Patent Publication Number: US-7588082-B2

Title: Downhole tool position sensing system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 60/701,688, entitled “Toolface Position Sensor and Correction System”, filed Jul. 22, 2005. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   BACKGROUND 
   Drilling a well involves using a drill bit inserted into the ground on a drill string. Also included on the drill string may be various tools for, performing tasks associated with drilling the wellbore. For example, when drilling a well, a drill operator often wishes to deviate a wellbore or control its direction to a given point within a producing formation. This operation is known as directional drilling. One example of this is for a water injection well in an oil field that is generally positioned at the edges of the field and at a low point in that field (or formation). 
   One type of drilling tool for drilling a deviated wellbore is a rotary steerable tool (RST) that controls the direction of a well bore. The RST tool uses an actuator, to manipulate the relative position of an inner sleeve with respect to an outer housing to orient the drill string in the desired drilling direction. The RST tool further includes a “brake” to lock the position of the inner sleeve relative to the outer housing once the desired relative position is obtained. A processor instructs the actuator to move the position of the direction of application of the force on the mandrel. The processor may also be used for determining when the direction of the force applied by the direction controller should be moved. The actuator in the outer housing may move the inner sleeve using a drive train with a very high gear ratio, for example 10,000:1. To determine the relative orientation of the inner sleeve to the outer housing, the RST tool uses the rotation of the motor and a known initial orientation of the inner sleeve to the outer housing to determine a “motor” reference position. As the motor turns, it energizes reference poles. The RST tool monitors and processes the energization of the reference poles, or “clicks”, to resolve the magnitude and direction the motor has turned. The RST tool uses the motor travel information, in addition to the known gear ratio between the inner sleeve and the actuator, to determine the position of the inner sleeve relative to the outer housing at any given time. 
   One issue that may occur is the ability of the RST tool to process the “clicks” of the motor reference poles. If an excessive external force is applied to the outer housing, the brake is designed to slip, which results in the motor and its drive train turning in that direction. Because the gearing ratio back to the motor may be over 10,000 to 1, the speed at which the end of the motor is spinning may create “clicks” faster than the processor may be able to process. Thus, the processor may miscount the number of “clicks”, resulting in the calculated versus actual position on the inner sleeve relative to the outer housing being out of sync. 
   Other types of downhole tools may also be included on the drill string. Additionally, other types of downhole tools may be comprised of a mandrel, an inner sleeve, and an outer housing. Still further, other downhole tools may include the use of a magnet on the inner sleeve as a “home position” and a magnetic sensor on the outer housing that detects the magnetic field of the magnet as it rotates relative to the sensor. However, such systems may only determine one position of the inner sleeve relative to the outer housing. Any positions other than the “home position” may not be detected. Additionally, a problem might arise if the magnetic sensor does not detect the magnet and the magnet never rotates past the sensor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed description of the embodiments, reference will now be made to the following accompanying drawings: 
       FIG. 1  is a cutaway side elevation view of a downhole tool in an inclined wellbore; 
       FIG. 2  is a side elevation view of the downhole tool of  FIG. 1 ; 
       FIG. 3  is a cross section view of the downhole tool of  FIGS. 1 and 2  taken at  3 - 3 ; 
       FIG. 4  illustrates a drive coupled to the inner sleeve of the downhole tool powered by a motor; 
       FIG. 5A  is a simplified perspective view of the inner sleeve of the downhole tool of  FIG. 1 ; 
       FIG. 5B  is a simplified perspective view of an alternative inner sleeve of the downhole tool of  FIG. 1 ; 
       FIG. 6  is an example output signal of a linear magnetic sensor for use with the downhole tool of  FIG. 1 ; 
       FIG. 7  are example output combinations for dual linear magnetic sensors for use in the downhole tool of  FIG. 5B ; 
       FIG. 8  is an exploded perspective view of an example electronics system for use with the downhole tools of  FIGS. 1-7 ; 
       FIG. 8A  is an elevation view of an example surface electronics system for use with the downhole tools of  FIGS. 1-7 ; and 
       FIG. 9  is an example linear signal output graph for two magnetic sensors illustrating signal threshold processing. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   In the drawings and description that follows, lice parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawing figures are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. The present invention is susceptible to embodiments of different forms. Specific embodiments are described in detail and are shown in the drawings, with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. Any use of any form of the terms “connect”, “engage”, “couple”, “attach”, or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the elements and may also include indirect interaction between the elements described. The various characteristics mentioned above, as well as other features and characteristics described in more detail below, will be readily apparent to those skilled in the art upon reading the following detailed description of the embodiments, and by referring to the accompanying drawings. 
