Nuclear magnetic resonance tool using switchable source of static magnetic field

• US 7,859,260 B2
• Filed: 07/27/2006
• Issued: 12/28/2010
• Est. Priority Date: 01/18/2005
• Status: Active Grant

• Current Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Original Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Inventors: Reiderman, Arcady
• Primary Examiner(s): Shrivastav; Brij B
• Application Number:
  US11/494,043
• Publication Number:
  US 20060255799A1
• Time in Patent Office: 1,615 Days
• Field of Search: 324/303, 324/300, 324/313
• US Class Current: 324/303
• CPC Class Codes: G01N 24/081 Making measurements of geologic samples, e.g. measurements of moisture, pH, porosity, permeability, tortuosity or viscosity
• CPC Class Codes: G01R 33/3808 Magnet assemblies for single-sided MR wherein the magnet assembly is located on one side of a subject only; Magnet assemblies for inside-out MR, e.g. for MR in a borehole or in a blood vessel, or magnet assemblies for fringe-field MR
• CPC Class Codes: G01R 33/381 using electromagnets
• CPC Class Codes: G01R 33/383 using permanent magnets
• CPC Class Codes: G01R 33/445 MR involving a non-standard magnetic field B0, e.g. of low magnitude as in the earth's magnetic field or in nanoTesla spectroscopy, comprising a polarizing magnetic field for pre-polarisation, B0 with a temporal variation of its magnitude or direction such as field cycling of B0 or rotation of the direction of B0, or spatially inhomogeneous B0 like in fringe-field MR or in stray-field imaging
• CPC Class Codes: G01R 33/5616 using gradient refocusing, e.g. EPI
• CPC Class Codes: G01V 3/32 operating with electron or nuclear magnetic resonance

Abstract

A nuclear magnetic resonance sensing apparatus and method for operating in an earth borehole comprises a source of switchable magnetic field to polarize nuclei in the region of interest, said source comprising a coil wound on a magnetic core having controllable residual magnetization. Maintaining the magnetization of the core during a polarization interval does not require steady current in the coil. Switching intensity and polarity of magnetization of the core causes precession of spin magnetic moments of the nuclei; the precession induces a signal indicative of nuclear magnetic resonance properties of earth formations.

Claims

1. A nuclear magnetic resonance (NMR) sensing apparatus configured for use in a borehole in an earth formation, the apparatus comprising:

a magnetic core having a residual magnetization, the magnetic core configured to produce a static magnetic field in a specified direction in the earth formation and polarizing nuclear spins in the specified direction;

a coil configured to provide adiabatic deviation of the nuclear spins from the specified direction;

a polarizing coil configured to produce a change of a static magnetic field in the specified direction and configured to produce precession of the nuclear spins about the specified direction; and

a coil configured to detect a signal resulting from the precession of nuclear spins.

2. The apparatus of claim 1 wherein the magnetic core is made of a permeable magnetic material having a magnetic property related to a dimension of the magnetic core.

3. The apparatus of claim 2 wherein the magnetic core further has a coercive force, which is substantially equal to the ratio of saturation magnetization and an effective permeability of the magnetic core in the direction of the magnetic field produced by the polarizing coil.

4. The apparatus of claim 2 wherein the permeable magnetic material has a substantially rectangular hysteresis loop in the direction of magnetic field produced by the polarizing coil.

5. The apparatus of claim 2 wherein said magnetic material has a coercive force in the range 1.5-8 kA/m.

6. The apparatus claim 2 wherein the permeable magnetic material is selected from the group consisting of (i) a ferrite, (ii) an electrically conductive crystalline magnetic metal ribbon, (iii) an electrically conductive magnetic tape, an (iv) an amorphous magnetic ribbon.

7. The apparatus of claim 6 wherein the magnetic field in the earth formation has a switching time, and wherein the permeable magnetic material comprises at least one of (i) a ribbon, and (ii) a tape having a thickness selected based on the switching time.

8. The apparatus of claim 1 wherein said static magnetic field in the earth formation has a switching time, and the polarizing coil has an inductance related to said switching time.

9. A nuclear magnetic resonance (NMR) sensing apparatus configured for use in a borehole in an earth formation, the apparatus comprising:

a magnetic core having a residual magnetization and configured to produce a static magnetic field in a specified direction in the earth formation and polarize nuclear spins in the direction, the specified direction being different from a direction of the earth'"'"'s magnetic field;

a polarizing coil configured to carry a current which reduces the magnetic field due to the core to substantially zero; and

a receiving coil configured to receive a signal resulting from precession of the nuclear spins about the direction of the earth'"'"'s magnetic field, the precession resulting from the reduction of the magnetic field due to the core.

10. The apparatus of claim 9 further comprising an auxiliary coil configured to substantially eliminate a remainder of the static magnetic field in the earth formation.

11. The apparatus of claim 9 further comprising an additional coil configured to spoil a signal from a fluid in the borehole , the additional coil configured to generate a substantially multi-pole type magnetic field with fast spatial attenuation.

12. The apparatus of claim 9 wherein the magnetic core is made of a permeable magnetic material having a magnetic property related to a dimension of the magnetic core.

13. The apparatus of claim 9 wherein the magnetic core is made of a permeable magnetic material having a substantially rectangular magnetic hysteresis loop in a direction of residual magnetization.

14. The apparatus of claim 9 wherein the receiving coil includes a multi-turn receiving coil and the polarizing coil comprises a substantially small number of turns.

15. The apparatus claim 13 wherein the permeable magnetic material is selected from the group consisting of (i) a ferrite, (ii) an electrically conductive crystalline magnetic metal ribbons, (iii) an electrically conductive crystalline tape, (iv) an magnetic amorphous ribbon, and (v) an amorphous tape.

16. A method for measuring a nuclear magnetic resonance property in a region of interest of an earth formation;

the method comprising;

producing a static magnetic field which polarizes nuclear spins in the region of interest using a source of switchable magnetic field, the source comprising a magnetic core having a controllable residual magnetization; and

inducing and detecting a signal indicative of the nuclear magnetic resonance property;

wherein inducing the signal further comprises

(i) simultaneously switching off the static magnetic field and applying a pulsed magnetic field to provide adiabatic deviation of polarization direction of the nuclei from direction of the switchable static magnetic field,

(ii) reapplying the static magnetic field, and

(iii) periodically reversing the polarity of the switchable static magnetic field to generate gradient spin echoes.

17. The method of claim 16 wherein inducing the signal further comprises switching off the static magnetic field and generating precession of the nuclear spins around the earth'"'"'s magnetic field.

18. The method of claim 17 further comprising using an auxiliary static magnetic field in order to correct for a residual inhomogeneous magnetic field remaining after switching off the static magnetic field.

19. The method of claim 17 further comprising generating an additional static magnetic field to spoil the signal from borehole fluids, said additional static magnetic field is a substantially multi-pole type magnetic field with fast spatial attenuation.