Patent Description:
Logging tool antennae are often formed by positioning coil windings about an axial section of the wellbore logging tool, such as a drill collar. A soft magnetic material is sometimes positioned beneath the coil windings to increase the efficiency and/or sensitivity of the logging tool antennae. The soft magnetic material facilitates a higher magnetic permeability path (i.e., a flux conduit) for the magnetic field generated by the coil windings, and helps shield the coil windings from adjacent drill collars and associated losses (e.g., eddy currents generated on the drill collars).

<CIT> discloses a shielding method and apparatus for an antenna disposed in an elongated support adapted for disposal within a borehole. <CIT> discloses an antenna assembly which includes a bobbin positionable about an outer surface of a tool mandrel, and a coil wrapped about an outer bobbin surface of the bobbin and extending longitudinally along at least a portion of the outer bobbin surface. <CIT> discloses tubulars for monitoring subsurface reservoir characteristics in directed orientations.

The following figures are included to illustrate certain aspects of the present disclosure, and should not be viewed as exclusive examples. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, without departing from the scope of this disclosure.

The present disclosure relates generally to wellbore logging tools used in the oil and gas industry and, more particularly, to the design of tilted coil antennas using stacked soft magnetic inserts and innovative antenna shields that improve gain, sensitivity, and efficiency of the tilted coil antennas.

Examples of the present disclosure describe improvements to the design of antenna assemblies used in resistivity logging tools for monitoring surrounding subterranean formations adjacent a drilled wellbore. The antenna assemblies described herein include tilted coil antennas that include a soft magnetic band to increase the inductance of the tilted coil antenna. The soft magnetic band includes a plurality of stacked inserts extending perpendicular to the tilted coil antenna and each stacked insert includes a plurality of rods positioned end-to-end. The rods included in each stacked insert are of a simple design and commercially available and, therefore, help reduce the cost of assembling and maintaining the antenna assemblies as compared to antenna assemblies having soft magnetic bands with inserts that exhibiting complex geometries that are difficult and expensive to manufacture.

Antenna assemblies described herein include a tilted coil antenna and an antenna shield positioned radially outward from the tilted coil antenna. The antenna shield defines a plurality of slots extending perpendicular to the windings of the tilted coil antenna and the plurality of slots is provided in two or more dissimilar lengths. The dissimilar lengths for the slots not only support operation of the antenna assemblies by minimizing attenuation of electromagnetic fields and preserving the dipole orientation of the tilted coil antenna, but also in preserving the mechanical integrity and strength to protect the tilted coil antenna.

<FIG> is a schematic diagram of an example drilling system <NUM> that may employ the principles of the present disclosure. As illustrated, the drilling system <NUM> may include a drilling platform <NUM> positioned at the surface and a wellbore <NUM> that extends from the drilling platform <NUM> into one or more subterranean formations <NUM>. In other examples, such as in an offshore drilling operation, a volume of water may separate the drilling platform <NUM> and the wellbore <NUM>.

The drilling system <NUM> may include a derrick <NUM> supported by the drilling platform <NUM> and having a traveling block <NUM> for raising and lowering a drill string <NUM>. A kelly <NUM> may support the drill string <NUM> as it is lowered through a rotary table <NUM>. A drill bit <NUM> may be coupled to the drill string <NUM> and driven by a downhole motor and/or by rotation of the drill string <NUM> by the rotary table <NUM>. As the drill bit <NUM> rotates, it creates the wellbore <NUM>, which penetrates the subterranean formations <NUM>. A pump <NUM> may circulate drilling fluid through a feed pipe <NUM> and the kelly <NUM>, downhole through the interior of drill string <NUM>, through orifices in the drill bit <NUM>, back to the surface via the annulus defined around drill string <NUM>, and into a retention pit <NUM>. The drilling fluid cools the drill bit <NUM> during operation and transports cuttings from the wellbore <NUM> into the retention pit <NUM>.

The drilling system <NUM> may further include a bottom hole assembly (BHA) coupled to the drill string <NUM> near the drill bit <NUM>. The BHA may comprise various downhole measurement tools such as, but not limited to, measurement-while-drilling (MWD) and logging-while-drilling (LWD) tools, which may be configured to take downhole measurements of drilling conditions. The MWD and LWD tools may include at least one resistivity logging tool <NUM>, which may comprise one or more antennas capable of receiving and/or transmitting one or more electromagnetic (EM) signals that are axially spaced along the length of the resistivity logging tool <NUM>. As described below, the resistivity logging tool <NUM> further comprises a plurality of stacked soft magnetic inserts used to enhance and/or shield the EM signals and thereby increase the azimuthal sensitivity of the resistivity logging tool <NUM>.

As the drill bit <NUM> extends the wellbore <NUM> through the formations <NUM>, the resistivity logging tool <NUM> may continuously or intermittently collect azimuthally-sensitive measurements relating to the resistivity of the formations <NUM>, i.e., how strongly the formations <NUM> opposes a flow of electric current. The resistivity logging tool <NUM> and other sensors of the MWD and LWD tools may be communicably coupled to a telemetry module <NUM> used to transfer measurements and signals from the BHA to a surface receiver (not shown) and/or to receive commands from the surface receiver. The telemetry module <NUM> may encompass any known means of downhole communication including, but not limited to, a mud pulse telemetry system, an acoustic telemetry system, a wired communications system, a wireless communications system, or any combination thereof. In certain examples, some or all of the measurements taken at the resistivity logging tool <NUM> may also be stored within the resistivity logging tool <NUM> or the telemetry module <NUM> for later retrieval at the surface upon retracting the drill string <NUM>.

At various times during the drilling process, the drill string <NUM> may be removed from the wellbore <NUM>, as shown in <FIG>, to conduct measurement/logging operations. More particularly, <FIG> depicts a schematic diagram of an example wireline system <NUM> that may employ the principles of the present disclosure. Like numerals used in <FIG> refer to the same components or elements and, therefore, may not be described again in detail. As illustrated, the wireline system <NUM> may include a wireline instrument sonde <NUM> that may be suspended in the wellbore <NUM> on a cable <NUM>. The sonde <NUM> may include the resistivity logging tool <NUM> described above, which may be communicably coupled to the cable <NUM>. The cable <NUM> may include conductors for transporting power to the sonde <NUM> and also facilitate communication between the surface and the sonde <NUM>. A logging facility <NUM>, shown in <FIG> as a truck, may collect measurements from the resistivity logging tool <NUM>, and may include computing and data acquisition systems <NUM> for controlling, processing, storing, and/or visualizing the measurements gathered by the resistivity logging tool <NUM>. The computing and data acquisition systems <NUM> may be communicably coupled to the resistivity logging tool <NUM> by way of the cable <NUM>.

