WO2007015087A1

WO2007015087A1 - Method of determining features of downhole apparatus

Info

Publication number: WO2007015087A1
Application number: PCT/GB2006/002873
Authority: WO
Inventors: Simon Benedict Fraser; William Brown-Kerr
Original assignee: Maxwell Downhole Technology Limited
Priority date: 2005-08-03
Filing date: 2006-08-02
Publication date: 2007-02-08
Also published as: GB0515949D0

Abstract

A method of determining a feature of downhole apparatus (12) comprises determining a variation in ambient magnetic field resulting from the presence of a downhole apparatus (12), and determining a feature of the apparatus from the variation. The method may involve comparing the features of the Earth's ambient magnetic field with features of the magnetic field affected by the presence of a downhole apparatus (12), or the presence of a particular feature of a downhole apparatus. The comparison may be used to identify the location of casing collars, as the field is affected differently by the presence of a single thickness wall of pipe between the joints, and the double thickness of material at the joints.

Description

METHOD OF DETERMINING FEATURES OF DOWNHOLE APPARATUS

FIELD OF THE INVENTION This in invention relates to a method and apparatus for determining a feature of a downhole apparatus, and in particular but not exclusively to a method and apparatus for determining a feature of metal bore-lining tubing. The invention further relates to a method and apparatus for determining the positioning of apparatus within a bore that has been lined with metal tubing.

BACKGROUND OF THE INVENTION

In the field of oil and gas exploration and production, a hole is first drilled and then lined with metal pipe, known as casing. This casing is then cemented in place, usually by pumping cement down through the pipe and around the base of the pipe and into the annulus between the pipe and the hole wall. This process of drilling and then running in and cementing lengths of pipe is typically repeated one or more times, with reducing diameters of pipe. The casing is formed of lengths (typically 10m long) of pipe with male threads at both ends. These pipe sections are then joined using cylindrical couplers with matching internal female threads. As a consequence, the thickness of material at the coupling is approximately two times the thickness of the casing pipe itself.

In many operations carried out in the well after the casing has been cemented in place it is desirable to know the location of the casing couplers in order that their presence does not interfere with the required operation. One example of such an operation is described below. Where it is necessary to drill a directional well it is common to use a device called a 'whipstock' to provide a tapered guide at the base of the well such that when the drill bit is lowered in to drill the next section of hole the drill bit will be pushed in a particular direction to cut through the casing and exit the existing hole at a desired azimuth, m order to control this azimuth it is necessary to run equipment above the whipstock whilst anchoring the whipstock in the well, to allow the azimuth to be selected. Typically this is performed using equipment fitted with a gyroscope. A device known as a casing collar locator (CCL) will often be fitted above the gyro tool. The CCL uses a set of coils and magnets to detect the 'collar' joints between each 10m section of casing. This information is then used to ensure that the selected drill bit exit point is not on a casing joint. Existing CCLs rely upon motion of the apparatus to generate an electromotive force (emf) to detect the collar position.

CCLs may of course be utilised in other operations, and US Patents Nos 5,626,192 and 6,305,467 (Cornell et al) disclose various forms of coiled tubing mounted joint locators.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a method of determining a feature of downhole apparatus, the method comprising: determining a variation in ambient magnetic field resulting from the presence of a downhole apparatus; and determining a feature of the apparatus from said variation.

According to a further aspect of the present invention there is provided sensing apparatus for use in determining a feature of a downhole apparatus, the sensing apparatus comprising: a sensor for sensing at least one feature of ambient magnetic field affected by a downhole apparatus; and means for comparing the sensed feature with ambient magnetic field from another location and determining a feature of the downhole apparatus based on the difference therebetween.

A preferred embodiment of the present invention operates by comparing the features of ambient magnetic field, typically features of the Earth's magnetic field, with features of the magnetic field which have been affected by the presence of a downhole apparatus, or the presence of a particular feature of a downhole apparatus. The comparison may be used to identify the location of, for example, casing collars, as the field is affected differently by the presence of a single thickness wall of pipe between the joints, and the double thickness of material at the joints. This offers the advantage over existing CCLs that the method and apparatus may operate using a stationary or slowly moving sensor, and do not rely on movement of the apparatus through the pipe. The information gained by the apparatus may then be utilised to determine an appropriate location for another apparatus, such as a whipstock, to ensure that a drill bit deflected by the whipstock will cut through the casing at a location other than at a casing collar. The invention will be described primarily in relation to the location of casing collars, for use in whipstock location, however those of skill in the art will recognise that the invention has utility in a wide variety of other applications. As well as identifying the presence of relatively thick sections of downhole pipe, aspects of the invention may also be employed to identify relatively thin sections of pipe, for example casing that has been eroded or expandable tubing that has not expanded evenly, resulting in differential thinning of the casing wall. Aspects of the invention may also be utilised to detect changes in pipe material or form, for example the presence of a profile or the presence of ovality. While it is preferred that the invention operate with reference to the Earth's magnetic field, in other embodiments a generated magnetic field may be utilised.

Furthermore, while reference is made primarily herein to casing, aspects of the invention have equal utility in other forms of downhole pipe or tube, including liner, completion tubing, and sand screen. The invention may also be useful in other areas where access to apparatus is restricted, for example in logging or mapping features of subsea tubing, such as risers and seabed pipelines. Certain aspects of the invention may also have application in relation to locating or identifying features of non-tubular objects.

The apparatus may be mounted on any suitable support, which may be coiled tubing, wireline or a string of pipe. The apparatus may be run into a bore together with other apparatus or devices, for example the apparatus may be run into casing together with a whipstock, for use in facilitating location of the whipstock. The apparatus may be integrated within apparatus adapted to be retained or fixed in the hole, or may be adapted to be retrievable.

The apparatus preferably includes means for storing or transmitting the information obtained by the sensor. Preferably, the information obtained by the sensor is transmitted to surface for analysis, and the apparatus thus may include a computer or the like suitably programmed to analyse the information transmitted to surface. Information may be transmitted by any suitable method, for example mud pulse telemetry, or by electromagnetic telemetry. Casing is typically of ferrous material, usually mild steel. This material has the effect of distorting the Earth's magnetic field. The distortion is proportional to the relative permeability of the casing material and to the thickness of this material. Consequently, the distortion varies where the material thickness changes. This variation in thickness occurs at each casing joint where the material thickness increases by a factor of around two. The effect will be pronounced at either edge of the connection at the point where the material thickness changes. Sensitive magnetic field sensors located within the pipe can measure this distortion of the Earth's magnetic field. The field will be diverted in the principle axis of the pipe and the flux will be preferentially directed in the walls of the pipe. Sensors having the required range and sensitivity can measure the variation in distortion. By applying appropriate digital signal processing the magnetic distortion can be analyzed. This information can then be transmitted to a surface computer system. The preferred apparatus uses flux-gate sensors, most preferably flux-gate magnetometers, these being among the most sensitive magnetic field detectors. However, it is possible that other devices such as AMR, GMR, Search Coil, Hall effect and other semiconductor devices could be utilized to provide similar results. Only a single sensor may be provided, which sensor may be moved axially through the casing. Alternatively, a multiplicity of sensors may be used in differential form to create a 'gradiometer'. This arrangement allows the data from identical sensors, spaced longitudinally, to be compared yielding very sensitive results. For the preferred apparatus these sensors would have a spacing greater than the length of a collar joint such that one sensor would always be outside the coupling at all times. The typical total field value of the naturally occurring Earth's magnetic field varies from place to place as does the local angle of dip (the angle of incident flux with respect to the Earth's surface) and declination (the amount that magnetic north varies from true north). However, an average value for total field is around 0.5 Gauss. This field is focused by the presence of ferrous material and can be much greater within the ferrous material itself. The apparent angle of dip and declination will also both be distorted. For the preferred application of the present invention, it is desirable to measure these three orthogonal axes of field and then interpret the distortion as it varies to identify the area where material thickness has changed. The field sensors are selected so as not to become saturated by the field and yet to have sufficient resolution to measure the distortion. Furthermore, it is preferable to reduce the transmitted data values to a minimum to make best use of the limited telemetry bandwidth likely to be available. The apparatus may be pre-programmed with the local ambient figures prevailing for field strength, dip and declination to aid in the process of recognizing distortion. A typical casing collar is 40cm long and so the data must be transmitted with enough frequency to give one measurement every 10cm if both edges of the collar are to be detected.

The apparatus may include a signaling system, and in one embodiment the signaling system may provide one data value every twenty seconds, such that the support string or pipe containing a measuring sonde may be moved at a rate of 0.3m/minute (or 18m/hour) during the measurement process. This means that around 30 minutes may be required to correctly establish the precise position of the apparatus with respect to a casing collar. It may also be possible to use the apparatus in a "real-time" mode whereby the signal is taken almost directly from the sensors and used to trigger a pulse from the telemetry system. This second method will only be attractive where it is easy to shift the support pipe relatively quickly and for the whipstock orienting application this will not always be so.

BRIEF DESCRIPTION OF THE DRAWING

These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawing, which is a diagrammatic illustration of an apparatus in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWING

The figure illustrates apparatus in accordance with a preferred embodiment of the present invention, in the form of a sensing apparatus 10 for use in identifying the location of casing collars 12 in a string of casing 14. The apparatus 10 is illustrated located in a casing-lined hole 15, adjacent a casing collar 12 which couples two casing sections 16,17 together. The apparatus 10 is mounted on a support string 18 which may also support other apparatus (not shown), such as a whipstock and gyro tool. The sensing apparatus 10 comprises a body 20 providing mounting for six longitudinally spaced sensors in the form of six identical flux-gate magnetometers 22. The sensors 22 are coupled to a processor 24, which outputs signals to a transmitter 26, signals being transmitted to surface from the transmitter 26. On surface, a receiver 28 detects and analyses the signals, to produce a human or machine-readable log 30.

In the illustrated embodiment, the six identical sensors 22 create a 'gradiometer', the data from the sensors 22 being compared to yield very sensitive results. To be effective, the sensors 22 have a total spacing greater than the length of a collar joint such that one sensor will always be outside the coupling. A typical casing collar is 40cm long, and in this embodiment the sensors are spaced by 10cm.

As described above, the apparatus 10 operates by detecting and analyzing distortions in the Earth's magnetic field produced by the presence of the mild steel casing 14. The ferrous material, which forms the casing 14, has the effect of distorting the

Earth's magnetic field, the field being diverted in the principle axis of the pipe and the flux preferentially directed in the walls of the pipe. The distortion is proportional to the relative permeability of the steel and the thickness of the steel. Consequently, the distortion varies where the material thickness changes, and in particular at each casing joint where the material thickness increases by a factor of around two. The effect is most pronounced at either edge of the connection, at the point where the material thickness changes. The magnetic field sensors 22, located within the pipe 14, can measure this distortion of the Earth's magnetic field.

The apparatus 10 is pre-programmed with the local figures prevailing for field strength, dip and declination to aid in the process of recognizing distortion. Within the casing 14, the field strength is focused by the presence of ferrous material and can be much greater within the ferrous material itself. The apparent angle of dip and declination will also both be distorted. The sensors 22 measure these three orthogonal axes of field and the apparatus 10 is used to interpret the distortion as it varies along the length of the casing 14, to identify the areas where material thickness changes.

As noted, the apparatus includes a signaling system, and in one embodiment the signaling system, in the form of the transmitter 26, provides one data value every twenty seconds. Accordingly, a measuring body or sonde 20 containing only a single sensor 22 may be moved at a rate of 0.3m/minute (or 18m/hour) during the measurement process. This means that around 30 minutes may be required to correctly establish the precise position of the apparatus with respect to a casing collar. However, with the illustrated arrangement of six spaced sensors 22, the body 20 may be moved more quickly. The data from the apparatus 10 may thus be used to identify the location of the collars 12 in the casing 14. Where the apparatus 10 is being used to assist in the location of a whipstock, which may be mounted on the same support string as the apparatus 10, this information may then be utilized in placing the whipstock in the casing to ensure that the drill bit which is subsequently deflected by the whipstock cuts through the casing 14 at a location spaced from a collar.

Those of skill in the art will recognize that the above-described embodiment is merely exemplary of the present invention and that various modifications and improvements may be made thereto, without departing from the present invention.

Claims

1. A method of determining a feature of downhole apparatus, the method comprising: determining a variation in ambient magnetic field resulting from the presence of a downhole apparatus; and determining a feature of the apparatus from said variation.

2. The method of claim 1, further comprising determining the ambient magnetic field in the absence of the downhole apparatus.

3. The method of claim 1 or 2, comprising determining features of the Earth's magnetic field which have been affected by the presence of at least one of a downhole apparatus or a particular feature of a downhole apparatus.

4. The method of claim 1, 2 or 3, comprising determining features of a generated magnetic field which have been affected by the presence of at least one of a downhole apparatus or a particular feature of a downhole apparatus.

5. The method of any of the preceding claims, wherein the downhole apparatus comprises a downhole tubular.

6. The method of claim 5, comprising determining a feature of at least one of casing, liner, completion tubing and sand screen.

7. The method of claim 5 or 6, comprising determining at least one of tubular wall thickness, tubular material and tubular form.

8. The method of any of claims 5 to 7, comprising determining the location a casing collar.

9. The method of any of the preceding claims, wherein the determination of said variation does not rely on movement of a sensor.

10. The method of any of the preceding claims, further comprising utilising the determined feature to determine an appropriate location for another activity.

11. The method of claim 10, comprising determining an appropriate location for a whipstock.

12. The method of any of the preceding claims, comprising running a magnetic field sensor into a bore.

13. The method of claim 12, comprising running the sensor on an elongate support.

14. The method of claim 13, wherein the elongate support is at least one of coiled tubing, wireline and a string of pipe.

15. The method of any of claims 12 to 14, further comprising running a sensor into a bore together with another apparatus.

16. The method of claim 15, wherein the other apparatus is a whipstock.

17. The method of any of claims 12 to 16, comprising retrieving the sensor from the bore.

18. The method of any of claims 12 to 16, comprising retaining the sensor in the bore.

19. The method of any of claims 12 to 18, comprising at least one of storing and transmitting information obtained by the sensor.

