NMR well logging at Schlumberger - Wiley Online Library

PART 4 NMR Well Logging at Schlumberger ROBERT L. KLEINBERG Schlumberger–Doll Research, Old Quarry Road, Ridgefield, Connecticut 06877; e-mail: kleinberg@ridgefield. sdr.slb.com KEY WORDS: NMR; NMR instrumentation; borehole NMR; petroleum exploration; Schlumberger CMR wireline tool; oil and gas industry

INTRODUCTION Ever since 1927, when the first electrical borehole measurements were made in the French oil field at Pechelbronn, Schlumberger has prided itself on bringing advanced technology to the petroleum industry (1,2). Thus it should be no surprise that in the early 1950’s, Schlumberger scientists already were working on designs for nuclear magnetic resonance borehole logging instrumentation (3). Nicolaas Bloembergen, retained as a consultant, authored a series of internal memos, a patent application filed in 1954 (4), and a paper on the Overhauser effect (5). The 1950’s was a very busy decade for Schlumberger. Work on a practical NMR borehole logging tool was crowded out as a plethora of electromagnetic, gamma ray, and neutron instruments were introduced (6). In the meantime, Chevron took the lead in NMR instrumentation (7). When Schlumberger returned to NMR, it found engineering the Chevron design difficult. After several not very successful attempts to build a commercially viable tool, Schlumberger introduced the NMT-B (nuclear magnetism tool) in 1979 (8), followed by the NMT-CB in 1987 (9). These tools Received 2 May 2001; revised 22 June 2001; accepted 22 June 2001. Concepts in Magnetic Resonance, Vol. 13(6), 396–403 (2001) © 2001 John Wiley & Sons, Inc.

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generated a lot of interest, but not much revenue. The problems with these tools were a case study in the collision of good physics with the peculiarities of the oil industry: • To save rig time, three or four different types of logging tools (resistivity, acoustic, gamma ray, neutron, etc.) normally are run simultaneously. However, the NMT was not combinable with other borehole logging tools because the coil polarization required the full output of the wireline truck’s generating capacity. Moreover, it was a difficult tool to use, requiring a specialist engineer. • The proton Larmor frequency in the Earth’s magnetic field varies considerably with geographical location, from 2.6 kHz in Canada to 1.0 kHz in Argentina. Because the signal level is approximately proportional to Larmor frequency, a measurement that was acceptable in Canada was unacceptably weak in Argentina. Ironically, unusual formation properties made Argentina one of the primary potential markets for the tool. • To kill the proton signal from the borehole, the fluid in it had to be doped with magnetite. Drilling fluids (“muds”) are remarkably complex mixtures of liquid, polymeric, and solid components. Occasionally, the addition of magnetite adversely affected the chemical and rheological properties of the drilling mud. To the NMR spectroscopist this may appear to be a technical detail. To the drilling engineer, it is an expensive disaster, and one not to be repeated. Despite these vexing problems, the promise of nuclear magnetic resonance could not be denied. Throughout the 1960’s and 1970’s, researchers at Mobil (see, e.g., (10)), Shell (see, e.g., (11)), Chevron (see, e.g., (12)), and other oil companies worked on understanding laboratory NMR measurements of fluids in rocks. The laboratory work showed that NMR relaxation time gave rather direct information about pore sizes and, therefore, hydraulic permeability. Among the myriad

NMR WELL LOGGING AT SCHLUMBERGER

oil reservoir properties measured by a myriad of borehole logging tools, a glaring omission was a fast and reliable measurement of hydraulic permeability. Thus NMR was seen as being able to provide important reservoir information that could be obtained in no other way.

INSPIRATION FROM LOS ALAMOS The energy crisis of the 1970’s gave U.S. national laboratories impetus to support nonnuclear energy-related projects. One result was the construction of a novel borehole NMR tool at Los Alamos National Laboratory (13–15). Although many patents describing NMR magnet-and-coil devices for borehole application had been issued in the decades since 1950 (3), most of those designs looked improbable and there was no indication that any had ever been built. In contrast, the Los Alamos design showed real insight into the physics of NMR measurement: it was fully characterized and described in the scientific literature, and it was demonstrated to function as intended. The Los Alamos group also analyzed their NMR signals to obtain a distribution of pore sizes (16), foreshadowing today’s most important use of NMR logging tools. Although the Los Alamos design was never commercialized, it stimulated the thinking of all who came after. One might think that the U.S. Department of Energy would have publicized these successes to show that tax dollars spent in the national laboratory system contributed to the common good. Strangely, however, the story seems to be unknown except to a few specialists. The work at Los Alamos coincided with a renewed effort to understand the NMR properties of rocks. Insights were borrowed from NMR studies of biological cells (17) and synthetic materials (18). At Schlumberger–Doll Research, new methods were brought to bear to understand NMR properties of rocks (19,20), while traditional petrophysical techniques were used to establish a new correlation between NMR relaxation time and fluid permeability (21).