<FIG> is a partial isometric view of an example wellbore logging tool <NUM>. The logging tool <NUM> may be the same as or similar to the resistivity logging tool <NUM> of <FIG> and, therefore, may be used in the drilling or wireline systems <NUM>, <NUM> depicted therein. The wellbore logging tool <NUM> is depicted as including an antenna assembly <NUM> that is positioned about a tool mandrel <NUM>, such as a drill collar or the like. The antenna assembly <NUM> may include a bobbin <NUM> and includes a coil <NUM> wrapped about the bobbin <NUM> and extending axially by virtue of winding along at least a portion of an outer surface of the bobbin <NUM>.

The bobbin <NUM> may structurally comprise a high temperature plastic, a thermoplastic, a polymer (e.g., polyimide), a ceramic, or an epoxy material, but could alternatively be made of a variety of other non-magnetic, electrically insulating/non-conductive materials. The bobbin <NUM> can be fabricated, for example, by additive manufacturing (i.e., 3D printing), molding, injection molding, machining, or other known manufacturing processes.

The coil <NUM> includes a plurality of consecutive "turns" (i.e. windings of the coil <NUM>) about the bobbin <NUM>, and typically will include at least a plurality (i.e. two or more) consecutive full turns, with each full turn extending <NUM>° about the bobbin <NUM>. In some examples, a pathway for receiving the coil <NUM> may be formed along the outer surface of the bobbin <NUM>. For example, one or more grooves or channels may be defined in the outer surface of the bobbin <NUM> to receive and seat the coil <NUM>. In other examples, however, the outer surface of the bobbin <NUM> may be smooth or even. The coil <NUM> can be concentric or eccentric relative to a tool axis <NUM> of the tool mandrel <NUM>.

As illustrated, a portion of the turns or windings of the coil <NUM> extend about the bobbin <NUM> at a winding angle <NUM> offset relative to the tool axis <NUM>. More specifically, the windings of the coil <NUM> on opposing sides of the bobbin <NUM> extend about the outer circumference of the bobbin <NUM> at the winding angle <NUM>. The windings, however, transition to perpendicular to the tool axis <NUM> at the top and bottom of the bobbin <NUM>, at which point the windings transition back to the winding angle <NUM> on opposing sides of the bobbin <NUM>. Successive windings of the coil <NUM> (i.e., one or more successive revolutions of the coil <NUM>) advance in a generally axial direction along at least a portion of the outer surface of the bobbin <NUM> such that coil <NUM> spans an axial length of the bobbin <NUM>. The antenna assembly <NUM> may be characterized and otherwise referred to as a "tilted coil" or "directional" antenna. In the illustrated example, the winding angle <NUM> is <NUM>°, by way of example, and could alternatively be any angle offset relative to the tool axis <NUM>, without departing from the scope of the disclosure.

<FIG> is a schematic side view of the wellbore logging tool <NUM> of <FIG>. When current is passed through the coil <NUM> of the antenna assembly <NUM>, a dipole magnetic field <NUM> may be generated that extends radially outward from the antenna assembly <NUM> orthogonal to the winding direction. As a result, the antenna assembly <NUM> may exhibit a magnetic field angle <NUM> with respect to the tool mandrel <NUM> and, since the winding angle <NUM> (<FIG>) is <NUM>°, the resulting magnetic field angle <NUM> will also be <NUM>° offset from the tool axis <NUM>. As will be appreciated, however, the magnetic field angle <NUM> may be varied by adjusting or manipulating the winding angle <NUM>.

<FIG> depicts a side view of an example antenna assembly <NUM>. The antenna assembly <NUM> may be similar in some respects to the antenna assembly <NUM> of <FIG> and, therefore, may be best understood with reference thereto, where like numerals represent like element not described again. As illustrated, the antenna assembly <NUM> includes the coil <NUM> wrapped about the tool mandrel <NUM> and, more particularly, within a saddle <NUM> defined on the tool mandrel <NUM>. The saddle <NUM> may comprise a portion of the tool mandrel <NUM> that exhibits a reduced-diameter as compared to the remaining portions of the tool mandrel <NUM>. Some or all of the components of the antenna assembly <NUM> may be arranged within the saddle <NUM>. While not shown in this example, the bobbin <NUM> (<FIG>) may alternatively be included to interpose the coil <NUM> and the tool mandrel <NUM> (i.e., the saddle <NUM>), as generally discussed above.

As illustrated, the windings of the coil <NUM> extend about the circumference of the tool mandrel <NUM> at the winding angle <NUM>, which is offset from the tool axis <NUM>, for example, by <NUM>°. As a result, the magnetic dipole moment <NUM> generated by the coil <NUM> may extend from the tool mandrel <NUM> at the magnetic field angle <NUM>, which is orthogonal to the winding angle <NUM> of the coil <NUM>. The directionality of the magnetic dipole moment <NUM> may generally correspond to the direction in which the coil <NUM> emits the dipole magnetic field <NUM> (<FIG>) when current is passed therethrough. In some applications, it may be desired for best results to have the magnetic dipole moment <NUM> offset from the tool axis <NUM> by <NUM>°, but the magnetic field angle <NUM> could alternatively be any angle between parallel and perpendicular to the tool axis <NUM> because of effects caused by the tool mandrel <NUM> or as a result of using a soft magnetic band, as described below.

<FIG> depicts another example of the antenna assembly <NUM>, which does not form part of the claimed invention. In the illustrated example, a soft magnetic band <NUM> interposes the coil <NUM> and the tool mandrel <NUM> within the saddle <NUM>. The soft magnetic band <NUM> may be configured to shield the coil winding <NUM> from eddy currents generated by the tool mandrel <NUM>, thereby increasing the azimuthal sensitivity of the antenna assembly <NUM> and/or increasing the efficiency or strength of the dipole magnetic field <NUM> (<FIG>) of the coil <NUM>.

