Source: http://www.google.com/patents/US6118272?dq=6,247,130
Timestamp: 2017-04-30 11:05:59
Document Index: 3653897

Matched Legal Cases: ['art.\n6', 'art.\n7', 'art 1', 'art 168', 'art 69', 'art 168', 'art 168', 'art 69']

Patent US6118272 - Nuclear magnetic resonance apparatus and method - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsA nuclear magnetic resonance apparatus including a magnet generating a static magnetic field in a first region containing materials to be analyzed. The magnet generates zero static magnetic field in a second region. The magnet has generally homogeneous magnetization along a longitudinal axis and is magnetized...http://www.google.com/patents/US6118272?utm_source=gb-gplus-sharePatent US6118272 - Nuclear magnetic resonance apparatus and methodAdvanced Patent SearchTry the new Google Patents, with machine-classified Google Scholar results, and Japanese and South Korean patents.Publication numberUS6118272 APublication typeGrantApplication numberUS 08/902,682Publication dateSep 12, 2000Filing dateJul 30, 1997Priority dateFeb 23, 1996Fee statusPaidAlso published asCA2196465A1, CA2196465C, US5712566, US5834936Publication number08902682, 902682, US 6118272 A, US 6118272A, US-A-6118272, US6118272 A, US6118272AInventorsGersh Zvi Taicher, Arcady ReidermanOriginal AssigneeWestern Atlas International, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (3), Referenced by (31), Classifications (10), Legal Events (3) External Links: USPTO, USPTO Assignment, EspacenetNuclear magnetic resonance apparatus and method
US 6118272 AAbstract
A nuclear magnetic resonance apparatus including a magnet generating a static magnetic field in a first region containing materials to be analyzed. The magnet generates zero static magnetic field in a second region. The magnet has generally homogeneous magnetization along a longitudinal axis and is magnetized substantially perpendicular to the axis. The apparatus includes means for generating a radio frequency magnetic field in the first region for exciting nuclei of the materials. The means for generating the radio frequency magnetic field includes an antenna disposed within the second region. The apparatus includes means for receiving a nuclear magnetic resonance signal from the excited nuclei. In a preferred embodiment, the means for generating and means for receiving include an antenna at least partially disposed within the second region. In a specific embodiment, the antenna consists of wire coils wound in planes perpendicular to the longitudinal axis of the instrument. A high permeability ferrite is disposed inside the wire coils of the antenna.
1. A nuclear magnetic resonance sensing apparatus, comprising:a magnet for inducing a static magnetic field within a first region containing materials to be analyzed, said magnet inducing substantially zero static magnetic field within a second region proximal to said magnet; means for generating a radio frequency magnetic field in said first region for exciting nuclei of said materials to be analyzed, said means for generating comprising an antenna disposed within said second region; and means for receiving a nuclear magnetic resonance signal from said excited nuclei. 2. The apparatus as defined in claim 1 wherein said antenna comprises a wire coil.
3. The apparatus as defined in claim 2 further comprising a high magnetic permeability ferrite disposed proximal to said wire coil.
4. The apparatus as defined in claim 3 further comprising a frequency control coil disposed proximal to said high magnetic permeability ferrite for selectively varying a static magnetic field level at said high magnetic permeability ferrite, said frequency control coil providing selective variation of the magnetic permeability of said high magnetic permeability ferrite thereby selectively controlling a tuning frequency of said antenna.
5. The apparatus as defined in claim 4 wherein said wire coil comprises a prepolarizing part and a main part, said main part being longer along an axis of said apparatus than said prepolarizing part.
6. The apparatus as defined in claim 5 wherein said prepolarizing part comprises a compensating coil for reducing magnetization of said prepolarizing part by said main part.
7. A method for nuclear magnetic resonance sensing comprising:inducing a static magnetic field of substantially equal amplitude within a first region containing materials to be analyzed, said static magnetic field comprising a second region having substantially zero static magnetic field; generating a radio frequency magnetic field within said first region for exciting nuclei of said materials, said step of generating performed from within said second region; and receiving nuclear magnetic resonance signals from said excited nuclei. 8. The method as defined in claim 7 wherein said step of receiving said nuclear magnetic resonance signal is performed from within said second region having substantially zero static magnetic field.
9. The method as defined in claim 8 wherein said step of generating is performed by an antenna including a high magnetic permeability ferrite disposed proximal to said antenna.
