Patent Publication Number: US-2002011358-A1

Title: Steerable drill string

Description:
FIELD OF THE INVENTION  
       [0001] The current invention is directed to an apparatus and method for steering a device through a passage, such as the steering of a drill string during the course of drilling a well.  
       BACKGROUND OF THE INVENTION  
       [0002] In underground drilling, such as gas, oil or geothermal drilling, a bore is drilled through a formation deep in the earth. Such bores are formed by connecting a drill bit to sections of long pipe, referred to as a “drill pipe,” so as to form an assembly commonly referred to as a “drill string” that extends from the surface to the bottom of the bore. The drill bit is rotated so that it advances into the earth, thereby forming the bore. In rotary drilling, the drill bit is rotated by rotating the drill string at the surface. In any event, in order to lubricate the drill bit and flush cuttings from its path, piston operated pumps on the surface pump a high pressure fluid, referred to as “drilling mud,” through an internal passage in the drill string and out through the drill bit. The drilling mud then flows to the surface through the annular passage formed between the drill string and the surface of the bore.  
       [0003] The distal end of a drill string, which includes the drill bit, is referred to as the “bottom hole assembly.” In “measurement while drilling” (MWD) applications, sensors (such as those sensing azimuth, inclination, and tool face) are incorporated in the bottom hole assembly to provide information concerning the direction of the drilling. In a steerable drill string, this information can be used to control the direction in which the drill bit advances.  
       [0004] Various approaches have been suggested for controlling the direction of the drill string as it forms the bore. The direction in which a rotating drill string is headed is dependent on the type of bit, speed of rotation, weight applied to the drill bit, configuration of the bottom hole assembly, and other factors. By varying one or several of these parameters a driller can steer a well to a target. With the wide spread acceptance of steerable systems in the 1980&#39;s a much higher level of control on the direction of the drill string was established. In the steerable system configuration a drilling motor with a bent flex coupling housing provided a natural bend angle to the drill string. The drill bit was rotated by the drilling motor but the drill string was not rotated. As long as the drill string was not rotated, the drill would tend to follow this natural bend angle. The exact hole direction was determined by a curvature calculation involving the bend angle and various touch points between the drill string and the hole. In this manner the bend angle could be oriented to any position and the curvature would be developed. If a straight hole was required both the drill string and the motor were operated which resulted in a straight but oversize hole.  
       [0005] There were several disadvantages to such non-rotating steerable drill strings. During those periods when the drill string is not rotating, the static coefficient of friction between the drill string and the borehole wall prevented steady application of weight to the drill bit. This resulted in a stick slip situation. In addition, the additional force required to push the non-rotating drill string forward caused reduced weight on the bit and drill string buckling problems. Also, the hole cleaned when the drill string is not rotating is not as good as that provided by a rotating drill string. And drilled holes tended to be tortuous.  
       [0006] Rotary steerable systems, where the drill bit can drill a controlled curved hole as the drill string is rotated, can overcome the disadvantages of conventional steerable systems since the drill string will slide easily through the hole and cuttings removal is facilitated.  
       [0007] Therefore it would also be desirable to provide a method and apparatus that permitted controlling the direction of a rotatable drill string.  
       SUMMARY OF THE INVENTION  
       [0008] It is an object of the current invention to provide a method and apparatus that permitted controlling the direction of a rotatable drill string. This and other objects is accomplished in a guidance apparatus for steering a rotatable drill string, comprising A guidance apparatus for steering a rotatable drill string through a bore hole, comprising (i) a housing for incorporation into the drill string, (ii) a movable member mounted in the housing so as to be capable of extending and retracting in the radial direction, the movable member having a distal end projecting from the housing adapted to engage the walls of the bore hole, (iii) a supply of a magnetorheological fluid, (iv) means for pressurizing the magnetorheological fluid, (v) means for supply the pressurized rheological fluid to the movable member, the pressure of the rheological fluid generating a force urging the movable member to extend radially outward, the magnitude of the force being proportional to the pressure of the Theological fluid supplied to the movable member, and (vi) a valve for regulating the pressure of the magnetorheological fluid supplied to the movable member so as to alter the force urging the movable member radially outward, the valve comprising means for subjecting the magnetorheological fluid to a magnetic field so as to change the shear strength thereof. In a preferred embodiment of the invention, the fluid is a magnetorheological fluid and the valve incorporates an electromagnetic for generating a magnetic field.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIG. 1 is a schematic diagram of a drilling operation employing a steerable rotating drill string according to the current invention.  
