Downhole detection

1. A system for use in downhole detection comprising:

a downhole arrangement defining a throughbore;

a tool deployable through the throughbore of the downhole arrangement;

a primary electromagnetic element provided on the deployable tool; and

a secondary electromagnetic element provided on the downhole arrangement,

wherein the primary and secondary electromagnetic elements are configurable for electromagnetic coupling therebetween,

wherein the downhole arrangement is configurable between a first configuration in which the secondary electromagnetic element is covered so as to prevent electromagnetic coupling between the primary and secondary electromagnetic elements and a second configuration in which the secondary electromagnetic element is accessible for electromagnetic coupling with the primary electromagnetic element.

2. A system according to claim 1 , wherein the downhole arrangement comprises a downhole tool.

3. A system according to claim 1 , wherein the downhole arrangement is configurable for at least one of fluid injection, stimulation, fracturing and production.

4. A system according to claim 1 , wherein the deployable tool comprises a shifting tool for actuating the downhole arrangement.

5. A system according to claim 1 , wherein the primary and secondary electromagnetic elements are configurable for electromagnetic coupling at a frequency in a range of 10 kHz to 1 MHz, 50 kHz to 500 kHz, or 100 kHz to 150 kHz.

6. A system according to claim 1 , comprising a controller which is configured to generate and apply an electrical signal to the primary electromagnetic element, to measure the electrical signal which results from any electromagnetic coupling between the primary electromagnetic element and the secondary electromagnetic element, and to determine a degree of electromagnetic coupling between the primary and secondary electromagnetic elements from the measured electrical signal.

7. A system according to claim 6 , wherein the controller is configured to determine a degree of electromagnetic coupling between the primary and secondary electromagnetic elements according to a shape of a frequency spectrum of the measured electrical signal.

8. A system according to claim 6 , wherein the controller is configured to determine a degree of electromagnetic coupling between the primary and secondary electromagnetic elements according to a shape or Q-factor of a resonant feature in the frequency spectrum of the measured electrical signal.

9. A system according to claim 6 , wherein the controller is configured to log the determined degree of electromagnetic coupling between the primary and secondary electromagnetic elements in a memory.

10. A system according to claim 6 , wherein the deployable tool comprises the controller.

11. A system according to claim 6 , wherein the controller is configured for communication uphole.

12. A system according to claim 6 , wherein the system comprises a power source for providing power to the controller.

13. A system according to claim 12 , wherein the deployable tool comprises the power source.

14. A system according to claim 6 , wherein the electrical signal exists on the primary electromagnetic element.

15. A system according to claim 6 , wherein the electrical signal exists on a tertiary electromagnetic element provided on the deployable tool as a result of any electromagnetic coupling between the primary electromagnetic element and the tertiary electromagnetic element via the secondary electromagnetic element.

16. A system according to claim 1 , wherein the primary electromagnetic element comprises a coil of wire arranged helically around a body portion of the deployable tool, the wire comprising an electrically conductive core surrounded by an electrically insulating outer layer.

17. A system according to claim 1 , wherein the primary electromagnetic element is encapsulated in a potting compound.

18. A system according to claim 1 , wherein the deployable tool comprises a primary enclosure and the primary electromagnetic element is housed within the primary enclosure.

19. A system according to claim 18 , wherein the primary enclosure is filled with a potting compound.

20. A system according to claim 18 , wherein the primary enclosure comprises a tubular base member and a tubular lid member, and the primary electromagnetic element is wrapped around the base member.

21. A system according to claim 20 , wherein at least the lid member of the primary enclosure is transparent to an electromagnetic field at a frequency in a range of 10 kHz to 1 MHz, 50 kHz to 500 kHz, or 100 kHz to 150 kHz.

22. A system according to claim 1 , wherein the secondary electromagnetic element is configured so that an electrical signal existing on the primary electromagnetic element when the primary and secondary electromagnetic elements are electromagnetically coupled has a predetermined frequency spectrum.

23. A system according to claim 22 , wherein the predetermined frequency spectrum has a predetermined shape.

24. A system according to claim 22 , wherein the predetermined frequency spectrum includes a resonant feature having a predetermined shape or a predetermined Q-factor.

25. A system according to claim 24 , wherein the resonant feature has a predetermined resonant frequency for a given temperature and/or pressure to which the secondary electromagnetic element is exposed.

26. A system according to claim 1 , wherein the secondary electromagnetic element extends around the throughbore of the downhole arrangement.

27. A system according to claim 1 , wherein the secondary electromagnetic element comprises a coil of wire arranged helically about the throughbore of the downhole arrangement, the wire comprising an electrically conductive core surrounded by an electrically insulating outer layer.