To facilitate this effect, the soft magnetic band <NUM> may comprise a soft magnetic material or any material that exhibits relatively high resistivity, high magnetic permeability, and low magnetic loss (e.g., hysteresis, magnetostriction, etc.). One suitable soft magnetic material that may be used includes ferrites, which generally comprise a composite mixture of a powder iron/ferrite material and a binder, such as a silicone-based rubber, an elastomer, an RTV, a polymer (such as polyimide), a ceramic, or an epoxy. The resulting mixture is molded and/or pressed into desired geometric shapes and configurations that conform to the shape of the soft magnetic band <NUM>. Other suitable soft magnetic materials that may be used in the soft magnetic band <NUM> include, but are not limited to, mu-metal, permalloy, metallic glass (metglass), or any combination of the foregoing.

The soft magnetic band <NUM> may comprise a generally annular ring that extends about the circumference of the tool mandrel <NUM> (e.g., within the saddle <NUM>) at a band angle <NUM>. In the illustrated example, the band angle <NUM> and the winding angle <NUM> are substantially the same such that the soft magnetic band <NUM> interposes the coil <NUM> and the tool mandrel <NUM> about the corresponding circumference of the tool mandrel <NUM>. To help maintain the directionality of the magnetic dipole moment <NUM> at <NUM>° relative to the tool axis <NUM>, the soft magnetic band <NUM> comprises a plurality of inserts <NUM>. Accordingly, in the illustrated example, the soft magnetic band <NUM> comprises a plurality of inserts <NUM> that form a discontinuous annular ring extending about the circumference of the tool mandrel <NUM> at the band angle <NUM>. As illustrated, the band angle <NUM> and the winding angle <NUM> are substantially parallel, but could alternatively be offset from parallel by +/-<NUM>°.

<FIG> is an isometric view of the soft magnetic band <NUM> of <FIG>. As illustrated, the inserts <NUM> exhibit a tilted cut shape and, therefore, may be referred to as "tilted" inserts <NUM>. Moreover, the inserts <NUM> are cut and otherwise formed axially and otherwise parallel to the tool axis <NUM>. Each insert <NUM> is separated from angularly adjacent inserts by a small gap <NUM> that prevents physical contact between angularly adjacent inserts <NUM>, and thereby prevents a continuous magnetic path between the adjacent inserts <NUM>. In some examples, the gap <NUM> may be filled with a material that exhibits a relative permeability (pr) of approximately <NUM>, which is equivalent to the permeability of free space or air (us). In such examples, for example, the inserts <NUM> may be positioned (inserted) within corresponding channels defined in the bobbin <NUM> (<FIG>) where the gap <NUM> is filled by separators provided by the bobbin <NUM>. In other examples, the gap <NUM> may not be filled with any particular material but may instead allow air to separate the adjacent inserts <NUM>. In any case, the gap <NUM> essentially serves as a non-magnetic insulator between the adjacent inserts <NUM>.

As illustrated, the inserts <NUM> comprise generally rectangular or parallelogram-shaped members (depending on where angularly located about the circumference of the tool mandrel <NUM>) separated by the gap <NUM>. Each insert <NUM> may have a length 510a and a width 510b, where the length 510a of each axially cut insert <NUM> extends substantially parallel to the tool axis <NUM>. As a result, the gap <NUM> separating each laterally adjacent insert <NUM> may be aligned axially with the tool axis <NUM> and otherwise run parallel thereto. Moreover, each insert <NUM> may exhibit an arcuate shape across (along) the width that conforms to the curvature of the tool mandrel <NUM> (<FIG>) and/or the saddle <NUM> (<FIG>).

Referring now to <FIG>, illustrated are isometric and side views, respectively, of another example of the antenna assembly <NUM>, which does not form part of the claimed invention. Similar to the examples of <FIG> and <FIG>, the antenna assembly <NUM> includes the coil <NUM> wrapped about the tool mandrel <NUM> and, in some examples, positioned within the saddle <NUM>. The windings of the coil <NUM> extend about the tool mandrel <NUM> at the winding angle <NUM> (<FIG>), which, as noted above, may be angularly offset from the tool axis <NUM> (<FIG>) by <NUM>°, but could alternatively be any angle offset from the tool axis <NUM>. As a result, the magnetic dipole moment <NUM> (<FIG>) generated by the coil <NUM> may extend from the tool mandrel <NUM> at the magnetic field angle <NUM> (<FIG>), which is orthogonal to the winding angle <NUM>.

Similar to the example of <FIG>, the soft magnetic band <NUM> radially interposes the coil <NUM> and the tool mandrel <NUM> (e.g., the saddle <NUM>), and the inserts <NUM> are separated by the gaps <NUM> (<FIG>), which extend substantially parallel to the tool axis <NUM>. Unlike the example of <FIG>, however, the soft magnetic band <NUM> extends about the circumference of the tool mandrel <NUM> (e.g., the saddle <NUM>) at a band angle <NUM> (<FIG>) that is orthogonal to the winding angle <NUM>. Accordingly, the soft magnetic band <NUM> may not only be characterized as "tilted" with respect to the tool axis <NUM>, but may also be referred to as a "reversed" soft magnetic band. In examples where the winding angle <NUM> is <NUM>° offset from the tool axis <NUM>, the band angle <NUM> may also be <NUM>° offset from the tool axis <NUM>, but angularly opposite the winding angle <NUM> along the tool axis <NUM> (i.e., <NUM>° offset from the winding angle <NUM>). Since the coil <NUM> and the soft magnetic band <NUM> are each wrapped about the circumference of the tool mandrel <NUM> in orthogonal directions, at least a portion of the coil <NUM> extends axially past the soft magnetic band <NUM> where the soft magnetic band <NUM> does not radially interpose the coil <NUM> and the tool mandrel <NUM>.

