Patent Publication Number: US-10782436-B2

Title: Guidance system for ranging using unbalanced magnetic fields

Description:
BACKGROUND 
     Magnetic ranging refers to well positioning that provides relative direction and distance of one well with respect to another. Several technologies for ranging from a ranging well to a remote casing in a target well are based upon launching a current at a known frequency from a power supply at the earth&#39;s surface down the casing of the target well and receiving a signal radiated from that casing in the ranging well. 
     The power supply at the surface typically employs a cable coupled to a weight bar (to provide downhole contact to the well casing) to deliver the current downhole so that magnetic fields can be generated surrounding the target well. The downhole contact between the weight bar and the casing results in the current flowing uphole through the casing. Sensors in the ranging well (e.g., drilling well) may measure the magnetic fields so that distance and direction between the target well and ranging well can be determined. 
     One problem with this method is that the current flowing uphole is in an opposite direction to the cable current direction. The magnetic field generated by each current flow has the effect of reducing the total magnetic field received at the sensors in the ranging well. Thus, it may be difficult to measure the resulting magnetic field in the ranging well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an example single wire guidance system incorporating a spiral configuration, according to aspects of the present disclosure. 
         FIG. 2  is a diagram of an example single wire guidance system incorporating the cable terminated downhole on the outside of a target well casing, according to aspects of the present disclosure. 
         FIG. 3  is a diagram of an example single wire guidance system incorporating the spiral configuration on the outside of the target well casing, according to aspects of the present disclosure. 
         FIG. 4  is a diagram of an example two cable system, according to aspects of the present disclosure. 
         FIG. 5  is a diagram of an example shielded cable with a metal exterior over an insulator, according to aspects of the present disclosure. 
         FIG. 6  is a diagram of an example shielded cable with a triangular metal core, according to aspects of the present disclosure. 
         FIG. 7  is a diagram of an example shielded cable with an insulator exterior, according to aspects of the present disclosure. 
         FIG. 8  is a diagram of an example shielded cable with a cylindrical conductive material wrapped with an insulated wire, according to aspects of the present disclosure. 
         FIG. 9  is a diagram of an example shielded cable with a rectangular conductive material wrapped by an insulated wire, according to aspects of the present disclosure. 
         FIG. 10  is a diagram of an example shielded cable apparatus for implementing a method for ranging, according to aspects of the present disclosure. 
         FIG. 11  is a plot showing the unbalanced magnetic field densities in the x-direction and the z-direction in accordance with the shielded cable apparatus of  FIG. 10 . 
         FIG. 12  is a plot showing the total magnetic field density at sensor point P in accordance with the shielded cable apparatus of  FIG. 10 . 
         FIG. 13  is a diagram of an example wireline system embodiment, according to aspects of the present disclosure. 
         FIG. 14  is a diagram of an example drilling rig system embodiment, according to aspects of the present disclosure. 
         FIG. 15  is a flowchart illustrating an example method for ranging between a target well and a ranging well using unbalanced magnetic fields, according to aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments described herein operate to provide information that assists in determining relative distance and direction of a well being drilled near at least one other well. For example, determining a location of a target well in relation to a ranging well. The ranging well may also be referred to as the drilling well. 
     A “target well” may be defined herein as a well, the location of which is to be used as a reference for the construction of another well. The other well may be defined as a “ranging well.” Other embodiments may reverse this terminology since the embodiments are not limited to any one well being the target well and any one well being the ranging well. The ranging may be used in steam assisted gravity drainage (SAGD), well intersection, relief well intersection, well avoidance, or any other usage where ranging, maintaining, avoiding, or intersecting between two wells is desirable. 
     As used herein, unbalanced magnetic fields are defined as two or more magnetic fields that have a different field pattern. For example, the magnetic fields may have different directions and/or different amplitudes. 
