Abstract:
A logging tool and method for winding a multi-component induction (MCI) antenna is presented. The method eliminates unwanted dipole signals that are created by the voltage drop that takes place in the transmitter. The antenna is made of at least two parts physically separated, one on each side of the mandrel. The winding method is performed in a way that creates at least four windings arranged to eliminate dipole signals attributable to an asymmetric voltage distribution. The midpoint of the conductive wire that for is the antenna windings may be electrically attached to the tool body. This method is suitable for the winding of the cross-components X and Y of the MCI coil arrays.

Description:
BACKGROUND 
     Drillers and oil field operators use electromagnetic logging tools to measure a wide variety of subsurface formation parameters including, e.g., resistivity and nuclear magnetic resonance (NMR) responses. Such tools often energize a transmitter antenna with an alternating current to emit electromagnetic energy through the borehole fluid and into the surrounding formations. The emitted energy interacts with the borehole and formation to produce signals that are detected and measured by one or more receiver antennas. By processing the detected signal data, a profile of the formation can be determined. Such information is useful for well exploration and production. 
     The antennas on these tools are generally formed as multi-loop coils of conductive wire. The conductive wire sustains a voltage drop when the transmitters are driven and the signals are induced into the receivers. Depending on the winding pattern for the coils, such voltage drops can create an unwanted dipole signal that adversely affects the measurements. In addition, such voltage drops can cause undesirable cross-coupling between the antennas, further contaminating the tool&#39;s measurements. These issues do not appear to have been adequately addressed in tools having transverse (X- and Y-) antennas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the various disclosed system and method embodiments can be obtained when the following detailed description is considered in conjunction with the drawings, in which: 
         FIG. 1  shows an illustrative logging while drilling environment; 
         FIG. 2  is a schematic cross-section of an illustrative electromagnetic logging tool; 
         FIG. 3A  is a top view of an illustrative antenna winding form; 
         FIG. 3B  is an isometric view of an illustrative antenna winding form; 
         FIG. 4A  is a top view of the form with an illustrative “x pattern” winding; 
         FIG. 4B  is an isometric view of the form with the illustrative “x-pattern”; 
         FIG. 5  shows an illustrative embodiment of the coil winding pattern when half-way complete; 
         FIG. 6  shows the illustrative coil winding pattern at an intermediate stage; 
         FIG. 7  shows the illustrative coil winding pattern when complete; and 
         FIG. 8  shows an illustrative representation of the antenna windings that are vertically offset. 
     
    
    
