Patent Publication Number: US-9404354-B2

Title: Closed loop well twinning methods

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     None. 
     FIELD OF THE INVENTION 
     Disclosed embodiments relate generally to methods for drilling subterranean wellbores and more particularly to closed loop methods for twinning subterranean wellbores. 
     BACKGROUND INFORMATION 
     In various well drilling operations it is desirable to estimate the location of a nearby wellbore. Examples of such operations include well intercept, well avoidance, well twinning, and relief well drilling operations. 
     Both passive and active magnetic ranging techniques are known in the oil field services industry. For example, U.S. Pat. Nos. 6,985,814 and 7,656,161 to McElhinney, disclose passive ranging methodologies for use in well twinning applications. The &#39;814 patent makes use of remanent magnetization in a target well casing string while the &#39;161 patent teaches a method for magnetizing the target well casing string prior to deployment in the target well. 
     U.S. Pat. No. 7,812,610 to Clark teaches a methodology in which a secondary electrical current is induced in the target wellbore casing string, e.g., via inducing a voltage across an insulative gap in the drill string located in the drilling wellbore. The secondary current in the target wellbore casing string further induces a magnetic field that may be measured in the drilling wellbore and used to estimate the location of the target. However, the need to stop drilling and make magnetic field measurements at three or more tool face angles can result in a time consuming drilling process. Further improvement is required. 
     SUMMARY 
     Closed loop methods for drilling a twin well are disclosed. The methods include rotary drilling the twin well with a drill string including a rotary steerable tool. An electrical current is induced in the target well while rotary drilling the twin well. The current may be induced in the target well, for example, by applying a voltage across an insulating gap in the BHA. The induced electrical current in turn induces a magnetic field about the target well that may be measured in the twin well. The measured magnetic field is processed while rotary drilling to obtain new rotary steerable tool settings which may be applied to change the drilling direction. 
     The disclosed embodiments may provide various technical advantages. For example, the disclosed methods may be used to steer the twin well automatically along a predetermined path with respect to the target well. No surface intervention is required. Such closed loop methods may therefore improve the efficiency of the drilling operation and significantly reduce the total time required to drill the twin well. The disclosed methods may further improve placement accuracy of the twin well with respect to the target well as the steering tool settings may be adjusted continually while drilling (e.g., at approximately 10 second intervals while drilling). 
     This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the disclosed subject matter, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts one example of a well twinning operation in which disclosed methods may be utilized. 
         FIG. 2  depicts a flow chart of one disclosed method embodiment. 
         FIG. 3  depicts a flow chart of another disclosed method embodiment. 
         FIG. 4  depicts one example of a method for computing a steering vector. 
         FIG. 5A  depicts a flow chart of yet another disclosed method embodiment. 
         FIG. 5B  depicts an example of a magnetic field power spectrum obtained while using the method shown on  FIG. 5A . 
         FIG. 6A  depicts a flow chart of still another disclosed method embodiment. 
         FIG. 6B  depicts another example of a magnetic field spectrum obtained while using the method shown on  FIG. 5B . 
         FIGS. 7A, 7B, and 7C  depict an example of the method embodiment show on  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts one example of a well twinning operation in which a twin well  20  is being drilled along a direction that is approximately parallel with a cased target well  40 . The bottom hole assembly (BHA) in the twin well  20  (also referred to herein as the drilling well) includes a drill bit  22  deployed below a rotary steerable tool  24 . In the example twinning operation depicted, the BHA further includes an electrical current generating tool  30  and a measurement while drilling (MWD) tool  26  including a magnetic field sensor  28 , for example, including a tri-axial magnetometer set. In the depicted embodiment, the MWD tool (and therefore sensor  28 ) is rotationally coupled with the drill string such that it rotates with the drill bit. The MWD tool  26  is further depicted as being deployed just above the drill bit  22 . In alternative embodiments, the magnetic field sensors may be deployed in the rotary steerable tool or higher up in the BHA (e.g., above the current generating tool  30 . The disclosed embodiments are not limited in this regard. 
     The electric current generating tool  30  may be a component of the MWD tool, such as in Schlumberger&#39;s E-Pulse or E-Pulse Express tool, or may be a stand alone tool. In the depicted embodiment, the electric current generating tool  30  includes an electrically insulating gap  32  across which a voltage may be applied to cause electric current  34  to flow along the length of the drill collar. It should be understood that the electric current generating tool  30  may use substantially any power supply configuration capable of generating the current  34  in the drill collar. The applied voltage may be an alternating (AC) voltage operating, for example, in a frequency range from about 0.1 to about 20 Hz. 
     When the twin well  40  is in close proximity with the target well  20  (e.g., within about 10 meters), a corresponding electric current may be induced in the target well. For example, in the depicted embodiment, applying a voltage across the insulating gap  32  causes electrical current to flow out into the formation to the target well  40 . The electrically conductive casing  42  in the target well  40  provides a path of low resistance which may support an axial current  36  in the target. This current  36  in the target well  40  in turn induces a magnetic field  38  in the formation that is proportional in strength to the magnitude of the current  36 . As described in more detail below, measurement of the magnetic field at magnetic field sensor  28  may enable a displacement vector including a distance and direction from the twin well to the target well to be computed. 
     It will be understood by those of ordinary skill in the art that the deployment depicted on  FIG. 1  is merely an example for the purpose of describing the disclosed embodiments set forth herein. For example, the disclosed method embodiments are not limited to the use of an electric current generating tool including an insulating gap. In other embodiments a toroid deployed about the drill string or an electromagnetic antenna may alternatively be used to induce an electric current in the target well casing. An induction device such as disclosed in U.S. Patent Publication 2012/0109527 may also be utilized. 
       FIG. 1  further includes a diagrammatic representation of a tri-axial magnetometer sensor set. By tri-axial it is meant that the magnetic field sensor includes three mutually perpendicular magnetic field sensors, designated as B x , B y , and B z . By convention, a right handed system is designated in which the z-axis magnetometer (B z ) is oriented substantially parallel with the borehole in the downhole direction as indicated (although disclosed embodiments are not limited by such conventions). The magnetometer set may therefore be considered as determining a plane (the x and y-axes) and a pole (the z-axis along the axis of the BHA). By convention, the magnetic field is taken to be positive pointing towards magnetic north. Moreover, also by convention, the y-axis is taken to be the toolface reference axis (i.e., magnetic toolface M equals zero when the y-axis is pointing towards the projection of magnetic north in the xy plane). Those of ordinary skill in the art will readily appreciate that the magnetic toolface M is projected in the xy plane and may be represented mathematically as: tan M=B x /B y . 
     It will be understood that the magnetometer set  28  is not necessarily deployed in MWD tool  26 , but may alternatively and/or additionally be deployed in the rotary steerable tool  24 . It will also be understood that the disclosed embodiments are not limited to the above described conventions for defining borehole coordinates. Those of ordinary skill in the art will be readily able to utilize other borehole coordinate conventions. Moreover, the disclosed embodiments are not limited to use with an offshore drilling rig as depicted. 
       FIG. 2  depicts a flow chart of one example of a method  100  for closed loop drilling of a twin well (such as that depicted on  FIG. 1 ). The twin well is rotary drilled at  102  using a drill string including a rotary steerable tool. Such rotary drilling may include circulating drilling fluid through the drill string, rotating the drill string at the surface using a top drive, rotary table, or other suitable drilling rig equipment, and advancing the drill string into the borehole as required by the rate of penetration of the subterranean formation. In the disclosed embodiments, a rotary steerable tool is used to control the direction of drilling of the twin well, e.g., via steering the drill bit while drilling. As is known to those of ordinary skill in the art, adjustment of various rotary steerable tool parameters enables the drilling direction to be changed in a predictable and controllable manner while drilling. 
     At  104  an electrical current is induced in the target well, for example via applying a voltage across an insulating gap in the BHA as described above with respect to  FIG. 1 . The induced current in turn induces a magnetic field that is measured at multiple tool face angles while the BHA rotates in the twin well at  106 . This may be accomplished for example, by measuring the magnetic field substantially continuously while drilling (e.g., at 10 millisecond intervals while drilling). The magnetic field measurements (at the multiple tool face angles) may then be used to compute new rotary steerable tool settings at  108 . For example, the magnetic field measurements may be used to compute a displacement vector (a distance and direction) between the twin and target wells which may in turn be compared with a desired displacement vector to obtain a steering vector, which may then by used to compute (or look up) the new settings. The new rotary steerable tool settings may alternatively be obtained derived directly from the magnetic field measurements, e.g., via an onboard look up table. The rotary steerable tool settings may then be adjusted as required at  110  while rotary drilling continues at  102 . 
     It will be understood that substantially any suitable rotary steerable tool may be used in the disclosed method embodiments. Various rotary steerable tool configurations are known in the art. For example, the PathMaker® rotary steerable system (available from PathFinder® a Schlumberger Company), the AutoTrak® rotary steerable system (available from Baker Hughes), and the GeoPilot® rotary steerable system (available from Sperry Drilling Services) include a substantially non-rotating outer housing employing blades that engage the borehole wall. Engagement of the blades with the borehole wall is intended to eccenter the tool body, thereby pointing or pushing the drill bit in a desired direction while drilling. A rotating shaft deployed in the outer housing transfers rotary power and axial weight-on-bit to the drill bit during drilling. Accelerometer and magnetometer sets may be deployed in the outer housing and therefore are non-rotating or rotate slowly with respect to the borehole wall. 
     The PowerDrive® rotary steerable systems (available from Schlumberger) fully rotate with the drill string (i.e., the outer housing rotates with the drill string). The PowerDrive® Xceed® makes use of an internal steering mechanism that does not require contact with the borehole wall and enables the tool body to fully rotate with the drill string. The PowerDrive® X5 and X6 rotary steerable systems make use of mud actuated blades (or pads) that contact the borehole wall. The extension of the blades (or pads) is rapidly and continually adjusted as the system rotates in the borehole. The PowerDrive® Archer® makes use of a lower steering section joined at a swivel with an upper section. The swivel is actively tilted via pistons so as to change the angle of the lower section with respect to the upper section and maintain a desired drilling direction as the bottom hole assembly rotates in the borehole. Accelerometer and magnetometer sets may rotate with the drill string or may alternatively be deployed in an internal roll-stabilized housing such that they remain substantially stationary (in a bias phase) or rotate slowly with respect to the borehole (in a neutral phase). To drill a desired curvature, the bias phase and neutral phase are alternated during drilling at a predetermined ratio (referred to as the steering ratio). 
       FIG. 3  depicts a flow chart of another example of a method  120  for closed loop drilling of a twin well. Method  120  is intended for use with a rotary steerable tool including a substantially non-rotating (or slowly rotating) outer blade housing. The magnetic field sensors are deployed in the blade housing and are therefore non-rotating (or slowly rotating) with respect to the borehole wall. 
     The twin well is rotary drilled at  122 . The rotary drilling operation may include circulating drilling fluid through the drill string, rotating the drill string at the surface, and advancing the drill string into the borehole as described above with respect to  FIG. 2 . In the disclosed embodiments, the rotary steerable tool is used to control the direction of drilling of the twin well. 
     At  124  an electrical current is induced in the target well, for example, via applying a voltage across an insulating gap in the twin well BHA as described above with respect to  FIG. 1 . The applied voltage may be an AC voltage, for example, having a frequency of about 10 Hz. The induced current in the target well in turn induces a magnetic field that is measured at  126  while rotary drilling continues. A band pass or high pass filter may optionally be applied to the magnetic field measurements at  128  to remove the earth&#39;s magnetic field (which is typically near DC). After a number of magnetic field measurements have been acquired, the measurements may be evaluated at  130  and  132  to determine whether or not at least three measurements have been obtained in a range of toolface angles greater than 180 degrees. If the housing in which the sensors are deployed has rotated at least 180 degrees then new rotary steerable tool settings may be computed at  134 . If not, then the method returns to  124  and makes additional magnetic field measurement(s). 
     In order to facilitate the acquisition of magnetic field measurements over a range of toolface angles, the rotary steerable tool may be controlled in a manner that permits slow rotation of the outer blade housing in the borehole. For example, the pressure (force) applied by at least one of the blades against the borehole wall may be sufficiently low so as to allow the housing to slowly rotate (e.g., at a rotation rate in a range from about 0.5 to about 5 RPM). U.S. Pat. No. 7,950,473, which is fully incorporated by reference herein, discloses techniques for controlling the rotation rate of the blade housing in a rotary steerable tool. 
     Computing new rotary steerable tool settings may include first computing a displacement vector (i.e., a distance and direction) between the twin well and the target well. The displacement vector may be used to determine a steering vector as described in more detail below with respect to  FIG. 4 . Alternatively, the new rotary steerable tool settings steering vector may be computed directly from the magnetic field measurements, for example, via processing measured magnetic field in combination with a look-up table to obtain new steering tool settings. The new rotary steerable tool settings may also be obtained directly from the displacement vector (e.g., via the use of a corresponding look-up table). 
     It will be understood that the induced magnetic field includes distorted and undistorted signal components and at least one noise component. The undistorted signal component is related to the induced magnetic field in the target well (and therefore to the relative position of the twin well with respect to the target well). The distorted signal component being is caused by distortion of the induced magnetic field by rotation of the magnetically permeable BHA. The noise component may result, for example, from the earth&#39;s magnetic field. In order to compute the displacement vector or the steering vector, the undistorted signal portion of the measured magnetic field may be isolated from the other components (i.e., the undistorted signal may be isolated from the distorted signal and from the earth&#39;s magnetic field). This may be accomplished, for example, via (i) obtaining three or more magnetic field measurements made over a range of toolface angles greater then 180 degrees, (ii) averaging the three or more measurements to obtain an average induced magnetic field (which may be taken to be the undistorted signal component), and (iii) estimating the distance and direction to the target well from the average induced magnetic field. In one embodiment, the three or more magnetic field measurements may be selected such that they are spaced at approximately equal tool face intervals (e.g., at approximately 120 degree intervals for three measurements, at approximately 90 degree intervals for four measurements, at approximately 60 degree intervals for six measurements, and so on). 
     The displacement vector between the twin well and the target well may be obtained from the undistorted signal component of the measured magnetic field vector. The magnitude of the measured magnetic field tends to be inversely related to the distance between the twin and target wells such that the magnitude increases with decreasing distance. The direction of the measured magnetic field vector indicates the relative direction between the twin and target wells. A displacement vector indicating the distance and direction between the two wells may be represented in magnetic units, for example, including the magnetic field strength and the direction of the vector or alternatively in spatial units including a physical distance and direction between the wells (e.g., a direction from the twin well to the target well). The displacement vector may be readily converted from magnetic units to spatial units, for example, using empirical or theoretical magnetic models, although such conversions are not required. 
       FIG. 4  depicts a view looking down the axes of the twin  20  and target  40  wells and illustrates one example of a methodology by which a steering vector may be obtained from the displacement vector. The displacement vector between the twin and target wells is shown at  52 .  FIG. 4  further depicts the desired (or planned) location of the twin well  20 ′ (located directly above the target well at a distance ‘d’ in this particular embodiment). A steering vector  54  may be obtained, for example, by subtracting the vector  56  between the desired location  20 ′ of the twin well and the target well from the measured displacement vector  52 . In this particular embodiment, the steering vector represents the displacement between the actual location of the twin well  20  and the desired location of the twin well  20 ′. 
     It will be understood that a one-axis cross-axial magnetic sensor may also be utilized to measure the induced magnetic field in the target well. For example, the one-axis sensor may be rotated one or more revolutions around the tool axis to obtain a peak AC signal direction (e.g., referenced with respect to gravity). The peak AC signal amplitude and direction may then be taken as a magnetic displacement vector and used to obtain the steering vector and/or new rotary steerable tool settings. 
       FIG. 5A  depicts a flowchart of yet another disclosed method embodiment  150 . Method  150  is intended for use with a rotary steerable tool that rotates with the drill string. Method  150  may be used with a rotary steerable tool in which the magnetic field sensors are deployed in a housing that is rotationally coupled with the drill string or alternatively in a roll-stabilized housing. The magnetic field sensors may also be deployed in a separate MWD tool deployed above or below the rotary steerable tool in the BHA. When deployed in a roll-stabilized housing, sensors may be stationary with respect to the borehole or rotate relatively slowly with respect to the borehole (as compared to the rotation rate of the BHA). Method  150  is similar to method  120  in that the twin well is rotary drilled at  152  using a BHA including a rotary steerable tool. The rotary drilling operation may include circulating drilling fluid through the drill string, rotating the drill string at the surface, and advancing the drill string into the borehole as described above with respect to  FIG. 2 . The rotary steerable tool is used to control the direction of drilling of the twin well. 
     At  154  an electrical current is induced in the target well, for example, via applying a voltage across an insulating gap in the twin well BHA as described above with respect to  FIG. 1 . The applied voltage may be an AC voltage, for example, having a frequency of about 10 Hz. The induced current in turn induces a magnetic field that is measured at  156  while rotary drilling continues. Magnetic field measurements may be made at substantially any suitable time interval during drilling (e.g., at 10 millisecond intervals—corresponding to a measurement frequency of 100 Hz). Upon acquiring a large number of measurements (e.g., 1000 measurements made over a 10 second time period or 6000 measurements made over a 60 second time period or some other suitable number of measurements), a band pass filter may be applied to the measurements at  158  to obtain the undistorted signal component of the magnetic field. For example a band pass filter having a narrow pass band around 10 Hz may be utilized when the voltage applied across the insulating gap has a frequency of 10 Hz. Those of ordinary skill in the art will readily be able to design suitable filters for substantially any suitable pass band. The obtained signal component may then be used to compute new rotary steerable tool settings at  160  which may then be applied at  162  to change the direction of drilling the twin well. 
       FIG. 5B  depicts a hypothetical example of a power spectrum of the magnetic field measurements made at  156  (those of ordinary skill in the art will readily appreciate that a power spectrum is a plot of power as a function of frequency). In the depicted embodiment, the applied voltage has a frequency of ω while the rotary steerable tool (including the sensors which may be deployed in the rotary steerable tool or elsewhere in the BHA) rotates with respect to the borehole at a frequency of ω o . Four peaks are indicated in the depicted spectrum. The earth&#39;s magnetic field is indicated at  202  centered at a frequency of ω o . First and second noise peaks (i.e., distorted signal peaks due to distortion of the induced magnetic field caused by rotation of the BHA) are depicted at  204  and  206 . These peaks are centered at corresponding frequencies ω−ω o  and ω+ω o  (i.e., the signal frequency ω modulated by the rotation rate of the BHA ω o ). The undistorted signal peak due to the induced magnetic field is depicted at  208  and shown centered at frequency ω. As described in more detail below with respect to  FIGS. 7A, 7B, and 7C , application of the filter at  158  is intended to remove the earth&#39;s magnetic field  202  as well as the distorted signal peaks  204  and  206  so as to isolate the undistorted signal peak  208 . 
       FIG. 6A  depicts a flowchart of still another disclosed method embodiment  180 . Method  180  is intended for use with a rotary steerable tool in which the magnetic field sensors are deployed in a roll-stabilized housing. Being deployed in a roll-stabilized housing the magnetic field sensors may be non-rotating with respect to the borehole (e.g., in the bias phase) or may rotate slowly with respect to the borehole (e.g., in the neutral phase). The rotation rate in the neutral phase is much less than that of the BHA and other rotary steerable tool components (e.g., in a range from about 1 to about 5 revolutions per minute). For example, in one embodiment the BHA may rotate at 120 revolutions per minute (2 Hz) while the sensors may rotate at −3 revolutions per minute (i.e., in the opposite direction as the BHA). The disclosed embodiments are of course not limited to any particular rotation rates of the BHA and roll-stabilized housing. 
     Method  180  is similar to method  120  in that the twin well is rotary drilled at  182  using a BHA including a rotary steerable tool. The rotary drilling operation may include circulating drilling fluid through the drill string, rotating the drill string at the surface, and advancing the drill string into the borehole as described above with respect to  FIG. 2 . The rotary steerable tool is used to control the direction of drilling of the twin well. In embodiments in which the roll-stabilized housing rotates at a non-zero rate with respect to the borehole, the roll-stabilized housing may initiate rotation at  184 . 
     An electrical current may be induced in the target well at  186 , for example, via applying a voltage across an insulating gap in the twin well BHA as described above with respect to  FIG. 1 . The applied voltage may be an AC voltage, for example, having a frequency of about 10 Hz. The induced current in turn induces a magnetic field that is measured at 188 while rotary drilling continues. As described above, magnetic field measurements may be made at substantially any suitable time interval during drilling (e.g., at 10 millisecond intervals—corresponding to a measurement frequency of 100 Hz). Upon acquiring a large number of measurements (e.g., 1000 measurements made over a 10 second time period), a band pass filter may be applied to the measurements at  190  to obtain (or isolate) the undistorted signal component of the magnetic field. For example a band pass filter having a narrow pass band around 10 Hz may be utilized when the voltage applied across the insulating gap has a frequency of 10 Hz. Those of ordinary skill in the art will readily be able to design suitable filters for substantially any suitable pass band. The obtained undistorted signal component may then be used to compute new rotary steerable tool settings at  192  which may be applied at  194  to change the direction of drilling. 
       FIG. 6B  depicts a hypothetical example of a power spectrum of the magnetic field measurements made at  188  when the tool is in the neutral phase (i.e., when the roll-stabilized housing rotates slowly with respect to the borehole). In the depicted embodiment, the applied voltage has a frequency of ω while the BHA rotates with respect to the borehole at a frequency of ω o . The magnetic field sensors rotate slowly (e.g., at −3 RPM) as compared to the BHA. Four peaks are indicated in the depicted spectrum. The earth&#39;s magnetic field is indicated at  212  and is centered at a near zero frequency owing to the slow rotation rate of the sensors (as compared to the spectrum depicted on  FIG. 5B  in which the earth&#39;s magnetic field is centered at the sensor/BHA rotation rate). First and second noise peaks (distorted signal peaks due to the rotation of the BHA) are depicted at  214  and  216 . These peaks are centered at corresponding frequencies ω−ω o  and ω+ω o  as described above with respect to  FIG. 5B . As depicted, the distorted signal peaks  214  and  216  are somewhat larger than those depicted on  FIG. 5B  at  204  and  206  since the BHA rotates with respect to the sensors in rotary steerable tool embodiments employing a roll-stabilized housing. The undistorted signal peak is depicted at  218  and shown centered at frequency ω. As described in more detail below with respect to  FIGS. 7A, 7B, and 7C , application of the filter at  190  is intended to remove the earth&#39;s magnetic field  212  as well as the noise peaks  214  and  216  so as to isolate the undistorted signal peak  218 . 
