of the formation materials, is also used to help in identifying formation water content and rock types. The dielectric constant, which is another electrical characteristic of a material, is largely
WELL LOGGING 151 determined by water content in the material. The dielectric tool operates at microwave frequencies in the range of a few megahertz to 1.1 GHz. Sonic or acoustic logging is accomplished by measuring the sound velocity in the formation from a transmitter to a sensor in the sonde. Sound velocity is an indicator of porosity and fluid content for a given type of rock. More sophisticated information can be gathered from acoustic arrays. Nuclear magnetic resonance, which is commonly used as a medical diagnostic called magnetic resonance imaging, relies on the nuclear magnetic moment of atoms (and molecules) in a strong magnetic field and the signal they emit when they return (relax) to their original state. The technique is effective in measuring properties related to fluids, including the saturated porosity of the formation and the size of the pore spaces containing fluids, which in turn provide information about the permeability of the formation. Because of the dropoff in magnetic field amplitude over distance, the attenuation of fields in geologic media, and the mud on the walls of the borehole, nuclear magnetic resonance gathers data only on media in very close proximity to the well bore. Direct formation pressure measurements are used to calibrate other measurements from which pressure can be inferred. The calibration enables cross comparisons of results directly and indirectly linked to the formation pressure. NUCLEAR WELL LOGGING METHODS There are a number of nuclear well logging tools that have been and still are important in the evaluation of hydrocarbon wells and reservoirs. While the recent interest in logging-while- drilling tools has changed the emphasis somewhat, interest in nuclear tools has remained as high as, or higher than, ever. The nuclear tools play roles in the determination of a number of the most important hydrocarbon well characteristics such as porosity, elemental composition, and whether or not oil or water is present. The nuclear tools of primary interest use either sources of gamma rays or neutrons. The one exception to this is the natural gamma-ray tool, which has no source and detects the natural gamma rays that are present in the rock formation outside the borehole. This tool primarily identifies the depth or distance along the borehole where shale layers exist that contain naturally occurring potassium-40 or radionuclides in the uranium and thorium decay chains. The single gamma-ray tool uses a relatively small source of cesium-137 (55 to 148 GBq [1.5 to 2 Ci]), which makes these Category 3 sources) that uses gamma-ray backscatter to infer formation density outside of the borehole. The gamma-ray source logging tool is an important tool. The source has the same measurement advantages as all radionuclide sources that have a long half-life and a known, well-defined gamma-ray energy. These advantages are: (1) they are very stable; (2) they do not require power supplies, which are often unstable and usually bulky; (3) they require very little space; (4) their radiation is monoenergetic and at an optimum energy for this application; (5) they do not require complex operational procedures; (6) they are relatively inexpensive; and (7) they emit radiation isotropically. To offset these advantages the disadvantages are: (1) they cannot be âturned off,â (2) the gamma-ray energy cannot be changed, and (3) they represent a potential radiation safety risk if somehow lost or misused. These gamma sources are metal capsules containing the radioactive material, most commonly cesium-137, but other radionuclides in some cases. As noted above, these sources have relatively low activity (IAEA Category 3). They are fabricated in a vitrified form because of the aggressive environment in the boreholes. All other common nuclear logging tools use neutron sources. Two types of neutron
