Systems and methods for NMR logging

• US 20050030021A1
• Filed: 04/30/2004
• Published: 02/10/2005
• Est. Priority Date: 05/02/2003
• Status: Active Grant

• Current Assignee: Earle Drack, Edward J. Harris III, George D. Goodman, Manfred G. Prammer, Sergey Knizhnik, Stefan K. Menger
• Original Assignee: Earle Drack, Edward J. Harris III, George D. Goodman, Manfred G. Prammer, Sergey Knizhnik, Stefan K. Menger
• Inventors: Knizhnik, Sergey, Menger, Stefan K., Harris, Edward J. III, Drack, Earle, Prammer, Manfred G., Goodman, George D.
• Granted Patent: US 7,463,027 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: G01V 3/32 operating with electron or nuclear magnetic resonance

Abstract

An NMR logging tool for conducting NMR measurements in a plurality of sensitive volumes ranging up to a meter from the tool. The tool comprises a magnetic assembly using one or more permanent magnets and at least one pole piece for extending a magnet pole and shaping the magnetic field to simulate a magnetic monopole in a sensitive volume within the formation. Different embodiments of a segmented antenna enable directional NMR logging. The tool embodiments and methods of their use are suitable for wireline or LWD logging, and can be used for directional drilling.

Claims

1. A system for nuclear magnetic resonance (NMR) logging of an underground formation traversed by a borehole, the system having a longitudinal axis and comprising:

two magnets polarized along the longitudinal axis, the magnets having like poles facing each other to generate in the underground formation a static magnetic field azimuthally symmetric with respect to the longitudinal axis, at least one of the facing magnetic poles being extended by one or more pole pieces with high permeability; and

an antenna assembly interposed between the two facing magnetic poles, the antenna assembly capable of operating at multiple frequencies and generating in a portion of the formation RF fields substantially perpendicular to the static magnetic field.

2. The system of claim 1, wherein at least one of the magnets is a permanent magnet.

3. The system of claim 1, wherein the antenna assembly comprises (i) a soft-magnetic core, (ii) at least one antenna coil wound around the core and capable of generating a first azimuthally non-directional oscillating magnetic field substantially orthogonal to the static magnetic field, and (iii) two or more directional antenna segments positioned around the core, each antenna segment generating oscillating magnetic fields orthogonal to the static magnetic field in one or more azimuthally focused sections of the underground formation surrounding the borehole.

4. The system of claim 3, wherein the antenna coil in (ii) is a solenoid coil.

5. The system of claim 3, wherein the two or more directional antenna segments are saddle coils.

6. The system of claim 5, wherein the saddle coils have planes orthogonal to each other.

7. The system of claim 1, wherein the two magnets are permanent magnets and comprise one or more of:

a AlNiCo magnet, a hard-ferrite magnet, a neodymium-iron-boron magnet, or a samarium cobalt magnet.

8. The system of claim 1, wherein the one or more pole pieces are made of one of:

ferrite, iron, or steel.

9. The system of claim 1, wherein the permeability of the one or more pole pieces is greater than about 100.

10. The system of claim 1, wherein both facing magnetic poles are extended by one or more pole pieces of high permeability.

11. The system of claim 3, wherein the soft-magnetic core is made of one of:

a soft ferrite, a ferrite polymer composite, a powdered iron or nickel, or a molypermalloy powder.

12. The system of claim 11, wherein the permeability of the soft-magnetic core is greater than 10.

13. The system of claim 1, wherein for different RF frequency the sensitive region of the system is in a range of about 10 cm to 100 cm in the formation.

14. The system of claim 1, wherein the antenna assembly comprises a soft-magnetic core and two or more directional antenna segments positioned around the core, each antenna segment generating oscillating magnetic fields orthogonal to the static magnetic field in one or more azimuthally focused sections of the underground formation surrounding the borehole, at least one antenna segment positioned around the core.

15. The system of claim 14, wherein comprising two, three or four directional antenna segments symmetrically disposed about the longitudinal axis of the system.