A NEW DESIGN FOR INSIDE-OUT NMR In the spring of 1985, Weng Chew, Douglas Griffin, and I found ourselves with time on our hands. We had just finished a project that was to mature

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into the successful Schlumberger oil base mud dipmeter (22) and we were looking for something to work on. Inspired by the new laboratory petrophysics and the novel Los Alamos design, we started thinking about new designs for borehole NMR apparatus. It was obvious from the outset that this project would be challenging. Reports from the Los Alamos group made it abundantly clear that small signal levels were going to be a major problem in designing a borehole NMR tool. This was no surprise, because every rule for designing NMR equipment was being broken. Laboratory NMR apparatus was moving to higher and higher frequencies, in part because the signalto-noise ratio scales as nearly the square of the Larmor frequency (23), but magnets simply cannot generate comparably high fields in external volumes. Moreover, what a laboratory spectroscopist would call the coil-filling factor—the efficiency with which RF fields are radiated to and from the sample—was absurdly low in the borehole situation, on the order of 10−2 . A design that pushed the sources of the static and RF fields as close as possible to the borehole wall would maximize both the coil-filling factor and the static magnetic field in the formation. That immediately suggested an instrument that contacted the borehole wall and projected the magnetic fields to one side. The first such idea was a Los Alamos-like magnet array, in which the magnets were canted toward the formation; see Fig. 4.1. However, moving the field sources closer to the formation sacrified resonated volume. To regain volume while still resonating only one side of the borehole, the volume of investigation had to be elongated. The pair of longitudinally magnetized bar magnets used in the Los Alamos design was abandoned in favor of a pair of slab magnets, elongated along the borehole axis and magnetized transversely; see Fig. 4.2. The idea of keeping the north poles in proximity to each other was retained from the Los Alamos concept. In the new design, there was a point inside the earth formation at which all spatial derivatives of the static magnetic field vanished. The length of the magnet array was obviously a critical parameter. The longer the magnets and RF antenna (which had not yet been designed), the larger the resonated volume and the larger the NMR signal. On the other hand, it is dangerous to make wall-engaging tools too long. Boreholes are often very rough, and a long wall-engaging tool face (“skid”) will lose contact with the wall when horizontal layers of rock are differentially eroded

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THE HISTORY OF NMR WELL LOGGING

Figure 4.1 First stage of evolution of the Schlumberger tool from the Los Alamos tool.

during drilling. A short skid is better able to follow the profile of the borehole and retain contact with the formation. Whereas it was obvious the NMR instrument would not look more than an inch or two into the formation, and because any amount of borehole fluid in the resonated volume would wreck the measurement, the skid had to be as short as possible. There is simply no way to rationally optimize the skid length, because the character of borehole rugosity differs from formation to formation. The only solution was to find other skid-type borehole logging tools with shallow depths of investigation that seemed to work in most wells. The Schlumberger electromagnetic propagation tool (EPT) fit that description. Its skid is 12 in. long, and that determined the length of the NMR skid. One idea that was quickly dismissed was to use coils to generate the static magnetic field. A sim-

Figure 4.2 Second stage of evolution: bar magnets to slabs (cross section).

ple calculation showed that power consumption would be prohibitive. After the experience with the NMT, Schlumberger was mindful of the problems associated with demanding more than a few hundred watts from the downhole power system. Superconducting magnets, even if the newly discovered high TC materials were used, required cooling systems that are hard to engineer. Certainly there was no point to undertaking that task unless absolutely necessary. Modern permanent magnet materials easily met all the requirements for an NMR tool without drawing any power or requiring a cooling system. A patent search was conducted to see if an instrument of the type contemplated already had been invented. By the mid-1980’s, an amazing number of patents had been issued for borehole NMR devices (3). Most of them described simple bar or horseshoe magnets. The design closest to that shown in Fig. 4.2 was patented by Schwede and was assigned to Schlumberger in 1970. It featured a magnet elongated along the borehole axis and magnetized transversely, resonating an elongated volume on one side of the borehole. However, north and south poles faced each other, and so failed to produce a line on which all spatial gradients of the static field vanished.