152 RADIATION SOURCE USE AND RADIATION sources are used: radionuclide sources1 and accelerator sources. Almost all of the radionuclide neutron sources are sealed sources that contain an alpha-emitting radionuclide mixed with beryllium or boron powder that is pressed and doubly encased in stainless steel. A beryllium or boron nucleus will absorb an alpha particle and emit a neutron with energy ranging from 0 to about 11 MeV with the average energy at about 4 MeV. The most commonly used radionuclide neutron sources are Category 2 or 3 Am-Be sources, although some plutonium-beryllium (Pu- Be) sources have been used in the past. A spontaneous fission source using californium-252 has been demonstrated as a replacement for Am-Be sources. This is discussed later in this chapter. Accelerator neutron sources are described in Chapter 4. They use an accelerated beam of ions to cause D-T fusion reactions in a target and typically produce 14.1-MeV neutrons, although with different targets they can be designed to cause different reactions that produce 2.45-MeV neutrons with much lower output. There are four main neutron source logging tools: 1. the neutron moderation tool, which primarily measures hydrogen content that can be related indirectly to porosity; 2. the C/O tool, which measures the ratio of carbon to oxygen, which can be indirectly related to oil and/or water content; 3. the formation neutron cross-section tool, which measures the neutron absorption cross section in both the borehole and the formation outside the borehole; and 4. the mineral or elemental analysis log, which measures parameters from which the chemical and mineral composition can be inferred. Radionuclide neutron sources are typically used for measurements of porosity and elemental analysis logs. The carbon to oxygen ratio (C/O) logs and neutron absorption logs (n-gamma logs) today typically use 14.1-MeV neutrons from D-T accelerators. Figure 9-2 shows a neutron logging tool, including the relationship and dimensions of the overall well logging tool to the much smaller Am-Be neutron source. Note that the well logging tool measures over 16 meters long and incorporates a complex set of detectors and electronics. Figure 9-3 is a photograph of a gamma-gamma density instrument, showing the source- handling device used to insert and remove the cesium-137 source. ALTERNATIVE TECHNOLOGIES X-ray generators offer one possible alternative to radionuclide radiation sources in well logging. Some work on this topic was reported in 1986 and 1987 by engineers at Schlumberger (Becker et al., 1987; King et al., 1987; Boyce et al., 1986). They investigated the use of a commercial radiography electron linac accelerator for well logging. Although such a source of x- rays technically would provide an alternative to gamma-ray radioisotope sources, the committee judges that they were then and still are impractical due to the large size of the device. One might hope that the size of these devices can and will eventually be reduced to make this a practical alternative to the use of cesium-137 sources. However, no further development work has been reported along these lines in the intervening 20 years. In addition, even if the size limitation could be ameliorated, there are a number of other disadvantages to this approach, including the fact that the source energy spectrum is continuous, not monoenergetic; and the source studied was unstable (King et al., 1987) and had to be normalized. The source emission 1 In well logging, these are commonly called âchemicalâ sources, but the committee here uses the term radionuclide sources for consistency.
WELL LOGGING 153 is forward scatterered, not isotropic, which could be an advantage, but would need to be factored into analysis of the result. As was already noted, there are at least four neutron source logging tools that are important in oil well logging: (1) the neutron moderation tool, (2) the mineral or elemental analysis tool, (3) the neutron cross-section tool, and (4) the C/O tool. The neutron moderation tool and the mineral or elemental analysis tool typically use the radionuclide neutron sources because these tools are more effective if the initial neutron energy is lower. Neutrons emitted by Am-Be sources have energies substantially lower than D-T fusion neutrons (average energies around 4 MeV versus 14.1 MeV). The C/O tool and the neutron cross-section tool normally use an accelerator D-T source because these tools use the gamma rays produced by neutron inelastic scattering, which favors higher neutron energies and is more easily measured if the neutron source is pulsed. a) b) FIGURE 9-2 Photograph of an Am-Be neutron source used in well logging (a) and a diagram of a sonde mounted on a drill for logging while drilling (b). SOURCE: (a) Image courtesy of Sandia National Laboratory (William Rhodes, III), (b) Provided by the committee. FIGURE 9-3 Atlas Densilog, which uses a vitrified cesium-137 source for wireline density measurements. The rod extending downward is a source handling device. The light-colored circles are windows for the source and detectors. SOURCE: Photo courtesy of Baker Hughes Inc.