Each insert <NUM> of the soft magnetic band <NUM> exhibits a unique cross-section that must conform to the curvature of the tool mandrel <NUM> and/or the saddle <NUM>. Consequently, it can be difficult and expensive to manufacture the inserts <NUM> since each insert <NUM> has to be custom made, which increases the overall manufacturing and assembling costs for the antenna assembly <NUM>. Soft magnetic band inserts that are difficult to manufacture on account of requiring complex geometries can be replaced with inexpensive, commercially available (i.e., off-the-shelf) soft magnetic band inserts that exhibit a simple geometry. As discussed below, using such commercially available soft magnetic band inserts with simple geometry can achieve the same or better antenna performance as compared to the example of <FIG>.

<FIG> is a side view of an example antenna assembly <NUM> that incorporates one or more principles of the present disclosure. The antenna assembly <NUM> may be similar in some respects to the examples of the antenna assembly <NUM> shown in <FIG>, <FIG>, and <FIG> and, therefore, may be best understood with reference thereto, where similar numerals refer to like elements or components not described again. Similar to the examples of the antenna assembly <NUM>, for example, the antenna assembly <NUM> includes the coil <NUM> wrapped about the tool mandrel <NUM> and, in at least some examples, positioned within the saddle <NUM>. The windings of the coil <NUM> extend about the tool mandrel <NUM> at the winding angle <NUM>, which, as noted above, may be angularly offset from the tool axis <NUM> by <NUM>°, but could alternatively be any angle offset from the tool axis <NUM>. As a result, the magnetic dipole moment <NUM> generated by the coil <NUM> extends from the tool mandrel <NUM> at the magnetic field angle <NUM>, which is orthogonal to the winding angle <NUM>. While the coil <NUM> is depicted as having only four consecutive windings, it will be appreciated that more or less than four windings may be employed in the antenna assembly <NUM>, without departing from the scope of the disclosure.

The antenna assembly <NUM> also includes a soft magnetic band <NUM> that radially interposes the coil <NUM> and the tool mandrel <NUM> (e.g., the saddle <NUM>). <FIG> is a side view of the antenna assembly <NUM> of <FIG> excluding the coil <NUM> to facilitate a better view of the features of the soft magnetic band <NUM>. The soft magnetic band <NUM> comprises a plurality of stacked inserts <NUM> angularly offset from each other to form a discontinuous annular ring that extends about the circumference of the tool mandrel <NUM> (e.g., within the saddle <NUM>) at the band angle <NUM>. The band angle <NUM> and the winding angle <NUM> (<FIG>) are substantially the same such that the soft magnetic band <NUM> interposes the coil <NUM> and the tool mandrel <NUM> about the entire circumference of the tool mandrel <NUM>.

Each stacked insert <NUM> is separated from angularly adjacent stacked inserts by a gap <NUM>, which is similar to the gap <NUM> discussed above with reference to <FIG>, <FIG>, and 6A-6B. Accordingly, the gaps <NUM> prevent physical contact between angularly adjacent stacked inserts <NUM>.

As illustrated, each stacked insert <NUM> includes a plurality of rods <NUM> (alternately referred to as "units") arranged and otherwise positioned end-to-end (i.e., "stacked") to cooperatively form a straight or substantially straight stacked insert <NUM>. The rods <NUM> may be made of any of the soft magnetic materials mentioned herein with respect to the soft magnetic band <NUM> of <FIG> and <FIG> including, but not limited to, ferrite, mu-metal, permalloy, metallic glass (metglass), or any combination thereof. As illustrated, three rods <NUM> may be arranged end-to-end to jointly (mutually) form a given stacked insert <NUM>. In other examples, however, more or less than three rods <NUM> (at least two) may be arranged end-to-end to form a given stacked insert <NUM>. Moreover, in some examples, a small gap may be formed between opposing ends of the rods <NUM>. In other examples, however, the opposing ends of one or more of the rods <NUM> in a given stacked insert <NUM> may come into contact and otherwise directly touch each other.

The rods <NUM> of each stacked insert <NUM> are arranged such that the stacked insert <NUM> extends substantially perpendicular to the windings of the radially adjacent coil <NUM> (<FIG>) at any given angular location about the circumference of the tool mandrel <NUM>. As a result, the soft magnetic band <NUM> helps to maintain the directionality of the magnetic dipole moment <NUM> (<FIG>) at <NUM>° relative to the tool axis <NUM> (<FIG>). As used herein, the phrase "substantially perpendicular" refers to a <NUM>° relative offset between the stacked insert <NUM> and the windings of the radially adjacent coil <NUM>, but also encompasses a +/-<NUM>° offset from perpendicular, without departing from the scope of the disclosure.

The rods <NUM> of each stacked insert <NUM> may comprise straight, cylindrical members that provide a circular or polygonal cross-section. In other words, each rod <NUM> may exhibit a cross-sectional shape that is circular, such as rounded, oval, or ovoid, or alternatively a cross-sectional shape that is polygonal, such as, triangular, rectangular (including square), pentagonal, etc. In the illustrated example the rods <NUM> are depicted as cylindrical members with a polygonal (e.g., rectangular) cross-section. In some examples, a given stacked insert <NUM> may comprise rods <NUM> having dissimilar cross-sectional shapes.

Each rod <NUM> has a length <NUM> that contributes to the total length <NUM> of the corresponding stacked insert <NUM>. The length <NUM> of a given rod <NUM> may or may not be the same as the length <NUM> of other rod(s) <NUM> in a corresponding stacked insert <NUM>. For example, the length <NUM> of any of the rods <NUM> can range between about <NUM> inch and <NUM> inches, but could alternatively be shorter than <NUM> inch or longer than <NUM> inches. In at least one example, the length <NUM> of the rods <NUM> will be less than half of the elliptical circumferences of the tool mandrel <NUM> (e.g., the saddle <NUM>) along the designed path of the stacked inserts <NUM>. Consequently, in such examples, the range of the length <NUM> of the rods <NUM> may depends on the diameter of the tool mandrel <NUM> (e.g., within the saddle <NUM>). The relatively short length <NUM> of the rods <NUM> allows the corresponding stacked insert <NUM> to roughly but substantially follow the curvature of the outer surface of the tool mandrel <NUM> (e.g., the saddle <NUM>) as it extends perpendicular to the coil <NUM>.