     DETAILED DESCRIPTION 
     The issues identified in the background are at least in part addressed by the disclosed coil winding methods and patterns for downhole logging tools. At least one tool embodiment includes a logging tool that has an antenna for transmitting or receiving signals with a magnetic field component orthogonal to the tool axis. The antenna includes at least two portions arranged on opposite sides of a tool axis, and is formed from a conductive wire arranged to form at least two windings on each of the at least two portions. In operation, the conductive wire sustains a voltage distribution along its length that is symmetric about a midpoint of the conductive wire. The windings are arranged so that each winding on each portion of the antenna is matched by another winding on that portion having an equal and opposite voltage distribution. Preferably, at each point the conductive wire in each winding is separated from a corresponding point of the conductive wire in the matched winding in a direction that is substantially only radial, the corresponding points having substantially opposite voltages. 
     One illustrative antenna has at least eight windings arranged to eliminate any dipole signal attributable to an unbalanced distribution of voltages along the length of the antenna wire. These eight windings are coupled in series between first and second driving terminals for the antenna. The eight windings include a first and second winding positioned on one side of the tool, a third and fourth winding positioned on an opposite side of the tool, a fifth and sixth winding substantially collocated with the third and fourth winding, and a seventh and eighth winding substantially collocated with the first and second winding. 
     To further assist the reader&#39;s understanding of the disclosed systems and methods, we describe an environment suitable for their use and operation in  FIG. 1 . A drilling platform  2  supports a derrick  4  having a traveling block  6  for raising and lowering a drill string  8 . A top drive  10  supports and rotates the drill string  8  as it is lowered through the wellhead  12 . A drill bit  14  is driven by a downhole motor and/or rotation of the drill string  8 . As bit  14  rotates, it creates a borehole  16  that passes through various formations. The drill bit  14  is just one piece of a bottom-hole assembly that typically includes one or more drill collars (thick-walled steel pipe) to provide weight and rigidity to aid the drilling process. Some of these drill collars may include logging instruments to gather measurements of various drilling parameters such as position, orientation, weight-on-bit, borehole diameter, resistivity, etc. Resistivity can be measured by electromagnetic logging tools, where the transmitter and receiver antennas are typically mounted with their axes along, or parallel to, the longitudinal axis of the tool. The tool orientation may be specified in terms of a tool face angle (rotational orientation), an inclination angle (the slope), and compass direction, each of which can be derived from measurements by magnetometers, inclinometers, and/or accelerometers, though other sensor types such as gyroscopes may alternatively be used. 
     The system further includes a tool  26  to gather measurements of formation properties from which formation boundaries can be identified as discussed further below. Using these measurements in combination with the tool orientation measurements, the driller can steer the drill bit  14  along a desired path  18  relative to boundaries  46 ,  48  using any one of various suitable directional drilling systems, including steering vanes, a “bent sub”, and a rotary steerable system. A pump  20  circulates drilling fluid through a feed pipe  22  to top drive  10 , downhole through the interior of drill string  8 , through orifices in drill bit  14 , back to the surface via the annulus around drill string  8 , and into a retention pit  24 . The drilling fluid transports cuttings from the borehole into the pit  24  and aids in maintaining the borehole integrity. Moreover, a telemetry sub  28  coupled to the downhole tools  26  can transmit telemetry data to the surface via mud pulse telemetry. A transmitter in the telemetry sub  28  modulates a resistance to drilling fluid flow to generate pressure pulses that propagate along the fluid stream at the speed of sound to the surface. One or more pressure transducers  30 ,  32  convert the pressure signal into electrical signal(s) for a signal digitizer  34 . Note that other forms of telemetry exist and may be used to communicate signals from downhole to the digitizer. Such telemetry may employ acoustic telemetry, electromagnetic telemetry, or telemetry via wired drill pipe. 
     The digitizer  34  supplies a digital form of the pressure signals via a communications link  36  to a computer  38  or some other form of a data processing device. Computer  38  operates in accordance with software (which may be stored on information storage media  40 ) and user input via an input device  42  to process and decode the received signals. The resulting telemetry data may be further analyzed and processed by computer  38  to generate a display of useful information on a computer monitor  44  or some other form of a display device. Such useful information includes formation properties such as resistivity. 
       FIG. 2  is a cross-section of an illustrative multi-component induction tool. Multi-component array induction tools use transmitter and receiver coil-antennas disposed to excite fields at three non-parallel (usually orthogonal) directions. The tool includes a metal tube  201  that defines a central bore which can be used as a fluid flow path (for logging while drilling embodiments) or as a wire guide (for wireline tool embodiments). An outer sleeve  203  surrounds the tool and keeps out the borehole fluid. The sleeve  203  is preferably nonconductive, but could have conductive elements. The mandrel between the tube  201  and the sleeve  203  could be mainly metal. The mandrel is designed to accommodate a pair of antennas  204  oriented along an x-axis and a pair of antennas  206  along a y-axis. The antennas are provided in pairs to maximize their sensing areas while at the same time preserving their symmetry. Though the antennas in each pair can be coupled in parallel, a series coupling may be preferred. 
       FIGS. 3A and 3B  show views of a mandrel  300  with grooves to accommodate the antenna windings. The windings for the antennas are preferably arranged so as to create a pattern of balanced voltages.  FIGS. 3A-3B  illustrate the beginning of the winding pattern, with a first winding beginning at  302  and ending at  304  of the top side of the mandrel  300 . This portion of the mandrel will have four windings, which is believed to be the minimum number required to cancel unwanted dipole signals and to minimize the coupling between the different antenna orientations, if symmetry is to be enforced while still feeding each portion of the mandrel from a single wiring port. (If wiring ports are provided on both sides of each portion, the minimum number of windings drops to four.) Because four windings will occupy this space, each of the four windings will use every fourth groove, i.e., three grooves are left open between the turns of each winding. 
       FIGS. 4A and 4B  show views of the mandrel  300  with a second winding added. Because this winding returns to the starting point of the last winding, it may be termed a “return winding”. In the second winding, the wires are wound back in the opposite direction from side  304  to side  302 , filling in the middle groove of the three available grooves. When the wires are wound back in the opposite direction, the second winding creates an “x-pattern” with the first winding as shown in  FIGS. 4A and 4B . As the wires pass over the top and bottom sides of the mandrel, they cross to form repetitive representations of the alphabet letter “x”. These two windings are represented in  FIG. 5  as winding A and winding B. Winding A starts from the first antenna terminal  502 , and winding B forms the x-pattern with winding A. From the end of winding B, the antenna wire runs diametrically across the tool to the opposite side of the mandrel. (In practice the wire may be routed around the outside of metal tube  201 , but for explanatory purposes this issue is neglected here.) 
       FIG. 5  shows that winding C begins on the mandrel at a point diametrically opposite the beginning of winding A. It should be noted that the polarity of each winding&#39;s response to a horizontal magnetic field is the same, i.e., winding C&#39;s response to a field in the x-direction should add to the response of windings A and B. Proceeding in this manner, windings C and D are added as illustrated in  FIG. 5 . The turns of return winding D cross the turns of winding C to create the same x-pattern observed for windings A and B. The antenna wire from winding D is routed to a grounding point  504  adjacent to the first antenna terminal  502 . 
     If the winding pattern were to halt at this point, the operation of the antenna would create an electrical field, since the average voltage of windings A and B is higher than the average voltage of windings C and D. To balance the voltages and cancel the undesired electrical field, the winding pattern continues with the addition of four more windings. From the grounding point  504 , the pattern continues with the addition of windings E and F as shown in  FIG. 6 . Windings E and F have the same x-pattern as observed previously, but they are offset from the x-pattern of windings E and F by one groove. The turns of windings C and D occupied every other groove, the remaining grooves are filled with the turns of windings E and F. 
     From the terminus of winding F, the antenna wire runs diametrically across the mandrel to add windings G and H as shown in  FIG. 7 . As with windings E and F, windings G and H form an x-pattern that is offset from the x-pattern of windings A and B by one groove. Winding H terminates at the second antenna terminal  506 . 
       FIG. 8  shows a schematic representation of the antenna windings with the windings vertically offset to provide some clarity. Starting from the first antenna terminal, a single antenna wire is routed to provide windings A and B, across the tool to provide windings C and D, back to a ground point, then back across the tool to providing windings E and F (slightly horizontally offset from C and D), and then routed to provide windings G and H (slightly offset from windings A and B) before reaching the second antenna terminal. If the voltage applied to the antenna is assumed to drop uniformly along the length of the antenna wire, the voltage in the windings can be expressed (where the x-axis always points to the right and ranges from 0 to L): 
                       V   A     ⁡     (   x   )       =     V   ⁡     (     1   -     x     4   ⁢   L         )                   V   B     ⁡     (   x   )       =     V   ⁡     (       1   2     +     x     4   ⁢   L         )                       V   C     ⁡     (   x   )       =     V   ⁡     (       1   4     +     x     4   ⁢   L         )                   V   D     ⁡     (   x   )       =     V   ⁡     (       1   4     -     x     4   ⁢   L         )                       V   E     ⁡     (   x   )       =     V   ⁡     (       -     1   4       +     x     4   ⁢   L         )                   V   F     ⁡     (   x   )       =     V   ⁡     (       -     1   4       -     x     4   ⁢   L         )                       V   G     ⁡     (   x   )       =     V   ⁡     (       -     1   2       -     x     4   ⁢   L         )                   V   H     ⁡     (   x   )       =     V   ⁡     (       -   1     +     x     4   ⁢   L         )                   
Observe that the voltage for the four windings A, B, G, H, and the voltage for the four windings C, D, E, F cancel:
 