154 RADIATION SOURCE USE AND RADIATION The neutron moderation tool measures the slowing down of neutrons by detecting the radiation (gamma or neutron) scattered back to the detector as the neutrons undergo collisions in the media. Higher energy neutrons penetrate more deeply into the media before much of the radiation is scattered back, and the intervening media shields the radiation to some extent. The high-energy neutrons emitted by the source are slowed to about thermal energy by the multiple scattering of the neutrons, primarily with hydrogen atoms, before they are detected by detectors that have their highest probability of detection with thermal energy neutrons. On average this may require anywhere from 15 to 200 scattering interactions per neutron from an Am-Be source (depending on the hydrogen content of the medium) and the average neutron path may be on the order of 50 cm. When 14.1-MeV neutrons are used, more scatters are required and the resulting average path length is longer, if only elastic scattering is of importance. As oil reserves are tapped (and formation saturation decreases) the neutron path length grows longer. To detect the same number of thermalized neutrons for the 14.1-MeV case, one requires a more intense source or a more sensitive detector: Because the neutrons penetrate more deeply and scatter farther away, the neutron detector would have to be placed at a greater distance from the source and the intensity of the signal diminishes as the inverse of the square of the distance. The same type of reasoning also applies to the elemental analysis tool except that in this case a longer distance would be required for neutron moderation before the (n,Î³) (pronounced ân-gammaâ) reaction occurs, which then produces a gamma ray that returns to the detector in the logging tool. So in both cases, a more intense source of neutrons or an improved detector would be required for the 14.1-MeV neutrons and a larger sample would be interrogated. However, elastic scattering is not the only mechanism of importance in slowing neutrons in well logging. Whenever oxygen and carbon are present (which is usually the case), inelastic scattering of high-energy neutrons becomes more important and neutron elastic scattering becomes less important. In this case, fewer elastic scatters may be required to thermalize neutrons for high-energy-source neutrons and the source-to-detector spacing required may be less for a higher energy neutron source. It is clear that this logging principle is complex. The elemental analysis tool relies on the (n,Î³) reaction in the geologic media, and that reaction is more probable at lower energies. In some materials, thermal neutron energies are best.2 Present accelerator sources are sealed and have a lifetime of several hundred hours; field service companies often bring two sources to a job in case one fails. Changing the sealed- source part of the accelerator is an added expense for oil-well logging companies, but this expense is justified because the pulsed, high-energy neutron source is advantageous. Note that there are some liabilities associated with accelerator sources, including the need to keep two tools on hand at any job site in case of tool failure and the regulatory and logistical burden caused by the dual-use nature of the D-T neutron generators (see Sidebar 9-1). Also, as mentioned in Chapter 4, D-T neutron generators do not obviate the need for radioactive waste management, although they constitute a lesser burden than the radionuclide radiation sources. 2 Thermal neutrons have energies corresponding to the temperature of the medium through which they travel. A typical average energy for thermal neutrons is around 0.025 eV. The average energy of neutrons from an Am-Be source is nearly 200 million times higher.
