SELECTIVE EXCITATION IN EARTH'S MAGNETIC FIELD NUCLEAR MAGNETIC RESONANCE WELL LOGGING TOOL

• US 20080007260A1
• Filed: 05/03/2007
• Published: 01/10/2008
• Est. Priority Date: 02/09/2004
• Status: Active Grant

• Current Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Original Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Inventors: Reiderman, Arcady
• Granted Patent: US 7,423,426 B2
• Time in Patent Office: Days
• 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 33/445 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 33/5617 using RF refocusing, e.g. RARE
• CPC Class Codes: G01V 3/32 operating with electron or nuclear magnetic resonance

Abstract

A downhole nuclear magnetic resonance method for measuring a property of a formation, the method includes applying a polarizing magnetic field to the formation, the field including a spatial distribution of magnitude; selecting an investigation region of the formation; decaying the polarizing magnetic field according to a decay shape to provide for substantially exciting only nuclei in the investigation region; and measuring signals induced by precession of the nuclei in the investigation region.

Claims

1. A downhole nuclear magnetic resonance method for measuring a property of a formation, the method comprising:

applying a polarizing magnetic field to the formation, the field comprising a spatial distribution of magnitude;

selecting an investigation region of the formation;

decaying the polarizing magnetic field according to a decay shape to provide for substantially exciting only nuclei in the investigation region; and

measuring signals induced by precession of the nuclei in the investigation region.

2. The method as in claim 1, wherein selecting comprises selecting the investigation region according to a desired depth from an edge of the borehole.

3. The method as in claim 2, wherein the desired depth exceeds about three inches.

4. The method as in claim 1, wherein decaying comprises applying a pulse of current, the current comprising an amplitude corresponding to a magnetic field close to a coercive force of a magnetic core and a polarity that corresponds to opposing the polarizing magnetic field.

5. The method as in claim 1, wherein decaying comprises applying a pulse of current, the current comprising an amplitude to drive a magnetic core into saturation and a polarity that opposes formation of a dipole in the core.

6. The method as in claim 1, further comprising counteracting distortions in the earth'"'"'s magnetic field due to magnetizing a magnetic core in a magnetic dipole, the counteracting comprising adjusting current in a coil.

7. A method for selecting a decay shape for decaying a polarizing magnetic field in nuclear magnetic resonance measurements, the method comprising:

evaluating a shape of a plurality of decaying magnetic fields; and

selecting the shape that produces a maximum magnetization of nuclei as a function of magnetic field intensity to provide the decay shape.

8. The method as in claim 7, wherein the maximum magnetization is greater than a magnetization immediately preceding and following the maximum magnetization.

9. The method as in claim 7, further comprising solving a Bloch equation with the shape as input to determine the maximum magnetization of the nuclei as a function of magnetic field intensity.

10. The method as in claim 7, further comprising correlating the maximum magnetization to an investigation region of a formation.

11. The method as in claim 10, wherein correlating comprises determining a signal induced in a receiver coil by precession of the nuclei as a function of distance from the coil, the precession resulting from decaying the polarizing magnetic field according to the decay shape.

12. The method as in claim 10, further comprising selecting the investigation region comprising the distance at which the signal is a maximum, wherein the signal immediately preceding and following the maximum is less than the maximum.

13. The method as in claim 7, further comprising the shape

$B_P$

( t ) = B P ⁢

 

⁢

0 1 + ( t τ

·

q ) q

where τ

represents a time constant of the shape, q represents a dimensionless decay rate parameter, and B_P0 is a magnitude of the polarizing magnetic field at time t=0.

14. The method as in claim 13, further comprising q=1.

15. A nuclear magnetic resonance instrument for measuring a property of a formation downhole, the instrument comprising:

a source adapted for applying a polarizing magnetic field to the formation, the field comprising a magnitude to provide a maximum precession amplitude of nuclei in an investigation region beyond a shallow region.

16. The instrument as in claim 15, wherein the source comprises a magnetic dipole.

17. The instrument as in claim 16, wherein the dipole comprises:

a hollow cylindrical magnetic core; and

two sets of windings, the two sets of windings disposed approximately 180 degrees apart on the circumference of the core.

18. The instrument as in claim 15, wherein the device comprises a processor.

19. The instrument as in claim 15, further comprising a coil, the coil comprising a coupling for coupling to an adjustable source of current, the coil providing the current to counteract distortions to earth'"'"'s magnetic field due to magnetization of a magnetic core.

20. The instrument as in claim 15, further comprising a thin magnetic shield exterior to the source.

21. A method for producing an instrument for measuring a property of a formation downhole, the method comprising:

selecting a source of a polarizing magnetic field; and

coupling the source to a processor, the processor comprising instructions for decaying the field.

22. The method as in claim 21, further comprising selecting a thin magnetic shield and placing the shield exterior to the dipole.

23. The method as in claim 21, further comprising selecting a coil for conducting an adjustable current, the current for counteracting distortions in the earth'"'"'s magnetic field due to magnetizing a magnetic core;

and coupling the coil to the a source of current.