 V   A ( x )+ V   B ( x )+ V   G ( x )+ V   H ( x )=0
 
 V   C ( x )+ V   D ( x )+ V   E ( x )+V F ( x )=0
 
Accordingly, no electric field is created between the two sets of windings.
 
     The foregoing winding pattern provides one orthogonally-oriented antenna along, e.g., the x-axis. The y-axis antenna can be wound in a similar manner that is rotated 90° from the first antenna. A z-axis antenna can be added by wrapping a co-axial coil with a two-layer winding pattern (having a first winding and a return winding over the first) around the mandrel over or beneath the windings for the x- and y-axis antennas. The disclosed arrangements offer voltage cancellation to eliminate a undesired electrical dipole signal and to eliminate cross-coupling between the orthogonal antennas. The positioning of the grounding point (physically, not electrically) near the antenna terminals provides further insurance against ground currents and static fluctuations. Electrically, the grounding point is midway along the length of the wire to ensure that winding voltages cancel out as described above. 
     Additional windings can be added by duplicating the given pattern and coupling it in parallel or series with the original. Some simplification is possible by eliminating two of the return windings and their matching windings, e.g., windings B, D, E, G could be eliminated to provide a balanced four-winding configuration at the cost of requiring additional wiring ports. So long as each winding is matched by a corresponding winding having an equal and opposite voltage distribution, the electrical field signals will be suppressed. The foregoing winding principles can be used for transmitting antennas, receiving antennas, and antennas that operate for both transmitting and receiving. It can be employed in resistivity logging tools, nuclear magnetic logging tools, or any downhole tool that employs an antenna oriented orthogonal to the longitudinal axis of the tool. These and other variations, equivalents, and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.