The rods <NUM> may be commercially available as an off-the-shelf item and may comprise standard sizes that can be purchased in the market from a variety of manufacturers and/or outlets. For example, the rods <NUM> may be purchased from CWS Bytemark of Orange, CA, USA, or Dexter Magnetic Technologies, Inc. of Elk Grove Village, IL, USA. As will be appreciated, using commercially available rods <NUM> to form the stacked inserts <NUM> may reduce costs in assembling and maintaining the antenna assembly <NUM> as compared to conventional or prior antenna assemblies where the soft magnetic band comprises inserts exhibiting complex geometries that are difficult and expensive to manufacture. For example, prior art soft magnetic bands can cost as much as US $<NUM> per antenna, while soft magnetic bands using the stacked inserts <NUM> described herein may cost only US $<NUM> per antenna. Moreover, as discussed below, employing the stacked inserts <NUM> may provide similar or better gain performance as compared to conventional or prior antenna assemblies with custom-made soft magnetic bands.

<FIG> depicts an isometric view of an example bobbin <NUM>. The bobbin <NUM> may be the same as or similar to the bobbin <NUM> described above with reference to <FIG> and, therefore, may be made of similar materials mentioned herein. While the stacked inserts <NUM> of the soft magnetic band <NUM> of <FIG> are shown positioned about the outer circumference of the mandrel <NUM> (e.g., the saddle <NUM>), the stacked inserts <NUM> may alternatively be positioned on and otherwise attached to the bobbin <NUM>. In turn, the bobbin <NUM> may be positioned about the outer circumference of the mandrel <NUM>, such as within the saddle <NUM>. In the illustrated example, the bobbin <NUM> may have a plurality of grooves or channels <NUM> defined on its inner radial surface <NUM>. Each channel <NUM> may be sized and otherwise configured to receive a single stacked insert <NUM> (<FIG>). In such examples, the coil <NUM> (<FIG>) would be wound about the outer radial surface <NUM> of the bobbin <NUM>.

<FIG> depicts an isometric view of another example of the bobbin <NUM> of <FIG>. In <FIG>, the stacked inserts <NUM> are shown arranged on the outer radial surface <NUM> of the bobbin <NUM>. The stacked inserts <NUM> may be at least partially received within corresponding channels <NUM> defined in the outer radial surface <NUM> of the bobbin <NUM>. In other examples, however, the outer radial surface <NUM> of the bobbin <NUM> may be smooth and the stacked inserts <NUM> may alternatively be arranged directly on the outer radial surface <NUM>. In such examples, the coil <NUM> (<FIG>) may be wound about the outer radial surface <NUM> of the bobbin <NUM> but radially supported by the stacked inserts <NUM>.

Referring again to <FIG>, it may be desired to protect the antenna assembly <NUM> (especially the coil <NUM>) from mechanical or operational damage during use. For example, an unprotected (unshielded) coil <NUM> may be damaged during wellbore drilling operations through prolonged exposure to wellbore cuttings and debris or by extensive contact with a wellbore wall as an associated drill string is moved within the wellbore. In some examples not forming part of the claimed invention, the coil <NUM> may be protected from mechanical damage by covering or otherwise coating all or a portion of the antenna assembly <NUM> with a non-magnetic, electrically insulating/non-conductive material such as, but not limited to, a polymer (e.g., PEEK), a polymer-ceramic blend, or a ceramic. This material may be added (deposited), for example, within the reduced diameter portion of the tool mandrel <NUM> defined by the saddle <NUM>. The material is electrically resistive and, therefore, can protect the antenna assembly <NUM> while not attenuating the electromagnetic fields transmitted or received.

The coil <NUM> is protected from mechanical damage by using or otherwise installing an antenna shield that axially spans the reduced diameter portion of the tool mandrel <NUM> defined by the saddle <NUM> and effectively covers the coil <NUM>. The antenna shield may be electromagnetically transmissive to allow transmission of electromagnetic signals. The electromagnetic transmissivity of the antenna shield is achieved by providing slots defined through the body of the antenna shield.

<FIG> is a side view of an antenna assembly <NUM>, not forming part of the claimed invention, that includes an antenna shield <NUM> used to protect the antenna assembly <NUM> (especially the underlying coil <NUM>). The antenna assembly <NUM> may be the same as or similar to any of the antenna assemblies <NUM> (<FIG>, <FIG>, and <FIG>) and <NUM> (<FIG>) described herein and, therefore, may be best understood with reference thereto, where similar numerals refer to like elements or components not described again. The antenna assembly <NUM> includes the coil <NUM> wrapped about the tool mandrel <NUM> and, in at least some examples, positioned within the saddle <NUM> (shown in dashed lines). The windings of the coil <NUM> extend about the tool mandrel <NUM> at the winding angle <NUM>.

The antenna assembly <NUM> also includes a soft magnetic band <NUM> radially interposing the coil <NUM> and the tool mandrel <NUM> (e.g., the saddle <NUM>). In the illustrated example, the soft magnetic band <NUM> is similar to the soft magnetic band <NUM> of <FIG>, but could alternatively comprise any of the soft magnetic band examples discussed herein or other soft magnetic band designs and configurations altogether, without departing from the scope of the disclosure.

The antenna shield <NUM> provides a circumferential encapsulation of the internal components of the antenna assembly <NUM> by extending about the tool axis <NUM>. More specifically, the antenna shield <NUM> is positioned radially outward from the coil <NUM> and the soft magnetic band <NUM>. As illustrated, the antenna shield <NUM> can axially span the axial length of the saddle <NUM> and is secured to (or otherwise engages) the tool mandrel <NUM> at its opposing axial ends. The antenna shield <NUM> may be designed such that a relatively smooth structural transition is achieved between the antenna shield <NUM> and the outer diameter of the tool mandrel <NUM> at the opposing axial ends of the antenna shield <NUM>.

In some examples, the antenna shield <NUM> can be formed of a nonconductive and/or non-metallic material, such as fiberglass or a polymer (e.g., polyether ether ketone or "PEEK"). In other examples, however, the antenna shield <NUM> can be made of a conductive and/or metallic material, such as stainless steel, a nickel-based alloy (e.g., MONEL®, INCONEL®, etc.), a chromium-based alloy, a copper-based alloy, or any combination thereof.