LABORATORY PROTOTYPE Armed with a basic idea, the “pulse nuclear magnetism tool” (PNMT) project officially started in the summer of 1985. Griffin and I continued on the project; Chew returned to his management responsibilities, and later left to take a professorship in the electrical engineering department of the University of Illinois in Urbana– Champaign. Chew was replaced by Apo Sezginer, a new Ph.D. electrical engineer from MIT. Masafumi Fukuhara from Schlumberger engineering in Fuchinobe, Japan, also was added. There was no clear division of responsibilities within this group—everyone worked on everything. However, Sezginer brought a highly disciplined approach to instrument design and quickly became the chief architect of the tool. Working within a set of predetermined housing diameters, he optimized the shapes, sizes, and positions of the magnets to give the largest possible field in the largest possible external volume. This could be done with considerable confidence because modern magnet materials can be modeled accurately.


Designing the megahertz RF antenna was much harder. Designs that theoretically were most efficient turned out to be controlled by parasitic effects that were hard to model accurately. Thus antenna development was very much a cut-andtry laboratory exercise, and all kinds of ideas were tried: many turns, few turns, coils made out of foil, coils made of litz wire, coils wound on ferrite, coils wound in various orientations, etc. Finally, coils were abandoned, and a simple half-coaxial antenna adopted. Even that idea went down some evolutionary dead ends before being perfected. Figure 4.3 shows the PNMT design at this stage. Given the signal-to-noise limitation, every means was exploited to make the measurement as efficient as possible. The antenna was filled with ferrite, which should have improved the signal level by about a factor of 3. However, it quickly was discovered that the ferrite was saturated, and thereby rendered useless, by the large static field generated by the magnets. This led to development of a ferrite-loaded antenna protected from the magnets with a permeable steel shell. Another important step was to make the antenna deadtime as short as possible, allowing the accumulation of spin echoes at a rapid rate. To do this, a large magnetoacoustic ringing signal had to be quelled by a combination of mechanical design and a phase alternated pulse sequence. The resulting laboratory prototype NMR instrument and the electromagnetic theory of the antenna are described elsewhere (24, 25). The PNMT group decided to build its own NMR spectrometer, because in the mid-1980’s there were no commercial NMR spectrometers that operated at the low Larmor frequencies—

Figure 4.3 Final configuration.

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around 1 MHz—that characterized the inside-out NMR concept. The consensus of informed opinion was that this task should be contracted out to an NMR engineering company. However, it was felt that the only way to really understand the measurement was to build the equipment inhouse. There were several novel circuits to design and a commercial AM radio station with a transmitter 1000 yd from the Schlumberger laboratory to compete with. The major capital purchase was a 1 kW, 10 kHz to 10 MHz, Class AB, unconditionally stable, 16-ft3 power amplifier that cost $18,000. After it was ordered but before it was received, Irving Lowe, the widely respected pioneer of NMR instrumentation, told the PNMT group that under no circumstances should a power amplifier be purchased from that manufacturer. The amplifier showed up and, after a few internal modifications, worked flawlessly. The spectrometer is described elsewhere (26). During that period, a number of visitors reviewed the nascent Schlumberger NMR tool. One visitor was John Deutch, MIT professor and later Assistant Secretary of Defense and then Director of the Central Intelligence Agency. A furious 20-min argument ensued over whether NMR measurements had anything to do with hydraulic permeability. Another visitor was the late Jerome Wiesner, science advisor to President Kennedy and president of MIT. Wiesner had worked on radar duplexers at the MIT Radiation Laboratory during World War II. The Schlumberger team knew this and casually mentioned that “we invented our own duplexer for the PNMT, superior to any of the standard radar designs.” “Let me see that schematic,” Wiesner said as he grabbed the circuit diagram. By late summer 1986 everything was together. In the best tradition of Schlumberger Research, the laboratory model sonde was square, “suitable for logging square boreholes.” It is shown in Fig. 4.4, sitting out on the lawn for its first “field test.” The first spin echoes were observed on 12 September 1986. Shortening the CPMG echo spacing to 0.16 ms and improving the efficiency of the antenna consumed much of the next year. To make sure the relative motion of the sensor and the sample would not make the NMR signal disappear and that speed corrections to the relaxation times were small and predictable, Fukuhara built a flow loop to circulate water through the sensed volume of the tool. By late 1987, the project was ready to go to Schlumberger Engineering in Houston.