     The present embodiments generate unbalanced magnetic fields so that Eq. (1) below will not be zero or too small to be measureable. One method for generating the unbalanced magnetic fields includes introducing different orientations of cable winding in or around the casing instead of a straight cable along the wellbore, as illustrated in  FIGS. 1-4  and discussed subsequently. Another method includes introducing magnetic shielding (e.g., high permeability mu-metal) in the cables, as illustrated in  FIGS. 5-9  and discussed subsequently. Other methods may combine the different orientations of cable windings with the magnetic shielding. For example, the high permeability mu-metal may be used in a spiral cable configuration. In another example, any of the cables of  FIGS. 5-9  may be used in any of the configurations of  FIGS. 1-4 . 
     To represent a magnetic field generated by a cable in a casing, the total current flowing in the cable may be represented by I C . The current flowing in the casing back to ground may be represented by I t (β) having an azimuthal angle β with respect to the target well. Consequently, with a separation R between sensors in the ranging well and the casing in the target well, the magnetic field H received at the sensors may be expressed by 
     
       
         
           
             
               
                 
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     Part of the casing current may disappear or be reduced due to lossy pipe properties and/or the current leaking to geological formations. In such situations, I C  in Eq. (2) is larger than the total casing current in all azimuthal directions. Eq. (2) may then be representative of a very weak or no magnetic field at Eq. (1) such that wireline sensors in the ranging well may not be able to measure the field and determine a distance and/or direction to the target well during a wireline operation. 
       FIG. 1  is a diagram of an example single wire guidance system incorporating a spiral configuration, according to aspects of the present disclosure This embodiment may use a typical cable  100  configured in a spiral and/or one of the subsequently discussed magnetically shielded cables, for example using high permeability mu-metals. 
       FIG. 1  shows a cable  100  spirally wound within a casing  102  of a target well. The spiral cable  100  (e.g., solenoid cable) is terminated downhole at the bottom of the casing  102  by a termination  107 . The termination  107  may be a weight bar that is in electrical contact with the casing  102  or some other electrically conductive termination between the cable  100  and the casing  102 . 
     The cable  100  is coupled to a power supply  110  on the surface of a formation  130  through which the target well and a ranging well  103  are drilled. The power supply  110  provides the current I C  through the cable  100 . The power supply ground  111  may be grounded to a well head, which is electrically connected to the casing, or to the geological formation  130 . The termination  107  between the spiral cable  100  and the casing  102  results in casing current I t (β) that returns to the power supply ground  111 . 
     The ranging well  103  may include sensors  105  (e.g., sensors included in a wireline logging tool or included in a drill string, e.g. as part of a bottom hole assembly (BHA)) to measure the magnetic field produced at the target well. The sensors  105  may include triaxial magnetometers or gradient sensors. The sensors  105  are located a distance R (see Eq. (1)) from the target well spiral cable  100 . 
     The spiral cable configuration  100  produces magnetic fields  121  in different directions as compared to the magnetic fields  120  from the casing current.  FIG. 1  defines that the z-direction is along the wellbore and the x-direction is in the direction from the drilling well to the target well. The magnetic fields  120  at the sensors with respect to the casing current I t (β) will be in the y-direction, whereas the magnetic fields  121  from spiral cabling  100  will be in both the y and z-directions. 
     The y-directional field from the spiral cabling  100  is typically similar but opposite in sign to the y-directional field  120  from casing current I t (β). Therefore, the total magnetic y-directional fields at the sensors  105  will disappear in Eq. (1). On the other hand, the more turns the spiral cabling  100  has, the more unbalanced the y-directional and z-directional fields will be. In one or more embodiments, such as when spiral cabling  100  that has many turns and/or a relatively large radius for each turn, Eq. (1) will not be valid. Thus it is possible to acquire a significant total field (both y-directional and z-directional fields) downhole from the cable current I C  in  FIG. 1  and such total field can be detected by different sensor configurations  105  in the ranging well  130 . 
       FIG. 2  is a diagram of an example single wire guidance system incorporating the cable terminated downhole on the outside of a target well casing, according to aspects of the present disclosure. The cable  200  may include a straight cable (i.e., non-spiral) or one of the cable embodiments of  FIGS. 5-9 . 
