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NMR logging tools
Compared to laboratory or clinical nuclear magnetic resonance (NMR), the dimensions of a typical borehole and the nature of continuous logging impose severe constraints on the physics, equipment, and operation of NMR logging tools. Unlike laboratory devices, logging measures an external sample by use of a weaker magnetic field while in motion relative to the sample.
The NML tool—the first generation of NMR logging (1960–1994)—was an Earth’s field device that measured the free-induction decay in the Earth’s magnetic field. Proton polarization (alignment) was achieved using a magnetic field produced by a coil energized with a strong direct current. Each experiment required several seconds to allow complete polarization. The power was turned off, and the same coil was used to receive the free-induction signal.
There were a number of problems inherent in this technology. NML devices measured the proton signal in the borehole fluid as well as in the surrounding formation. Magnetite powder was circulated into the mud system to cancel the borehole hydrogen signal thereby preventing it from overwhelming the formation signal. This was a time-consuming process. The low intensity of the Earth’s magnetic field (0.5 gauss) resulted in a low S/N. The NML had a large dead time (i.e., time between the cessation of the static magnetic field and the first measurement), approximately 20 ms; pore fluids with relaxation times less than the dead time were not measurable. The initial signal amplitude had to be extrapolated backwards from the subsequent free-induction decay data. The NML could not distinguish between oil and water or measure total porosity; however, the dead time was close to the T2 cutoff established for irreducible water saturation. The NML could detect movable fluids and provided a measurement called the Free Fluid Index (FFI). An expanded discussion of the NML technology can be found in Brown and Neuman and Brown.
Unlike conventional magnetic resonance imaging (MRI) devices in which the sample is placed inside the stationary coil, a borehole-logging device investigates a sample (rock volume) that is outside the device itself while moving along the borehole. This setup has been called the "inside-out" NMR problem because the geometry of the magnets and coils is inverse to that used in laboratory-NMR spectrometers. To obtain useful measurements, a logging tool must generate a large, radially symmetric static magnetic field and also a high-frequency oscillating magnetic field. Each must be capable of penetrating one or more inches into the formation surrounding the borehole. The diameter of a typical well limits the size of the permanent magnets that can be used and, therefore, the strength of the magnetic field that can be generated by a logging device. In comparison with laboratory-NMR and clinical-NMR devices, which may operate at 10 MHz and the static magnetic fields of which are commonly in the range 1 to 2 Tesla (high field), modern logging tools and laboratory spectrometers designed for petroleum investigations are considered low-field devices. They operate at or below 2 MHz and generate relatively weak (< 200 gauss) and inhomogeneous magnetic fields (i.e., gradients up to 20 gauss/cm). By comparison, the NML operating frequency was only 2 kHz, and the Earth’s magnetic field is only 0.5 gauss. These factors limit borehole investigation to protons (hydrogen) and the use of relaxation data only. Chemical shifts (widely used in the biological field) are not observable. Furthermore, to compensate for the lower S/N that results from low-field intensity, logging tools must acquire more echoes and/or stack data to improve S/N. As mentioned previously, with the introduction of pulse-echo tools, T2 became the primary acquisition mode because it permitted faster logging—a major factor in high-cost wells.
This new design provides a number of operational advantages over the NML:
- Using permanent magnets rather than electromagnets reduces the surface-power requirement.
- Focusing the sensitive region of the magnetic field at some distance into the formation eliminates the requirement for suppressing the mud signal.
- Using an RF pulse from a coil tuned to the Larmor frequency ensures that only nuclei in the sensitive region are in resonance.
- Controlling the pulse duration means shorter dead times that, in turn, allow a better estimate of initial decay amplitude (porosity) measurement for short T2 components (bound-fluid evaluation).
- Enabling more sophisticated pulse sequences allows for measurement of additional rock and fluid properties.
One initially unwanted product of this design is the creation of gradients in the static magnetic field that causes molecular diffusion. The strength of the magnetic field gradient, G, is controlled by tool design and configuration (i.e., tool size and tool frequency); by environmental conditions such as formation temperature; and by internal gradients induced by the applied field, B0. Subsequent characterization of these gradients has enabled the in-gradient diffusion to be used for hydrocarbon typing.
From the beginning, a debate in NMR logging has been whether to use a centralized or eccentered tool design. There are two different wireline designs in current commercial service: #Numar (now part of Halliburton) magnetic resonance imaging tool (MRIL), a centralized mandrel device
- Schlumberger combinable magnetic resonance (CMR) tool and the Baker Atlas MREX eccentered devices, both of which require contact with the borehole wall.
The general designs of both the MRIL (mandrel-type) and CMR (pad-type) have evolved over the past decade with the addition of new capabilities and faster logging speeds, made possible by improved electronics and improved data acquisition and processing. Computalog is field testing a prototype mandrel device: the Nuclear Magnetic Resonance Tool (NMRT) developed by NPF Karotazh.
A centralized NMR logging tool like the MRIL must be longer than a pad device, simply to contain magnets of sufficient strength to project the required magnetic field, through the borehole and into the formation. This factor results in a greater sensitivity to borehole salinity than a pad device, which can exclude the mud from its measurement. The MRIL design generates relatively thin (1 to 2 mm) sensitive volumes, but the reduced S/N that accompanies these volumes is compensated by the vertical size of the sensitive area. It also generates a relatively high magnetic gradient. In contrast, a contact device, such as the CMR, can use smaller magnets and electronics, which provide higher vertical resolution but a shallower depth of investigation (DOI) and greater sensitivity to borehole rugosity. In addition to the standard permanent magnets, some designs now include "prepolarization" magnets, which are added to ensure full polarization at typical logging speeds.
The latest wireline NMR-logging tools operate simultaneously in several RF frequencies to measure (image) multiple sample volumes. In the presence of a gradient magnetic field, pulses with different frequencies will cause protons in different (and parallel) regions of space (i.e., measurement or sensitive volumes) to resonate. Cycling through several frequencies excites protons in different cylindrical volumes, allowing measurements to be made more quickly. If the frequencies of multifrequency measurements are very close together, then the sensitive volumes are very close together; and, for practical purposes, the rocks sampled can be considered to be the same. This principle is the same as that used for slice selection in medical MRI imaging. These tools acquire multiple echo trains using different values of TW, TE, and variable magnetic gradients (G) in a single logging pass. The time between measurements made at multiple frequencies can be as little as the time of an echo train, and the time between measurements made at a single frequency is essentially the time needed to repolarize (TW). The thickness of these sensitive volumes may be as small as 1 mm. Furthermore, recent advances in tool design now permit cost-effective T1 acquisition. Table 1 summarizes the capabilities, advantages, and disadvantages of T1 and T2 acquisition. These differences influenced the designs of the LWD tools discussed in the next section. Multifrequency operation provides measurements at multiple DOIs (typically 1 to 4 in.). This allows invasion effects to be accounted for in the data interpretation, thus enabling determination of near-wellbore fluid saturation and oil properties at high resolution.
Downhole NMR spectrometer
Contamination of hydrocarbon reservoirs by oil-based mud (OBM) and synthetic oil-base mud (SOBM) is a significant problem for accurate reservoir and fluid analysis in wells in which OBMs are used, especially offshore. Direct knowledge or an estimate of the OBM’s NMR characteristics is required to distinguish it from connate oil in fluid samples obtained by formation testers. Acquisition of T1, T2, and D0 (the self-diffusion coefficient) permits evaluation of connate oil, oil viscosity, and gas/oil ratio. Furthermore, in costly drilling environments, real-time acquisition of NMR properties permits immediate, rather than delayed, fluid evaluation. Information on NMR fluid properties is also valuable for the interpretation of NMR wireline and LWD logs.
The recently introduced downhole NMR spectrometer, incorporated into a formation-testing tool, can obtain NMR measurements of OBM contamination directly, on live samples at in-situ conditions. As the testing tool pumps fluid from the reservoir into the borehole or sample chamber, the spectrometer—using a measurement time of 30 seconds—measures the hydrogen index (HI), T1, T2, and diffusion. The T1 measurement is made while flowing; T2 and diffusion are static measurements. The T1 distribution is important in differentiating between highly refined OBM filtrates and native oil. The T1 characteristics of these common filtrates are measured and cataloged so that the data from oil-based filtrates can be distinguished from native hydrocarbons.
NMR-log data are presented in a variety of formats designed to emphasize specific aspects of the data and thus enable rapid visual interpretation of movable and immovable fluids, porosity, and permeability. Data interpretation is further enhanced when additional log and core information are also included in the log presentation.
The T2 distribution is typically displayed in waveform presentation, image (variable-density log, VDL) format, and bin-distribution plot. Each T2 format represents the distribution of the porosity over T2 values, and hence, over the pore sizes (Figs.1 and 2).
Fig.1 – Log presentation illustrating some of the formats used to present T2 distributions: Track 1, a plot of the cumulative amplitudes from the binned T2-distribution; Track 2, shallow, medium, and deep resistivity log curves; Track 3, a color VDL image of the binned T2 distribution; Track 4, a waveform presentation of the same information. The displayed T2 distribution typically corresponds to binned amplitudes for exponential decays that may range from 0.5 to 1,024 ms. Logging data are much noisier than laboratory data, and the T2 distribution shown on well logs is comparatively coarse.
NMR tool mnemonics
Table 2 presents a cross reference of logging-tool output-data mnemonics for the different service companies currently offering NMR-logging services.
|D0||=||molecular diffusion coefficient, gauss/cm|
|G||=||field-strength gradient, gauss/cm|
|T1||=||longitudinal relaxation time, seconds|
|T2||=||transverse relaxation time, seconds|
|TE||=||CMPG interecho spacing, seconds|
|TW||=||polarization (wait) time, seconds|
- Dunn, K.-J., Bergman, D.J., and LaTorraca, G.A. ed. 2002. Nuclear Magnetic Resonance—Petrophysical and Logging Applications, Vol. 32. New York: Handbook of Geophysical Exploration: Seismic Exploration, Pergamon Press.
- Brown, R.J.S. and Neuman, C.H. 1982. The Nuclear Magnetism Log--A Guide For Field Use. The Log Analyst 23 (5): 4. SPWLA-1982-vXXIIIn5a1.
- Brown, R.J.S. 2001. The Earth's-field NML development at Chevron. Concepts in Magnetic Resonance 13 (6): 344-366. http://dx.doi.org/10.1002/cmr.1020
- Cooper, R.K. and Jackson, J.A. 1980. Remote (Inside-Out) NMR. I. Remote Production of a Region of Homogeneous Magnetic Field. J. Magn. Reson. 41: 400–405.
- Burnett, L.J. and Jackson, J.A. 1980. Remote (Inside-Out) NMR. II. Sensitivity of NMR Detection for External Samples. J. Magn. Reson. 41: 406–410.
- Jackson, J.A., Burnett, L.J., and Harmon, F. 1980. Remote (Inside-Out) NMR. III. Detection of Nuclear Magnetic Resonance in a Remotely Produced Region of Homogeneous Magnetic Field. J. Magn. Reson. 41: 411–421.
- Jackson, J.A. 1984. Nuclear Magnetic Resonance Well Logging. The Log Analyst 25 (5): 16. SPWLA-1984-vXXVn5a3.
- Jackson, J.A. 2001. Los Alamos NMR well logging project. Concepts in Magnetic Resonance 13 (6): 368-378. http://dx.doi.org/10.1002/cmr.1022
- Kleinberg, R.L. and Jackson, J.A. 2001. An introduction to the history of NMR well logging. Concepts in Magnetic Resonance 13 (6): 340-342. http://dx.doi.org/10.1002/cmr.1018
- Miller, M.N., Paltiel, Z., Gillen, M.E. et al. 1990. Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, 23-26 September. SPE-20561-MS. http://dx.doi.org/10.2118/20561-MS
- Chandler, R.N., Drack, E.O., Miller, M.N. et al. 1994. Improved Log Quality With a Dual-Frequency Pulsed NMR Tool. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, 25-28 September. SPE-28365-MS. http://dx.doi.org/10.2118/28365-MS
- Prammer, M.G., Drack, E.D., Bouton, J.C. et al. 1996. Measurements of Clay-Bound Water and Total Porosity by Magnetic Resonance Logging. Presented at the SPE Annual Technical Conference and Exhibition, Denver, 6-9 October. SPE-36522-MS. http://dx.doi.org/10.2118/36522-MS
- Drack, E., Chandler, R., Bouton, I. et al. 1999. Theory and Operation of a New, Multi-Volume NMR Logging System. Presented at the SPWLA Annual Logging Symposium, Oslo, Norway, 30 May–3 June. SPWLA-1999-DD.
- Prammer, M.G., Bouton, J., Drack, E.D. et al. 2001. A New Multiband Generation of NMR Logging Tools. SPE Res Eval & Eng 4 (1): 59-63. SPE-69670-PA. http://dx.doi.org/10.2118/69670-PA
- Kleinberg, R.L., Sezginer, A., Griffin, D.D. et al. 1992. Novel NMR apparatus for investigating an external sample. J. Magn. Reson. 97 (3): 466-485. http://dx.doi.org/10.1016/0022-2364(92)90028-6
- Morriss, C.E., Kenyon, W.E., Tutunjian, P.N. et al. 1993. Field Test of an Experimental Pulsed Nuclear Magnetism Tool. Presented at the SPWLA Annual Logging Symposium, Calgary, 13–16 June. SPWLA-1993-GGG.
- Freedman, R., Morriss, C.E., Flaum, C. et al. 1997. Measurement of Total NMR Porosity Adds New Value to NMR Logging. Presented at the SPWLA Annual Logging Symposium, Houston, 15–18 June. SPWLA-1997-OO.
- Mirth, C.C., Willis, D., Hurlimann, M. et al. 1999. An Improved NMR Tool for Faster Logging. Presented at the SPWLA Annual Logging Symposium, Oslo, Norway, 30 May–3 June. SPWLA-1999-CC.
- Heaton, N.J., Freedman, R., Karmonik, C. et al. 2002. Applications of a New-Generation NMR Wireline Logging Tool. Presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, 29 September-2 October. SPE-77400-MS. http://dx.doi.org/10.2118/77400-MS
- DePavia, L., Heaton, N., Ayers, D. et al. 2003. A Next-Generation Wireline NMR Logging Tool. Presented at the SPE Annual Technical Conference and Exhibition, Denver, 5-8 October. SPE-84482-MS. http://dx.doi.org/10.2118/84482-MS
- Minh, C.C., Bordon, E., Hurliman, M. et al. 2005. Field Test Results of the New Combinable Magnetic Resonance Autotune Logging Tool. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, 9-12 October. SPE-96759-MS. http://dx.doi.org/10.2118/96759-MS
- Chen, S., Fang, S., Zhang, G. et al. 2003. MR Explorer Log Acquisition Methods: Petrophysical-Objective-Oriented Approaches. Presented at the SPWLA Annual Logging Symposium, Galveston, Texas, USA, 22–25 June. SPWLA-2003-ZZ.
- Khamatdinov, R., Mityushin, E., Murtsovkin, V. et al. 2003. Field Test of a New Nuclear Magnetic Resonance Tool. Presented at the SPWLA Annual Logging Symposium, Galveston, Texas, USA, 22–25 June. SPWLA-2003-AAA.
- Coates, G.R., Xiao, L.Z., and Prammer, M.G. 1999. NMR Logging: Principles and Applications, 234. Houston: Halliburton Energy Services.
- Bonnie, R.J.M., Akkurt, R., Al-Waheed, H. et al. 2003. Wireline T1 Logging. Presented at the SPE Annual Technical Conference and Exhibition, Denver, 5-8 October. SPE-84483-MS. http://dx.doi.org/10.2118/84483-MS
- Heaton, N.J., Minh, C.C., Kovats, J. et al. 2004. Saturation and Viscosity From Multidimensional Nuclear Magnetic Resonance Logging. Presented at the SPE Annual Technical Conference and Exhibition, Houston, 26-29 September. SPE-90564-MS. http://dx.doi.org/10.2118/90564-MS
- Bonnie, R.J.M., Galford, J., Fox, P. et al. 2004. Advanced NMR Acquisition Has Improved the Assessment of Hydrocarbons in Place in the Gulf of Mexico. Presented at the SPWLA 45th Annual Logging Symposium, Noordwijk, The Netherlands, 6–9 June. SPWLA-2004-II.
- Kleinberg, R.L. and Vinegar, H.J. 1996. NMR Properties of Reservoir Fluids. The Log Analyst 37 (6): 20–32.
- Prammer, M.G., Bouton, J., and Masak, P. 2001. The Downhole NMR Fluid Analyzer. Presented at the SPWLA Annual Logging Symposium, Houston, 16–20 June. SPWLA-2001-N.
- Bouton, J., Prammer, M.G., Masak, P. et al. 2001. Assessment of Sample Contamination by Downhole NMR Fluid Analysis. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, 30 September-3 October. SPE-71714-MS. http://dx.doi.org/10.2118/71714-MS.
- Masak, P.C., Prammer, M.G., Drack, E. et al. 2002. Field Test Results and Applications of the Downhole Magnetic Resonance Fluid Analyzer. Presented at the SPWLA 43rd Annual Logging Symposium, Oiso, Japan, 2–5 June. SPWLA-2002-GGG.
- Akkurt, R., Fransson, C.-M., Witkowsky, J.M. et al. 2004. Fluid Sampling and Interpretation with the Downhole NMR Fluid Analyzer. Presented at the SPE Annual Technical Conference and Exhibition, Houston, 26-29 September. SPE-90971-MS. http://dx.doi.org/10.2118/90971-MS
Noteworthy papers in OnePetro
Akkurt, R., Marsala, A. F., Seifert, D., Al-Harbi, A., Buenrostro, C., Kruspe, T., … Kroken, A. (2009, January 1). Collaborative Development of a Slim LWD NMR Tool: From Concept to Field Testing. Society of Petroleum Engineers. doi:10.2118/126041-MS
DePavia, L., Heaton, N., Ayers, D., Freedman, R., Harris, R., Jorion, B., … Garcia, S. (2003, January 1). A Next-Generation Wireline NMR Logging Tool. Society of Petroleum Engineers. doi:10.2118/84482-MS
Prammer, M. G., Bouton, J., Drack, E. D., Miller, M. N., & Chandler, R. N. (2001, February 1). A New Multiband Generation of NMR Logging Tools. Society of Petroleum Engineers. doi:10.2118/69670-PA
Heaton, Nicholas James, Jain, Vikas, Boling, Brian, Oliver, David,Degrange, Jean-Marie, Ferraris, Paolo, Hupp, Douglas, Prabawa, Hendrayadi,Torres Ribeiro, Mauro, Vervest, Edwin, Stockden, Ian - New Generation Magnetic Resonance While Drilling 160022-MS SPE Conference Paper - 2012
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