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Figure 4.4 “Field test” of the PNMT prototype, “suitable for logging square holes.”

It is ironic that, during this period, the Schlumberger group also invented a tool similar to what would became the NUMAR MRIL (27). Sezginer proposed a cylindrical ferrite magnet, magnetized transversely and wrapped with a coil whose windings were distributed according to the cosine function. The tool would be centralized in the borehole and resonate a cylindrical shell. The cosine coil form already had been cut when it was noted that only a thin shell, of thickness about 1 mm, would be resonated. It was believed that the lateral motion of the tool during logging would ruin the measurement, so the idea was dropped without filing a patent application.

MEASUREMENT TECHNIQUE: T 1 VERSUS T2 Building the hardware was only part of developing a viable tool. The development of the measurement technique also had its pitfalls and surprises. Until the late 1980’s, all laboratory experiments on rocks were based on measurements of T1 , avoiding T2 . The reason for this is simple. Rocks filled with oil and/or water are granular porous media with a significant content of paramagnetic ions in the solid phase and a relatively nonmagnetic liquid phase. Thus there are strong internal magnetic field gradients, and the transverse relaxation of the fluids is dominated by molecular diffusion in these gradients (28). This makes T2 hard to interpret. In contrast, T1 is generally linearly proportional to pore size and thus embodies the most interesting petrophysical information.

Unfortunately, determination of the longitudinal magnetization decay requires a time-consuming series of inversion-recovery measurements. Due to the inherently low signal-to-noise ratio—limited primarily by the low magnetic field intensity generated by the tool in the surrounding rock formation—there was no way a magnetic resonance logging tool could measure T1 by inversion recovery while moving at commercially acceptable speeds in the borehole. In utter desperation, the fast-inversion-recovery-CPMG (FIR-CPMG) pulse sequence was invented; it measures T1 and T2 simultaneously, faster than any of 60 other published methods (29). It was expected that T2 would be a useless by-product of this method and would be discarded. When simultaneous T1 and T2 measurements were made on rocks using the laboratory prototype, it was found, to the considerable surprise of the experimenters, that T1 and T2 were highly correlated (30). Systematic investigation by Kleinberg and Horsfield (31) showed that transverse relaxation due to internal gradients was negligible at proton resonance frequencies of 5 MHz and below. Fortuitously, the borehole tools are not able to generate high static magnetic fields and, therefore, do not generate large internal magnetic field gradients in rocks. Static field gradients due to the tool magnets either are known apparatus constants or are negligible. A CPMG determination of T2 is much faster than any method of measuring T1 . Since the logging tool operated at about 2 MHz, T2 became the measurement of choice, and all the old petrophysical correlations with T1 were restated in terms of T2 (32). By a curious twist of fate, the apparatus property that prevents efficient measurement of T1 —low magnetic field strength—is exactly the same property that permits meaningful measurement of T2 . An additional reason to measure T2 in preference to T1 was discovered by Ridvan Akkurt when he worked at Schlumberger in 1989–1990. Akkurt modeled the response of the tool, running the FIR-CPMG pulse sequence, to layered formations of various descriptions. He found that T1 results were erratic, depending on the exact position of the tool with respect to a bed boundary at the start of a pulse sequence. In contrast, the computed T2 logs were well behaved, varying smoothly and consistently at bed boundaries (30,33). This confirmed T2 as the primary borehole measurement.