28. A system according to claim 1 , wherein the secondary electromagnetic element comprises a capacitor electrically connected between two ends of the core.

29. A system according to claim 1 , wherein the secondary electromagnetic element is encapsulated in a potting compound.

30. A system according to claim 1 , wherein the downhole arrangement comprises a secondary enclosure and the secondary electromagnetic element is housed within the secondary enclosure.

31. A system according to claim 30 , wherein the secondary enclosure is filled with a potting compound.

32. A system according to claim 30 , wherein the secondary enclosure comprises a tubular base member and a tubular lid member which define a annular cavity therebetween, the annular cavity having a radial extent selected so as to avoid electromagnetic coupling between the secondary electromagnetic element and a body portion of the downhole arrangement.

33. A system according to claim 32 , wherein the secondary electromagnetic element is wrapped around the base member.

34. A system according to claim 32 , wherein at least the base member of the secondary enclosure is transparent to an electromagnetic field at a frequency in a range of 10 kHz to 1 MHz, 50 kHz to 500 kHz, or 100 kHz to 150 kHz.

35. A system according to claim 1 , wherein the secondary electromagnetic element comprises a plurality of coils of wire distributed axially along the downhole arrangement, each coil of wire being unconnected electrically to the other coils of wire, and each coil of wire extending helically around the throughbore of the downhole arrangement, the wire of each coil of wire comprising an electrically conductive core surrounded by an electrically insulating outer layer, and each coil of wire configured such that electromagnetic coupling of each coil with the primary electromagnetic element provides a corresponding measured electrical signal having a characteristic frequency spectrum.

36. A system according to claim 1 , wherein the secondary electromagnetic element comprises an electrically conductive ring extending around the throughbore of the downhole arrangement.

37. A system according to claim 36 , wherein the ring is defined by an annular portion of the downhole arrangement.

38. A system according to claim 36 , wherein the secondary electromagnetic element comprises a plurality of rings distributed axially along a longitudinal axis of the downhole arrangement, each ring extending around the throughbore of the downhole arrangement.

39. A system according to claim 38 , wherein each ring is configured such that electromagnetic coupling of each ring with the primary electromagnetic element provides a corresponding measured electrical signal having a characteristic frequency spectrum.

40. A system according to claim 39 , wherein the configuration of a ring is defined at least in part by an inner profile of the ring.

41. A system according to claim 39 , wherein the configuration of a ring is defined at least in part by an inner diameter of the ring.

42. A system according to claim 39 , wherein the configuration of a ring is defined at least in part by a material from which the ring is formed.

43. A system according to claim 39 , wherein the configuration of a ring is defined at least in part by an axial extent of the ring.

44. A system according to claim 39 , wherein the configuration of each ring is selected from a finite set of different ring configurations.

45. A system according to claim 44 , wherein the finite set of different ring configurations comprises a plurality of data ring configurations and a checking ring configuration which is different from the data ring configurations.

46. A system according to claim 45 , wherein the plurality of rings defines a series of rings in which a checking ring configured according to the checking ring configuration is included repeatedly.

47. A system according to claim 46 , wherein the plurality of rings defines the series of rings in which the checking ring configured according to the checking ring configuration is included with a regular axial periodicity.

48. A system according to claim 46 , wherein alternate rings in the series of rings are the checking rings configured according to the checking ring configuration.

49. A system according to claim 44 , wherein the plurality of rings defines a series of rings in which a series of ring configurations is repeated at least once.

50. A system according to claim 44 , wherein the secondary electromagnetic element comprises an axially inner plurality of rings, a first axially outer plurality of rings, and a second axially outer plurality of rings, and wherein first and second series of ring configurations of the first axially outer plurality of rings and the second axially outer plurality of rings are different to one another and to a third series of ring configurations of the axially inner plurality of rings.

51. A system according to claim 1 , wherein the downhole arrangement comprises a cover member; and wherein the secondary electromagnetic element and the cover member are moveable relative to one another so as to selectively configure the downhole arrangement between the first and second configurations.

52. A system according to claim 51 , wherein the downhole arrangement is configurable between the first and second configurations on actuation or de-actuation of the downhole arrangement.

53. A system according to claim 51 , wherein the secondary electromagnetic element is arranged radially outwardly of the cover member relative to a longitudinal axis of the throughbore of the downhole arrangement.

54. A system according to claim 51 , wherein the cover member extends at least partially between the secondary electromagnetic element and the throughbore of the downhole arrangement in the first configuration and the cover member is at least partially withdrawn from between the secondary electromagnetic element and the throughbore of the downhole arrangement in the second configuration.