20. The method of any of claims 12 to 19, comprising transmitting the information obtained by the sensor to surface for analysis.

21. The method of claim 20, further comprising analyzing the information transmitted to surface to determine said feature of the apparatus.

22. The method of claim 20 or 21, comprising transmitting the information to surface by at least one of mud pulse telemetry and by electromagnetic telemetry.

23. The method of any of the preceding claims, comprising determining a feature of an apparatus comprising ferrous material.

24. The method of claim 23, comprising determining a distortion of the Earth's magnetic field due to the presence of said ferrous material.

25. The method of claim 24, wherein the distortion is proportional to at least one of the relative permeability of the material and to the thickness of this material.

26. The method of claim 25, comprising detecting variations in the distortion where the material thickness changes.

27. The method of any of claims 24 to 26, comprising applying digital signal processing to analyze the magnetic field distortion.

28. The method of any of the preceding claims, comprising using a flux-gate sensor to detect magnetic field distortion.

29. The method of any of the preceding claim 28, comprising using a flux-gate magnetometers to detect magnetic field distortion.

30. The method of any of the preceding claims, comprising using at least one of AMR, GMR, Search Coil and Hall effect sensors to detect magnetic field distortion.

31. The method of any of the preceding claims, comprising providing at least one magnetic field sensor and moving said sensor axially through a downhole tubular.

32. The method of any of the preceding claims, comprising providing a plurality of axially spaced magnetic field sensors.

33. The method of claim 32, comprising analyzing data provided by comparing data from longitudinally spaced identical sensors.

34. The method of claim 32 or 33, wherein the sensors have a spacing greater than the length of a feature to be detected.

35. The method of any of the preceding claims, comprising determining at least one of the total field value, the local angle of dip and the local angle of declination of the naturally occurring Earth's magnetic field and comparing the determined value with a corresponding distorted value within a ferrous tubular.

36. The method of claim 35, comprising determining the total field value, the local angle of dip and the local angle of declination of the naturally occurring Earth's magnetic field and comparing the determined values with a corresponding distorted value within a ferrous tubular.

37. The method of any of the preceding claims, comprising pre-determining local ambient magnetic field values.

38. Sensing apparatus for use in determining a feature of a downhole apparatus, the sensing apparatus comprising: a sensor for sensing at least one feature of ambient magnetic field affected by a downhole apparatus; and means for comparing the sensed feature with ambient magnetic field from another location and determining a feature of the downhole apparatus based on the difference therebetween.

39. The apparatus of claim 38, wherein the sensor is adapted to be mounted on an elongate support.

40. The apparatus of claims 38 or 39, wherein the sensor is adapted to be run into a bore with another apparatus.

41. The apparatus of claim 40, wherein the sensor is adapted to be run into a bore with a whipstock.

42. The apparatus of any of claims 38 to 41, further means for at least one of storing and transmitting information obtained by the sensor.

43. The apparatus of any of claims 38 to 42, further comprising a transmitter for transmitting the information obtained by the sensor to surface for analysis.

44. The apparatus of claim 43, further comprising means for analyzing the information transmitted to surface to determine said feature of the apparatus.

45. The apparatus of claim 43 or 44, comprising at least one of a mud pulse telemetry transmitter and an electromagnetic telemetry transmitter.

46. The apparatus of any of claims 38 to 45, wherein the sensor is adapted to determine a distortion of the Earth's magnetic field due to the presence of ferrous material.

47. The apparatus of claim 46, comprising means for applying digital signal processing to analyze the sensed magnetic field distortion.

48. The apparatus of any of claims 38 to 47, comprising a flux-gate magnetometer.

49. The apparatus of any of claims 38 to 48, comprising at least one of an AMR,

GMR, Search Coil and Hall effect sensor or other semiconductor device.

50. The apparatus of any of claims 38 to 49, comprising a plurality of axially spaced magnetic field sensors.

51. The apparatus of claim 50, comprising means for analyzing data provided by comparing data from longitudinally spaced identical sensors.

52. The apparatus any of claims 38 to 51, comprising means for determining at least one of the total field value, the local angle of dip and the local angle of declination of the naturally occurring Earth's magnetic field and means for comparing the determined value with a corresponding distorted value within a ferrous tubular.

53. The apparatus of any claims 38 to 52, comprising means for storing predetermined local ambient magnetic field values.

54. The apparatus of any of claims 38 to 53, wherein the apparatus is adapted to be retained or fixed in a bore.

55. The apparatus of any of claims 38 to 53, wherein the apparatus is adapted to be retrievable from a bore.

56. The apparatus of any of claims 38 to 55, further comprising a signalling system adapted to provide a data value at predetermined intervals.

57. The apparatus of any of claims 38 to 56, wherein the apparatus is adapted such that a signal from the sensor triggers a pulse from a telemetry system.