NMR PROPERTIES OF ROCKS While hardware development was proceeding, there was an acute awareness of the need to develop methods to understand the data the logging tool would collect. There was a sense of minor frustration because it was suspected that a lot of the needed information was locked away in confidential oil company files. Lacking the key to unlock those files, Schlumberger launched a major effort to measure and understand NMR properties of rocks. Four distinct avenues were pursued: petrophysical correlations, studies of the mechanism of relaxation at the pore–grain interface, studies of diffusion in porous media, and imaging. The study of petrophysical correlations was led by William Kenyon, Christian Straley, and James Howard. A major theme was understanding how nonexponential magnetization decays of rocks related to quantities of interest to the oil industry. NMR-derived pore size distributions of rocks were compared to thin-section images and mercury porosimetry measurements (34). A very fruitful approach was to compare NMR relaxation time distributions before and after centrifugation of water-filled rocks. This established a correlation between borehole NMR measurements and capillary-bound (nonproducible) water in oil reservoirs, the basis of the most widespread practical application of the borehole NMR tools (35). Estimation of the fraction of capillary-bound water has been extended more recently to rocks in which water molecules can sample a bimodal pore size distribution before being relaxed (36). It is well known that NMR relaxation times of bulk fluids vary strongly with temperature (37). The PNMT group was afraid that the petrophysical correlations developed by their colleagues on the basis of laboratory measurements at 40◦ C would be useless at hydrocarbon reservoir temperatures, which can be as high as 175◦ C. A simple but effective high temperature wind tunnel and a pressure vessel capable of maintaining water in its liquid state at high temperature were constructed by Lawrence Latour and used to measure the temperature dependence of T1 and T2 of water-filled rocks between 25◦ and 175◦ C. To the amazement of the Schlumberger group, little or no temperature dependence was found, at least not in the small group of randomly selected rocks used in the investigation (38). This was the central observation that led to a theory of NMR relaxation of fluids at a fluid–solid interface (39,40).

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The third major focus of the NMR group at Schlumberger in the late 1980’s and early 1990’s was on restricted diffusion of fluids in porous media. A large effort was devoted to research in this area. P. Sen, P. P. Mitra, and L. M. Schwartz led the effort on the theoretical side, while E. J. Fordham, L. L. Latour, M. D. Hurlimann, and I were the principal contributors on the experimental side. Imaging of rocks during drainage or filtration was pursued in large measure by E. J. Fordham, L. M. Schwartz, and C. Straley, as well as by many groups outside of Schlumberger. A review of the extensive results of these investigations is outside the scope of this history.

COMMERCIAL TOOLS The prototype PNMT tool built at Schlumberger– Doll Research was designed strictly for laboratory use. It could not survive immersion in a bathtub, let alone in a borehole filled with hot salt water or oil. For the purposes of building borehole prototypes and for eventual commercialization, the project was transferred to Schlumberger Engineering in Houston in 1988. Engineering retained the Research design while introducing many refinements. The center magnet shown in Fig. 4.3 was not essential and was removed to make room for electronic circuits. The magnets were segmented and the magnetization vector of each segment was adjusted to optimize the magnetic field profile. The antenna efficiency was improved and the deadtime was reduced. There was a steady improvement in the electronics. Roughly speaking, the signal-to-noise ratio improved by 1 dB per year; there is a story connected with each decibel. The experimental prototype of the renamed combinable magnetic resonance (CMR) tool logged its first client well in Lea County, NM on 15 December 1991. A year-long North American field test campaign followed (41). Four copies of the engineering prototype tool went into worldwide service early in 1995, followed rapidly by a full production run. Practical experience in the oilfield prompted important improvements in both measurement technique and interpretation of borehole NMR data. These efforts were led in large measure by Robert Freedman and Chris Morriss in Houston, and Charles Flaum in Ridgefield. Harold Vinegar of Shell was responsible for many ingenious and practical innovations. Martin Hurlimann definitively modeled the spin dynamics of the tool. A

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second-generation tool, of the same basic design but with longer magnets and an improved measurement procedure, went into service in 2000 (42). Ever since the Schlumberger brothers invented well logging in the early part of the twentieth century, it has been necessary to stop drilling and extract miles of drill pipe before the well logging instruments can be lowered into the borehole on a cable. The expensive drilling rig stood idle while the measurements were being made. In the 1990’s, logging-while-drilling made its appearance. Sections of drill pipe immediately above the drill bit are replaced by measuring instruments in very strong steel housings. Data are stored in computer memory, which is uploaded whenever the drill bit is returned to the surface for replacement. There is also a very limited real-time telemetry capability. NMR is the latest technology to be implemented in logging-while-drilling systems. Measurements are very difficult due to the violent motion and severe shocks associated with the drilling process, but some simple NMR measurements can be made (43). Schlumberger continues to improve its NMR technology. The veterans of the 1980’s have been augmented by new groups of researchers in both Ridgefield and Houston. Recent advances in instrument design, measurement methodology, and signal processing, not yet published, inspire humility in this author.

