Patent Publication Number: US-8118114-B2

Title: Closed-loop control of rotary steerable blades

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of co-pending, commonly assigned U.S. patent application Ser. No. 12/332,911 entitled C LOSED -L OOP  P HYSICAL  C ALIPER  M EASUREMENTS AND  D IRECTIONAL  D RILLING  M ETHOD , which is in turn a continuation-in-part of commonly-assigned U.S. patent application Ser. No. 11/595,054 (now U.S. Pat. No. 7,464,770) entitled C LOSED -L OOP C   ONTROL OF  H YDRAULIC  P RESSURE IN A  D OWNHOLE  S TEERING  T OOL.    
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to downhole tools, for example, including directional drilling tools such as three-dimensional rotary steerable tools (3DRS). More particularly, embodiments of this invention relate to closed-loop control of rotary steerable blades and steering methods utilizing such control. 
     BACKGROUND OF THE INVENTION 
     Directional control has become increasingly important in the drilling of subterranean oil and gas wells, for example, to more fully exploit hydrocarbon reservoirs. Downhole steering tools, such as two-dimensional and three-dimensional rotary steerable tools, are commonly used in many drilling applications to control the direction of drilling. Such steering tools commonly include a plurality of force application members (also referred to herein as blades) that may be independently extended out from and retracted into a housing. The blades are disposed to extend outward from the housing into contact with the borehole wall. The direction of drilling may be controlled by controlling the magnitude and direction of the force or the magnitude and direction of the displacement applied to the borehole wall. In rotary steerable tools, the housing is typically deployed about a shaft, which is coupled to the drill string and disposed to transfer weight and torque from the surface (or from a mud motor) through the steering tool to the drill bit assembly. 
     In general, the prior art discloses at least two types of directional control mechanisms employed with rotary steerable tool deployments. U.S. Pat. Nos. 5,168,941 and 6,609,579 to Krueger et al disclose examples of rotary steerable tool deployments employing a first type of directional control mechanism. The direction of drilling is controlled by controlling the magnitude and direction of a side (lateral) force applied to the drill bit. This side force is created by extending one or more of a plurality of ribs (referred to herein as blades) into contact with the borehole wall and is controlled by controlling the pressure in each of the blades. The amount of force on each blade is controlled by controlling the hydraulic pressure at the blade, which is in turn controlled by proportional hydraulics or by switching to the maximum pressure with a controlled duty cycle. Krueger et al further disclose a hydraulic actuation mechanism in which each steering blade is independently controlled by a separate piston pump. A control valve is positioned between each piston pump and its corresponding blade to control the flow of hydraulic fluid from the pump to the blade. During drilling each of the piston pumps is operated continuously via rotation of a drive shaft. 
     U.S. Pat. No. 5,603,386 to Webster discloses an example of a rotary steerable tool employing a second type of directional control mechanism. Webster discloses a mechanism in which the steering tool is moved away from the center of the borehole via extension (and/or retraction) of the blades. The direction of drilling may be controlled by controlling the magnitude and direction of the offset between the tool axis and the borehole axis. The magnitude and direction of the offset are controlled by controlling the position of the blades. In general, increasing the offset (i.e., increasing the distance between the tool axis and the borehole axis) tends to increase the curvature (dogleg severity) of the borehole upon subsequent drilling. Webster also discloses a hydraulic mechanism in which all three blades are controlled via a single pump and pressure reservoir and a plurality of valves. In particular, each blade is controlled by three check valves. The nine check valves are in turn controlled by eight solenoid controlled pilot valves. Commonly assigned, co-pending U.S. patent application Ser. No. 11/061,339 employs hydraulic actuation to extend the blades and a spring biased mechanism to retract the blades. Spring biased retraction of the blades advantageously reduces the number of valves required to control the blades. The &#39;339 application is similar to the Webster patent in that only a single pump and/or pressure reservoir is required to actuate the blades. 
     The above described steering tool deployments are known to be commercially serviceable. Notwithstanding, there is room for improvement of such tool deployments and directional drilling methods, especially for smaller diameter steering tool deployments (e.g., having a tool diameter of less than about 8 inches). For example, in deployments utilizing the first type of control mechanism, directional control is related to many factors including weight and stiffness of the BHA, borehole inclination, and formation harness or softness. Therefore, obtaining a consistent and predictable borehole curvature can be difficult. Deployments utilizing the second type of control mechanism require accurate position sensors and physical caliper measurements. Moreover the total force exerted against the borehole is typically not controlled. Too much force can lead to excessive drag while too little force can lead to housing roll (rotation of the blade housing in the borehole). Therefore there exists a need for improved directional drilling methods in rotary steerable deployments. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the need for improved drilling methods for use in rotary steerable deployments. Aspects of this invention include a steering tool having a controller configured to provide closed-loop control of blade pressure and position. In one exemplary embodiment, the controller is configured to execute a directional control methodology in which the drilling direction is controlled via control of the blade positions. The pressure in each of the blades is also maintained within a predetermined range of pressures. Such a deployment tends to advantageously prevent borehole friction from becoming excessively high while at the same time tends to reduce housing roll via maintaining at least minimum blade pressure in each of the blades. Moreover adequate blade contact with the borehole wall is all ensured which tends to promote accurate borehole caliper measurements. 
     In another exemplary embodiment, the controller is configured to correlate blade pressure measurements and blade position measurements during drilling. The correlation may then be utilized as part of a secondary directional control scheme in the event of a downhole failure to one or more of the blade position or pressure sensors. The correlation is utilized, for example, to select predetermined blade pressures suitable to achieve desired blade positions (e.g., to achieve a desired tool face and offset of the steering tool housing). These embodiments tend to advantageously provide stable and reliable directional control and therefore provide a suitable backup directional control mechanism in the event of one or more sensor failures. The invention therefore has the potential to save considerable rig time. 