     [0010]FIG. 2 is a cross-section taken through line II-II shown in FIG. 1 showing the steering of the drill string using a guidance module according to the current invention.  
     [0011]FIG. 3 is a transverse cross-section through the guidance module shown in FIG. 1.  
     [0012]FIG. 4 is a longitudinal cross-section taken through line IV-IV shown in FIG. 3.  
     [0013]FIG. 5 is a view of one of the covers of the guidance module viewed from line V-V shown in FIG. 3.  
     [0014]FIG. 6 is a transverse cross-section through the guidance module taken through line VI-VI shown in FIG. 3.  
     [0015]FIG. 6 a  is a cross-section taken through circular line VIa-VIa shown in FIG. 6 showing the arrangement of the valve and manifold section of the guidance module if it were split axially and laid flat.  
     [0016]FIG. 7 is a transverse cross-section through the guidance module taken through line VII-VII shown in FIG. 3.  
     [0017]FIG. 8 is a transverse cross-section through the guidance module taken through line VIII-VIII shown in FIG. 3.  
     [0018]FIG. 9 is a transverse cross-section through the guidance module taken through line IX-IX shown in FIG. 3 (note that FIG. 9 is viewed in the opposite direction from the cross-sections shown in FIGS.  6 - 8 ).  
     [0019]FIG. 10 is an exploded isometric view, partially in cross-section, of a portion of the guidance module shown in FIG. 3.  
     [0020]FIG. 11 is a longitudinal cross-section through one of the valves shown in FIG. 3.  
     [0021]FIG. 12 is a transverse cross-section through a valve taken along line XII-XII shown in FIG. 11.  
     [0022]FIG. 13 is a schematic diagram of the guidance module control system.  
     [0023]FIG. 14 is a longitudinal cross-section through an alternate embodiment of one of the valves shown in FIG. 3.  
     [0024]FIG. 15 is a transverse cross-section through a valve taken along line XV-XV shown in FIG. 14.  
     [0025]FIG. 16 shows a portion of the drill string shown in FIG. 1 in the vicinity of the guidance module. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0026] A drilling operation according to the current invention is shown in FIG. 1. A drill rig  1  rotates a drill string  6  that, as is conventional, is comprised of a number of interconnected sections. A drill bit  8 , which preferably has side cutting ability as well as straight ahead cutting ability, at the extreme distal end of the drill string  6  advances into an earthen formation  2  so as to form a bore  4 . Pumps  3  direct drilling mud  5  through the drill string  6  to the drill bit  8 . The drilling mud  5  then returns to the surface through the annular passage  130  between the drill string  6  and the bore  4 .  
     [0027] As shown in FIGS. 1 and 2, a guidance module  10  is incorporated into the drill string  6  proximate the drill bit  8  and serves to direct the direction of the drilling. As shown in FIGS. 3 and 4, in the preferred embodiment, the guidance module  10  has three banks of pistons  12  slidably mounted therein spaced at 120° intervals, with each bank of pistons comprising three pistons  12  arranged in an axially extending row. However, a lesser number of piston banks (including only one piston bank) or a greater number of piston banks (such as four piston banks) could also be utilized. In addition, a lesser number of pistons could be utilized in each of the banks (including only one piston per bank), as well as a greater number. Moreover, the piston banks need not be equally spaced around the circumference of the drill string.  