The antenna shield <NUM> also includes a plurality of slots <NUM> defined through the body of the antenna shield <NUM>. The slots <NUM> facilitate electromagnetic transmissivity of the antenna shield <NUM> by providing areas where electromagnetic signals can penetrate the antenna shield <NUM> to be received or transmitted. In the illustrated example, each slot <NUM> is formed in the shape of a rectangle, but could alternatively exhibit other shapes, without departing from the scope of the disclosure. Each slot <NUM> has a length <NUM> and a width <NUM>, and is separated from angularly adjacent slots <NUM> by a separation gap <NUM>. The separation gap <NUM> may or may not be uniform between all angularly adjacent slots <NUM>. The slots <NUM> are formed in the antenna shield <NUM> such that each slot <NUM> extends perpendicular or substantially perpendicular to the radially adjacent coil <NUM> at any given angular location about the circumference of the tool mandrel <NUM>. Consequently, the length <NUM> of each slot <NUM> extends perpendicular to the radially adjacent windings of the coil <NUM>. As used herein, the phrase "substantially perpendicular" refers to a <NUM>° relative offset between the slots <NUM> and the radially adjacent coil <NUM>, but also encompasses a +/-<NUM>° offset from a truly perpendicular relationship, without departing from the scope of the disclosure.

The length <NUM> of each slot <NUM> is constant (the same) and the pattern of the slots <NUM> (including the magnitude of the separation gaps <NUM>) is constant about the entire circumference of the antenna shield <NUM>. The slots <NUM> cooperatively form a discontinuous annular ring that extends about the circumference of the antenna shield <NUM> at a slot angle <NUM>. The slot angle <NUM> and the winding angle <NUM> are substantially the same such that the slots <NUM> are arranged radially outward from the coil <NUM> at any given angular location about the circumference of the tool mandrel <NUM>.

The slots <NUM> help reduce the gain loss from the antenna shield <NUM>, while also reserving the winding (tilt) angle <NUM> of the coil <NUM>. The slots <NUM> of <FIG>, however, are not optimized to provide maximum gain while maintaining acceptable mechanical integrity and strength for the antenna shield <NUM>. Rather, the slot design shown in <FIG> constitutes a uniform pattern of similarly sized and arranged slots <NUM> about the circumference of the antenna shield <NUM>.

<FIG> is a side view of the antenna assembly <NUM> that includes an example of the antenna shield <NUM> that incorporates one or more principles of the present disclosure. As will be discussed below, one way to increase the gain sensitivity of the antenna assembly <NUM> is to increase the length of the slots <NUM> in the antenna shield <NUM>. However, due to mechanical constraints of the structure of the antenna shield <NUM>, it is prohibitive to simply increase the length of all the slots <NUM> to a maximum, which might adversely affect the structural integrity of the antenna shield <NUM> and render it unsuitable for downhole use.

Similar to the example of <FIG>, each slot <NUM> of the antenna shield <NUM> of <FIG> is separated from angularly adjacent slots <NUM> by the separation gap <NUM>. Unlike the example of <FIG>, however, the slots <NUM> of the antenna shield <NUM> of <FIG> are defined and otherwise provided in at least two dissimilar lengths extending perpendicular to the winding angle <NUM> (<FIG>). More specifically, as illustrated, one or more slots <NUM> exhibit a first length 1002a and one or more additional (other) slots <NUM> exhibit a second length 1002b, where the first length 1002a is longer than the second length 1002b. The magnitude (size) of the first and second lengths 1002a,b may depend on the material used for the antenna shield, the axial length of the coil <NUM>, and other structural parameters of the antenna shield <NUM>. Having at least two dissimilar lengths 1002a,b helps minimize the gain loss through the antenna shield <NUM> but also maintains the structural integrity and strength of the antenna shield <NUM>.

In principle, and based on conclusions derived from the tests shown in <FIG>, the slots <NUM> having the first length 1002a should be as long as possible under the condition that all such slots <NUM> should not merge at one end and otherwise maintain a minimum distance at one end. The slots <NUM> having the second length 1002b may interpose the longer slots <NUM> and may also be as long as possible under the condition that all such slots <NUM> should not merge with the longer slots <NUM> and also maintain a minimum distance between the longer slots <NUM>.

In some examples, as illustrated, the slots <NUM> may be defined (arranged) in a slot pattern where the first and second lengths 1002a,b alternate about the circumference of the antenna shield <NUM> in a one-to-one ratio. More particularly, a slot <NUM> having the second length 1002b interposes each angularly adjacent pair of slots <NUM> having the first length 1002a, or vice versa, where a slot <NUM> having the first length 1002a interposes each angularly adjacent pair of slots <NUM> having the second length 1002b.

In other examples, however, the slots <NUM> may be defined in a slot pattern where the first and second lengths 1002a,b alternate about the circumference of the antenna shield <NUM> in a two-to-two ratio. In such examples, two slots <NUM> having the first length 1002a may be succeeded by two slots <NUM> having the second length 1002b in a continuous alternating pattern about the circumference of the antenna shield <NUM>.

In yet other examples, the slots <NUM> may be defined in a slot pattern where the first and second lengths 1002a,b alternate about the circumference of the antenna shield <NUM> in a two-to-one ratio. In such examples, two or more slots <NUM> having the second length 1002b may interpose each angularly adjacent pair of slots <NUM> having the first length 1002a, or vice versa. In even further examples, it is contemplated herein that the slots <NUM> having the first and second lengths 1002a,b may be provided in a random slot pattern that may or may not repeat about the circumference of the antenna shield <NUM>.

While the antenna shield <NUM> of <FIG> shows slots <NUM> having only two dissimilar lengths 1002a,b, it is contemplated herein that the antenna shield <NUM> provide slots <NUM> having three or more dissimilar lengths. In such examples, the pattern of the slots <NUM> having three or more dissimilar lengths may be uniform and repeat about the circumference of the antenna shield <NUM>, or may otherwise be provided in a random pattern that may or may not repeat, without departing from the scope of the disclosure.