     In one aspect the present invention includes a downhole steering tool configured to operate in a borehole. The steering tool includes at least three blades deployed on a housing. The blades are disposed to extend radially outward from the housing and engage a wall of the borehole such that engagement of the blades with the borehole wall is operative to eccenter the housing in the borehole. A hydraulic module includes a fluid chamber disposed to provide pressurized fluid to each of the plurality of blades, the pressurized fluid operative to extend the blades. Each of the blades includes at least a first valve in fluid communication with high pressure fluid and at least a second valve in fluid communication with low pressure fluid. Each of the blades further includes a pressure sensor disposed to measure a fluid pressure in the blade and a position sensor disposed to measure a radial position of the blade. The steering tool further includes a controller configured to (i) lock at least one of the blades in a predetermined radially extended position by closing both the corresponding first and second valves, (ii) receive pressure measurements for each of the locked blades from the corresponding pressure sensors; and (iii) radially further extend or retract at least one of the locked blades by opening the corresponding first valve when the corresponding pressure measurement is less than a first predetermined threshold or opening the corresponding second valve when the corresponding pressure is greater than a second predetermined threshold. 
     In another aspect, the invention includes a method of directional drilling. The steering tool described in the preceding paragraph is first coupled with a drill string and rotated in a borehole. Each of the blades is extended to a corresponding first predetermined radial position. At least one of the blades is locked at the corresponding predetermined radial position by closing the corresponding first and second valves. A hydraulic pressure is then measured in each of the locked blades using the corresponding pressure sensors. The method further includes extending or retracting at least one of the locked blades by opening the corresponding first valve(s) when the corresponding measured pressure is less than a predetermined minimum threshold or opening the corresponding second valve(s) when the corresponding measured pressure is greater than a predetermined maximum threshold. 
     In still another aspect invention includes a downhole steering tool configured to operate in a borehole. The steering tool includes at least three blades deployed on a housing. The blades are disposed to extend radially outward from the housing and engage a wall of the borehole such that engagement of the blades with the borehole wall is operative to eccenter the housing in the borehole. Each of the blades includes a corresponding blade pressure sensor disposed to measure a pressure in the blade and a corresponding position sensor disposed to measure a radial position of the blade. The steering tool further includes a controller configured to (i) receive radial position measurements from each of the position sensors at a plurality of measured depths while drilling a subterranean borehole, (ii) receive corresponding pressure measurements from the pressure sensors, (iii) correlate the pressure measurements and the position measurements, (iv) use said correlation to select a set of blade pressures for achieving desired blade positions during drilling, and (v) apply the set of blade pressure to the blades. 
     In yet another aspect, the invention includes a method of directional drilling. The steering tool described in the preceding paragraph is first coupled with a drill string and rotated in a borehole. A radial position of each of the blades is measured at a plurality of measured depths while drilling. Corresponding hydraulic pressures are measured in each of the blades. The measured positions and pressures are then correlated and the correlation used to select a set of blade pressures for achieving desired blade radial positions during drilling. The set of blade pressures is then applied to the blades. This method is preferably, although not necessarily, used in response to a failure of at least one of the blade position sensors. 
     In a further aspect, the present invention includes a method of directional drilling. The method includes rotating a drill string in a borehole, the drill string including a rotary steerable tool having at least three blades deployed on a rotary steerable housing. The blades are disposed to extend radially outward from the housing and engage a wall of the borehole such that engagement of the blades with the borehole wall is operative to eccenter the housing in the borehole. The method further includes measuring a radial position and a corresponding blade pressure for each of the blades at a plurality of measured depths while drilling and correlating the measured radial positions and the corresponding measured blade pressures. The method further includes using the correlation to select either (i) a set of blade pressures for achieving a desired set of blade positions or (ii) a set of blade positions for achieving a desired set of blade pressures and applying either the set of blade pressures or the set of blade positions selected in to the blades. 
     The foregoing has outlined rather broadly the features of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other methods, structures, and encoding schemes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts a drilling rig on which exemplary embodiments of the present invention may be deployed. 
         FIG. 2  is a perspective view of one exemplary embodiment of the steering tool shown on  FIG. 1 . 
         FIGS. 3A and 3B  depict schematic diagrams of an exemplary hydraulic control module employed in exemplary embodiment of the steering tool shown on  FIG. 2 . 
         FIG. 4  depicts one exemplary method embodiment of the present invention in flowchart form. 
         FIG. 5  depicts another exemplary method embodiment of the present invention in flowchart form. 
         FIG. 6  depicts still another exemplary method embodiment of the present invention in flowchart form. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIGS. 1 through 3B , it will be understood that features or aspects of the embodiments illustrated may be shown from various views. Where such features or aspects are common to particular views, they are labeled using the same reference numeral. Thus, a feature or aspect labeled with a particular reference numeral on one view in  FIGS. 1 through 3B  may be described herein with respect to that reference numeral shown on other views. 
       FIG. 1  illustrates a drilling rig  10  suitable for utilizing exemplary downhole steering tool and method embodiments of the present invention. In the exemplary embodiment shown on  FIG. 1 , a semisubmersible drilling platform  12  is positioned over an oil or gas formation (not shown) disposed below the sea floor  16 . A subsea conduit  18  extends from deck  20  of platform  12  to a wellhead installation  22 . The platform may include a derrick  26  and a hoisting apparatus  28  for raising and lowering the drill string  30 , which, as shown, extends into borehole  40  and includes a drill bit  32  and a steering tool  100  (such as a three-dimensional rotary steerable tool). In the exemplary embodiment shown, steering tool  100  includes a plurality of blades  150  (e.g., three) disposed to extend outward from the tool  100 . The extension of the blades  150  into contact with the borehole wall is intended to eccenter the tool in the borehole, thereby changing an angle of approach of the drill bit  32  (which changes the direction of drilling). Exemplary embodiments of steering tool  100  further include hydraulic  130  and electronic  140  control modules ( FIG. 2 ) configured to provide closed-loop control of system and/or blade hydraulic pressures. Drill string  30  may further include a downhole drilling motor, a mud pulse telemetry system, and one or more additional sensors, such as LWD and/or MWD tools for sensing downhole characteristics of the borehole and the surrounding formation. The invention is not limited in these regards. 