       FIGS. 7A, 7B, and 7C  depict one example of the application of method  180 . In this particular example, the BHA rotation rate was 60 revolutions per minute (1 Hz). The rotation rate of the roll-stabilized housing was −3 revolutions per minute. The induced magnetic field had a frequency of 10 Hz.  FIG. 7A  is similar to  FIG. 6B  in that it depicts a plot of the power spectral density of the magnetic field measurements made at  188 . The earth&#39;s magnetic field component is shown at  222  having a center frequency at about 0 Hz. The noise peaks caused by BHA distortion are depicted at  224  and  226  having center frequencies of 9 and 11 Hz (i.e., modulated at frequencies of 10−1 and 10+1 Hz). The signal component is depicted at  228  having a center frequency of 10 Hz.  FIG. 7B  depicts one example of a finite impulse response (FIR) filter having a center frequency of 10 Hz and a bandwidth (i.e., a pass band) of 1 Hz from 9.5 to 10.5 Hz. In the depicted filter embodiment the frequency is normalized such that unity represents 50 Hz (and such that the center frequency of 0.2 corresponds to 10 Hz).  FIG. 7C  depicts the undistorted signal component obtained upon filtering the data depicted on  FIG. 7A  with the FIR filter depicted on  FIG. 7B . The obtained undistorted signal component  228 ′ may be processed as described above to obtain a displacement vector and/or a steering vector. It will be understood that the disclosed embodiments are not limited to the use of an FIR filter. Other types of digital filters (e.g., infinite impulse response filters) and even analog filters may be utilized. 
     The filter (e.g., the FIR filter) may be applied, for example, to the x- and y-axis magnetic field measurements (e.g., at 10 second intervals including 1000 measurements each). In a closed loop well twinning operation, the demand toolface and the steering ratio of the rotary steerable tool (the ratio of the bias and neutral phases) may be automatically adjusted in a closed loop manner based on the magnitudes of the filtered x- and y-axis magnetic field measurements at 10 Hz. For example, a look-up table may be constructed based on a mathematical model and certain steering strategy considerations. The x- and y-axis magnetic field measurements may then be evaluated with the look up table to obtain new steering tool settings (e.g., bias and neutral phase times and ratio). 
     It will be understood that while not shown in  FIG. 1 , BHAs and/or rotary steerable tools suitable for use with the disclosed embodiments generally include at least one electronic controller. Such a controller may include signal processing circuitry including a digital processor (a microprocessor), an analog to digital converter, and processor readable memory. The controller may also include processor-readable or computer-readable program code embodying logic, including instructions for making, processing, and filtering magnetic field measurements. One skilled in the art will also readily recognize the aforementioned filtering operations may be applied using either hardware or software mechanisms. 
     A suitable controller may include a timer including, for example, an incrementing counter, a decrementing time-out counter, or a real-time clock. The controller may further include multiple data storage devices, various sensors, other controllable components, a power supply, and the like. The controller may also optionally communicate with other instruments in the drill string, such as telemetry systems that communicate with the surface or an EM (electro-magnetic) shorthop that enables the two-way communication across a downhole motor. It will be appreciated that the controller is not necessarily located in the rotary steerable tool, but may be disposed elsewhere in the drill string in electronic communication therewith. Moreover, one skilled in the art will readily recognize that the multiple functions described above may be distributed among a number of electronic devices (controllers). 
     In one example embodiment, a closed loop method for drilling a twin well along a predetermined path with respect to a target well, the target well being cased with a metallic liner, the method comprising: (a) rotary drilling the twin well using a drill string including a drill bit, a current generating tool, a rotary steerable tool, and a magnetic field sensor; (b) inducing an electrical current in the target well liner using the current generating tool while rotary drilling in (a), said induced electrical current resulting in a magnetic field about the target well; (c) making a plurality of magnetic field measurements using the magnetic field sensor while rotary drilling in (a); 
     (d) processing the plurality of magnetic field measurements made in (c) to obtain new rotary steerable tool settings; and (e) changing a direction of rotary drilling using the new steering tool settings obtained in (d). 
     Although closed loop well twinning methods and certain advantages thereof have been described in detail, it should be understood that various changes, substitutions and alternations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.