     [0028] Preferably, the pistons  12  are selectively extended and retracted during each rotation of the drill string so as to guide the direction of the drill bit  8 . As shown in FIG. 2, the first bank of pistons  12 ′, which are at the 90° location on the circumference of the bore  4 , are extended, whereas the second and third banks of pistons  12 ″ and  12 ′″, which are at the 210° and 330° locations, respectively, are retracted. As a result, the first bank of pistons  12 ′ exert a force F against the wall of the bore  4  that pushes the drill bit  8  in the opposite direction (i.e., 180° away in the 270° direction). This force changes the direction of the drilling. As shown in FIG. 1, the drill bit is advancing along a curved path toward the 90° direction. However, operation of the pistons  12  as shown in FIG. 2 will cause the drill bit to change its path toward the 270° direction.  
     [0029] Since the drill string  6  rotates at a relatively high speed, the pistons  12  must be extended and retracted in a precise sequence as the drill string rotates in order to allow the pistons to continue to push the drill string in the desired direction (e.g., in the 270° direction). For example, as shown in FIG. 2, after the pistons  12 ′ in the first piston bank reach the 90° location, at which time they are fully extended, they must begin retracting so that they are fully retracted by the time the drill string rotates 120° so as to bring them to the 330° location. The pistons  12 ″ in the second piston bank, however, must begin extending during this same time period so that they are fully extended when they reach the 90° location. The pistons  12 ′″ in the third piston bank remain retracted as the drill string  6  rotates from the 330° location to the 210° location but then begin extending so that they too are fully extended when they reach the 90° location. Since the drill string  6  may rotate at rotational speeds as high as 25° RPM, the sequencing of the pistons  12  must be controlled very rapidly and precisely. According to the current invention, the actuation of the pistons  12  is controlled by magnetorheological valves, as discussed further below.  
     [0030] Alternatively, the guidance module  10  could be located more remotely from the drill bit so that operation of the pistons  12  deflects the drill pipe and adds curvature to the bottom hole assembly, thereby tilting the drill bit. When using this approach, which is sometimes referred to as a “three point system,” the drill bit need not have side cutting ability.  
     [0031] A preferred embodiment of the guidance module  10  is shown in detail in FIGS.  3 - 13 . As shown best in FIGS. 3 and 4, the guidance module  10  comprises a housing  14 , which forms a section of drill pipe for the drill string, around which the three banks of pistons  12  are circumferentially spaced. Each bank of pistons  12  is located within one of three recesses  31  formed in the housing  14 . Each piston  12  has a arcuate distal end for contacting the surface of the bore  4 . However, in some applications, especially larger diameter drill strings, it may be desirable to couple the distal ends of the pistons together with a contact plate that bears against the walls of the bore  4  so that all of the pistons  12  in one bank are ganged together. Each piston  12  has a hollow center that allows it to slide on a cylindrical post  18  projecting radially outward from the center of a piston cylinder  19  formed in the bottom of its recess  31 .  
     [0032] The radially outward movement of the pistons  12  in each piston bank is restrained by a cover  16  that is secured within the recess  31  by screws  32 , shown in FIG. 5. Holes  27  in the cover  16  allows the distal ends of the pistons to project radially outward beyond the cover. In addition, in the preferred embodiment, four helical compression springs  20  are located in radially extending blind holes  21  spaced around the circumference of each piston  12 . The springs  20  press against the cover  16  so as to bias the pistons  12  radially inward. Depending on the magnitude of the force urging the pistons  12  radially outward, which is applied by a magnetorheological fluid as discussed below, the pistons may be either fully extended, fully retracted, or at an intermediate position. Alternatively, the springs  20  could be dispensed with and the magnetorheological fluid relied upon exclusively to extend and retract the pistons  12 .  
     [0033] Three valve manifold recesses  33  are also spaced at 120° intervals around the housing  14  so as to be axially aligned with the recesses  31  for the piston banks but located axially downstream from them. A cover  17 , which is secured to the housing  14  by screws  32 , encloses each of the valve manifold recesses  33 . Each cover  17  forms a chamber  29  between it and the inner surface of its recess  33 . As discussed below, each of the chambers  31  encloses valves and manifolds for one of the piston banks.  