16. A system for NMR logging of an underground formation traversed by a borehole, the system having a longitudinal axis and comprising:

means for imparting a static magnetic field in the formation surrounding the borehole, the static magnetic field being substantially radially-oriented with respect to the longitudinal axis in a portion of the formation;

means for applying a first oscillating magnetic field having magnetic field lines substantially orthogonal to the static magnetic field in substantially all azimuthal directions;

means for applying one or more directional oscillating magnetic fields being substantially orthogonal to the static magnetic field in corresponding one or more angular sectors of a volume in the formation surrounding the system; and

means for directionally sensitive detection of NMR signals from the formation surrounding the system.

17. The system of claim 16, wherein the means for imparting a static magnetic field comprise at least two tubular magnetic configurations having longitudinal polarization and positioned with like poles facing each other.

18. The system of claim 17, wherein each magnetic configuration comprises a permanent magnet, and a plurality of pole pieces longitudinally extending one or more poles thereof.

19. The system of claim 16, wherein the means for imparting a static magnetic field comprises one permanent magnet and a plurality of pole pieces longitudinally extending one or more poles thereof, to simulate a magnetic monopole in a region of the formation surrounding the system.

20. The system of claim 16, wherein the means for applying a first oscillating magnetic field comprise at least one solenoid coil.

21. The system of claim 16, wherein the means for applying one or more directional oscillating magnetic fields comprises a first and second saddle coils.

22. The system of claim 20, wherein the first and second saddle coils have planes orthogonal to each other.

23. The system of claim 16, wherein the means for applying one or more directional oscillating magnetic fields comprises a plurality of antenna segments formed of solenoids, and disposed symmetrically about the longitudinal axis of the tool.

24. The system of claim 23, wherein at least two adjacent antenna segments are separated by a conducting layer to reduce mutual detuning between the at least two adjacent antenna segments.

25. A method for NMR logging a borehole in a geologic formation by means of a tool having longitudinal axis, the method comprising the steps of:

imparting a static magnetic field having substantially radial orientation with respect to the tool axis in a volume of the formation surrounding the borehole;

imparting a plurality of oscillating magnetic fields substantially orthogonal to the static magnetic field in the volume of the formation in which the static magnetic field has substantially radial orientation, the plurality of oscillating magnetic fields being azimuthally directional;

detecting NMR signals from the formation in a plurality of directionally sensitive antenna segments; and

processing of the detected NMR signals to obtain directionally sensitive information about at least one property of the formation.

26. The method of claim 25, wherein the plurality of directionally sensitive antenna segments detect NMR signals from angular sectors of approximately 90 degrees.

27. An antenna assembly for directional NMR logging of a borehole in a underground formation by means of a logging tool having a longitudinal axis, the assembly comprising:

a soft magnetic core longitudinally disposed around the axis;

at least one solenoid coil wound around the core for producing an axially-oriented and azimuthally symmetric oscillating magnetic fields in the formation surrounding the borehole; and

at least two saddle coils disposed substantially orthogonally with respect to each other around the core and capable of receiving azimuthally asymmetric NMR signals from a sensitive region in the formation.

28. A magnet configuration for use in NMR logging of a geologic formation surrounding a borehole, the magnet configuration comprising:

a single permanent magnet polarized along a longitudinal axis substantially parallel to the axis of the borehole;

at least one pole piece made of highly permeable material(s) extending from one or both poles of the magnet to shape its magnetic field in a manner as to simulate in a portion of the formation a substantially radial magnetic field.

29. The configuration of claim 28, wherein the at least one pole piece is made of ferromagnetic material and is attached to the magnet pole using one of:

adhesive bonding, studs or bolts.

30. The configuration of claim 28 further comprising at least one antenna placed around a pole piece, the antenna capable of generating oscillating magnetic fields substantially orthogonal to the static magnetic field of the magnet in a portion of the formation.