       FIG. 2  illustrates the casing  202  surrounded by an insulating concrete layer  213 . The cable  200  is embedded in the insulating concrete layer  213  and coupled to the casing at a cable termination point  207 . The termination point  207  may be any location along the target well casing  202  according to various embodiments. 
     The cable  200  is further coupled to a power supply  210  on the surface of the formation  230  through which the wells are drilled. The power supply  210  provides the current I C  through the cable  200 . The power supply ground  211  may be grounded to a well head, which is electrically connected to the casing, or to the geological formation  230 . 
     The power supply  210  supplies the current I C  through the cable  200  to the termination point  207 . The current then returns to ground on the casing as represented by return current I t (β). 
     The ranging well  203  includes sensors  205  (e.g., magnetometers, gradient sensors) that are located a distance R from the center of the target well casing  202 . The sensors  205  may be included in a wireline logging tool or included in a drill string, e.g. part of a BHA. 
     Using one of the cables of  FIGS. 5-9 , current may be delivered to the end of the target well to generate larger unbalanced total fields as compared to the fields due to the same current of the typical straight cable. The resulting magnetic fields are received at the sensors  205  in ranging well owing to the advantages of unbalance magnetic fields between current I C  at the cable  200  and the current I t (β) at the casing  202 . 
       FIG. 3  is a diagram of an example single wire guidance system incorporating the spiral configuration on the outside of the target well casing, according to aspects of the present disclosure. This embodiment adopts similar wiring methods as illustrated in  FIGS. 5-9  as cable winding methods integrated with the insulating cement layer  313 . 
     One end of the spirally wound cable  300  is coupled to a power supply  310  on the surface. The target well and the ranging well  303  are drilled into a geological formation  330 . The power supply  310  is further grounded to either the well head of the target well or the geological formation  330 . The power supply  310  supplies the cable current I C . 
     The spiral cable  300  is terminated on the casing  302  at a termination point  307 . The termination point  307  is shown at the bottom of the casing  302  but may be located anywhere on the casing  302 . The connection of the cable  300  to the casing  302  enables the casing current I t (β) to return to the power supply ground. 
     The cable  300  of  FIG. 3  may be a typical cable (with conductor and insulator inside) or a magnetically shielded cable. The cement layer  313  provides both insulation of the cable  300  from the casing  302  as well as stabilization of the cable  300  with respect to the casing  302 . The system of  FIG. 3  provides the unbalanced magnetic fields as described previously. Since the cable  300  is permanently installed with cement  313  around the casing  302 , the cable may be accessible anytime and can be used for other purposes. For example, the cable  300  may be built with fibers for well monitoring purposes. 
       FIG. 4  is a diagram of an example two cable system, according to aspects of the present disclosure. Both the current source cable  400  and the current return cable  411  are located within the target well casing  402 . 
     The system of  FIG. 4  includes the current source cable  400  coupled to an output of a power supply  410 . The current return cable  411  is coupled to the power supply&#39;s return. 
     As in previous embodiments, the target well casing  402  and the ranging well  403  are located in a geological formation  430 . The ranging well  403  includes sensors  405  located a distance R from the center of the target well casing  402 . 
     A first cable  400  of the two cables is a magnetically shielded cable such as those shown in  FIGS. 5-9 . The second cable  411  of the two cables is normal, unshielded cable. The cables  400 ,  411  have no connection to the target well casing  402  so that no casing current will be generated. The sensors  405  in the ranging well  403  measure the unbalanced magnetic fields  420  where a majority of the magnetic fields are generated from the normal, unshielded cable. Either one of the current source cable  400  or the current return cable  411  may be the magnetically shielded cable as long as the other cable is the unshielded cable. 
       FIGS. 5-9  illustrate various embodiments for magnetically shielded cables. These embodiments may utilize a cable with: (a) intrinsic magnetic shielding materials, (b) intrinsic solenoid wiring or extrinsic spiral winding around a cylindrical material (such as mu-metal), or (c) other intrinsic or extrinsic wiring orientations (e.g., cylindrical, triangular, rectangular, or other shapes for wiring). A mu-metal may be defined as a nickel-iron alloy. 