10. The method as defined in claim 9 further comprising selectively varying a static magnetic field level in said high magnetic permeability ferrite, thereby selectively varying magnetic permeability of said high magnetic permeability ferrite so that a tuning frequency of said antenna is selectively varied and a frequency of said radio frequency magnetic field is selectively varied.
11. The method as defined in claim 7 wherein said step of generating is performed by a first antenna which is substantially orthogonal to a second antenna used to perform said step of receiving.
12. The method as defined in claim 11 further comprising the step of adjusting responses of said first and of said second antennas to have substantially zero mutual inductance.
This is a division of application Ser. No. 08/606,089 filed on Feb. 23, 1996, now U.S. Pat. No. 5,712,566.
After the 90 degree pulse ends, the antenna is typically electrically connected to a receiver, which detects and measures voltages induced in the antenna by precessional rotation of the spin axes of the hydrogen nuclei. The precessional rotation generates RF energy at a frequency proportional to the strength of the magnetic field applied by the magnet, this frequency being referred to as the Larmor frequency. The constant of proportionality for the Larmor frequency is known as the gyromagnetic ratio (yo). The gyromagnetic ratio is unique for each different chemical elemental isotope. The spin axes of the hydrogen nuclei gradually "dephase" because of inhomogeneities in the magnet's field and because of differences in the chemical and magnetic environment within the earth formation. Dephasing results in a rapid decrease in the magnitude of the voltages induced in the antenna. The rapid decrease in the induced voltage is referred to as the free induction decay (FID). The rate of FID is typically referred to by the notation T2*. The FID decay rate consists of a first component, referred to as "true T2", which is due to internal molecular environmental effects, and a second component resulting from microscopic differences in magnetic field gradients and inhomogeneities in the earth formation. The effects of the second component can be substantially removed by a process referred to as spin-echo measurement.
Another drawback to the apparatus of the Taicher et al '713 patent is that the antenna must be connected to complicated, difficult to build tuning circuitry in order to establish an operating frequency for the RF pulses and to receive the spin-echo emitted energy at that same frequency. It can be desirable to operate the antenna at a plurality of substantially different frequencies in order to measure properties of the earth formation at a plurality of radial distances from the axis of the NMR logging tool. Operating the antenna of the apparatus in the Taicher et al '713 patent at substantially different frequencies can be difficult and expensive, as the antenna cannot be retuned to a different frequency during operation except by connection to different transmitter and receiver circuits each having different tuned electrical characteristics.
FIG. 2 shows the NMR probe 42 in more detail. The NMR probe 42 preferably comprises a generally cylindrical, permanent- or electro- magnet assembly 60. The magnet assembly 60 can include at least one permanent magnet 62, which preferably has a substantially circular cross section and is generally elongated along a magnet axis 80. The magnet axis 80 is preferably positioned coaxially with the longitudinal axis 76 of the wellbore (22 in FIG. 1). Alternatively, a plurality of permanent magnets may be used to make up the magnet assembly 60. For clarity of the description of the invention, the one or more permanent magnets 62 will be considered together and referred to as permanent magnet 62, and their common axis 80 and the collocated axis of the wellbore (22 in FIG. 1) will be jointly identified herein as the longitudinal axis, shown at 78.
FIG. 4 shows, in general form, the NMR probe 42 and a functional block diagram of the NMR well logging apparatus according to the present invention. A transmitter/receiver (T/R) matching circuit 45 can be disposed within the housing 43. The T/R matching circuit 45 typically includes a series of resonance capacitors (not shown separately), a transmitter/receiver switch (not shown separately) and both "to-transmitter" and "to-receiver" matching circuitry. The T/R matching circuit 45 can be coupled both to a radio frequency (RF) power amplifier 74 and to a receiver preamplifier 73. While shown as located inside the housing 43 the T/R matching circuit 45, the RF power amplifier 74 and the receiver preamplifier 73 alternatively may be located outside the housing 43 within the top centralizer (56 in FIG. 1) or within the NMR electronics cartridge (44 in FIG. 1). The locations of the T/R matching circuit 45, the RF power amplifier 74 and the receiver preamplifier 73 are not to be construed as a limitation on the invention.