31. The magnetic configuration of claim 29, wherein the at least one antenna is a coil wound around the pole piece.

32. A method for determining an NMR property of formation surrounding a borehole with the aid of a logging device, comprising the steps of:

providing a magnetic configuration producing a substantially radial static magnetic field into a portion of the formation;

providing a plurality of directionally sensitive antenna segments disposed along a longitudinal axis of the logging device generating RF pulsed magnetic fields in the formation, the RF fields being substantially orthogonal to the static magnetic field;

receiving NMR signals from a sensitive region of the formation at two adjacent antenna segments; and

determining a property of the formation in a direction based on orientation of both the adjacent antenna segments.

33. The method of claim 32, wherein the plurality of antenna segments are disposed substantially symmetrically about the longitudinal axis of the logging device.

34. The method of claim 32, wherein at least one of the plurality of antenna segments comprises (i) a soft-magnetic core and (ii) at least one antenna coil located outside the soft-magnetic core.

35. The method of claim 32, wherein at least four antenna segments are disposed along the longitudinal axis of the logging device.

36. The method of claim 32, wherein the direction based on orientation of both the adjacent antenna segments for the property of the formation is approximately intermediate between the orientation of each of the adjacent antenna segments.

37. The method of claim 32, wherein the provided magnet configuration comprises two magnets polarized along a longitudinal axis of the logging device, the magnets having like poles facing each other.

38. The method of claim 32 further comprising the step of energizing fewer than all in the plurality of directionally sensitive antenna segments.

39. The method of claim 38, wherein two adjacent antenna segments are energized to provide RF magnetic fields.

40. The method of claim 32 further comprising the step of providing a plurality of RF pulses at corresponding plurality of frequencies to probe different sensitive regions of the formation.

41. The method of claim 32, wherein the magnetic configuration simulates a magnetic monopole about the plurality of antenna segments.

42. The method of claim 32 further comprising determining a directional property of the formation from signals received in four symmetrically placed antenna segments in accordance with:

$\left[\begin{array}{c}N\\ E\\ S\\ W\end{array}\right]-\left[\begin{array}{c}1001\\ 1100\\ 0110\\ 0011\end{array}\right]\times <br><br>\left[\begin{array}{c}\mathrm{NE}\\ \mathrm{SE}\\ \mathrm{SW}\\ \mathrm{NW}\end{array}\right]\to <br><br>\min ,\mathrm{for}<br><br>\left[\begin{array}{c}N\\ E\\ S\\ W\end{array}\right]\ge <br><br>0;<br><br>$

wherein N, E, S, and W, respectively stand for a value of the property in each of four symmetrically placed directions and NE, SE, SW, and NW stand for respective quantities related to the signals detected in four symmetric antennas segments facing four intermediate directions with reference to the four symmetric directions.

43. An apparatus for directionally sensitive NMR logging in a borehole for collecting directionally sensitive formation data comprising:

a segmented antenna comprising a plurality of independently addressable antenna segments;

a magnet simulating a monopole static magnetic field in a volume surrounding the borehole; and

means for transmitting signals at a plurality of frequencies from a plurality of segments in the segmented antenna for obtaining NMR signals from a plurality of volumes such that the location of each volume corresponds to a particular frequency and the plurality of segments in the segmented antenna.

44. The apparatus of claim 43, wherein a signal at the particular frequency from the plurality of frequencies is transmitted from the plurality of segments in the segmented antenna in a specified time interval.

45. The apparatus of claim 43, wherein the signal at the particular frequency is transmitted from at least two adjacent segments in the segmented antenna.

46. The apparatus of claim 43 further comprising at least one standoff for providing a separation between the apparatus away from a wall of the borehole.

47. The apparatus of claim 43 further comprising at least one bow-spring for holding the apparatus in place in a borehole to provide an apparatus eccentralization and to maintain contact with a borehole wall in a desired direction.