       FIG. 5  is a diagram of an example shielded cable with a metal exterior over an insulator, according to aspects of the present disclosure. The illustrated embodiment includes a metal enclosure  501  (e.g., steel) to protect the conductor  504  and shielding  503 . An insulator  502  insulates the conductor  504  and shielding  503  from the metal enclosure  501 . The shield  503  (e.g., metal, mu-metal) is wrapped around the metal conductor  504  (e.g., copper). In an embodiment, the shield  503  is spirally wrapped around the metal conductor  504 . 
       FIG. 6  is a diagram of an example shielded cable with a triangular metal core, according to aspects of the present disclosure. The illustrated embodiment includes a triangular metal core  602  (e.g., metal, mu-metal) around which is wrapped a metal conductor  603  (e.g., copper) that provides magnetic shielding of the metal core  602 . An insulator  601  encloses the cable for protection of the cable as well as insulation of the conductor  602  and shielding  603  from other metal contact. The metal conductor  603  may be spirally wrapped around the metal core  602 . 
       FIG. 7  is a diagram of an example shielded cable with an insulator exterior, according to aspects of the present disclosure. The illustrated embodiment includes a cylindrical metal core  702  (e.g., metal, mu-metal) around which is wrapped a metal conductor  703  (e.g., copper) that provides magnetic shielding of the metal core  702 . An insulator  701  encloses the cable for protection of the cable as well as insulation of the metal conductor  702  and shielding  703  from other metal contact. The metal conductor  703  may be spirally wrapped around the metal core  702 . 
       FIG. 8  is a diagram of an example shielded cable with a cylindrical conductive material wrapped with an insulated wire, according to aspects of the present disclosure. The illustrated embodiment includes a metal core  802  (e.g., copper) enclosed by an insulator  801  to form an insulated wire. The insulated wire is spirally-wrapped around a cylindrical conductive material  800  (e.g., metal, mu-metal) to form the cable. The insulated wire may be spirally wrapped around the cylindrical conductive material  800 . 
       FIG. 9  is a diagram of an example shielded cable with a rectangular conductive material wrapped by an insulated wire, according to aspects of the present disclosure. The illustrated embodiment includes a metal core  902  (e.g., copper) enclosed by an insulator  901  to form an insulated wire. The insulated wire is wrapped around a rectangular conductive material  900  (e.g., metal, mu-metal) to form the cable. The insulated wire may be spirally wrapped around the rectangular conductive material  900 . 
     The various shapes and compositions of the embodiments illustrated by  FIGS. 5-9  are for purposes of illustration only. Other shapes and compositions may be used for magnetically shielded cables. 
       FIG. 10  is a diagram of an example shielded cable apparatus for implementing a method for ranging, according to aspects of the present disclosure. Parameters from this apparatus, as modeled using the following parameter assumptions, are incorporated into Eq. (1) in order to generate the plots of  FIGS. 11 and 12 . 
       FIG. 10  shows that the cable  1000  has two sections  1001 ,  1002  with spiral wiring. Each section  1001 ,  1002  is assumed to have a length of L. In addition, there is a separation distance of S between the two spiral wiring sections  1001 ,  1002 . 
     One spiral wiring  1001  is in a counterclockwise direction and the other spiral wiring  1002  is in clockwise direction. The inner conductor  1005  may be used as a current inject path and the outer conductor  1001 ,  1002  may be used as a current return path. Which conductor is the current return path and which conductor is the current injection path is interchangeable. An injection path can be a return path by changing the current direction (i.e., applying positive voltage to one path and negative voltage to the other path). 
     The following parameter assumptions are used for modeling the apparatus of  FIG. 10  only to generate the plots of  FIGS. 11 and 12  and are not limiting on any other examples herein: radius of the spiral cable=2.54 centimeters, L=4 meters, S=L, the injected current is 1 Amp, and the density of the spiral cable is D=N/L=200=number of turns/meter. The sensor (e.g., magnetometer, wireline tool) is assumed to be 5 meters away in the x direction from the cable. Z=0 meter for point P that represents the position of the sensor between the two spiral wiring sections  1001 ,  1002 . 