REFERENCES 1. Allaud L, Schlumberger MM. The history of a technique. New York: Wiley; 1977. 2. Bowker GC. Science on the run: Information management and industrial physics at Schlumberger 1920–1940. Cambridge, MA: MIT Press; 1994. 3. Jackson JA, Mathews M. Nuclear magnetic resonance bibliography. Log Analyst 1993; 34(3):35–69. 4. Bloembergen N. Paramagnetic resonance precession method and apparatus for well logging. U.S. Patent 3,242,422, 1966. 5. Codrington RS, Bloembergen N. Overhauser effect in manganese solutions in low magnetic fields. J Chem Phys 1958; 29:600–604. 6. Segesman FF. Well logging method. Geophys 1980; 45:1667–1684. 7. Brown RJS, Gamson BW. Nuclear magnetism logging. Petrol Trans AIME 1960; 219:199–207 8. Herrick RC, Couturie SH, Best DL. An improved nuclear magnetism logging system and its application to formation evaluation. Paper 8361, Society of Petroleum Engineers, 1979.

9. Chandler RN, Kenyon WE, Morriss CE. Reliable nuclear magnetism logging—With examples in effective porosity and residual oil saturation. 28th SPWLA Annual Logging Symposium, 1987, Paper C. 10. Woessner DE. NMR spin echo self diffusion measurements on fluids undergoing restricted diffusion. J Phys Chem 1963; 67:1365–1367. 11. Loren JD, Robinson JD. Relations between pore size, fluid and matrix properties, and NML measurements. Soc Petrol Eng J 1970; 10:268–278. 12. Timur A. Producible porosity and permeability of sandstones investigated through nuclear magnetic resonance principles. Log Analyst 1969; 10(1): 3–11. 13. Cooper RK, Jackson JA. Remote (inside-out) NMR. I. Remote production of a region of homogeneous magnetic field. J Magn Reson 1980; 41:400–405. 14. Burnett LJ, Jackson JA. Remote (inside-out) NMR. II. Sensitivity of detection for external samples. J Magn Reson 1980; 41:406–410. 15. Jackson JA, Burnett LJ, Harmon JF. Remote (inside-out) NMR. III. Detection of nuclear magnetic resonance in a remotely produced region of homogeneous magnetic field. J Magn Reson 1980; 41:411–421. 16. Brown JA, Brown LF, Jackson JA, Milewski JV, Travis BJ. NMR logging tool development: Laboratory studies of tight gas sands and artificial porous material. Paper 10813, Society of Petroleum Engineers, 1982. 17. Brownstein KR, Tarr CE. Importance of classical diffusion in NMR studies of water in biological cells. Phys Rev A 1979; 19:2446–2453. 18. Gallegos DP, Smith DM. A NMR technique for the analysis of pore structure: Determination of continuous pore distributions. J Coll Interface Sci 1988; 122:143–153. 19. Lipsicas M, Banavar JR, Willemsen J. Surface relaxation and pore sizes in rocks—A nuclear magnetic resonance analysis. Appl Phys Lett 1986; 48:1544– 1546. 20. Banavar JR, Schwartz LM. Probing porous media with nuclear magnetic resonance. In: Klafter J, Drake, JM, editors. Molecular dynamics in restricted geometries. New York: Wiley; 1989. 21. Kenyon WE, Day PI, Straley C, Willemsen JF. A three-part study of NMR longitudinal relaxation properties of water-saturated sandstones. Soc Petrol Eng Form Eval 1988; 3:622–636; Erratum. 1989; 4:8. 22. Kleinberg RL, Chew WC, Griffin DD. Noncontacting electrical conductivity sensor for remote, hostile environments. IEEE Trans Instrum Meas 1989; 38:22–26. 23. Hoult DI, Richards RE. The signal to noise ratio of the nuclear magnetic resonance experiment. J Magn Reson 1976; 24:71–85.