WELL LOGGING 155 SIDEBAR 9-1 Sealed D-T Neutron Generator Systems Are Nuclear-Related Dual-Use Technology Dual-use technologies are those that have both commercial and military or proliferation applications. Because a sealed pulsed neutron generator (one with no external vacuum system) can be used as a trigger for a nuclear explosive device, it is dual-use nuclear equipment and subject to the Export Administration Regulations, administered by the Department of Commerce. Federal regulations impose requirements on both domestic and international shipments and use of these accelerator-based neutron sources. Domestic shipments must be tracked. The regulations require that International shipments have an export license for each movement of a device outside the country, restrict the number of such shipments to and from each country, and impose both administrative costs and often delays in shipment. The regulations are the U.S. mechanism for implementing an international system of nonproliferation measures called the export control regime. One service company, Schlumberger, markets accelerator neutron porosity tools: a wireline tool called APS and an LWD tool called Ecoscope. APS was introduced in the early 1990s and has gained modest acceptance. It relies on detection of epithermal neutrons, and so the results are not readily comparable with results found with other tools. Ecoscope was brought to the field in 2005 and utilizes a different detector array which, with analysis, provides results that are more directly comparable to those from other tools (H. Evans, QSA Global, Verbal presentation to the Committee on Radiation Source Use and Replacement, Irvine, CA, February 1, 2007). Note that the neutron flux output from these tools is reported to be higher by a factor of 5 to 10 than that from Am-Be sources in their radionuclide-source counterpart tools. Schlumberger declined to quote the price of its tools for this report. Further, the company sells services using the tools, not the tools themselves. Anecdotal reports of costs for D-T tools that are for sale by other companies range between $40,000 and $50,000, which provides a rough estimate of the cost of the D-T porosity tools, although this range does not account for differences in the detector costs and the supporting software. Another interesting possibility is the replacement of Am-Be with californium-252 spontaneous fission sources. Investigations into using californium-252 for well logging date back to the late 1960s, but to the committeeâs knowledge, only Pathfinder Energy Services provides such a tool for LWD (see Valant-Spaight et al., 2006). Californium-252 sources (described in Chapter 2) emit fission neutrons and gamma rays with a neutron energy spectrum skewed lower (average neutron energy of about 2.14 MeV) than the Am-Be sources (average neutron energy of about 4.18 MeV). The Pathfinder tool uses a 0.41-GBq (0.011-Ci) californium-252 neutron source, which is a Category 4 source, instead of a 300-GBq (8-Ci) Am-Be source to emit neutrons at the same rate (1.8 Ã 107 neutrons/s) after 3.5 years. Valant-Spaight et al. (2006) report that the californium-252 source emits a factor of 15 lower gamma-ray exposure rate than the equivalent Am-Be source (although this may refer to the equivalent Am-Be source at the initial neutron emission rate, which is 740 GBq or 20 Ci). The californium-252 source costs approximately $6,000, and the tool costs in the range of $100,000 (Mitchell Ferren, Oak Ridge National Laboratory, personal communication with M. D. Lowenthal, June 13, 2007; W. Schultz, Pathfinder Energy Services, personal communication to M. D. Lowenthal, June 21, 2007). Both the californium-252 sources and the D-T accelerator sources have shorter working lives than the Am-Be sources. The californium-252 sources have a half-life of 2.65 years and need replacement after 4 or 5 years (they begin with a larger neutron output to enable longer operation). The operating life of pulsed neutron generators varies but typically might be around 500 hours. The Am-Be source, by contrast, has a half-life of about 433 years, and so it only needs to be replaced when it requires reencapsulation, which the manufacturers recommend as every 15 years.
156 RADIATION SOURCE USE AND RADIATION There is some incentive for well loggers to switch away from Am-Be sources. After the U.S. domestic supply from the Department of Energyâs (DOEâs) Isotopes Program was exhausted, REVISS was left as the only international supplier of americium sources, and the price of a well logging source climbed dramatically. Anecdotal reports of the costs are in the range of $80,000 to $100,000 each, with a two-year lead time on orders. There is a stock of Am- Be sources at Los Alamos National Laboratory collected through the Offsite Source Recovery Project. Some in the well logging industry have sought to have DOE recycle these sources, making them available to well loggers. The sources are planned to be disposed and no decision has been made within DOE to recycle them. In addition, the new U.S. NRC requirements for handling Category 2 sources has caused several companies to consider redesigning their tools to use sources just below the Category 2 threshold (0.6 TBq or 16 Ci for Am-Be). Even with the high cost of Am-Be sources, both the D-T accelerator neutron source and the californium-252 neutron source face a significant obstacle to being adopted more broadly within the well logging industry: The analyses of well logging data rely on a large body of data that