     [0034] According to the current invention, the guidance module  10  contains a supply of a magnetorheological fluid. Magnetorheological fluids are typically comprised of non-colloidal suspensions of ferromagnetic or paramagnetic particles, typically greater than 0.1 micrometers in diameter. The particles are suspended in a carrier fluid, such as mineral oil, water or silicone oil. Under normal conditions, magnetorheological fluids have flow characteristics of a convention oil. However, in the presence of a magnetic field, the particles become polarized so as to be organized into chains of particles within the fluid. The chains of particles act to increase the fluid shear strength or flow resistance of the fluid. When the magnetic field is removed, the particles return to an unorganized state and the fluid shear strength or flow resistance of the fluid returns to its previous value. Thus, the controlled application of a magnetic field allows the fluid shear strength or flow resistance of a magnetorheological fluid to be altered very rapidly. Magnetorheological fluids are described in U.S. Pat. No. 5,382,373 (Carlson et al.), hereby incorporated by reference in its entirety. Suitable magnetorheological for use in the current invention are commercially available from Lord Corporation of Cary, N.C.  
     [0035] A central passage  42  is formed in the housing  14  through which the drilling mud  5  flows. A pump  40 , which may be of the Moineau type, and a directional electronics module  30  are supported within the passage  42 . As shown best in FIGS. 4 and 6, the pump  40  has an outlet  54  that directs the magnetorheological fluid outward through a radially extending passage  74  formed in the housing  14 . From the passage  74 , the magnetorheological fluid enters a supply manifold  62 ′ formed in the chamber  29 ′ that is axially aligned with the bank of pistons  12 ′. Two other supply manifolds  62 ″ and  62 ′″ are formed within the chambers  29 ″ and  29 ′″ so as to be axially aligned with the other two banks of pistons  12 ″ and  12 ′″, respectively. From the supply manifold  62 ′, the magnetorheological fluid is divided into three streams.  
     [0036] As shown in FIG. 4, the first stream flows through opening  66 ′ into tubing  51 ′ and then to a first supply valve  70 ′. As shown in FIGS. 4 and 8, the second stream flows through a circumferentially extending supply passage  78  formed in the housing  14  to the second supply manifold  62 ″. As shown in FIGS. 4 and 6 a , from the supply manifold  62 ″ the second stream of magnetorheological fluid flows through opening  66  into tubing  51 ″ and then to a second supply valve  70 ″. Similarly, the third stream flows through circumferentially extending supply passage  80  to the third supply manifold  62 ′″, then through opening  66 ′″ into tubing  51 ′″ and then to a third supply valve  70 ′″. The supply valves  70  are discussed more fully below.  
     [0037] As shown in FIGS. 4 and 6 a  , sections of tubing  53  are connected to each of the three supply valves  70  and serve to direct the magnetorheological fluid from the supply valves to three axially extending supply passages  22  formed in the housing  14 . Each supply passage  22  extends axially underneath one bank of pistons  12  and then turns 180° to form a return passage  24 , as shown best in FIG. 10. As shown in FIGS. 3 and 4, radial passages  23  direct the magnetorheological fluid from the each of the supply passages  22  to the cylinders  19  in which the pistons  12  associated with the respective bank of pistons slide.  
     [0038] As shown in FIGS. 4 and 6 a  , the return passage  24  for each bank of pistons  12  delivers the magnetorheological fluid to a section of tubing  57  disposed within the chamber  29  associated with that bank of pistons. The tubing  57  directs the fluid to three return valves  71 , one for each bank of pistons  12 . From the return valves  71 , sections of tubing  55  direct the fluid to openings  68  and into three return manifolds  64 . As shown in FIG. 9, passages  79  and  83  direct the fluid from the return manifolds  64 ′ and  64 ′″ to the return manifold  64 ″ so that return manifold  64 ″ receives the fluid from all three piston banks. As shown in FIG. 7, from the return manifold  64 ″, the fluid is directed by passage  76  to the inlet  56  for the pump  40  where it is recirculated to the pistons  12  in a closed loop.  