Another particular advantage of the present invention is the presence of a substantially constant static magnetic field amplitude and static field amplitude gradient in the materials to be analyzed within the sensitive volume 58. This feature can be used for direct measurement of the diffusion coefficient of liquid present in the material to be analyzed, as explained for example in C. P. Slichter, Principles of Magnetic Resonance, Appendix G, Springer Verlag Berlin Heidelberg, N.Y., 1980. The amplitude gradient of the static magnetic field can be used to generate a diffusion measurement particularly by adjusting the frequency of the RF magnetic field, as previously explained, to first generate the sensitive volume 58 where the static magnetic field has a gradient which exceeds internal magnetic field gradients of the materials to be analyzed. A static field gradient which will perform according to this aspect of the invention can be about 30 Gauss/cm. The NMR signal can be received from this same sensitive volume 58 at the same frequency. The sensitive volume 58 can then be moved by adjusting the RF magnetic field frequency to be positioned where the static magnetic field is generally less than the internal gradients in the materials to be analyzed, generally corresponding to a static field gradient of about 5 Gauss/cm.
S=w m Asv (B1 /I1)l                         (1)
where m and Asv, respectively, represent the nuclear magnetization and the cross sectional area of the sensitive volume (58 in FIG. 1), I1 represents the magnitude of the current flowing in the transceiver antenna 67, the oscillating frequency of the current is represented by w and l represents the effective length of the transceiver antenna 67. For simplicity of the discussion, m and B1 are assumed to be substantially homogeneous within the sensitive volume 58.
By substituting m=x B0 /μ0 ; where x represents the nuclear magnetic susceptibility of hydrogen nuclei within the sensitive volume 58, w=y B0, where B0 represents the static magnetic field generated by the permanent magnet (62 in FIG. 2) and described in equation (1), it is therefore possible to derive the following expression for S:
S=(y x/&#956;0)B0 2 (B1 /I1)Asv l (2)
The NMR signal thus acquired is therefore directly proportional to the sensitive volume 58 in the earth formation (26 in FIG. 1). The geometry of the sensitive volume 58 is determined by the existence of a resonance condition. In pulsed NMR, the resonance condition is typically met when the deviation of the static magnetic field magnitude B0 (R) from its value B0 (Rsv), corresponding to the central frequency of the current energizing the transceiver antenna 67 (B0 (R)=w/y), is no greater then half the magnitude of the RF magnetic field B1 induced by passing current through the transceiver antenna 67, expressed as shown in equation (3):
B0 (R)-B0 (Rsv)&#8806;B1 /2           (3)
B0 (R0)-B0 (Ri)&#8806;B1         (5)
where R0 and Ri represent, respectively, the outer and inner radii of the sensitive volume 58. As a practical matter R0 -Ri)<<Rexc.
The current flowing in the transceiver antenna 67 may be expressed as I1 =(P1 /r)1/2, where P1 represents the peak power of the RF pulse energizing the antenna 67, r represents the active part of the antenna 67 impedance. Therefore: r=w L/Q=y B0 L/Q. Substituting for equation (2) yields the expression:
S=(&#960;X/&#956;0)(y B0)1/2 (P1 Q/L)1/2 (B1 /I1)2 Rsv 2 l                          (9)
Nrms =(4kT&#916;fr)1/2                          (10)
where Δf represents the receiver bandwidth. The bandwidth is typically about y B1 /2π for a matched receiver; k represents Boltzmann's constant; and T represents the absolute temperature.
S/N=[(2kT)-1/2 &#960;3/2 (X/&#956;0)(B0 /y)1/4 Rsv 2 ][(B1 /I1)3/2 P1 1/4 (Q/L)3/4 l]                                                        (11)
B1 =qm (i l/4&#960;)/[R2 +(l/2)2 ]3/2(12)
wherein qm =μ0 μrod (πd2 /4)I1 n/l. In equation (12), qm represents the effective magnetic charge, μ0 represents the magnetic permeability of free space, μrod represents the magnetic permeability of the ferrite rod (shown as 68 in FIG. 2); d represents the diameter of the ferrite rod 68, I1 represents the current flowing in the transceiver antenna 67, n represents the number of coil turns in the transceiver antenna's 67 coil windings (66 in FIG. 2), l represents the transceiver antenna 67 length, and R represents the radius of the sensitive volume (shown as 58 in FIG. 2).