55. A system according to claim 1 , wherein the downhole arrangement comprises a further secondary electromagnetic element configurable for electromagnetic coupling with the primary electromagnetic element, wherein the further secondary electromagnetic element is accessible for electromagnetic coupling with the primary electromagnetic element regardless of a status of the downhole arrangement.

56. A system according to claim 55 , wherein the further secondary electromagnetic element comprises one or more of the same features as the secondary electromagnetic element.

57. A system according to claim 56 , wherein the further secondary electromagnetic element is configured for location downhole of the secondary electromagnetic element.

58. A system according to claim 1 , wherein the primary electromagnetic element comprises an electrical conductor and/or a coil of wire comprising an electrically conductive core surrounded by an electrically insulating outer layer.

59. A system according to claim 1 , wherein the secondary electromagnetic element comprises an electrical conductor and/or a coil of wire comprising an electrically conductive core surrounded by an electrically insulating outer layer.

60. A downhole arrangement for use in downhole detection, the downhole arrangement defining a throughbore for deployment of a deployable tool therethrough and the downhole arrangement comprising secondary electromagnetic element configurable for electromagnetic coupling with a primary electromagnetic element provided on the deployable tool, wherein the downhole arrangement is configurable between a first configuration in which the secondary electromagnetic element is covered so as to prevent electromagnetic coupling between the primary and secondary electromagnetic elements and a second configuration in which the secondary electromagnetic element is accessible for electromagnetic coupling with the primary electromagnetic element.

61. A system for use in downhole detection comprising:

a downhole arrangement defining a throughbore;

a tool deployable through the throughbore of the downhole arrangement;

a primary electromagnetic element provided on the deployable tool; and

a secondary electromagnetic element provided on the downhole arrangement,

wherein the primary and secondary electromagnetic elements are configurable for electromagnetic coupling therebetween,

wherein the secondary electromagnetic element comprises a plurality of electrically conductive rings distributed axially along a longitudinal axis of the downhole arrangement, each ring extending around the throughbore of the downhole arrangement, and

wherein each ring is configured such that electromagnetic coupling of each ring with the primary electromagnetic element provides a corresponding measured electrical signal having a characteristic frequency spectrum.

62. A system for use in downhole detection comprising:

a downhole arrangement defining a throughbore;

a tool deployable through the throughbore of the downhole arrangement;

a primary electromagnetic element provided on one of the downhole arrangement and the deployable tool; and

a secondary electromagnetic element provided on the other of the downhole arrangement and the deployable tool; and

a controller,

wherein the primary and secondary electromagnetic elements are configurable for electromagnetic coupling therebetween, and

wherein the controller is configured to measure an electrical signal which varies with a degree of electromagnetic coupling between the primary and secondary electromagnetic elements and to determine the degree of electromagnetic coupling between the primary and secondary electromagnetic elements from measurements of the electrical signal across a range of frequencies.

63. A system according to claim 62 , wherein the primary and secondary electromagnetic elements are configurable for electromagnetic coupling at a frequency in a range of 10 kHz to 1 MHz, 50 kHz to 500 kHz, or 100 kHz to 150 kHz.

64. A system according to claim 62 , comprising a controller which is configured to generate and apply an electrical signal to the primary electromagnetic element, to measure the electrical signal which results from any electromagnetic coupling between the primary electromagnetic element and the secondary electromagnetic element, and to determine a degree of electromagnetic coupling between the primary and secondary electromagnetic elements from the measured electrical signal.

65. A system according to claim 64 , wherein the electrical signal exists on the primary electromagnetic element.

66. A system according to claim 64 , wherein the electrical signal exists on a tertiary electromagnetic element provided on the deployable tool as a result of any electromagnetic coupling between the primary electromagnetic element and the tertiary electromagnetic element via the secondary electromagnetic element.

67. A system according to claim 64 , wherein the secondary electromagnetic element is configured so that the electrical signal has a predetermined frequency spectrum.

68. A system according to claim 67 , wherein the controller is configured to determine the frequency spectrum of the measured electrical signal and to determine the degree of electromagnetic coupling between the primary and secondary electromagnetic elements from the determined frequency spectrum of the measured electrical signal.

69. A system according to claim 67 , wherein the predetermined frequency spectrum has a predetermined shape.

70. A system according to claim 67 , wherein the predetermined frequency spectrum includes a resonant feature.

71. A system according to claim 70 , wherein the resonant feature has a predetermined resonant frequency for a given temperature and/or pressure to which the secondary electromagnetic element is exposed.

72. A system according to claim 70 , wherein the resonant feature has a predetermined shape and/or a predetermined Q-factor.