External hollow antenna

Abstract

A beacon assembly located at a downhole end of a drill string proximate a boring tool. The beacon assembly transmits data to an above-ground receiver. The beacon has a housing with a housing wall located between its sensors, such as gradiometers, accelerometers, and other orientation sensors, and an antenna assembly. The antenna assembly has a protective covering made of electromagnetically transparent material.

Claims

1. A downhole tool comprising: a housing; an antenna surrounding the housing and configured to be attached to a sensor; a conductive, non-magnetic shield disposed between the housing and the antenna; and a non-conductive, non-magnetic tube disposed between the shield and the antenna; and a magnetically transparent sleeve surrounding the antenna and having an outer surface, in which the outer surface of the sleeve constitutes an outermost surface of the entire periphery of at least a portion of the downhole tool.

2. A system, comprising: a borehole; a drill string situated within the borehole; the downhole tool of claim 1 coupled to the drill string; and an above-ground tracker configured to receive data from the downhole tool.

3. The downhole tool of claim 1 in which the antenna comprises an antenna coil.

4. The downhole tool of claim 1 in which the housing is part of a body element, the body element having a pair of opposed ends, in which at least one end is configured to form a threaded connection with a drill string.

5. The downhole tool of claim 1 in which the housing has longitudinally offset first and second portions, the first portion having a maximum cross-sectional dimension greater than that of the second portion, in which the second portion is at least partially surrounded by the antenna.

6. The downhole tool of claim 5 further comprising a sensor situated within a hollow region of the first portion of the housing.

7. The downhole tool of claim 6 in which the sensor is an orientation sensor.

8. The downhole tool of claim 1 in which the downhole tool comprises one and only one conductive, non-magnetic shield.

9. The downhole tool of claim 1 in which the antenna is electrically insulated from the housing.

10. A system, comprising: the downhole tool of claim 1 positioned beneath a ground surface; an electromagnetic signal emitted from the downhole tool towards the ground surface; and a tracker positioned at the ground surface and configured to receive the electromagnetic signal.

11. A downhole tool comprising: a housing having first and second portions, the first portion having a maximum cross-sectional dimension greater than that of the second portion; in which the first portion is configured to house a beacon assembly, and in which the second portion is configured so that an antenna may at least partially surround the second portion; one and only one conductive, shield surrounding the second portion of the housing, in which the shield has the shape of a tubular sleeve having a continuous outer surface; and a non-conductive, non-magnetic tube surrounding the shield.

12. The downhole tool of claim 11 further comprising a sensor situated within a hollow region of the first portion of the housing.

13. The downhole tool of claim 12 in which the sensor is an orientation sensor.

14. The downhole tool of claim 12 in which the sensor is a magnetic gradiometer.

15. The downhole tool of claim 11 further comprising an antenna at least partially surrounding the second portion of the housing.

16. The downhole tool of claim 15 in which the antenna comprises an antenna coil.

17. The downhole tool of claim 15 in which the shield is disposed between the housing and the antenna, and the tube is disposed between the shield and the antenna.

18. A downhole tool, comprising: an elongate conductive housing having a hollow internal region and an external surface; a sensor disposed within the internal region of the housing; an antenna formed from a conductive wire coiled into a cylindrical shape, the antenna supported by the external surface of the housing, in electrically insulated relationship therewith; and a magnetically transparent sleeve surrounding the antenna and having an outer surface, in which the outer surface of the sleeve constitutes an outermost surface of the entire periphery of at least a portion of the downhole tool.

19. The downhole tool of claim 18 in which the sensor is longitudinally offset from the antenna.

20. The downhole tool of claim 18 in which the sensor is a magnetic gradiometer.

21. A system, comprising: a drill string configured to advance along an underground path from a ground-level entry point to a ground-level exit point; and the downhole tool of claim 18 carried by the drill string.