       FIG. 11  is a plot showing the unbalanced magnetic field densities in the x-direction and the z-direction in accordance with the shielded cable apparatus of  FIG. 10 .  FIG. 12  is a plot showing the total magnetic field density at sensor point P in accordance with the shielded cable apparatus of  FIG. 10 . Both show Amps/meter (A/m) for the magnetic field in the z-direction. It can be seen that the maximum field is approximately 0.57 A/m (or ˜716 nT) for the illustrated embodiment. This is a relatively significant magnetic field for a typical ranging application. 
     The embodiments of  FIGS. 10, 11, and 12  are only for purposes of illustration of a typical ranging embodiment. Other embodiments may have different parameters that generate different magnetic fields at the sensor location P. 
       FIG. 13  is a diagram showing a wireline system  1364  and  FIG. 14  is a diagram showing a drilling rig system  1464 . The systems  1364 ,  1464  may thus comprise portions of a wireline logging tool body  1320 , including the above-described sensors, as part of a wireline logging operation or of a down hole tool  1424 , including the above-described sensors, as part of a down hole drilling operation. 
       FIG. 13  illustrates a well that may be used as a ranging well or a target well. In this case, a drilling platform  1386  is equipped with a derrick  1388  that supports a hoist  390 . If this well is used as the target well, the sensors in the wireline logging tool  1320  and the illustrated cable may be replaced with one or more of the previously discussed embodiments (e.g., spiral cable, intrinsic magnetic shielding materials, intrinsic solenoid wiring or extrinsic spiral winding around a cylindrical material (such as mu-metal), or other intrinsic or extrinsic wiring orientations (e.g., cylindrical, triangular, rectangular, or other shapes for wiring)). 
     Drilling oil and gas wells is commonly carried out using a string of drill pipes connected together so as to form a drillstring that is lowered through a rotary table  1310  into a wellbore or borehole  1312 . Here it is assumed that the drillstring has been temporarily removed from the borehole  1312  to allow a wireline logging tool  1320 , such as a probe or sonde, to be lowered by wireline or logging cable  1374  (e.g., slickline cable) into the borehole  1312 . Typically, the wireline logging tool  1320  is lowered to the bottom of the region of interest and subsequently pulled upward at a substantially constant speed. In one or more embodiments, the borehole  1312  of  FIG. 3  may represent a ranging well to the target well of  FIG. 14 . When this well is used as a ranging well, the wireline logging tool  1320  may include the sensors to measure the magnetic field produced from the target well. 
     During the upward trip, at a series of depths, various instruments may be used to perform measurements on the subsurface geological formations  1314  adjacent to the borehole  1312  (and the tool body  1320 ), including measurements of the magnetic field produced at the target well. The wireline data may be communicated to a surface logging facility  392  for processing, analysis, and/or storage. The logging facility  1392  may be provided with electronic equipment, such as a controller, for various types of signal processing. The controller  1396  may be coupled to the ranging tool and configured to determine and decouple the total magnetic field to a relative range and direction from the ranging well to the target well. Similar formation evaluation data may be gathered and analyzed during drilling operations (e.g., during LWD/MWD operations, and by extension, sampling while drilling). 
     In some embodiments, the tool body  1320  is suspended in the wellbore by a wireline cable  1374  that connects the tool to a surface control unit (e.g., comprising a workstation  1354 ). The tool may be deployed in the borehole  1312  on coiled tubing, jointed drill pipe, hard wired drill pipe, or any other suitable deployment technique. 
     Referring to  FIG. 14 , it can be seen how a system  1464  may also form a portion of a drilling rig  1402  located at the surface  1404  of a well  1406 . The drilling rig  1402  may provide support for a drillstring  1408 . The drillstring  1408  may operate to penetrate the rotary table  1310  for drilling the borehole  1312  through the subsurface formations  1314 . The drillstring  1408  may include a drill pipe  1418  and a bottom hole assembly  1420 , perhaps located at the lower portion of the drill pipe  1418 . 
