Downhole Nuclear Magnetic Resonance Sensor Using Anisotropic Magnetic Material

• US 20170010378A1
• Filed: 02/09/2016
• Published: 01/12/2017
• Est. Priority Date: 03/11/2015
• Status: Active Grant

• Current Assignee: Halliburton Energy Services Incorporated (Halliburton Company)
• Original Assignee: Halliburton Energy Services Incorporated (Halliburton Company)
• Inventors: Reiderman, Arcady, Li, Lilong
• Granted Patent: US 10,890,684 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: E21B 47/00 Survey of boreholes or wells monitoring pressure or flow of drilling fluid E21B21/08
• CPC Class Codes: G01R 33/3678 involving quadrature drive or detection, e.g. a circularly polarized RF magnetic field
• 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 33/383 using permanent magnets
• CPC Class Codes: G01V 3/14 operating with electron or nuclear magnetic resonance
• CPC Class Codes: G01V 3/32 operating with electron or nuclear magnetic resonance

Abstract

A nuclear magnetic resonance (NMR) sensor including a magnet that generates a static magnetic field, a soft magnetic core secured to the magnet, and an antenna extending at least partially about the soft magnetic core and generating a radio frequency (RF) magnetic field. A magnetic permeability of the soft magnetic core in a direction of the static magnetic field is less than the magnetic permeability of the soft magnetic core in a direction of the RF magnetic field.

Claims

1. A nuclear magnetic resonance (NMR) sensor, comprising:

a magnet that generates a static magnetic field;

a soft magnetic core secured to the magnet; and

an antenna extending at least partially about the soft magnetic core and generating a radio frequency (RF) magnetic field, wherein a magnetic permeability of the soft magnetic core in a direction of the static magnetic field is less than the magnetic permeability of the soft magnetic core in a direction of the RF magnetic field.

2. The NMR sensor of claim 1, wherein the direction of the static magnetic field is orthogonal to the direction of the RF magnetic field.

3. The NMR sensor of claim 1, wherein the magnet comprises:

a central magnetic piece having a first axial end and a second axial end opposite the first axial end;

a first magnetic end piece positioned at the first axial end; and

a second magnetic end piece positioned at the second axial end.

4. The NMR sensor of claim 3, wherein the magnet is a permanent magnet and the central magnetic piece and the first and second magnetic end pieces each comprise one or more permanent magnetic pieces.

5. The NMR sensor of claim 1, wherein the antenna comprises a transversal-dipole antenna for generating the RF magnetic field.

6. The NMR sensor of claim 1, wherein the soft magnetic core comprises an isotropic magnetic material having at least one annular gap defined thereon to reduce the magnetic permeability of the soft magnetic core in the direction of the static magnetic field.

7. The NMR sensor of claim 1, wherein the soft magnetic core comprises an anisotropic magnetic material.

8. The NMR sensor of claim 1, wherein an axially extending non-magnetic gap is defined on at least a portion of an outer surface of the soft magnetic core.

9. A well system, comprising:

a wellbore drilled through one or more subterranean formations; and

a nuclear magnetic resonance (NMR) sensor positioned in the wellbore for obtaining NMR data of the one or more subterranean formations, the NMR sensor including;

a magnet that generates a static magnetic field;

a soft magnetic core secured to the magnet; and

an antenna extending at least partially about the soft magnetic core and generating a radio frequency (RF) magnetic field, wherein a magnetic permeability of the soft magnetic core in a direction of the static magnetic field is less than the magnetic permeability of the soft magnetic core in a direction of the RF magnetic field.

10. The well system of claim 9, further comprising a drill string extended into the wellbore, wherein the NMR sensor forms part of the drill string.

11. The well system of claim 9, further comprising a cable extended into the wellbore, wherein the NMR sensor is attached to the cable.

12. The well system of claim 9, wherein the magnet comprises:

a central magnetic piece having a first axial end and a second axial end opposite the first axial end;

a first magnetic end piece positioned at the first axial end; and

a second magnetic end piece positioned at the second axial end.

13. The well system of claim 9, wherein the antenna comprises a transversal-dipole antenna for generating the RF magnetic field, a direction of the RF magnetic field being orthogonal to a direction of the static magnetic field.

14. The well system of claim 9, wherein the soft magnetic core comprises an isotropic magnetic material and at least one annular gap is defined on the soft magnetic core to reduce the magnetic permeability of the soft magnetic core in the direction of the static magnetic field.

15. The well system of claim 9, wherein the soft magnetic core comprises an anisotropic magnetic material.

16. The well system of claim 9, wherein an axially extending non-magnetic gap is defined on at least a portion of an outer surface of the soft magnetic core and is filled with a non-magnetic material.

17. A method, comprising:

conveying a nuclear magnetic resonance (NMR) sensor into a wellbore drilled through one or more subterranean formations; and

obtaining NMR data from the wellbore using the NMR sensor by generating a static magnetic field using a magnet of the NMR sensor, and generating a radio frequency (RF) magnetic field using an antenna extending at least partially about a soft magnetic core that is secured to the magnet,

wherein a magnetic permeability of the soft magnetic core in a direction of the static magnetic field is less than the magnetic permeability of the soft magnetic core in a direction of the RF magnetic field.

18. The method of claim 17, wherein generating the RF magnetic field comprises generating the RF magnetic field in the direction that is orthogonal to the direction of the static magnetic field and by using the antenna that comprises a transversal-dipole antenna to produce circular polarized excitation of the one or more subterranean formations.

19. The method of claim 17, wherein generating the static magnetic field comprises generating the static magnetic field using the magnet that comprises:

a central magnetic piece having a first axial end and a second axial end opposite the first axial end;

a first end magnetic piece positioned at the first axial end; and

a second end magnetic piece positioned at the second axial end.

20. The method of claim 17, further comprising reducing a shorting of a magnetic flux in the soft magnetic core with an axially extending non-magnetic gap defined on at least a portion of an outer surface of the soft magnetic core.