22. The system of claim 21 further comprising an above-ground tracker configured to receive data from the downhole tool.

23. A system, comprising: a downhole tool comprising: a housing; an antenna surrounding the housing and configured to be attached to a sensor; a conductive, non-magnetic shield disposed between the housing and the antenna; a non-conductive, non-magnetic tube disposed between the shield and the antenna; and a magnetically transparent sleeve surrounding the antenna; in which the downhole tool is positioned beneath a ground surface; an electromagnetic signal emitted from the downhole tool towards the ground surface; and a tracker positioned at the ground surface and configured to receive the electromagnetic signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional side view of a downhole tool having an external antenna.

(2) FIG. 2 is a perspective view of a beacon assembly of the downhole tool of FIG. 1.

(3) FIG. 3 is a perspective sectional view of the antenna assembly of the downhole tool of FIG. 1.

(4) FIG. 4 is a partial sectional end view of the downhole tool, showing the antenna assembly of the downhole tool.

(5) FIG. 5 is a cross-sectional side view of an alternative embodiment of the antenna assembly of the downhole tool with the antenna coil shown un-sectioned for clarity.

(6) FIG. 6 is a perspective sectional view of another embodiment of the antenna assembly of a downhole tool having an insulating gap between a housing wall and a shield.

(7) FIG. 7 is a cross-sectional view of another embodiment of a downhole tool having an external antenna disposed about both a housing wall and a beacon assembly.

(8) FIG. 8 is an illustration of a horizontal directional drilling operation.

DETAILED DESCRIPTION

(9) Horizontal Directional Drilling (HDD) applications typically employ a subsurface tracking beacon and a walk-over tracking receiver to follow the progress of a horizontal borehole. An example of a walkover receiver and method for use thereof is shown in U.S. Pat. No 8,497,684 issued to Cole, et. al., the contents of which are incorporated herein by reference. The tracking beacon contains devices to measure pitch, roll (bit angle), beacon battery voltage, beacon temperature, and a variety of other physical parameters. Measured information is transmitted by the beacon using a modulated electromagnetic signal. Transmission of the beacon's signal typically involves an internal antenna consisting of multiple wire turns wrapped around a ferrite rod. The surface tracking receiver contains electronic elements which receive and decode the modulated signal. The surface tracking receiver also detects the signal's field characteristics and measures the beacon's emitted signal amplitude to estimate the beacon's depth and location.

(10) In some cases, the beacon measurements of interest are magnetic field measurements. Certain applications require the use of magnetic field gradiometers, which are instruments used to determine a magnetic field's rate of change along a certain path. Magnetic field gradiometers essentially involve magnetic field measurements separated by a known distance along some axis. Construction of a magnetic field gradiometer in the HDD industry is complicated, not only by the limited axial and radial space available for sensor placement, but also by the need to communicate measurements to the surface receiver by a magnetic field transmission. The lack of space makes it desirable to package beacon electronics elements as densely as possible, but the presence of the antenna's ferrite rod near a gradiometer's magnetic field sensors is known to be capable of disturbing the gradiometer's measurement capability. In the case of the most sensitive sensors, the proximity of a ferrite rod to any of the sensing elements can produce undesirable measurement degradation.

(11) Further, conventional beacon antennas will be inside a beacon housing that attenuates the magnetic field because the beacon housing is conductive and magnetically permeable. To reduce this effect, slots are often provided in the beacon housing. However, limitations include differences in the strength based upon the orientation of the housing, attenuation, and may require specifically clocked housings for accurate measurements.

(12) The present invention packages the antenna away from sensors and outside of the beacon housing. The invention may also be used with a downhole generator that may be integral with the beacon for power, which could be housed in a common housing. The beacon may be used with a single or dual-member drill string. The beacon could also be used with a drive shaft going through the beacon to drive a downhole tool such as in a coiled tubing application.