In any of the slot pattern scenarios described herein, the separation gap <NUM> between angularly adjacent slots <NUM> of any length 1002a,b may be uniform or instead vary about the circumference of the antenna shield <NUM>, without departing from the scope of the disclosure.

The soft magnetic band <NUM> at least partially visible through the slots <NUM> in the antenna shield <NUM> of <FIG> may comprise the soft magnetic band <NUM> of <FIG>. Accordingly, the soft magnetic band <NUM> may include the plurality of stacked inserts <NUM> (<FIG>) extending perpendicular to the coil <NUM>, and simultaneously extending parallel to the slots <NUM>. In some examples, the gap <NUM> (<FIG>) separating each stacked insert <NUM> from angularly adjacent stacked inserts may have the same or similar magnitude (size) as the separation gap <NUM> that separates angularly adjacent slots <NUM>. The stacked inserts <NUM> are radially misaligned with the slots <NUM>.

Altering different parameters of the antenna shield <NUM> affects the gain sensitivity of the antenna assembly <NUM>. Example shield parameters that can be altered (manipulated) to increase the gain sensitivity of the antenna assembly <NUM> include, but are not limited to, the number of slots <NUM> included in the antenna shield <NUM>, the width <NUM> (<FIG>) of each slot <NUM>, and the length <NUM> (<FIG>) of each slot <NUM>. Through testing, the inventors have determined that the most efficient way to increase the gain sensitivity of the antenna assembly <NUM> is to maximize the length <NUM> of the slots <NUM>. <FIG> graphically depict testing data that supports this conclusion.

More particularly, <FIG> depict test results obtained by varying the aforementioned shield parameters on an antenna assembly similar to the antenna assembly <NUM> of <FIG> and with the antenna shield similar to the antenna shield <NUM> of <FIG>. Tests were performed while exciting the coil <NUM> (<FIG>) of the antenna assembly <NUM> at several frequencies: <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>.

<FIG> provides a series of plots depicting test data resulting from increasing the number of slots defined in the antenna shield. Data were obtained from two dissimilar slot designs: <NUM>) slots having a length of <NUM> inches (in. ) and a width of <NUM> in. , and <NUM>) slots having a length of <NUM> in. and a width of <NUM> in. As shown in the plots of <FIG>, increasing the number of slots in the antenna shield results in a corresponding increase of gain percentage sensitivity.

<FIG> provides a series of plots depicting test data resulting from increasing the width of the slots defined in the antenna shield. Data were obtained from three antenna shield designs: <NUM>) an antenna shield with twelve slots having a length of <NUM> in. , <NUM>) an antenna shield with twenty-four slots having a length of <NUM> in. , and <NUM>) an antenna shield with twelve slots having a length of <NUM> in. As shown in the plots of <FIG>, increasing the width of the slots in a given antenna shield results in a corresponding increase of gain percentage sensitivity.

<FIG> provides a series of plots depicting test data resulting from increasing the length of the slots defined in the antenna shield. Data were obtained from one antenna shield design that including twelve slots having a width of <NUM> in. As shown in the plots of <FIG>, increasing the length of the slots in the antenna shield results in a corresponding increase of gain percentage sensitivity.

The data and findings from <FIG> are summarized in the following Table <NUM>:.

From Table <NUM>, it can be concluded that the most efficient way to increase the gain sensitivity of an antenna assembly is to maximize the length of the slots defined in the antenna shield.

<FIG> is a table <NUM> showing comparative test results obtained from three variations in antenna shield designs applied to the same antenna assembly. Tests were performed while exciting the coil of each antenna assembly at several frequencies: <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. The antenna shield designs are compared against a dipole response for an air coil antenna, as shown in the second column. The air coil antenna is a coil antenna operating without an accompanying tool mandrel, soft magnetic band, or antenna shield and the dipole response results from coil excitation. The gain for the air coil antenna (magnetic dipole) is used as reference to calculate the gain percentage of the other antenna designs (Gain = GainDesign / GainDipole). As shown in the results of the first column, the dipole response at each frequency is <NUM>.

The antenna shield designs are further compared against performance of a base first antenna assembly without an antenna shield, as shown in the third column. The first antenna assembly is the same as the antenna assembly <NUM> of <FIG>, and is used as the base antenna assembly for each of the second, third, and fourth antenna assemblies.

The second antenna assembly includes an antenna shield that is substantially similar to the antenna shield <NUM> of <FIG>, which includes a plurality of tilted slots each having a length of length of <NUM> in. and a width of <NUM> in. The third antenna assembly includes an antenna shield having twelve tilted slots, where each slot has a length of length of <NUM> in. and a width of <NUM> in. The fourth antenna assembly includes an antenna shield that is substantially similar to the antenna shield <NUM> of <FIG>, which includes twenty-four tilted slots provided with two dissimilar lengths of <NUM> in. and <NUM> in. , and with a width of <NUM> in.

The data in table <NUM> indicate that adding an antenna shield with tilted slots to the first antenna assembly results in a gain decrease, as shown by the data obtained from the second, third, and fourth antenna assemblies. The gain performance for the fourth antenna assembly, however, was largely superior to the gain performance of the second and third antenna assembly designs, especially at higher frequencies (e.g., <NUM> and <NUM>). It can be concluded, then, that having two or more dissimilar lengths of slots (long and short) in an antenna shield can increase the gain performance of a given antenna assembly while simultaneously maintaining the structural integrity of the antenna shield.

<FIG> is a table <NUM> showing comparative test results obtained from two variations in antenna shield designs in conjunction with varying the design of the underlying soft magnetic band of an antenna assembly. Similar to table <NUM> of <FIG>, the antenna shield designs are again compared against a dipole response for an air coil antenna, as shown in the second column. Moreover, tests were again performed while exciting the coil of a given antenna assembly at several frequencies: <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>.

The antenna shield designs are also compared against two base antenna assembly designs, shown in the third column as a first antenna assembly and a fourth antenna assembly. The first antenna assembly is the same as the antenna assembly <NUM> of <FIG>, and is used as the base antenna assembly for the second and third antenna assemblies. The fourth antenna assembly is the same as the antenna assembly <NUM> of <FIG>, which includes the soft magnetic band having a plurality of stacked inserts, and is used as the base antenna assembly for the fifth and sixth antenna assemblies.