     [0039] In operation, the pressure of the rheological fluid supplied to the cylinders  19  for each bank of pistons  12  determines the magnitude of the radially outward force that the pistons in that bank exert against the springs  20  that bias them radially inward. Thus, the greater the pressure supplied to the pistons  12 , the further the pistons extend and the greater the radially outward force F that they apply to the walls of the bore  4 . As discussed below, the pressure supplied to the pistons is controlled by the supply and return valves  70  and  71 , respectively.  
     [0040] A supply valve  70  is shown in FIGS. 11 and 12. The valve  70  is electromagnetically operated and preferably has no moving parts. The valve  70  comprises an inlet  93  to which the supply tubing  51 , which is non-magnetic, is attached. From the inlet  93 , the Theological fluid flows over a non-magnetic end cap  89  enclosed by an expanded portion  86  of tubing  57 . From the end cap  89 , the rheological fluid flows into an annular passage  94  formed between a cylindrical valve housing  87 , made from a magnetic material, and a cylindrical core  92 . The core  92  is comprised of windings  99 , such as copper wire, wrapped around a core body  91  that is made from a magnetic material so as to form an electromagnet. From the annular passage  94 , the rheological fluid flows over a second end cap  90  enclosed within an expanded section of the tubing  53 , both of which are made from a non-magnetic material, and is discharged from the valve  20 . Preferably, the magnetic material in the valve  70  is iron. A variety of materials may be used for the non-magnetic material, such as non-magnetic stainless steel, brass, aluminum or plastic. The return valves  71 , which in some applications may be dispensed with, are constructed in a similar manner as the supply valves  70 .  
     [0041] When electrical current flows through the windings  99 , a magnetic field is developed around the core  92  that crosses the flow path in the passage  94  in two places at right angles. The strength of this magnetic field is dependent upon the amperage of the current supplied to the windings  99 . As previously discussed, the shear strength, and therefore the flow resistance, of the magnetorheological fluid is dependent upon the strength of the magnetic field—the stronger the field, the greater the shear strength.  
     [0042]FIGS. 14 and 15 show an alternate embodiment of the supply and return valves  70  and  71 . In this embodiment, the valve body consists of a rectangular channel  104  made from a magnetic material and having non-magnetic transition sections  106  and  108  at its inlet and outlet that mate with the tubing sections  51 ,  53 ,  55  and  57 . The channel  104  is disposed within an electro-magnet formed by a C-shaped section of magnetic material  102  around which copper windings  110  are formed.  
     [0043]FIG. 16 shows the portion of the drill string  6  in the vicinity of the guidance module  10 . In addition to the pump  40  and directional electronics module  30 , previously discussed, the guidance module  10  also includes a motor  116 , which is driven by the flow of the drilling mud and which drives the pump  40 , a bearing assembly  114 , and an alternator  112  that provides electrical current for the module.  
     [0044] According to the current invention, actuation of the pistons  12  is controlled by adjusting a magnetic field within the valves  70  and  71 . Specifically, the magnetic field is created by directing electrical current to flow through the windings  99 . As previously discussed, this magnetic field increases the shear strength, and therefore the flow resistance, of the rheological fluid.  
     [0045] As shown in FIGS. 11 and 13, the flow of electrical current to the windings  99  in each of the valves  70  and  71  is controlled by a controller  13 , which preferably comprises a programmable microprocessor, solid state relays, and devices for regulating the amperage of the electrical current. Preferably, the controller  30  is located within the directional electronics module  30 , although it could also be mounted in other locations, such as an MWD tool discussed below.  
     [0046] As shown in FIG. 4, the directional electronics module  30  may include a magnetometer  123  and an accelerometer  124  that, using techniques well known in the art, allow the determination of the angular orientation of a fixed reference point A on the circumference of the drill string  6  with respect to the circumference of the bore hole  4 , typically north in a vertical well or the high side of the bore in a inclined well, typically referred to as “tool face”. For example, as shown in FIG. 2, the reference point A on the drill string is located at the 0° location on the bore hole  4 . The tool face information is transmitted to the controller  13  and allows it to determine the instantaneous angular orientation of each of the piston banks—that is, the first bank of pistons  12 ′ is located at the 90° location on the bore hole  4 , etc.  