     The bottom hole assembly  1420  may include drill collars  1422 , a down hole tool  1424 , and a drill bit  1426 . The drill bit  1426  may operate to create the borehole  1312  by penetrating the surface  1404  and the subsurface formations  1314 . The down hole tool  1424  may comprise any of a number of different types of tools including sensors used to measure magnetic fields, as described previously, MWD tools, LWD tools, and others. The sensors may be used to measure the magnetic fields and relay the information to a controller  1396  that may then control the direction and depth of the drilling operation in order to range to the target well. 
     During drilling operations, the drillstring  1408  (perhaps including the drill pipe  1418  and the bottom hole assembly  1420 ) may be rotated by the rotary table  1310 . Although not shown, in addition to, or alternatively, the bottom hole assembly  1420  may also be rotated by a motor (e.g., a mud motor) that is located down hole. The drill collars  1422  may be used to add weight to the drill bit  1426 . The drill collars  1422  may also operate to stiffen the bottom hole assembly  1420 , allowing the bottom hole assembly  1420  to transfer the added weight to the drill bit  1426 , and in turn, to assist the drill bit  1426  in penetrating the surface  1404  and subsurface formations  1314 . 
     During drilling operations, a mud pump  1432  may pump drilling fluid (sometimes known by those of ordinary skill in the art as “drilling mud”) from a mud pit  1434  through a hose  1436  into the drill pipe  1418  and down to the drill bit  1426 . The drilling fluid can flow out from the drill bit  1426  and be returned to the surface  1404  through an annular area  440  between the drill pipe  1418  and the sides of the borehole  1312 . The drilling fluid may then be returned to the mud pit  1434 , where such fluid is filtered. In some embodiments, the drilling fluid can be used to cool the drill bit  1426 , as well as to provide lubrication for the drill bit  1426  during drilling operations. Additionally, the drilling fluid may be used to remove subsurface formation cuttings created by operating the drill bit  1426 . 
     The workstation  1354  and the controller  1396  may include modules comprising hardware circuitry, a processor, and/or memory circuits that may store software program modules and objects, and/or firmware, and combinations thereof. The workstation  1354  and controller  1396  may be configured to control the direction and depth of the drilling, by executing instructions, in order to perform ranging from a target well using the method for ranging using unbalanced magnetic fields as described subsequently. For example, the controller  1396  may be configured to determine and decouple the total magnetic field to a relative range and direction from the ranging well to the target well. For example, in some embodiments, such modules may be included in an apparatus and/or system operation simulation package, such as a software electrical signal simulation package, a power usage and distribution simulation package, a power/heat dissipation simulation package, and/or a combination of software and hardware used to simulate the operation of various potential embodiments. 
       FIG. 15  is a flowchart showing an embodiment of a method for ranging between a target well and a ranging well using unbalanced magnetic fields. In block  1501 , a current is injected, by a power supply, downhole through an injection path (e.g., spiral cable). The current may be a direct current or some form of alternating current (e.g., clock signal, sine wave). 
     In block  1503 , a return path is provided for the current. The return path may be the target well casing or another cable. The return path is coupled to the ground of the power supply. 
     In block  1505 , the unbalanced magnetic fields from the injection path and the return path are measured. The measurement may be accomplished from the ranging well during a wireline operation as shown in  FIG. 13  or a MWD/LWD operation as shown in  FIG. 14  and discussed previously. 
     In block  1507 , the total magnetic field is measured. The total magnetic field is received at the sensors with the presence of current in the injection path and the return path. Since the first and the second magnetic fields are unbalanced, the sensors pick up a total magnetic field strong enough to determine the relative distance and direction between the target well and the drilling well. 
     In block  1509 , a relative distance and direction of the ranging well to the target well is determined based on the total magnetic field. The range from the well can then be used to steer the ranging well during the drilling operation. When the magnetic field increases, the ranging well is getting closer to the target well. When the magnetic field decreases, the ranging well is getting farther from the target well. In addition, design of gradient sensors with tri-axial component measurements can be utilized to directly determine the relative distance. 