NMR WELL LOGGING AT SCHLUMBERGER 24. Kleinberg RL, Sezginer A, Griffin DD, Fukuhara M. Novel NMR apparatus for investigating an external sample. J Magn Reson 1992; 97:466–485. 25. Sezginer A, Griffin DD, Kleinberg RL, Fukuhara M, Dudley DG. RF sensor of a novel NMR apparatus. J Electromagn Waves Appl 1993; 7:13–30. 26. Griffin DD, Kleinberg RL, Fukuhara M. Low frequency NMR spectrometer. Meas Sci Technol 1993; 4:968–975. 27. Taicher Z, Coates G, Gitartz Y, Berman L. A comprehensive approach to studies of porous media (rocks) using a laboratory spectrometer and logging tool with similar operating characteristics. Magn Reson Imaging 1994; 12:285–289. 28. Glasel JA, Lee KH. On the interpretation of water nuclear magnetic resonance relaxation times in heterogeneous systems. J Amer Chem Soc 1974; 96:970–978. 29. Sezginer A, Kleinberg RL, Fukuhara M, Latour LL. Very rapid simultaneous measurement of nuclear magnetic resonance spin-lattice relaxation time and spin-spin relaxation time. J Magn Reson 1991; 92:504–527. 30. Kleinberg RL, Straley C, Kenyon WE, Akkurt R, Farooqui SA. Nuclear magnetic resonance of rocks: T1 vs T2 . Paper 26470, Society of Petroleum Engineers, 1993. 31. Kleinberg RL, Horsfield MA. Transverse relaxation processes in porous sedimentary rock. J Magn Reson 1990; 88:9–19. 32. Straley C, Rossini D, Vinegar H, Tutunjian P, Morriss C. Core analysis by low field NMR. Paper SCA9404, Society of Core Analysts, 1994. 33. Akkurt R. Effects of motion in pulsed NMR logging. Ph.D. Dissertation, Colorado School of Mines, 1990. 34. Kenyon WE, Howard JJ, Sezginer A, Straley C, Matteson A, Horkowitz K, Ehrlich R. Pore size distribution and NMR in microporous cherty sandstones. 30th Annual SPWLA Logging Symposium, 1989, Paper LL. 35. Straley C, Morriss CE, Kenyon WE, Howard JJ. NMR in partially saturated rocks: Laboratory insights on free fluid index and comparison with borehole logs. 32nd Annual SPWLA Logging Symposium, 1991, Paper CC. 36. Ramakrishnan TS, Schwartz LM, Fordham EJ, Kenyon WE, Wilkinson DJ. Forward models for nuclear magnetic resonance in carbonate rocks.

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39th Annual SPWLA Logging Symposium, 1998, Paper SS. Bloembergen N, Purcell EM, Pound RV. Relaxation effect in nuclear magnetic resonance absorption. Phys Rev 1948; 73:679–712. Latour LL, Kleinberg RL, Sezginer A. Nuclear magnetic resonance properties of rocks at elevated temperatures. J Colloid Interface Sci 1992; 150:535–548. Kleinberg RL, Kenyon WE, Mitra PP. Mechanism of NMR relaxation of fluids in rock. J Magn Reson A 1994; 108:206–214. Foley I, Farooqui SA, Kleinberg RL. Effect of paramagnetic ions on NMR relaxation of fluids at solid surfaces. J Magn Reson A 1996; 123:95–104. Morriss CE, MacInnis J, Freedman R, Smaardyk J, Straley C, Kenyon WE, Vinegar HJ, Tutunjian PN. Field test of an experimental pulsed nuclear magnetism tool. 34th Annual SPWLA Logging Symposium, 1993, Paper GGG. McKeon D, Cao Minh C, Freedman R, Harris R, Willis D, Davies D, Gubelin G, Oldigs R, Hurlimann M. An improved NMR tool design for faster logging. 40th Annual SPWLA Logging Symposium, 1999, Paper CC. Speier P, Crary S, Kleinberg RL, Flaum C. Reducing motion effects on magnetic resonance bound fluid estimates. 40th Annual SPWLA Logging Symposium 1999, Paper II.

SUMMARY OF PART 4 Following the Los Alamos project, Schlumberger also developed a new magnet/RF coil configuration. Their magnet geometry evolved from the Los Alamos configuration, but is an accepted tool that produces a long, slender region of homogeneous B0 field inside the formation. Schlumberger has long been a pioneer in well logging technology and has successfully marketed their new combined magnetic resonanceTM logging tool in the petroleum industry. These new logging tools led to the development of new data acquisition and handling techniques that took advantage of their capabilities. These are described in Part 5.