(13) With reference now to the figures in general and FIGS. 1 and 8 in particular, shown therein is a downhole tool 10. The downhole tool 10 is connected on a first end 12 to a drill bit 13 and a second end 14 to a drill string 11. A drilling machine 2 positioned at a ground surface 3 rotates and thrusts the drill string 11 forward underground to form a borehole 4. The drill string 11 advances underground from a ground-level entry point 5 to a separate ground-level exit point. An operator 6 tracks the position of the downhole tool 10 underground using an above ground tracker 7. As shown in FIG. 1, the tool 10 is adapted to connect to a dual member drill string 11 comprising an inner member 11a and an outer member 11b, though a single member drill string may be utilized with the proposed invention without departing from its spirit. The tool 10 may connect to the drill string 11 at a threaded connection or other known connection at its second end 14. The tool 10 comprises a front tool body 16, a beacon assembly 18, and an antenna assembly 20. The tool 10 comprises a housing wall 21 which is preferably located about a periphery of the beacon assembly 18 but inside the antenna assembly 20. The beacon assembly 18 may allow fluid to pass through the center portion of the tool 10 forming an internal passage 13 of the drill string 11 or with an annulus between the inner member 11a and outer member 11b of a dual member drill string.

(14) The housing wall 21 preferably has a varying diameter creating a first portion 21a and second portion 21b , such that the diameter of the housing wall 21 when encasing the beacon assembly 18 (first portion 21a) is greater than the diameter of the housing wall when within the antenna assembly 20 (second portion 21b). A shoulder may be created between the first portion 21a and the second portion 21b , or the transition may be tapered or gradual. The housing wall 21 may comprise an opening, or feedthrough 104 (FIG.5) for the antenna coil 100 (FIG. 5), to traverse between the antenna assembly 20 and the beacon assembly 18.

(15) The front tool body 16 allows fluid flow from within the drill string 11 to a drill bit or other tool as well as transmission of rotation from the inner member 11a to the drill bit. The beacon assembly 18 comprises a magnet motor 22 and a generator assembly 24. As relative rotation occurs between the inner member 11a and outer member 11b of the drill string 11, components of the downhole tool 10 also rotate relative to one another due to connection made at stem weldment. An exemplar generator assembly 24 utilizing a dual-member drill string 11 may be found in U.S. Pat. No. 6,739,413, issued to Sharp, et. al., the contents of which are incorporated herein by reference.

(16) The antenna assembly 20 comprises an antenna 26 and a protective casing 29. The antenna 26 transmits signals generated by the beacon assembly 18 as will be described in further detail with reference to FIGS. 3-5. The protective casing 29 is preferably a magnetically transparent sleeve, a material that has a relative permeability of substantially unity. The casing 29 may comprise cast urethane, plastics, ceramics, or other materials that provide structural protection but create little or no interference with the signal of the antenna 26.

(17) With reference now to FIG. 2 the beacon assembly 18 is shown in greater detail. The beacon assembly 18 may be rotationally locked to the inner member 11a (not shown). The generator assembly 24 comprises stator poles 30, bobbins 32, and a back plate 34. The stator poles 30, when rotated relative to magnet motor 22 (FIG. 1) through fluid flow or relative rotation of the inner 11a and outer 11b drill members, generate a current to power the tool 10. Alternatively, power for the tool 10 may also be provided by wireline or batteries.

(18) The beacon assembly 18 further comprises a sensor assembly 40. The back plate 34 helps to isolate the generator assembly 24 from the sensor assembly 40. The sensor assembly 40 comprises a board 42, a sensor 44, and a program port 46. The board 42 provides structural and electrical connectivity for the sensor 44 and program port 46. The board 42 may be curved to match the shape of the beacon assembly 18. The sensor 44 comprises one or more sensors for determining an orientation of the downhole tool 10. Such sensors 44 may comprise one or more yaw, pitch, roll, tension, force, conductivity, or other sensors. For example, an accelerometer may be utilized. The program port 46 allows a user to access data and configure the sensors 44. Further, while the use of sensors 44 is one advantageous use of the antenna assembly 20 (FIG. 3), another transmission source could be utilized with the antenna assembly disclosed below.

(19) The antenna assembly (FIG. 3) may also connect to the beacon sensors 44 through port 46. A locating key 48 may be utilized to lock the clock position of the beacon assembly 18 to the antenna assembly 20 (FIG. 3). In this way, a feedthrough 104 (FIG. 5) may be placed between the sensor assembly 40 and the antenna assembly 20 through the housing wall 21 (FIG. 3). As shown, a center tube 49 passes through the beacon assembly 18 to provide fluid flow and optionally provide rotational torque from the drill string 11 (FIG. 1).

(20) With reference to FIG. 3, the antenna 26 comprises an end support 50, a support tube 52, at least one ferrite rod 54, a nonconductive tube 56 and a shield 58. The end support 50 provides an insulating support for the antenna 26 within the tool 10 so that electromagnetic interference of the housing wall 21 at the ends of the antenna 26 is minimized. Further, any electromagnetic interference between the antenna 26 and sensors 44 is also minimized. The support tube 52 is disposed about the housing wall 21 and locates the ferrite rods 54 within the antenna assembly 20. The shield 58 is preferably highly conductive, non-magnetic. Aluminum may be used in the shield 58, as could other materials such as copper. Preferably, the shield covers the end support 50. There may be a further insulator between the shield 58 and the housing wall 21. The nonconductive magnetic field layer, or tube 56 is located between the shield 58 and ferrite rods 54 and insulates them from each other. Further, the tube 56 may be a non-magnetic material such as plastic. Without the nonconductive tube 56 or similar structure, the magnetic field would be pushed outward but some eddy currents would flow within the housing wall 21. The tube 56 may be a hollow cylinder, or may be comprised of multiple pieces with nonconductive, non-magnetic properties.

(21) The ferrite rods 54 are located between the plastic tube 56 and protective casing 29 and magnify signal strength of the beacon signals corresponding to readings of the beacon assembly 18. An antenna coil 100 (FIG. 5) may be provided about the ferrite rods 54 to transmit the beacon signals. Further, as shown in FIG. 5, the antenna coil 100 may be utilized without ferrite rods. The antenna coil 100 is preferably a single layer to minimize its profile, but a multi-layer antenna may be used. The sensor 44 may be disposed within the coil 100 and within the wall 21.

(22) With reference now to FIG. 4, the antenna assembly 20 is shown in cross section. The housing wall 21 is removed for clarity. As shown, the antenna assembly 20 comprises twenty-five ferrite rods 54, though other numbers of rods may be used. Additionally, the ferrite rods 54 themselves may be removed and elements of the housing wall 21 may be used with an antenna coil. The antenna coil 100 may be also utilized about the ferrite rods. In general, the arrangement of the antenna assembly 20 from inside to outside is housing wall 21 (FIG. 3), shield 58, tube 56, ferrite rods 54, antenna coil 100 (FIG. 5), protective casing 29. An insulating gap or material 59 may be utilized between the housing wall 21 and shield 58 as shown in FIG. 6. Further, the plastic tube 56 may be replaced with a layer of any non-conductive material, such as air.

(23) In operation, the antenna assembly 20 of FIG. 4 operates when current passes through the antenna coil 100 to generate a magnetic field corresponding to beacon readings. The field passes through the tube 56 and permeates the shield 58 according to skin depth rules. The eddy current induced in the shield 58 will push the magnetic field out away from the tool 10, minimizing power loss. The insulating gap 59 prevents eddy currents from reaching the housing wall 21.

(24) In FIG. 1, the antenna assembly 20 and beacon assembly 18 are shown with linear displacement for clarity. One of skill in the art will appreciate that these assemblies may be placed at any location longitudinally relative to one another without critically impairing the spirit of this invention. In fact, the antenna assembly 20 may be disposed about a portion of the housing wall 21 that is disposed about the beacon assembly 18 as shown in FIG. 7.

(25) With reference now to FIG. 5, an alternative embodiment of the antenna assembly 20 is shown. The antenna assembly 20 comprises a housing wall 21 with a first, large diameter portion 21a and a recessed, second portion 21b . The recessed portion 21b is covered, or filled, with a protective casing 29. The antenna coil 100 is wrapped around the housing wall 21 and within the protective casing 29. The protective casing 29 may comprise a urethane material or other magnetically transparent material. The antenna coil 100 is connected to the beacon assembly 18 (FIG. 1) through the feedthrough 104. The feedthrough 104 may comprise small radial holes made in the housing wall 21.

(26) One skilled in the art will appreciate that the embodiments contained herein may be modified without departing from the spirit of the invention contained herein. For example, alternative sensors or antenna arrangements, and materials may be utilized.