   Referring initially to  FIGS. 1-4 , there is shown a downhole tool  10  in the form of an RST tool for directional drilling shown in an inclined wellbore.  FIG. 1  illustrates the low-side  2   a  of the wellbore  2 , defined as the side of the wellbore nearest the center of the earth. The low-side  2   a  is on the left-hand side of the overall wellbore  2 . 
   The downhole tool  10  is shown attached to an upper adapter sub  4 , which would in turn be attached to a drill string (not shown). The adapter sub  4  is located at the upper end of the downhole tool  10 , i.e. the end of the downhole tool  10  which is closest to the opening of wellbore  2 . The adapter sub is attached to an inner rotatable mandrel  11 . For the purposes of this description, the relative terms upper and lower are defined with respect to the wellbore  2 , the upper end of the wellbore  2  being the open end, the lower end being the drilling face. 
   The adapter sub  4  serves to connect the drill string to the inner rotatable mandrel  11 . However, the adapter sub  4  may not be necessary if the drill string pipe threads match the downhole tool  10  threads. 
   The mandrel  11  has an elongate central part  11   a  that extends almost the whole length of the tool  10 . At either end, the central part of the mandrel  11   a  is connected to an upper mandrel section  11   b  and a lower mandrel section  11   c . The upper part  11   b  of the mandrel  11  is attached to upper adapter sub  4 . The lower part  11   c  of the mandrel  11  is attached directly to a drill bit  7 . In practice a lower adapter sub may be located between the mandrel and drill bit  7  if the threads differ between the mandrel  11  and drill bit  7 . The lower part  11   c  also need mot be connected directly to the drill bit  7 , but may be connected to additional drill string or other downhole tools, such as a mud motor. 
   An inner sleeve  12  is located about at least a portion of the mandrel  11  and has an eccentric bore. The mandrel  11  is free to rotate within the inner sleeve  12 . In practice, bearing surfaces may be present between the mandrel  11  and the inner sleeve  12  to allow rotation of the mandrel  11 . The inner sleeve  12  of the example has two parts, an upper part  12   a  and a lower part  12   d . In the downhole tool  10  of  FIG. 1 , both the upper part  12   a  and the lower part  12   d  have an eccentric bore for receiving the mandrel  11 . The upper part  12   a  is located close to the top end of the downhole tool  10  and the lower part  12   d  is located towards the lower part of the downhole tool  10 . The upper and lower parts of the inner sleeve  12  are spaced apart from one another along the length of the mandrel  11 . However, it should be appreciated that inner sleeve  12  may be one part surrounding at least a portion of the length of the mandrel  11 . 
   The downhole tool  10  also includes an outer housing  13 . In the example of  FIG. 1 , the outer housing  13  houses the middle part  11   a  of the mandrel  11 . The upper  12   a  and lower  12   d  pails of the inner sleeve are located at the upper and lower ends of the housing  13  respectively, such that the housing  13  only covers a portion of each of the upper and lower parts of the inner sleeve  12   a ,  12   d . The inner sleeve  12  may be turned freely within an area, by a drive means (not shown), inside the outer housing. The outer housing  13  may be eccentric on its outside, resulting in a “heavier” side. This heavier side of the outer housing  13  is referred to as the “biasing portion”  20 . 
   The biasing portion  20  of the outer housing  13  forms the heavy side of the outer housing  13  and may be manufactured as a part of the outer housing  13 . The outer housing  13  is freely rotatable under gravity such that the biasing portion  20  will bias itself toward the low side of the wellbore  2 . In operation, the position of the inner sleeve  12  is manipulated with respect to the position of the biasing portion  20  of the outer housing. Therefore, the inner sleeve  11  is moveable with respect to the outer housing  13 . 
     FIG. 2  is external view of the downhole tool  10  without the upper adapter sub  4  or drill bit  7 . The upper and lower parts  11   b  and  11   c  of the mandrel are respectively located at the top and bottom of the downhole tool  10 . Adjacent the upper and lower parts  11   b  and  11   c  of the mandrel  11  are located the upper and lower parts  12   a  and  12   d  of the inner sleeve  12 . Viewed from the outside, the outer housing  13  is located between the upper  12   a  and lower  12   d  parts of the inner sleeve  12 . As explained with reference to  FIG. 1 , the upper and lower parts of the inner sleeve  12  are partially located within the housing  13 . 
   Stabilizer blades  21  are located on the outside of the outer housing  13 . In this particular example, three stabilizer blades  21  are located around the circumference of the outer housing  13 . The stabilizer blades  21  may be elongate and aligned parallel with the rotation axis of the downhole tool  10 . The stabilizer blades  21  may also be positioned at 90 degree intervals from one another. As there are only three stabilizer blades shown in the example of  FIG. 2 , the stabilizer blades  21  do not extend around the entire circumference of the outer housing  13 . The stabilizer blades  21  are arranged so that there is a first blade 180 degrees away from the biased portion  20 , with two stabilizer blades  21  positioned on either side of the first stabilizer blade  21 . The stabilizer blades  21  serve to counter any reactionary rotation on the part of the outer housing  13  caused by bearing friction between the rotating mandrel  11  and the inner sleeve  12  and to center the outer housing  13  within the borehole  2 . Three secondary stabilizer blades  14  are located around the lower part  11   c  of mandrel  11 . These stabilizer blades  14  may be arranged symmetrically around the circumference of the mandrel  11  with 120 degrees between each stabilizer blade  14 . 
     FIG. 2  shows the principle axis of wellbore  2  as C/L W  and the rotation axis of the bit (or drill string) as C/L D . The rotation axis of the drill string and the principle axis of the wellbore  2  will not always be parallel to one another, as when the downhole tool  10  effects a change in the desired drilling direction. The rotation axis and the principle axis are offset by the eccentricity of the inner sleeve  12  in  FIG. 2 . 
     FIG. 3  shows a cross section of the downhole tool  10  through line  3 - 3  of  FIG. 2 . In  FIG. 3 , the biased portion  20  of the outer housing  13  locates itself at the low side of the wellbore  2 . The stabilizer blades  21  located on the circumference of the outer housing  13  are arranged such that the middle stabilizer blade  21  is located against the high side of the wellbore  2  with the other two stabilizer blades  21  located on the right and left sides of the wellbore  2 . The inner sleeve  12  is located within the bore of the outer housing  13 . Previously, the inner sleeve  12  has been described in terms of two parts, an upper  12   a  and a lower part  12   d.    FIG. 3  just shows the upper part  12   a  of the inner sleeve  12  shown in the example of  FIG. 1 . However, it will be appreciated by those skilled in the art that the lower part  12   d  of the sleeve  12  could also be used in this cross section. The inner sleeve  12  is eccentrically bored. The mandrel  11 , or more correctly, the central part of the mandrel  11   a  is located within the bore of the inner sleeve  12 . The inner sleeve  12  can be rotated with respect to the biased portion  20  of the outer housing  13  thus changing the force on the mandrel  11 . 
   In  FIG. 4 , the actuator, which may be an electric or hydraulic motor or other means, is located within a cavity  27  within the biased portion  20  of the outer housing  13 . Within this cavity is also located a pinion gear  25  associated with the actuator. The teeth on the pinion gear  25  are capable of inter-engaging with the teeth on the ring gear  26  such that movement of the pinion  25  effects movement of the inner sleeve  12  with respect to the outer housing  13 . The power supply may be provided by a battery that is also located within the biased portion  20  or, the rotation of the mandrel  11  may be used to rotate the pinion  25 . 
   Because the teeth of the ring gear  26  and the pinion  25  interact, the inner sleeve  12  and the outer housing  13  are locked in position with respect to one another once the pinion  25  becomes stationary. The RST tool  10  may further include a “brake” to lock the position of the inner sleeve  12  relative to the outer housing  13  once the desired relative position is obtained. 
   In order to change the drilling direction, the actuator must be actuated and told by how much to move the inner sleeve  12 . Such information may be signaled from an electronics system  40  that includes a processor either included in the downhole tool  10  itself or located on the surface but in communication with the downhole tool  10  through any suitable telemetry means, such a telemetry system that is part of a bottom-hole-assembly that in turn communicates with the surface. Further, as discussed below, the downhole tool  10  includes a method of signaling the surface to confirm the position of the inner sleeve  12  relative to the outer housing  13 . 
   The actuator in the outer housing  13  may move the inner sleeve  12  using a drive train including the ring gear  26  and the pinion  25  having a 10,000:1 gear ratio. Thus, it takes 10,000 revolutions of the actuator/pinion  25  to rotate the ring gear  26 /inner sleeve  12  one complete rotation. 
   Referring now to  FIGS. 5A-9 , the RST tool  10  operation thus uses the known orientation of the outer housing  13  and the relative orientation of the inner sleeve  12  to the outer housing  13  to control the drilling direction. To verify the relative orientation of the inner sleeve  12  to the outer housing  13 , the RST tool  10  uses a magnetic position sensing system. As illustrated in  FIGS. 5A and 5B , the magnetic position sensing system includes more than one selected positions  42  spaced around the outer surface of the inner sleeve  12  and organized in at least one “set”. Each set includes at least one selected position  42  placed about a given plane of the inner sleeve  12 . Each of the selected positions  42  includes either a magnet with a North pole orientation  44 , a magnet with a South pole orientation  46 , or no magnet at all. At least two of the selected positions  42  include either North or South pole magnets  44 ,  46 , whether they be in one set or more than one set. The magnetic flux of each of the North and South pole magnets  44 ,  46  is sufficient to overcome the Earth&#39;s ambient magnetic field. 
   The magnetic position sensing system also includes at least one magnetic sensor  48  for each corresponding set of selected positions  42 . The magnetic sensor  48  is capable of sensing at least one of the amplitude and polarity of the magnetic field for the selected positions  42 . For example, the magnetic sensor(s)  48  may be a linear, bipolar Hall Effect sensors. As a further example, more than one magnetic sensor  48  may be used where the magnetic sensors  48  are all non-bipolar, all bipolar, or a combination of bipolar and non-bipolar sensors. The magnetic sensor(s)  48  may be located in the outer housing  13  and may be situated in a stainless steel or other magnetically transparent pressure vessel such that the magnetic sensor(s)  48  is(are) isolated from the borehole pressure. As such, there will be material between the magnetic sensor(s)  48  and the North and South pole magnets  44 ,  46  located on the inner sleeve  12 . This intervening material should, as far as possible, be magnetically transparent. In other words, the magnetic field should pass through this material without becoming deflected or distorted. Materials that exhibit these properties include austenitic stainless steels and other non-ferrous material. 
   As illustrated in  FIG. 8 , the magnetic sensor(s)  48  is/are in communication with the electronics system  40  and transmit a signal indicative of the sensed magnetic field. As illustrated, the electronics system  40  is located in the downhole tool  10  itself. As mentioned previously, however, the electronics system  40  may also be located on the surface and be in communication with the downhole tool  10  through any suitable telemetry system as illustrated in  FIG. 8A . 
   As illustrated in  FIG. 6 , the downhole tool  10  includes an electronics system  40  for processing the sensor signal to determine a “magnet” reference position of the inner sleeve relative to the outer housing. As the inner sleeve  12  rotates relative to the outer housing  13 , the North and South pole magnets  44 ,  46  pass by the magnetic sensor(s)  48 . Each magnetic sensor  48  then produces a signal corresponding to at least one of the amplitude and orientation of the sensed magnetic field. If the magnetic sensor  48  is bipolar, as a North pole magnet  44  passes by the magnetic sensor  48 , the magnetic sensor  48  signal amplitude increases in the North pole direction and then returns to baseline, which is indicative of the naturally occurring magnetic field without the affect of a North or South pole magnet  44 ,  46 . As a South pole magnetic field is sensed by a passing South pole magnet  46 , the amplitude of the signal increases in the South pole direction and then returns to baseline. If the magnetic sensor  48  only senses the amplitude of the magnetic field, then the signal will still increase with an increase in magnetic flux, but will only increase in one direction, not indicating polarity. With the location of the selected positions  42  known, the electronics system  40  then processes this signal to determine the position of the inner sleeve  12  relative to the outer housing  13  as the inner sleeve  12  rotates with respect to the outer housing  13 . The selected positions  42  may be uniformly or non-uniformly spaced about the inner sleeve  12 . The magnetic signal thus presents a coding for an operating logic that the electronics system  40  uses to process the signal and determine the position of the inner sleeve  12  relative to the outer housing  13 . For example, the selected positions may be spaced 180 degrees apart in the example of  FIG. 5A  and include a North pole magnet  44  at one selected position  42  and a South pole magnet  46  at the other selected position  42 . For such an example, the following coding would result: 
                   TABLE 1                  Sensor/Magnet Coding from FIG. 5A                         Toolface   Magnet Sensor   Output Voltage                                     0       +1   1.50       180   degrees   −1   3.50                    
As shown, there are only two positions because only two positions may actually be sensed. A “null” selected position  42  (where there is no magnet) will produce the same magnetic signal as when sensing a non-selected position with no magnet and so may not be used to give a positive indication of position.
 
   As discussed and as illustrated in  FIG. 5B , the downhole tool  10  may also include more than one set of selected positions  42  on the outer surface of the inner sleeve  12 . Again, each selected position may include either a North pole oriented magnet  44 , a South pole oriented magnet  46 , or no magnet. In the example shown in  FIG. 5B , for each set of selected positions, there is a corresponding bipolar magnetic sensor  48  capable of sensing the amplitude and polarity of the magnetic field for the selected positions  42 . The electronics system  40  processes the signals from the magnetic sensors  48  according to the possible signal combinations from the sensors  48  as illustrated in  FIG. 7 . Or, in tabular form, the resulting coding is as follows: 
                   TABLE 2                  Sensor/Magnet Combinations                             Toolface   Magnet Sensor 1   Magnet Sensor 2   Output Voltage                                         0       0   +1   0.50       45   Right   +1   +1   1.00       90   Right   +1   0   1.50       135   Right   +1   −1   2.00       180       0   −1   2.50       135   Left   −1   −1   3.00       90   Left   −1   0   3.50       45   L   −1   +1   4.00                    
As illustrated, the selected positions  42  are uniformly spaced. However, it should be appreciated that the selected positions  42  may also not be uniformly spaced. As can be shown from Tables 1 and 2, because there are only three possibilities for the magnet orientations (North, South, or no magnet), the total number of selected positions detectable for a given sensor/magnet configuration is the number of sensor states to the power of the number of sensors, minus one. Thus, for the example shown in  FIG. 5B , there are two bipolar sensors  48 , each having three sensor states so the total number of possible selected positions is three squared minus one, or eight as shown in Table 2.
 
   As illustrated in  FIG. 9 , sensor signal thresholds may also be set that negate the effect of the Earth&#39;s magnetic field and that serve as limit switches. These limit switches may be employed as a means of logic control within the electronics system  40 . For example, if the magnetic sensors  48  are not exactly aligned, or the selected positions of each set of selected positions are not exactly aligned, the magnetic sensors ( 48 ) may prematurely signal a North pole/no magnet combination, when in fact, the inner sleeve  12  is only a small degree of rotation away from a North pole/North pole combination. Therefore, the electronics system  40  only processes the sensor signals if the amplitude of at least one signal is greater than a first selected threshold  50 , or trigger threshold. Once at least one signal rises above the first selected threshold  50 , the electronics system  40  then processes that signal and drops the signal threshold for all the magnetic sensor signals to a second selected threshold  52 , where the second selected threshold is lower than the first selected threshold  50 . Likewise, the electronics system  40  must also determine when to return to the decreased processing mode. Thus, once the electronics system  40  determines that any magnetic signal drops below the second selected threshold, the electronics system  40  stops processing all of the signals from the magnetic sensors  48 . The electronics system  40  then raises the threshold back up to the first selected threshold  50  for triggering the processing the next time a magnetic signal rises above the trigger threshold  50 . 
   Alternatively, the magnetic position sensing system illustrated in  FIGS. 1-9  may also be used in cooperation with a motor reference position sensing system as previously discussed. As discussed the motor is used to move the inner sleeve  12  relative to the outer housing  13 . The motor energizes reference poles as the motor rotates relative to the reference poles, the energization of a reference pole transmitting a signal, or “click”. The electronics system  40  may also be capable of processing the “clicks” from the energization of the reference poles for determining a “motor” reference position of the inner sleeve  12  relative to the outer housing  13 . The electronics system  40  may also be capable of comparing the “motor” reference position of the inner sleeve  12  relative to the outer housing  13  with the “magnet” reference position determined from the processing of the signals from the magnetic sensors  48 . As previously discussed, the “motor” reference system, while possibly being more precise, has the potential to have the “motor” reference position to be out of sync with the actual relative position of the inner sleeve  12  relative to the outer housing  13 . If the “magnet” reference position differs front the “motor” reference position by more than a selected amount, the electronics system  40  may then “reset” the “motor” reference position to be that of the “magnet” reference position. The “motor” reference position system may then continue to monitor the position of the inner sleeve  12  relative to the outer housing  13  as previously described. This combination provides redundancy to the determination of the position of the inner sleeve  12  relative to the outer housing in case of failure of one of the measuring systems. The combination also provides the potentially more accurate position determination of the “motor” reference system with the reliability of the “magnet” reference system. 
   While specific embodiments have been shown and described, modifications can be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments as described are exemplary only and are not limiting. Many variations and modifications are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.