B1 /I1 &#8733;l-3 [1+(2R/l)2 ]-3/2(13)
B1 /I1 &#8776;0.18&#956;0 nl-1 [1+(2R/l)2 ]-3/2                                                (15)
L=(0.35 &#956;0 &#960;/4)n2 l                        (17)
S/N&#8733;l-5/4 [1+(2R/l)2 ]-9/4           (18)
Referring once again to FIG. 2, the relative dimensions of the transceiver antenna 67 should be selected in order to optimize S/N. The ratio of antenna length l to the radius of the sensitive volume 58 should be in a range of approximately 3-5. The diameter of the ferrite rod 68 should no be so large as to ensure that μrod <<μ. For typical values of μ, which can be in the range from 1500-2000, the l/d ratio of the ferrite rod 68 should generally not exceed 40. The ferrite rod 68 diameter is approximately limited to the diameter of the hole 83 in the permanent magnet 62. It is also important to note that the diameter of the ferrite rod 68 should be as large as practical within the limits of the diameter of the hole 83 to minimize magnetic flux density in the ferrite rod 68 and consequently to minimize specific power loss (maximize the Q of the transceiver antenna 67) when the RF pulses are conducted through the transceiver antenna 67. Magnetization dynamics in ferrite materials causing power loss in oscillating fields are discussed, for example in A. Reiderman, Magnetic Characterization of Recording Media, UB Ac. Sc. USSR, Part 1, p. 37, 1990.
Assuming first that the magnet assembly 61 is long enough so that end effects may be neglected, the magnetostatic analysis may be reduced to a two-dimensional problem. For those skilled in the art of magnetic field synthesis from permanent magnet sources, it should be apparent that there is substantially zero magnetic field inside a circular cylindrical hole in a circular cylindrical permanent magnet which is magnetized uniformly perpendicular to the cylindrical axis. For example, in Manlio G. Abele, Structure of Permanent Magnets, John Wiley & Sons, pp. 42-66, 1993 it is shown that the field inside a permanent magnet cylinder magnetized perpendicularly to its axis is uniform and is equal to Br /2, where Br represents the remanence magnetization of the permanent magnet material. The hole 83 in the main permanent magnet 61 may be represented as a superposition of two permanent magnet cylinders of the same magnet material being magnetized to the same value of Br but having opposite magnetization directions. Since each of the superimposed magnets in this representation produces the same magnetic field strength, equal to Br /2, but in opposite directions, there exists substantially zero magnetic field strength inside the hole 83. Furthermore, when the permanent magnet cylinder 62 and the hole 83 are coaxial with each other, the magnetic field direction outside the permanent magnet 62, having the hole 83 as shown in FIG. 2 is the same as for a solid cylindrical permanent magnet. Only the field strength is reduced in proportion to reduction of the cross sectional area of the magnet assembly 61 by including the hole 83.
To keep the length of the magnet 61 as short as is practical, it is preferable to compensate end effects by using the end magnets 63, 64 as shown in FIG. 2 and previously described herein. FIG. 8 shows a graphic representation of the effect of the end magnets (63, 64 in FIG. 2) on the magnetic field inside the magnet hole (83 in FIG. 2) as well as outside the magnet (62 in FIG. 2) at a 12 inch diameter sensitive volume (58 in FIG. 2). The graph of FIG. 8 represents the magnetic field generated by the magnet assembly 61 which is especially suitable for use in slim bore holes. The permanent magnet 62 for use in slim wellbores can have a 6.6 cm diameter and 100 cm length. The permanent magnet 62 can be formed from ferrite permanent magnet material such as sold under trade name "Spinalor" and manufactured by Ugimag, 405 Elm St., Valparaiso, Ind., or sold under trade name "Permadure" and manufactured by Philips, 230 Duffy Ave., Nicksville, N.Y. The magnet material described herein has 0.42 T remanence induction. The top end magnet 63 and the bottom end magnet 64 can also be 6.6 cm diameter cylinders about 18.5 cm in length and placed at a distance of 3.5 cm from the ends of the main magnet 62. The end magnets 63, 64 can be made from a permanent magnet material such as neodymium-iron-boron material sold under trade name "Ugistab" and manufactured by Ugimag, 405 Elm St., Valparaiso, Ind. or sold under trade name "Vacodym" and manufactured by Vacuumschmeize GMBH, 9/7 Rhenaniastrasse St., Berlin, Germany. The neodymium-iron-boron material typically has a remanence induction of about 1T.
As is understood by those skilled in the art, determination of properties of interest of the earth formations (such as 26 in FIG. 1) require that an NMR well logging instrument be able to measure short duration values of a magnetic resonance parameter referred to as T2. Some nuclear magnetic resonance phenomena decay in amplitude very quickly, as is understood by those skilled in the art. In order to measure these short duration events, the NMR well logging apparatus should have as short "dead time" as ia practical. Dead time of an NMR logging system is affected by, among other things, magnetoacoustic interaction which may produce an unwanted signal in the transceiver antenna (such as 67 in FIG. 2). This section of the description of the preferred embodiment will explain how the NMR logging apparatus of the present invention reduces the effects of magnetoacoustic interaction to reduce the dead time.
Different types of magnetoacoustic interaction may produce a parasitic signal in the NMR antenna. Antenna wiring and other metal parts of the NMR probe (42 in FIG. 2) can be affected by the permanent magnet's (62 in FIG. 2) magnetic field and the RF field generated by passing RF pulses through the transceiver antenna 67. These fields can produce spurious "ringing" which is well known to those skilled in the art as "coil disease". This type of ringing is excited by the Lorenz force. As explained in E. Fukushima et. al., Spurious Ringing in Pulse NMR, J. Magn. Res. v. 33, pp. 199-203, 1979, the efficiency of conversion of RF radiation into acoustical waves, and vice versa, is directly proportional to square of the static magnetic field intensity at the location of the antenna.
A spurious signal generated by the permanent magnet 62, which continues to vibrate upon cessation of the RF pulse is a direct consequence of the inverse effect of magnetostriction. Two features of the present invention substantially reduce ringing of the magnet 62. First, the radial dependence of the RF field strength, as previously explained herein, is relatively small when compared with that of prior art NMR logging instruments. The relatively small radial dependence is a result of the use of the longitudinal dipole antenna with the ferrite rod (67 and 68 in FIG. 2). Second is the use of an orthogonal receiver coil. In the present invention, the additional receiver coil (70 in FIG. 2) is substantially orthogonal to the transceiver coil 67 and so meets this requirement. FIG. 9 shows a graph of the radial dependence of the RF field strength for the longitudinal (transceiver antenna 67), at curve 9-1, and for the transversal dipole (additional receiver coil 70) antenna at curve 9-2, from which it is apparent that the RF field affecting the permanent magnet 62 does not significantly exceed B1 at radius R=Rsv. That magnitude of RF magnetic field is typically not sufficient to effectively excite acoustic waves. The orthogonal receiver antenna (additional receiver coil 70) in the preferred embodiment of the invention substantially removes coupling of the additional receiver coil (70 in FIG. 2) with parasitic magnetic flux due to the inverse effect of magnetostriction.
The second set of conditions includes wellbores having nominal diameters between about 4 inches and 7 inches. The NMR probe 42 external diameter in this example can be about 33/8 inches. The present example includes a wellbore 22 having a diameter of about 5 inches, and sensitive volume 58 diameters of about 7 inches and 12 inches. The permanent magnet 62 axial length in this example can be about 80 cm. This axial length for the permanent magnet 62 provides about 45 cm length of axially equal magnetic field strength. Referring now to FIG. 10, an arrangement of a high vertical resolution RF antenna is presented. The main part 168A of the transceiver antenna 67 can be about 15 cm length and 1 cm diameter. The prepolarizing part 69 of the antenna 67 can be about 7.5 cm length and 1 cm diameter and is typically placed at a distance about 1 cm from the main part 168A. A compensating receiver coil 71 serves to compensate of the magnetizing effect of the main part 168A on the prepolarizing part 69. FIG. 11 shows a graph of the spatial distribution of the effective RF field (orthogonal to static magnetic field component of RF field) and the antenna receiving sensitivity function which is presented in the form of the RF field distribution. The effect of compensating the receiver coil 66 is also illustrated.
FIG. 13 shows a cross-sectional view of the arrangement shown in FIG. 12 to better explain the relative placement of the components of the present embodiment of the invention. The magnet 62A is shown generally eccentered in the wellbore 22A so as to be impressed against the wall of the wellbore 22A. The sensitive volume 58A is typically selected, by appropriate selection of RF frequency for the power pulses conducted through the transceiver antenna 67A, to be at a depth into the earth formation 26A of about 5 cm from the wellbore wall. Geometrical considerations in selection of appropriate frequency include first that the sensitive volume radius (Rsv) should exceed the quantity 2Rbh -Rm wherein Rbh represents the wellbore radius and Rm represents the radius of the magnet 62A. Second, the effective diameter da of the additional receiver antenna 70A can be approximately equal to the quantity 2Rsv -Rm. Axial length considerations for the magnet 62A, the transceiver antenna 67A and the additional receiver antenna 70A can be substantially the same as in the first embodiment of the invention.
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