     [0047] Preferably, the drill string  6  also includes an MWD tool  118 , shown in FIG. 16. Preferably, the MWD tool  118  includes an accelerometer  120  to measure inclination and a magnetometer  121  to measure azimuth, thereby providing information on the direction in which the drill string is oriented. However, these components could also be incorporated into the directional electronics module  30 . The MWD tool  118  also includes a mud pulser  122  that uses techniques well known in the art to send pressure pulses from the bottom hole assembly to the surface via the drilling mud that are representative of the drilling direction sensed by the directional sensors. As is also conventional, a strain gage based pressure transducer at the surface (not shown) senses the pressure pulses and transmits electrical signals to a data acquisition and analysis system portion of the surface control system  12  where the data encoded into the mud pulses is decoded and analyzed. Based on this information, as well as information about the formation  2  and the length of drill string  6  that has been extended into the bore  4 , the drilling operator then determines whether the direction at which the drilling is proceeding should be altered and, if so, by what amount.  
     [0048] Preferably, the MWD tool  118  also includes a pressure pulsation sensor  97  that senses pressure pulsations in the drilling mud flowing in the annular passage  30  between the bore  4  and the drill string  6 . A suitable pressure pulsation sensor is disclosed in U.S. patent application Ser. No. 09/086,418, filed May 29, 1999, entitled “Method And Apparatus For Communicating With Devices Downhole in a Well Especially Adapted For Use as a Bottom Hole Mud Flow Sensor,” hereby incorporated by reference in its entirety. Based on input from the drilling operator, the surface control system  12  sends pressure pulses  126 , indicated schematically in FIG. 13, downhole through the drilling mud  5  using a pressure pulsation device  132 , shown in FIG. 1. The pulsations  126  are sensed by the pressure sensor  97  and contain information concerning the direction in which the drilling should proceed. The information from the pressure sensor  97  is directed to the guidance module controller  13 , which decodes the pulses and determines, in conjunction with the signals from the orientation sensors  120  and  121  and the tool face sensors  123  and  124 , the sequence in which the pistons  12  should be extended and, optionally, the amount of the change in the pressure of the rheological fluid supplied to the pistons  12 .  
     [0049] The controller  13  then determines and sets the current supplied to the supply and return valves  70  and  71 , respectively, thereby setting the strength of the magnetic field applied to the rheological fluid, which, in turn, regulates the pressure of the rheological fluid and the force that is applied to the pistons  12 . For example, with reference to FIG. 2, if the surface control system  12  determined that the drilling angle should be adjusted toward the 270° direction on the bore hole  4  and transmitted such information to the controller  13 , using mud flow telemetry as discussed above, the controller  13  would determine that the pistons in each piston bank should be extended when such pistons reached the 90° location.  
     [0050] According to the current invention, the force exerted by the pistons  12  is dependent upon the pressure of the rheological fluid in the piston cylinders  19 , the greater the pressure, the greater the force urging the pistons radially outward. This pressure is regulated by the supply and return valves  70  and  71 .  
     [0051] If it is desired to decrease the rheological fluid pressure in the cylinders  19  associated with a given bank of pistons  12 , current is applied (or additional current is applied) to the windings of the valve  70  that supplies rheological fluid to that bank of pistons so as to create (or increase) the magnetic field to which the rheological fluid is subjected as it flows through the valve. As previously discussed, this magnetic field increases the fluid shear strength and flow resistance of the rheological fluid, thereby increasing the pressure drop across the valve  70  and reducing the pressure downstream of the valve, thereby reducing the pressure of the rheological fluid in the cylinders  19  supplied by that valve. In addition, the current to the windings in the return valve  71  associated with that bank of pistons is reduced, thereby decreasing the fluid shear strength and flow resistance of the return valve  71 , which also aids in reducing pressure in the cylinders  19 .  