48. The apparatus of claim 43, wherein the apparatus is a mandrel type apparatus.

49. A method for obtaining a directionally-sensitive NMR log in a borehole of a geologic formation, comprising:

producing a static magnetic field and a plurality of RF magnetic fields having different resonance frequencies at an NMR logging device, the static and the plurality of RF magnetic fields having mutually orthogonal components in an investigation region in the formation surrounding the logging device;

receiving NMR signals from the formation at a plurality of directionally sensitive antenna segments of the NMR logging device;

processing the received NMR signals to determine at least one formation property value associated with a particular spatial direction with respect to the logging device, wherein said at least one formation property is determined based on NMR signals received in two or more adjacent antenna segments.

50. The method of claim 49, further comprising the step of switching frequencies such that different volumes in the formation are investigated from the same antenna segment by receiving an NMR echo train in response to a corresponding transmission of a train of pulses.

51. A method for obtaining an azimuthally-sensitive log in a borehole traversing a formation comprising

placing a standoff for separating a segmented antenna section in a wireline tool from a wall of the borehole;

providing a plurality of segments in an antenna, wherein at least one segment is capable of transmitting directionally sensitive pulsed magnetic field signals into the formation;

providing a magnetic field simulating a magnetic monopole in a volume of the formation surrounding the borehole; and

switching frequencies such that different volumes in the formation are investigated from the same antenna segment by receiving an NMR echo train in response to a corresponding transmission of a train of pulses.

52. A method of drilling a borehole in a desired direction in a geologic formation, the method comprising:

(i) receiving NMR signals from the formation at one or more directionally sensitive antenna segments of an NMR logging tool attached to a drill string;

(ii) computing at least one formation property value associated with a predetermined portion of the geologic formation based on NMR signals received at the directionally sensitive antenna segment(s); and

(iii) repeating step (ii) at least once and directing a drill bit on the drill string in a direction determined by one or more of;

an increase in the value of the property, a decrease in the property, and maintaining a value of the property within a set of bounds.

53. The method of claim 52 further comprising the step of providing an NMR logging tool having at least two directionally sensitive antenna segments that are symmetrically disposed about a longitudinal axis of the logging tool.

54. The method of claim 52, wherein the number of antenna segments is two, three or four.

55. The method of claim 54, wherein at least two antenna segments are solenoids.

56. The method of claim 55, wherein the solenoid antenna segments are symmetrically disposed about a longitudinal axis of the tool.

57. The method of claim 54, wherein at least two antenna segments are longitudinal strips.

58. A method for NMR logging a borehole in a geologic formation, comprising the steps of:

lowering a magnet assembly to a location in the borehole;

using the lowered magnet assembly for conducting NMR measurements in;

(i) at least one shallow region of the formation surrounding the borehole, which shallow region may be subject to borehole fluid invasion; and

(ii) at least one deep region of the formation surrounding the borehole, the deep region being substantially free of borehole fluid invasion; and

combining NMR measurements from the at least one shallow region of the formation and NMR measurements from the at least one deep region of the formation to determine petrophysical properties of the formation.

59. The method of claim 58, wherein a shallow region of the formation extends to about 30 cm away from the magnet assembly.

60. The method of claim 58, wherein a deep region of the formation extends to about 1 m away from the magnet assembly.

61. The method of claim 58, wherein the sensitive volumes for the shallow and the deep region NMR measurements are non-overlapping.

62. The method of claim 58, wherein the magnet assembly is part of a logging tool.

63. The method of claim 58, wherein NMR measurements from at least one deep region of the formation and NMR measurements from at least one shallow region of the formation are combined to obtain a borehole fluid invasion profile of the borehole.

64. The method of claim 58, wherein petrophysical properties of the formation determined from the combined measurements comprise porosity and hydrocarbon saturation(s), and wherein shallow region NMR measurements are used primarily for formation porosity estimation.

65. The method of claim 58 further comprising the step of modeling petrophysical parameters of the formation for at least one of:

different locations along the length of the borehole, and different depths of investigation for the same location in the borehole.