has been accumulated for the porosity logs using Am-Be sources. These data would be less useful in analyzing the results from 14.1-MeV neutrons from D-T accelerators and fission neutrons from californium-252 sources until a substantial database using these sources accumulates. Even if these sources hold the promise of providing superior information, this database inertia must be overcome. Both D-T and californium-252 source replacements are now being studied by Monte Carlo simulation and, to some extent, by in-hole experiments, demonstrations, and operations with tools on the market, but more will be needed to make these replacements more broadly attractive. Schlumberger would argue that its Ecoscope tool is already being accepted. Others point out that the well log databases are not static: No two geological fields are the same; conditions change even in the same field; the same genre of tools from different vendors can give significantly different interpretations of petrophysical parameters (Badruzzaman, 2002); and new technologies are adopted because they add valuable information, and any new tool, even with the same old radionuclide sources, requires extensive calibration before (and after) it is fielded. Even accepting all these points, however, the committee observes a barrier to acceptance of a replacement tool that gives a measurement that differs from the old one. Field service companies report that upon introduction of a new tool, their customers ask if the tool can provide a result that looks like the old measurement. There is not an obvious role for the federal government in overcoming this and other obstacles to implementation of these alternatives. The industry itself, however, can form industry working groups, called Special Interest Groups under the Society of Petrophysicists and Well Log Analysts, to investigate questions and establish common practices across the industry. Such a group already exists concerning nuclear well logging tools. If so tasked, the group could develop new reference standards (measure standard signals from known reference rock formations) for these replacement tools, examine the response of these tools relative to the Am-Be tools, and explore any differences in response when the replacement tools are used in combination with other nuclear and nonnuclear well logging tools. Detector Technology The well logging industry continuously monitors developments in detector technology. Although this technology is being researched heavily by a rather large industry, advances appear to be relatively slow, especially for those that would drastically affect hydrocarbon well logging. Well logging for hydrocarbon recovery requires very rugged detectors that can be exposed to and will operate in the harsh environment of the well borehole. As noted in Chapter 4, so far the primary detectors being used are the sodium-iodide (NaI) scintillation detector for the natural gamma-ray and density backscatter gamma-ray tools and the helium-3 (He-3) gas
WELL LOGGING 157 proportional counters for neutron detection. The latter (He-3) detector replaced the gas proportional BF3 detectors perhaps 10 to 15 years ago. There is some recent use of bismuth germinate (BGO) scintillation detectors in place of the NaI detectors, but the trade-offs apparently do not greatly favor this replacement at present. Although the BGO detector has higher density and effective atomic number, which gives better total and full-energy peak detection capability, the temperature response and scintillation efficiency characteristics still favor NaI detectors. However, BGO is not the only higher density scintillation detector that is used as an alternative to NaI. A gadolinium silicate crystal (Gd2SiO5 or GSO) is available, which is slightly less dense than BGO but has better temperature characteristics than BGO (Roscoe et al., 1991). For more detail on detector technology, one should refer to Chapter 4 of this report, the standard text by Knoll (2000), and articles in the journal Nuclear Instruments and Methods, Parts A and B. FINDING AND RECOMMENDATION Finding: Accelerator neutron sources and californium-252 sources show promise as potential replacements for americium-beryllium sources in neutron well logging tools. However, there are technical obstacles for these replacement sources and they are at a disadvantage based on the extensive experience and data accumulated with americium- beryllium sources. Recommendation: The Society of Petrophysicists and Well Log Analysts should task an industry working group, called a Special Interest Group (SIG) such as the Nuclear Logging SIG, to address the technical obstacles to implementing replacements for the americium-beryllium sources used in well logging and the challenges of data interpretation. The group should decide what obstacles are most important, but the issues might include development of new reference standards for these replacement tools, examination of the response of these tools relative to the americium-beryllium tools, and exploration of any differences in response when the replacement tools are used in combination with other nuclear and nonnuclear well logging tools. Replacement of the cesium 137 sources used in well logging would be difficult because well loggers desire monoenergetic gamma rays, compact sources, and robust source forms for the cesium sources. These sources are also vitrified and not easily dispersed and are of Category 3 intensity. Thus, the committee judges that replacement of these sources is not a priority.