     [0052] Correspondingly, if it is desired to increase the rheological fluid pressure in the cylinders  19  associated with a given bank of pistons  12 , current is reduced (or cut off entirely) to the windings of the valve  70  that supplies rheological fluid to that bank of pistons so as to reduce (or eliminate) the magnetic field to which the rheological fluid is subjected as it flows through the valve. As previously discussed, this reduction in magnetic field decreases the fluid shear strength and flow resistance of the rheological fluid, thereby decreasing the pressure drop across the valve  70  and increasing the pressure downstream of the valve, thereby increasing the pressure of the rheological fluid in the cylinders  19  supplied by that valve. In addition, the current to the windings in the return valve  71  associated with that bank of pistons is increased, thereby increasing the fluid shear strength and flow resistance of the return valve  71 , which also aids in increasing pressure in the cylinders  19 . Since the pressure generated by the pump  40  may vary, for example, depending on the flow rate of the drilling mud, optionally, a pressure sensor  125  is incorporated to measure the pressure of the rheological fluid supplied by the pump and this information is supplied to the controller  13  so it can be taken into account in determining the amperage of the current to be supplied to the electromagnetic valves  70  and  71 . In addition, the absolute pressure of the magnetorheological fluid necessary to actuate the pistons  12  will increase as the hole get deeper because the static pressure of the drilling mud in the annular passage  130  between the bore  4  and the drill string  6  increases as the hole get deeper and the column of drilling mud get higher. Therefore, a pressure compensation system can be incorporated into the flow path for the magnetorheological fluid to ensure that the pressure provided by the pump is additive to the pressure of the drilling mud surrounding the guidance module  10 .  
     [0053] Thus, by regulating the current supplied to the windings of the supply and return valves  70  and  71 , respectively, the controller  13  can extend and retract the pistons  12  and vary the force F applied by the pistons to the wall of the bore  4 . Thus, the direction of the drilling can be controlled. Moreover, by regulating the current, the rate at which the drill bit changes direction (i.e., the sharpness of the turn), sometimes referred to as the “build rate,” can also be controlled.  
     [0054] In some configurations, the drilling operator at the surface provides instructions, via mud flow telemetry as discussed above, to the controller  13  as to the amount of change in the electrical current to be supplied to the electromagnetic valves  70  and  71 . However, in an alternative configuration, the drilling operator provides the direction in which the drilling should proceed. Using a feed back loop and the signal from the directional sensors  120  and  121 , the controller  13  then varies the current as necessary until the desired direction is achieved.  
     [0055] Alternatively, the drilling operator could provide instructions, via mud flow telemetry, concerning the location to which the drill should proceed, as well as information concerning the length of drill string that has been extended into the bore  4  thus far. This information is then combined with information from the direction sensors  120  and  121  by the controller  13 , which then determines the direction in which the drilling should proceed and the directional change necessary to attain that direction in order to reach the instructed location.  
     [0056] In all of the embodiments described above the transmission of information from the surface to the bottom hole assembly can be accomplished using the apparatus and methods disclosed in the aforementioned U.S. patent application Ser. No. 09/086,418, filed May 29, 1999, entitled “Method And Apparatus For Communicating With Devices Downhole in a Well Especially Adapted For Use as a Bottom Hole Mud Flow Sensor,” previously incorporated by reference in its entirety.  
     [0057] In another alternative, the controller  13  can be preprogrammed to create fixed drilling direction that is not altered during drilling.  
     [0058] Although the use of a magnetorheological fluid is preferred, the invention could also be practiced using electrorheological fluid. In such fluids the shear strength can be varied by using a valve to apply an electrical current through the fluid.  
     [0059] Although the invention has been described with reference to a drill string drilling a well, the invention is applicable to other situations in which it is desired to control the direction of travel of a device through a passage, such as the control of drilling completion and production devices. Accordingly, the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.