The second and fifth antenna assemblies each include an antenna shield that is substantially similar to the antenna shield <NUM> of <FIG>, where the antenna shield has twenty-four tilted slots and each slot has the same uniform length. The third and sixth antenna assemblies each include an antenna shield that is substantially similar to the antenna shield <NUM> of <FIG>, where the antenna shield has tilted slots provided with two dissimilar lengths (long and short).

The data in table <NUM> provide a comparison of the gain performance between a base antenna assembly design with a conventional soft magnetic band (i.e., the first through third antenna assemblies) and one with stacked inserts (i.e., the fourth through sixth antenna assemblies). Table <NUM> indicates that the performance of the base antenna assembly design with the conventional soft magnetic band and without an antenna shield (i.e., the first antenna assembly) is superior to the antenna assembly design with stacked inserts without an antenna shield (i.e., the fourth antenna assembly). However, when the antenna shield is included, both antenna assembly designs exhibit substantially similar performance at each frequency. This may prove useful for drilling operations that require an antenna assembly to obtain measurements while drilling. Inclusion of the antenna shield in drilling operations may be required to protect the coil from wear and damage caused by drilling debris and movement of a drill string.

<FIG> is a table <NUM> showing comparative test results obtained from variations in antenna assembly designs having stacked inserts of a soft magnetic band aligned and misaligned with the slots of an antenna shield. Similar to tables <NUM> and <NUM> of <FIG> and <FIG>, respectively, the antenna shield designs are again compared against a dipole response for an air coil antenna, as shown in the second column. Tests were performed while exciting the coil of the given antenna assemblies at several frequencies: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>.

Table <NUM> provides comparative test data from first, second, third, and fourth antenna assemblies. Each antenna assembly includes a soft magnetic band comprising a plurality of stacked inserts, similar to the antenna assembly <NUM> of <FIG>. Moreover, each antenna assembly is tested in conjunction with an antenna shield similar to the antenna shield <NUM> of <FIG>, where the tilted slots of the antenna shield are provided with two dissimilar lengths (long and short). The antenna shield used with the first and second antenna assemblies has twenty-four tilted slots with two dissimilar lengths, and the antenna shield used with the third and fourth antenna assemblies has thirty-six tilted slots with two dissimilar lengths. The data in Table <NUM> also reflects measurements obtained when the stacked inserts of the soft magnetic band of each antenna assembly are misaligned or aligned with the tilted slots. It is observed from the test results provided in Table <NUM> that when the stacked inserts are aligned radially with the tilted slots, such that the stacked inserts are exposed through the radially adjacent tilted slots, the gain performance of the particular antenna assembly is superior as compared to the antenna assemblies where the stacked inserts are radially misaligned with the tilted slots.

<FIG> is a table <NUM> showing comparative test results obtained from variations in antenna assembly designs having a soft magnetic band comprising stacked inserts with varying numbers of rods. Similar to tables <NUM>, <NUM>, and <NUM> of <FIG>, respectively, the antenna shield designs are again compared against a dipole response for an air coil antenna, as shown in the second column. Moreover, tests were performed while exciting the coil of the given antenna assemblies at several frequencies: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>.

Table <NUM> provides comparative test data from first, second, third, and fourth antenna assemblies. Each antenna assembly includes a soft magnetic band comprising a plurality of stacked inserts, similar in some respects to the antenna assembly <NUM> of <FIG>. Moreover, each antenna assembly is tested in conjunction with an antenna shield, similar to the antenna shield <NUM> of <FIG>, where the tilted slots of the antenna shield are provided with two dissimilar lengths (long and short). The antenna shield used with the first and second antenna assemblies has twenty-four tilted slots with two dissimilar lengths, and the antenna shield used with the third and fourth antenna assemblies has thirty-six tilted slots with two dissimilar lengths.

In the testing, the number of stacked inserts and rods in the soft magnetic band of each antenna assembly was varied to determine its effect on gain performance. The soft magnetic band of the first antenna assembly, for example, includes forty-eight stacked inserts, where each stacked insert includes three rods. The soft magnetic band of the second antenna assembly includes thirty-six stacked inserts, where each stacked insert includes four rods. Consequently, the stacked inserts of the second antenna assembly are longer that the stacked inserts of the first antenna assembly. The soft magnetic band of the third antenna assembly includes thirty-six stacked inserts, where each stacked insert includes three rods. Lastly, the soft magnetic band of the fourth antenna assembly includes thirty-six stacked inserts, where each stacked insert includes four rods. Consequently, the stacked inserts of the second and fourth antenna assemblies are longer that the stacked inserts of the first and third antenna assemblies.

Claim 1:
An antenna assembly (<NUM>), comprising:
a tool mandrel (<NUM>) having a tool axis (<NUM>);
a coil (<NUM>) including a plurality of windings wrapped about the tool mandrel (<NUM>), wherein portions of the plurality of windings are wrapped about the tool mandrel at a winding angle (<NUM>) offset from the tool axis (<NUM>);
an antenna shield (<NUM>) secured to the tool mandrel (<NUM>) and positioned radially outward from the coil (<NUM>), wherein the antenna shield (<NUM>) defines a plurality of slots (<NUM>) extending perpendicular to the coil (<NUM>) at any angular location about a circumference of the tool mandrel (<NUM>) and the plurality of slots (<NUM>) comprise slots of different lengths, characterized in that the antenna assembly further comprises:
a soft magnetic band (<NUM>, <NUM>) radially interposing the coil (<NUM>) and the tool mandrel (<NUM>) and extending about a circumference of the tool mandrel (<NUM>), wherein the soft magnetic band (<NUM>, <NUM>)
includes a plurality of stacked inserts (<NUM>) extending perpendicular to the coil (<NUM>) about the circumference of the tool mandrel (<NUM>) and each stacked insert (<NUM>) includes a plurality of rods (<NUM>) positioned end-to-end, wherein each stacked insert (<NUM>) is separated from angularly adjacent stacked inserts (<NUM>) by a gap (<NUM>), and wherein the plurality of stacked inserts (<NUM>) are radially misaligned with the plurality of slots (<NUM>).