     Example 1 is a method for ranging between a target well and a ranging well, the method comprising: generating a downhole current through a current injection path, wherein the current injection path generates a first magnetic field; receiving a return current through a return path, wherein the return path generates a second magnetic field, wherein the first and second magnetic fields are unbalanced with respect to each other; and measuring the first and second magnetic fields. 
     In Example 2, the subject matter of Example 1 can further include measuring the total magnetic fields from the first and second magnetic fields. 
     In Example 3, the subject matter of Examples 1-2 can further include decoupling the total magnetic field to a relative distance and direction from the ranging well to the target well. 
     In Example 4, the subject matter of Examples 1-3 can further include the injection path and the return path are exchangeable. 
     In Example 5, the subject matter of Examples 1-4 can further include wherein generating the downhole current through the current injection path comprises generating the downhole current through a spiral cable. 
     In Example 6, the subject matter of Examples 1-5 can further include wherein the spiral cable is coupled to a casing of the target well such that the casing is the return path. 
     In Example 7, the subject matter of Examples 1-6 can further include wherein the spiral cable is located inside or outside of the casing of the target well. 
     In Example 8, the subject matter of Examples 1-7 can further include wherein generating the downhole current through the current injection path comprises generating the downhole current through an intrinsically magnetically shielded cable. 
     Example 9 is a system for ranging between a target well and a ranging well, the system comprising: a current injection path associated with a target well casing, wherein the current injection path is configured to generate a first magnetic field; a return path coupled to the current injection path, wherein the return path is configured to generate a second magnetic field such that the first and second magnetic fields are unbalanced with respect to each other; and the current injection path and current return path are exchangeable. 
     In Example 10, the subject matter of Example 9 can further include wherein the current injection path or the current return path comprises a spiral cable. 
     In Example 11, the subject matter of Examples 9-10 can further include, wherein the spiral cable is embedded in concrete around the exterior of the target well casing. 
     In Example 12, the subject matter of Examples 9-11 can further include, wherein the current injection path or the current return path comprises the target well casing. 
     In Example 13, the subject matter of Examples 9-12 can further include wherein the spiral cable comprises a high permeability mu-metal cable. 
     In Example 14, the subject matter of Examples 9-13 can further include wherein the spiral cable comprises a mu-metal wire wrapped around a conductive core. 
     In Example 15, the subject matter of Examples 9-14 can further include wherein the spiral cable comprises a conductive wire wrapped around a mu-metal core. 
     In Example 16, the subject matter of Examples 9-15 can further include wherein the spiral cable comprises a core having a shape of one of a triangle, a cylinder, or a rectangle. 
     In Example 17, the subject matter of Examples 9-16 can further include wherein the current injection path is a first cable and the return path is a second cable. 
     In Example 18, the subject matter of Examples 9-17 can further include wherein the first cable is a spiral cable and the second cable is a straight cable. 
     In Example 19, the subject matter of Examples 9-18 can further include wherein the current injection path is a spiral cable located inside or outside of the target well casing and the return path is the target well casing. 
     Example 20 is a system comprising: a target well comprising a casing; a power supply coupled to the casing and configured to launch a current downhole through an injection path and receive a return current from a return path, wherein the injection path generates a first magnetic field and the return path generates a second magnetic field that is unbalanced with respect to the first magnetic field; a ranging tool in a ranging well, the ranging tool configured to measure a total field from the first and second unbalanced magnetic fields; and a controller coupled to the ranging tool, the controller configured to determine and decouple the total magnetic field to a relative range and direction from the ranging well to the target well. 
     In Example 21, the subject matter of Example 20 can further include wherein the injection path or the return path comprises a spiral cable that terminates at the casing. 
     In Example 22, the subject matter of Examples 20-21 can further include wherein the power supply is grounded via a well head of the target well or a geological formation disposed proximate thereto. 
     The accompanying drawings that form a part hereof, show by way of illustration, and not of limitation, specific embodiments in which the subject matter may be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled.