Magnetic Field Deflector in an Induction Resistivity Tool

Abstract

A downhole induction resistivity assembly comprises a mandrel. Disposed around the mandrel are coils of wire disposed circumferentially around magnetic field concentrators. The coils of wire and the magnetic field concentrators are disposed on an outer diameter of the mandrel. A magnetic field deflector, of magnetic permeability greater than the mandrel, is disposed intermediate the coils and the mandrel.

Description

BACKGROUND OF THE INVENTION

Electric resistivity of a downhole formation is often measured from a wireline in a well bore to analyze formation parameters. Induction resistivity tools induce a magnetic field into the formation; and thus, are different from laterolog resistivity systems, where an electric current is passed through the formation.

U.S. Pat. No. 6,677,756 to Fanini, et al, which is herein incorporated by reference for all that it contains, discloses an induction tool for formation resistivity evaluations. The tool provides electromagnetic transmitters and sensors suitable for transmitting and receiving magnetic fields in radial directions.

U.S. Pat. No. 6,359,438 to Bittar, which is herein incorporated by reference for all that it contains, discloses a resistivity tool for use in an LWD system that includes a transmitter array with multiple transmitters positioned above a pair of receivers. The transmitters are selectively energized, causing current to be induced in the collar of the tool.

U.S. Pat. No. 6,577,129 to Thompson, et al, which is herein incorporated by reference for all that it contains, discloses an electromagnetic wave propagation resistivity borehole logging system comprising multiple groups of electromagnetic transmitter-receiver arrays operating at three frequencies.

U.S. Pat. No. 6,538,447 to Bittar, which is herein incorporated by reference for all that it contains, discloses a multi mode resistivity tool for use in a logging-while-drilling system that includes an asymmetric transmitter design with multiple transmitters capable of generating electromagnetic signals at multiple depths of investigation.

U.S. Pat. No. 7,141,981 to Folbert, et al, which is herein incorporated by reference for all that it contains, discloses a resistivity logging tool suitable for downhole use that includes a transmitter, and two spaced apart receivers. The measured resistivities at the two receivers are corrected based on measuring the responses of the receivers to a calibration signal.

U.S. Pat. No. 6,218,842 to Bittar, et al, which is herein incorporated by reference for all that it contains, discloses a resistivity tool for use in LWD systems that includes an asymmetric transmitter design with multiple transmitters capable of generating EM signals at multiple frequencies.

U.S. Pat. No. 5,045,795 to Gianzero, et al, which is herein incorporated by reference for all that it contains, discloses a coil array which is installed on a MWD drill collar for use in a resistivity logging system. The drill collar is provided with upper and lower coil support rings. These are toroids which support individual coil segments, and are connected by suitable magnetic shorting bars. The coil segments and shorting bars inscribe a specified solid angle or azimuthal extent.

U.S. Pat. No. 5,606,260 to Giordano, et al, which is herein incorporated by reference for all that it contains, discloses a microdevice is provided for measuring the electromagnetic characteristics of a medium in a borehole. The microdevice includes at least one emitting or transmitting coil (31), and at least one receiving coil (41,51). The microdevice generates an A.C. voltage at the terminals of the transmitting coil and measures a signal at the terminals of the receiving coil. The microdevice also includes an E-shaped electrically insulating, soft magnetic material circuit serving as a support for each of the coils and which is positioned adjacent to the medium in the borehole.

U.S. Pat. No. 6,100,696 to Sinclair, which is herein incorporated by reference for all that it contains, discloses a directional induction logging tool that is provided for measurement while drilling. This tool is preferably placed in a side pocket of a drill collar, and it comprises transmitter and receiver coils and an electromagnetic reflector.

U.S. Pat. No. 6,163,155 to Bittar, et al, which is herein incorporated by reference for all that it contains, discloses a downhole method and apparatus for simultaneously determining the horizontal resistivity, vertical resistivity, and relative dip angle for anisotropic earth formations.

U.S. Pat. No. 6,476,609 to Bittar, et al, which is herein incorporated by reference for all that it contains, discloses an antenna configuration in which a transmitter antenna and a receiver antenna are oriented in nonparallel planes such that the vertical resistivity and the relative dip angle are decoupled.

U.S. Pat. No. 6,900,640 to Fanini, et al, which is herein incorporated by reference for all that it contains, discloses a tool that provides electromagnetic transmitters and sensors suitable for transmitting and receiving magnetic fields in radial directions that are orthogonal to the tool's longitudinal axis with minimal susceptibility to errors associated with parasitic eddy currents induced in the metal components surrounding the transmitter and receiver coils.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the present invention a downhole induction resistivity assembly comprises a mandrel. Disposed around the mandrel are coils of wire disposed circumferentially around magnetic field concentrators. The coils of wire and the magnetic field concentrators are disposed on an outer diameter of the mandrel. A magnetic field deflector, which comprises a magnetic permeability greater than the mandrel, may be disposed intermediate the coils and the mandrel. The magnetic field concentrator may comprise a ferrite core.

In some embodiments of the invention, the magnetic field deflector may comprise a mu-metal, Hipernum, HyMu-80, permalloy, a magnetically soft alloy or sheet metal, or any material or alloy with a permeability greater than the mandrel. The deflector material may contain nickel, iron, manganese, molybdenum, silicon, magnetic material, carbon, or any combination thereof. The material may further comprise an alloy that comprises between 70% to 85% nickel and between 10% to 20% iron. The deflector may be annealed. The deflector may comprise a material with a higher permeability value than the mandrel. The material my further comprise having a magnetic permeability that is at least 100 times greater than the permeability of the mandrel.

In some embodiments, the magnetic field deflector may be disposed circumferentially at least once around the mandrel. The deflector may be intermediate transmitter coils, bucking coils, or receiver coils. The deflector may be disposed under a portion of the coils. The positioning of the deflector may comprise spanning a space between a transmitter coil and a receiver coil. The deflector may be disposed circumferentially around the entire length of the mandrel. The deflector may comprise a sheet of material or wire/cable wrapper circumferentially around the mandrel. Strips of a magnetic deflector may be disposed circumferentially around the mandrel adjacent to an end of a receiver coil.

In some embodiments, the mandrel may comprise a magnetic material. The mandrel may further comprise sections of magnetic and nonmagnetic material. A magnetic section may be position proximate a transmitter or receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of an embodiment of a downhole induction resistivity assembly.

FIG. 2 is a perspective diagram of an embodiment of induction resistivity assembly.

FIG. 3 a is a perspective diagram of an embodiment of an induction transmitter.

FIG. 3 b is a cross-sectional diagram of an embodiment of an induction transmitter.

FIG. 4 a is a perspective diagram of a downhole tool string.

FIG. 4 b is a perspective diagram of an embodiment of a magnetic field deflector.

FIG. 5 a is a perspective diagram of an embodiment of an induction transmitter or receiver.

FIG. 5 b is a perspective diagram of an embodiment of a magnetic field deflector.

FIG. 6 is a perspective diagram of an embodiment of a magnetic field deflector.

FIG. 7 is a perspective diagram of an embodiment of a magnetic field deflector.

FIG. 8 is a cross-sectional diagram of an embodiment of a magnetic field deflector.

FIG. 9 is a cross-sectional diagram of an embodiment of a magnetic field deflector.

FIG. 10 is a perspective diagram of an embodiment of a magnetic deflector disposed on a mandrel.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a downhole tool string 101 may be suspended by a 10 derrick 102. The tool string may comprise one or more downhole components 100, linked together in a tool string 101 and in communication with surface equipment 103 through a downhole network. The network may enable high-speed communication between devices connected to the tool string, and the network may facilitate the transmission of data between sensors and sources. The data gathered by the downhole instrumentation may be processed downhole, may be transmitted to the surface for processing, may be filtered downhole and then transmitted to the surface for processing, may be compressed downhole and then transmitted to the surface for processing or combinations thereof. In some embodiments, the data may be stored downhole and dumped to uphold equipment when the tool string is tripped out of the wellbore.

FIG. 2 is an embodiment of a tool string component 100. The tool string component may comprise an induction transmitter 201 and a plurality of induction receivers 202 and 203. The receivers 202 and 203 may be placed in a variety of orientations with respect to each other and to the transmitter 201. The induction transmitter 201 is adapted to send an induction signal into the formation, which generates an induced field in the formation surrounding the well bore. The induction receivers 202 and 203 are adapted to sense various attributes of the induced formation field. These attributes may include among others, some or all of the following: frequency, amplitude, or phase. The transmitter and the receivers may be powered by batteries, a turbine generator, or from the downhole network. The receivers may also be passive. In some embodiments, there may be several induction transmitters located along the length of the tool string component. In some embodiments, the additional transmitters may be used to calibrate measurements, such as is common in borehole compensation techniques.

FIG. 3 a is a perspective diagram of an embodiment of an induction transmitter. The transmitters 300 may comprise coils of electrically conductive material 304 wrapped around a magnetic field concentrator 305. The coil wrapped field concentrators 305 may be disposed on the outer diameter of a mandrel. The mandrel may be a drill pipe, tool string component, or combination thereof. The magnetic field concentrator 305 may comprise a magnetically conducting core where a wire is wrapped around its diameter or a magnetically conducting annular ring where the wire is disposed within a ring's recess.

FIG. 3 b is a cross-sectional diagram of an embodiment of an induction transmitter. The transmitter coils 301 may be disposed circumferentially around the mandrel 302. A magnetic field deflector 303 may be disposed circumferentially intermediate the mandrel 302 and the transmitter coils 301. The magnetic field deflector 303 may comprise a material that has a higher magnetic permeability than that of the mandrel 302. The deflector 303 may comprise mu-metal, Hipernum, HyMu-80, permalloy, a magnetically soft alloy or sheet metal. In some embodiments, the deflector 303 may comprise any material that has a relative magnetic permeability at least 100 times greater than the magnetic permeability of the mandrel 302. The deflector 303 may comprise alloys of iron and nickel. The deflector 303 may be annealed. Annealing of the magnetic field deflector increases the magnetic permeability of the material by aligning the grains of the metal. The higher permeability may provide a path of least resistance for a magnetic field around the mandrel, thereby shielding the mandrel from the magnetic field. In some embodiments, the deflector may be segmented.

As the coils of the transmitters carry an electric current, its associated magnetic field is concentrated by the magnetic concentrators. The coils and magnetic concentrators work together to control the parameters of the induced field. The deflector is disposed more centrally to the mandrel's central axis than both the coils and the magnetic concentrators and may insulate, or even isolate, the mandrel from the induced magnetic field. Preferably, the coils are wrapped around the magnetic field concentrator.

FIG. 4 a is a perspective diagram of a downhole tool string component. As an induction transmitter 400 produces a magnetic field 401, a current is induced in the mandrel 302. The induced current 402 can be measured by a spectrum analyzer 403, which may be attached to the ends of the mandrel 302. The current 403 in the mandrel 302 produces a magnetic field that can be picked up by the receiver coils on the drill pipe, thereby interfering with acquiring resistivity measurements.

FIG. 4 b discloses a magnetic field deflector positioned intermediate the transmitter coils 400 and the mandrel 302 that reduces or eliminates the induced current in the mandrel 302. The spectrum analyzer 403 may not detect a current 402 induced on the mandrel 302. Decreasing or eliminating the magnetic field produced by the mandrel 302 increases the sensitivity of the receiver coils to magnetic fields emanating from the formation.

FIG. 5 a is a perspective diagram of an embodiment of an induction transmitter or receiver. The induction transmitter or receiver 501 may comprise wrapping an electrically conductive wire 502 around a magnetic field concentrator 503. The magnetic field concentrator 503 may comprise a ferrite core. As the coil of wires induce a magnetic field, the field is concentrated by the ferrite core 503, and the field is concentrated in the core 503.

FIG. 5 b is a perspective diagram of an embodiment of a magnetic field deflector 504. The magnetic field deflector 504 may be disposes circumferentially around the mandrel 302. The magnetic field deflector 504 may be disposed intermediate at least one coil 502 and the mandrel 302. The deflector 504 may span more or less area than the area under the transmitter or receiver coils 501.

FIG. 6 is a perspective diagram of an embodiment of a magnetic field deflector 601. In this embodiment the deflector 601 may comprise a wire or cable 601 wrapped circumferentially at least once around the mandrel 302. The deflector 601 may be disposed intermediate the induction transmitter or receiver 501 and the mandrel 302. The deflector 601 coils may be spaced apart or tightly wound. In some embodiments, the wire or cable turns are in electrical communication with each other so the entire deflector acts as a sheet.

FIG. 7 is a perspective diagram of an embodiment of a magnetic field deflector. A deflector 701 may be disposed circumferentially around the mandrel 302 such that the deflector 701 is adjacent the ends of the induction transmitter or receiver 501.

FIG. 8 is a cross-sectional diagram of an embodiment of a magnetic field deflector. A transmitter coil may comprise a bucking coil 801. The bucking coil 801 induces a magnetic field 802 which pushes a magnetic field 803 produced by other transmitter coils 804 into the formation 805. A deflector 303 may be disposed intermediate the mandrel 302 and the bucking coil 801.

FIG. 9 is a cross-sectional diagram of an embodiment of a magnetic field deflector 303. The magnetic field lines 1001 depicted in diagram will follow a path a least resistance. The magnetic field deflector 303 comprises a material of higher magnetic permeability than the mandrel such that the magnetic field lines 1001 travel preferentially through the deflector 303 instead of the mandrel 302.

FIG. 10 is a perspective diagram of an embodiment of a magnetic field deflector 1101 disposed on a mandrel 303. The deflector 1101 may be disposed circumferentially around the mandrel 303 at least once. In the figure multiple layers are depicted. Additional layers provide more shielding and would decrease the induced field in the mandrel 303.

In some embodiments, the magnetic deflector may be electrically isolated from the mandrel. The magnetic deflector may also be segmented axially or circumferentially around the outer diameter of the mandrel.

Whereas the present invention has been described in particular relation to the drawings attached hereto, it should be understood that other and further modifications apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.

Claims

1. A downhole induction resistivity assembly, comprising: a mandrel;a coil of wire disposed circumferentially around a magnetic field concentrator;wherein the coil and magnetic field concentrator are disposed on an outer diameter of the mandrel;a magnetic field deflector disposed intermediate the mandrel and the coil;wherein the magnetic field deflector comprises a magnetic permeability greater than that of the mandrel.

2. The assembly of claim 1, wherein the magnetic field deflector may comprise a mu-metal, Hipernum, HyMu-80, permalloy, a magnetically soft alloy or sheet metal.

3. The assembly of claim 1, wherein the magnetic field deflector comprises an alloy which may contain nickel, iron, manganese, molybdenum, silicon, or carbon.

4. The assembly of claim 3, wherein the alloy comprises between 70% to 85% nickel and between 10% to 20% iron.

5. The assembly of claim 1, wherein the magnetic field deflector comprises a magnetically conductive material that is annealed.

6. The assembly of claim 1, wherein the magnetic field deflector is disposed circumferentially at least once around the mandrel.

7. The assembly of claim 1, wherein the magnetic field deflector is disposed intermediate a bucking coil and the mandrel.

8. The assembly of claim 1, wherein a magnetic field line generated by a transmitter wraps around the mandrel through the magnetic field deflector.

9. The assembly of claim 1, wherein the magnetic field deflector is disposed circumferentially around the mandrel spanning a space between a transmitter coil and a receiver coil.

10. The assembly of claim 1, wherein the magnetic field deflector comprises a sheet disposed circumferentially around the mandrel.

11. The assembly of claim 1, wherein the magnetic field deflector is disposed under a portion of the coils.

12. The assembly of claim 1, wherein the magnetic field deflector is disposed circumferentially around the entire length of the mandrel.

13. The assembly of claim 1, wherein the mandrel comprises a magnetic material.

14. The assembly of claim 15, wherein the mandrel comprises sections of magnetic and nonmagnetic material.

15. The assembly of claim 16, wherein the magnetic section of the mandrel is positioned proximate a transmitter or receiver.

16. The assembly of claim 1, wherein the magnetic field deflector is disposed circumferentially around the mandrel adjacent to an end of a receiver coil.

17. The assembly of claim 1, wherein the magnetic field deflector comprises a material with a higher mu value than the mandrel.

18. The assembly of claim 1, wherein the coil adjacent the mandrel comprises transmitter coils and receiver coils.

19. The assembly of claim 1, wherein the magnetic field concentrator comprises a ferrite core.