     It will be understood by those of ordinary skill in the art that methods and apparatuses in accordance with this invention are not limited to use with a semisubmersible platform  12  as illustrated in  FIG. 1 . This invention is equally well suited for use with any kind of subterranean drilling operation, either offshore or onshore. While exemplary embodiments of this invention are described below with respect to rotary steerable embodiments (e.g., including a shaft disposed to rotate relative to a housing), it will be appreciated that the invention is not limited in this regard. The invention is equally well suited for use with substantially any suitable downhole steering tools that utilize a plurality of blades to steer the drill bit. 
     Turning now to  FIG. 2 , one exemplary embodiment of steering tool  100  from  FIG. 1  is illustrated in perspective view. In the exemplary embodiment shown, steering tool  100  is substantially cylindrical and includes threaded ends  102  and  104  (threads not shown) for connecting with other bottom hole assembly (BHA) components (e.g., connecting with the drill bit at end  104  and upper BHA components at end  102 ). The steering tool  100  further includes a housing  110  and at least one blade  150  deployed, for example, in a recess (not shown) in the housing  110 . Steering tool  100  further includes hydraulics  130  and electronics  140  modules (also referred to herein as control modules  130  and  140 ) deployed in the housing  110 . In general (and as described in more detail below with respect to  FIGS. 3A and 3B ), the control modules  130  and  140  are configured for measuring and controlling the relative positions of the blades  150  as well as the hydraulic system and blade pressures. Control modules  130  and  140  may include substantially any devices known to those of skill in the art, such as those disclosed in U.S. Pat. No. 5,603,386 to Webster or U.S. Pat. No. 6,427,783 to Krueger et al. 
     To steer (i.e., change the direction of drilling), one or more of blades  150  are extended and exert a force against the borehole wall. The steering tool  100  is moved away from the center of the borehole by this operation, altering the drilling path. It will be appreciated that the tool  100  may also be moved back towards the borehole axis if it is already eccentered. To facilitate controlled steering, the rotation rate of the housing is desirably less than 0.1 rpm during drilling, although the invention is not limited in this regard. By keeping the blades  150  in a substantially fixed position with respect to the circumference of the borehole (i.e., by preventing rotation of the housing  110 ), it is possible to steer the tool without constantly extending and retracting the blades  150 . Non-rotary steerable embodiments are thus often only utilized in sliding mode. In rotary steerable embodiments, the tool  100  is constructed so that the housing  110 , which houses the blades  150 , remains stationary, or substantially stationary, with respect to the borehole during directional drilling operations. The housing  110  is therefore constructed in a rotationally non-fixed (of floating) fashion with respect to a shaft  115  ( FIGS. 3A and 3B ). The shaft  115  is connected with the drill string and is disposed to transfer both torque and weight to the bit. It will be understood that the invention is not limited to rotary steerable embodiments. 
     In general, increasing the offset (i.e., increasing the distance between the tool axis and the borehole axis) tends to increase the curvature (dogleg severity) of the borehole upon subsequent drilling. In the exemplary embodiment shown, steering tool  100  includes full-gauge near-bit stabilizer  120 , and is therefore configured for “point-the-bit”steering in which the direction (tool face) of subsequent drilling tends to be in the opposite direction (or nearly the opposite; depending, for example, upon local formation characteristics) of the offset between the tool axis and the borehole axis. The invention is not limited to the mere use of a near-bit stabilizer. It is equally well suited for “push-the-bit” steering in which there is no full-gauge near-bit stabilizer and the direction of subsequent drilling tends to be in the same direction as the offset between the tool axis and borehole axis. Those of skill in the art will readily recognize that push-the-bit steering can be equally well achieved with no near-bit stabilizer or an under-gauge near-bit stabilizer. 
     With reference now to  FIGS. 3A and 3B , one exemplary embodiment of hydraulic module  130  is schematically depicted.  FIG. 3A  is a simplified schematic of the hydraulic module  130  showing only a single blade  150 A.  FIG. 3B  shows each of the three blades  150 A,  150 B, and  150 C as well as certain of the electrical control devices (which are in electronic communication with electronic control module  140 ). Hydraulic module  130  includes a hydraulic fluid chamber  220  including first and second, low and high pressure reservoirs  226  and  236 . In the exemplary embodiment shown, low pressure reservoir  226  is modulated to wellbore (hydrostatic) pressure via equalizer piston  222 . Wellbore drilling fluid  224  enters fluid cavity  225  through filter screen  228 , which is deployed in the outer surface of the non-rotating housing  110 . It will be readily understood to those of ordinary skill in the art that the drilling fluid in the borehole exerts a force on equalizer piston  222  proportional to the wellbore pressure, which thereby pressurizes hydraulic fluid in low pressure reservoir  226 . 
     Hydraulic module  130  further includes a piston pump  240  operatively coupled with drive shaft  115 . In the exemplary embodiment shown, pump  240  is mechanically actuated by a cam  118  formed on an outer surface of drive shaft  115 , although the invention is not limited in this regard. Pump  240  may be equivalently actuated, for example, by a swash plate mounted to the outer surface of the shaft  115  or an eccentric profile formed in the outer surface of the shaft  115 . In the exemplary embodiment shown, rotation of the drive shaft  115  causes cam  118  to actuate piston  242 , thereby pumping pressurized hydraulic fluid to high pressure reservoir  236 . Piston pump  240  receives low pressure hydraulic fluid from the low pressure reservoir  226  through inlet check valve  246  on the down-stroke of piston  242  (i.e., as cam  118  disengages piston  242 ). On the upstroke (i.e., when cam  118  engages piston  242 ), piston  242  pumps pressurized hydraulic fluid through outlet check valve  248  to the high pressure reservoir  236 . 
     It will be understood that the invention is not limited to any particular pumping mechanism. As stated above, the invention is not limited to rotary steerable embodiments and thus is also not limited to a shaft actuated pumping mechanism. In other embodiments, an electric powered pump may be utilized, for example, powered via electrical power generated by a mud turbine and/or supplied by batteries. 
     Hydraulic fluid chamber  220  further includes a pressurizing spring  234  (e.g., a Belleville spring) deployed between an internal shoulder  221  of the chamber housing and a high pressure piston  232 . As the high pressure reservoir  236  is filled by pump  240 , high pressure piston  232  compresses spring  234 , which maintains the pressure in the high pressure reservoir  236  at some predetermined pressure above wellbore pressure. Hydraulic module  130  typically (although not necessarily) further includes a pressure relief valve  235  deployed between high pressure and low pressure fluid lines. In one exemplary embodiment, a spring loaded pressure relief valve  235  opens at a differential pressure of about 750 psi, thereby limiting the pressure of the high pressure reservoir  236  to a pressure of about 750 psi above wellbore pressure. However, the invention is not limited in this regard. 
     With continued reference to  FIGS. 3A and 3B , extension and retraction of the blades  150 A,  150 B, and  150 C are now described. The blades  150 A,  150 B, and  150 C are essentially identical and thus the configuration and operation thereof are described only with respect to blade  150 A. Blades  150 B and  150 C are referred to below in reference to exemplary methods in accordance with this invention. Blade  150 A includes one or more blade pistons  252 A deployed in corresponding chambers  244 A, which are in fluid communication with both the low and high pressure reservoirs  226  and  236  through controllable valves  254 A and  256 A, respectively. In the exemplary embodiment shown, valves  254 A and  256 A include solenoid controllable valves, although the invention is not limited in this regard. 
     While the invention is described with reference to a rotary steerable tool in which the blades are hydraulically actuated, it will be understood that the invention is not limited to any particular blade extension/retraction mechanism. In another suitable embodiment, the blades may be actuated with a ramp mechanism, for example, powered via electrical power generated by a mud turbine. 
     Referring again to the exemplary embodiment depicted on  FIGS. 3A and 3B , blade  150 A may be extended (radially outward from the tool body) by opening valve  254 A and closing valve  256 A, thereby allowing high pressure hydraulic fluid to enter chamber  244 A. As chamber  244 A is filled with pressurized hydraulic fluid, piston  252 A is urged radially outward from the tool, which in turn urges blade  150 A outward (e.g., into contact with the borehole wall). When blade  150 A has been extended to a desired (predetermined) position, valve  254 A may be closed, thereby “locking” the blade  150 A in position (at the desired extension from the tool body). The blade is considered to be locked in position when both valves  254 A and  256 A are closed. 
     In order to retract the blade (radially inward towards the tool body), valve  256 A is open (while valve  254 A remains closed). Opening valve  256 A allows pressurized hydraulic fluid in chamber  244 A to return to the low pressure reservoir  226 . Blade  150 A may be urged inward (towards the tool body), for example, via spring bias and/or contact with the borehole wall. In the exemplary embodiment shown, the blade  150 A is not drawn inward under the influence of a hydraulic force, although the invention is not limited in this regard. 
     Hydraulic module  130  may also advantageously include one or more sensors, for example, for measuring the pressure and volume of the high pressure hydraulic fluid. In the exemplary embodiment shown on  FIG. 3B , sensor  262  is disposed to measure hydraulic fluid pressure in reservoir  236 . Likewise, sensors  272 A,  272 B, and  272 C are disposed to measure hydraulic fluid pressure at blades  150 A,  150 B, and  150 C, respectively. Position sensor  264  is disposed to measure the displacement of high pressure piston  232  and therefore the volume of high pressure hydraulic fluid in reservoir  236 . Position sensors  274 A,  274 B, and  274 C are disposed to measure the displacement of blade pistons  252 A,  252 B, and  252 C and thus the extension of blades  150 A,  150 B, and  150 C. In one exemplary embodiment of the invention, sensors  262 ,  272 A,  272 B, and  272 C each include a pressure sensitive strain gauge, while sensors  264 ,  274 A,  274 B, and  274 C each include a potentiometer having a resistive wiper, however, the invention is not limited in regard to the types of pressure and volume sensors utilized. For example, in an alternative embodiment, electrical current consumption of an electromechanical motor may be used to sense blade pressure. Moreover, pressurized fluid volume (or alternatively the extension of the blades) may be measured using flow meters. 
     In the exemplary embodiments shown and described with respect to  FIGS. 3A and 3B , hydraulic module  130  utilizes pressurized hydraulic oil in reservoirs  226  and  236 . The artisan of ordinary skill will readily recognize that the invention is not limited in this regard and that pressurized drilling fluid, for example, may also be utilized to extend blades  150 A,  150 B, and  150 C. 
     During a typical directional drilling application, a steering command may be received at steering tool  100 , for example, via drill string rotation encoding. Exemplary drill string rotation encoding schemes are disclosed, for example, in commonly assigned U.S. Pat. Nos. 7,222,681 and 7,245,229. In prior art directional drilling methods, new blade positions are calculated based on the received steering command and each of the blades  150 A,  150 B, and  150 C are then independently extended and/or retracted to the appropriate position (as measured by position sensors  274 A,  274 B, and  274 C). Two of the blades (e.g., blades  150 B and  150 C) are commonly locked into position as described above (e.g., valves  254 B,  254 C,  256 B, and  256 C are closed). The third blade (e.g., blade  150 A) preferably remains “floating” (i.e., open to high pressure hydraulic fluid via valve  256 A) in order to maintain a grip on the borehole wall so that housing  110  does not rotate during drilling. 
     While such prior art drilling methods are commercially serviceable, there remains a need for further improvements. For example, as described above in the Background Section, such methods do not typically provide control over the force exerted by the blades on the borehole wall. Too much force has been observed to result in excessive frictional drag between the blades and the borehole wall, which tends to reduce the rate of penetration during drilling. Too little force can result in blade housing roll (excessive rotation of housing  110  in the borehole), which makes directional control more difficult owing to the need to constantly extend and retract the blades. Excessive rotation of the housing can also cause damage to the blades (due to tangential forces acting on the blades). 
     With reference now to  FIG. 4 , one exemplary directional drilling method embodiment  300  in accordance with the present invention is depicted in flowchart form. At  302  a downhole tool (such as tool  100 ) is deployed in a subterranean borehole and drilling commences (e.g., via rotating the drill string). At  304 , each of the blades is independently extended (or retracted) to a corresponding predetermined radial position (e.g., calculated based on predetermined target tool face and offset values and a measured borehole caliper). At least one blade, and preferably each of the blades, is further locked at its corresponding radial position, e.g., via closing corresponding valves  254  and  256 . At  306  the hydraulic pressure is measured in each of the locked blades, e.g., using corresponding pressure sensors  272 . At  308 , each of the blade pressures measured in  306  is compared with a predetermined target pressure range. The predetermined target pressure range includes both an upper pressure threshold and a lower pressure threshold. While the invention is not limited to any particular pressure values, the target pressure range is typically selected to have a lower threshold value that is sufficiently high enough to resist housing roll and an upper threshold value that is sufficiently low enough to prevent excessive frictional drag between the blades and borehole wall. In one exemplary embodiment the target pressure is in the range from about 200 to about 700 psi above hydrostatic wellbore pressure. 
     It will be appreciated that a serviceable target pressure range may be selected based on substantially any suitable measured or expected borehole and tool parameters. Moreover the target pressure range may be selected using rule-based intelligence. Such “smart” control systems may be configured to control the target pressure range based on drilling performance and/or other steering tool measurements. For example, a failure to achieve a particular dogleg severity may trigger a controller to increase the upper threshold in the pressure range. Alternatively, excessive housing roll (e.g., as measured via a change in gravity tool face of the housing) may trigger a controller to increase the lower threshold in the pressure range. Moreover, the target pressure range may be selected from a look-up table relating various drilling parameters to the pressure range. 
     The frictional force of the blades on the borehole wall may be measured directly and used as an alternative and/or additional control parameter in determining a suitable target pressure range. For example, conventional strain gauges may be deployed above and below blade housing  110  ( FIG. 2 ) and utilized to measure the near-bit weight-on-bit at both locations. It will be understood that the difference between the two weight-on-bit measurements (the weight supported by the blades) is directly proportional to the frictional force of the blades on the borehole wall. Excessive weight-on-bit loss at the blades (the difference between the two weight-on-bit measurements) may thus be used to trigger a controller to reduce the upper threshold in the target pressure range. 
     It will further be appreciated that numerous other borehole and/or tool parameters may be utilized to select a desired target pressure range. For example, the target pressure range may also be determined based on various measured parameters such as borehole inclination, borehole caliper, borehole curvature, LWD formation measurements, bending moments, hydraulic fluid pressure fluctuations, BHA vibration, and the like. Borehole curvature may be determined, for example, from longitudinally spaced inclination and/or azimuth measurements (e.g., at first and second longitudinal positions on the drill string) as disclosed in commonly assigned U.S. Pat. No. 7,243,719. Predetermined build rates, turn rates, DLS, and steering tool offset (the predetermined distance between the center of the borehole and the tool axis) may also utilized to determine pressure thresholds. LWD formation measurements may be used, for example, to identify known formations in which frictional forces tend to be excessive. Exemplary LWD measurements include, for example, formation density, resistivity, and various sonic velocities (also referred to reciprocally as slownesses). 
     It will be still further appreciated that the position-based and/or force-vector-based (pressure-vector-based) steering methods disclosed herein may further be utilized to follow pre-determined well plans, pre-determined target inclinations and/or azimuths, and/or pre-determined geological characteristics in a closed-loop manner. Such “high-level” close-loop control of the target position and/or force-vector (pressure-vector) parameters are well known in the art. 
     With continued reference to  FIG. 4 , if the pressure in each of the blades is within the target range, the controller typically waits a predetermined time (e.g., 1 second) before repeating steps  306  and  308  as indicated at  312 . If the measured pressure in any of the blades is outside of the predetermined target range, then the corresponding blade is either extended or retracted at  310  (e.g., via opening either valve  254  or  256 ) until the measured pressure in that blade is within the target range. For example, if the target pressure in the blade is greater than the upper threshold, then the blade may be retracted via opening valve  256 . Conversely, when the target pressure in the blade is less than the lower threshold the blade may be extended via opening valve  254 . After the blade pressure has returned to the target range, the blade is again typically locked in position via closing valves  254  and  256 . 
     It will be appreciated that the invention is not limited to embodiments in which a single hydraulic system controls all three blades (e.g., as depicted in  FIG. 3 ). In one alternative embodiment, the tool may have an independent hydraulic system for each blade. Nor is the invention limited to tool embodiments utilizing solenoid controllable valves. In one alternative embodiment, servo-valves may be utilized to control the target pressure on each blade. The use of servo-valves may be advantageous in certain tool embodiments in that a servo-valve can be continuously adjusted to positions between fully open and fully closed. As such, the use of servo-valves enables the flow rate of the hydraulic fluid to be controlled and may therefore reduce the frequency of valve actuation (as compared to a binary valve which is either open or closed). Notwithstanding, the invention is not limited in these regards. 
     Extension or retraction of one or more of the blades in  310  (in order to maintain the blade pressure within the target range) may sometimes change the tool face and offset of the drilling tool in the borehole (depending upon the degree of extension or retraction required). Therefore it may be advantageous in certain applications to calculate new predetermined blade positions  314  when any of the locked blades have been extended or retracted in  310 . New predetermined blade positions may be calculated, for example, via measuring the new blade positions, calculating the borehole caliper, and then calculating the new predetermined positions based on the borehole caliper. After calculating the new predetermined blade positions in  314 , the controller may return to steps  304  so as to extend (or retract) the blades to the new predetermined positions. 
     The new predetermined blade positions may be calculated at  314 , for example, as follows. The new blade positions are typically first measured and used to calculate a borehole caliper, for example, using equations known to those of ordinary skill in the art. The center location of the borehole in Cartesian coordinates may be calculated, for example, using the following equations: 
     
       
         
           
             
               
                 
                   
                     
                       X 
                       C 
                     
                     = 
                     
                       
                         
                           
                             
                               
                                 
                                   ( 
                                   
                                     
                                       Y 
                                       3 
                                     
                                     - 
                                     
                                       Y 
                                       2 
                                     
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     
                                       Y 
                                       3 
                                     
                                     - 
                                     
                                       Y 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     
                                       Y 
                                       2 
                                     
                                     - 
                                     
                                       Y 
                                       1 
                                     
                                   
                                   ) 
                                 
                               
                               + 
                             
                           
                         
                         
                           
                             
                               
                                 
                                   ( 
                                   
                                     
                                       Y 
                                       2 
                                     
                                     - 
                                     
                                       Y 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     
                                       X 
                                       3 
                                       2 
                                     
                                     - 
                                     
                                       X 
                                       1 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                               - 
                               
                                 
                                   ( 
                                   
                                     
                                       Y 
                                       3 
                                     
                                     - 
                                     
                                       Y 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     
                                       X 
                                       2 
                                       2 
                                     
                                     - 
                                     
                                       X 
                                       1 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                         
                       
                       
                         2 
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           X 
                                           3 
                                         
                                         - 
                                         
                                           X 
                                           1 
                                         
                                       
                                       ) 
                                     
                                     ⁢ 
                                     
                                       ( 
                                       
                                         
                                           Y 
                                           2 
                                         
                                         - 
                                         
                                           Y 
                                           1 
                                         
                                       
                                       ) 
                                     
                                   
                                   - 
                                 
                               
                             
                             
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         X 
                                         2 
                                       
                                       - 
                                       
                                         X 
                                         1 
                                       
                                     
                                     ) 
                                   
                                   ⁢ 
                                   
                                     ( 
                                     
                                       
                                         Y 
                                         3 
                                       
                                       - 
                                       
                                         Y 
                                         1 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       Y 
                       C 
                     
                     = 
                     
                       
                         
                           
                             
                               
                                 
                                   ( 
                                   
                                     
                                       X 
                                       3 
                                     
                                     - 
                                     
                                       X 
                                       2 
                                     
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     
                                       X 
                                       3 
                                     
                                     - 
                                     
                                       X 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     
                                       X 
                                       2 
                                     
                                     - 
                                     
                                       X 
                                       1 
                                     
                                   
                                   ) 
                                 
                               
                               + 
                             
                           
                         
                         
                           
                             
                               
                                 
                                   ( 
                                   
                                     
                                       X 
                                       2 
                                     
                                     - 
                                     
                                       X 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     
                                       Y 
                                       3 
                                       2 
                                     
                                     - 
                                     
                                       Y 
                                       1 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                               - 
                               
                                 
                                   ( 
                                   
                                     
                                       X 
                                       3 
                                     
                                     - 
                                     
                                       X 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     
                                       Y 
                                       2 
                                       2 
                                     
                                     - 
                                     
                                       Y 
                                       1 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                         
                       
                       
                         2 
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           X 
                                           3 
                                         
                                         - 
                                         
                                           X 
                                           1 
                                         
                                       
                                       ) 
                                     
                                     ⁢ 
                                     
                                       ( 
                                       
                                         
                                           Y 
                                           2 
                                         
                                         - 
                                         
                                           Y 
                                           1 
                                         
                                       
                                       ) 
                                     
                                   
                                   - 
                                 
                               
                             
                             
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         X 
                                         2 
                                       
                                       - 
                                       
                                         X 
                                         1 
                                       
                                     
                                     ) 
                                   
                                   ⁢ 
                                   
                                     ( 
                                     
                                       
                                         Y 
                                         3 
                                       
                                       - 
                                       
                                         Y 
                                         1 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     where X C  and Y C  represent the center location of the borehole in the Cartesian coordinate reference frame of the downhole tool  100 . The center location of the tool is defined to be (0,0) in this reference frame. The contact points of blades  1 ,  2 , and  3  (e.g., blades  150 A,  150 B, and  150 C) with the borehole wall are represented in Cartesian coordinates as (X 1 ,Y 1 ), (X 2 ,Y 2 ), and (X 3 ,Y 3 ) respectively. These contact points may be calculated, for example, from the above described blade position (extension) measurements and a corresponding gravity tool face measurement. The radius and/or the diameter of the borehole may further be calculated, for example, as follows: 
     
       
         
           
             
               
                 
                   Radius 
                   = 
                   
                     
                       Diameter 
                       2 
                     
                     = 
                     
                       
                         
                           
                             
                               ( 
                               
                                 
                                   X 
                                   1 
                                 
                                 - 
                                 
                                   X 
                                   C 
                                 
                               
                               ) 
                             
                             2 
                           
                           + 
                           
                             
                               ( 
                               
                                 
                                   Y 
                                   1 
                                 
                                 - 
                                 
                                   Y 
                                   C 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                       
                       2 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     Equations 1 and 2 have been selected to minimize downhole processing time and are therefore well suited for use with downhole microcontrollers having limited processing power. Equation 1, for example, includes only subtraction, multiplication, and division steps (and no trigonometric functions). The invention is of course not limited by these equations. The artisan of ordinary skill in the art will readily be able to derive similar mathematical expressions for computing borehole caliper using blade position measurements as an input. Nor is the invention limited in any way to the reference frame in which the borehole caliper is represented. Those of ordinary skill in the art will readily be able to compute the borehole caliper in substantially any suitable reference frame or convert the borehole caliper from one reference frame to another (e.g., from Cartesian coordinates to polar coordinates and/or from a tool reference frame to a borehole reference frame). 
     The new blade positions may then be calculated, for example, as follows:
 
 C   i =√{square root over ( a   2   +b   2 +2 ab  cos α i )}  Equation 3
 
     where C i  represents the predetermined blade position of the corresponding i th  blade (e.g., blade  150 A,  150 B, or  150 C), a represents the target offset value, and b represents the borehole radius (e.g., as computed in Equation 2). The parameter α i  is in units of radians and is related to the target tool face angle (the direction of the target offset) and the measured tool face angle (e.g., the measured gravity tool face) of the i th  blade and is represented mathematically as follows: 
     
       
         
           
             
               α 
               i 
             
             = 
             
               π 
               - 
               
                 γ 
                 
                   i 
                   ⁢ 
                   
                       
                   
                 
               
               - 
               
                 arcsin 
                 ⁢ 
                 
                   
                     a 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     sin 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       γ 
                       i 
                     
                   
                   b 
                 
               
             
           
         
       
     
     where γ i  represents the difference between the target tool face angle and the measured tool face angle of the i th  blade. 
     It will be appreciated that the invention is not limited by the above described equations. Those of ordinary skill in the art will readily be able to compute blade positions based on the borehole caliper and a target tool face and offset using known trigonometric relationships. Similar equations may also be expressed in different coordinate systems (e.g. Cartesian Coordinates). 
     With continued reference to  FIG. 4 , it may be advantageous in certain embodiments of the invention to allow a “hysteresis” in the upper and lower pressure thresholds of the target range to reduce the frequency of valve actuation. This may be accomplished, for example, by using first and second unequal upper and lower thresholds. For example, first and second upper thresholds of 700 psi and 650 psi and first and second lower thresholds of 200 psi and 250 psi may be utilized. In such an exemplary embodiment, valve  254  is opened when the blade pressure drops below 200 psi, but is not closed until the blade pressure exceeds 250 psi. Likewise, valve  256  opened when the blade pressure exceeds 700 psi, but is not closed until the blade pressure drops below 650 psi. The artisan of ordinary skill in the art will readily appreciate that this 50 psi “hysteresis” tends to advantageously reduce the frequency of valve actuation. A hysteresis may also be achieved by implementing a predetermined time delay between the opening and closing of valves  254  and  256 . For example, a delay of about one or two seconds often provides sufficient hysteresis. It will be appreciated that the invention is not limited in these regards. 
     With still further reference to  FIG. 4 , it will be appreciated that the predetermined positions to which the blades are extended in  304  can be frequently updated during drilling. The predetermined positions may be changed, for example, in response to a change in the gravity tool face of the housing  110 . The predetermined positions may also be changed in order to change the direction of drilling, for example, in response to receiving a new steering tool command from the surface or in response to various sensor measurements utilized in closed-loop and/or geosteering applications. The invention is not limited in these regards. 
     As described above, accurate blade position measurements are typically required in steering deployments utilizing a blade position control scheme (the second type of directional control mechanism discussed in the Background Section). The Webster Patent discloses a rotary steerable tool in which each blade is fitted with a sensor (such as a potentiometer) for measuring the displacement of the blade. While such deployments have been utilized commercially for many years, potentiometers are known to be susceptible to mechanical wear and failure in demanding downhole environments. Such failures commonly result in the need to trip out, which results in a significant loss in rig time. In order to avoid tripping out (and the associated loss of rig time), there is a need for a backup steering methodology to overcome the loss of one or more blade position sensors. 
     With reference now to  FIG. 5 , another exemplary directional drilling method embodiment  400  in accordance with the present invention is depicted in flowchart form. Method  400  is intended to overcome the above described failure of a blade sensor and therefore may potentially (and advantageously) save considerable rig time in the event of such failures. Method  400  is similar to method  300  (depicted in  FIG. 4 ) in that it includes deploying the steering tool in the borehole at  402 . The radial position of each of the blades is measured in  404  (e.g., using position sensors  274 ) and the corresponding pressure in each of the blades is measured in  406  (e.g., using pressure sensors  272 ). At  408 , the blade positions and the measured pressures are then correlated. Such position and pressure measurement and their correlation continues during drilling. For example, the controller may generate a lookup table that includes measured blade pressures as a function of predetermined or measured blade positions achieved during drilling at least a portion of a subterranean borehole. Such a correlation between the blade positions and measured pressures may then be used in steering decisions in the event of a sensor failure. For example, predetermined blade pressures may be selected in  410  for achieving desired blade positions (i.e., for achieving a desired tool face and offset of the steering tool housing in the borehole). At  412 , the predetermined pressures are applied to each of the blades in order to achieve the desired blade positions. In this way directional drilling may continue despite the failure of one or more blade position sensors. 
     It will be appreciated that the correlation may include other steering tool and/or borehole parameters, such as borehole inclination and dogleg severity. For example, in a horizontal borehole, the blades typically need to support the weight of the BHA. Therefore, more force (pressure) may be required to achieve a particular build or drop rate in a horizontal borehole than in a vertical borehole. Moreover, it has been observed that a greater blade force (pressure) is required in order to make a course change than to maintain a particular course. For example, when the drilling direction is changed in order to build inclination (for example from a neutral position having an offset equal to 0 inches to a non-neutral position having an offset equal to 0.2 inches), the steering tool blades initially require the application of more force. However, once the steering tool enters the curved section of the borehole, less force is needed to maintain the non-neutral offset (e.g., the 0.2 inch offset). 
     It will be appreciated that raw sensor data may also be sent to the surface and raw control signals may be downlinked to the downhole computer via a telemetry or data-link system (e.g., a wired drilling string). By using high-speed two-way telemetry, exemplary embodiments of the invention may be implemented entirely on a surface computer. 
     With reference now to  FIG. 6 , another exemplary directional drilling method embodiment  500  in accordance with the present invention is depicted in flowchart form. Method  500  depicts one exemplary embodiment by which the blade pressures may be controlled in block  412  of method  400 . Predetermined blade pressures are applied at  502 . The blade pressures are then measured at  504 . If the measured blade pressure is greater than an upper threshold at  506  (e.g., 10 psi above the predetermined pressure), then valve  256  is opened at  508  so as to decrease the pressure in the blade. Valve  256  may then be closed when the pressure drops below the predetermined value. If the measured blade pressure is less than a lower threshold at  510  (e.g., 10 psi below the predetermined pressure), then valve  254  is opened at  512  so as to increase the pressure in the blade. Valve  254  may then be closed when the pressure rises above the predetermined value. 
     In certain embodiments it may be advantageous to implement method  500  with a duty cycle so as to conserve pressurized hydraulic fluid. For example, method  500  may be implemented for a first duration (e.g., 30 seconds) so as to achieve a stable force vector (a stable blade pressure in each of the blades). The blades may then be locked in place for a second duration (e.g., 30 seconds) via closing valves  254  and  256 . The use of such a duty cycle has been found to advantageously enable high pressure reservoir  236  to remain appropriately charged with high pressure fluid while at the same time providing for stable and reliable directional control. It will be appreciated that the invention is not limited to the use of a duty cycle, to any particular duty cycle (e.g., 50% as described above), or to any particular time durations. 
     Methods  400  and  500  have been found to advantageously provide stable and reliable directional control and therefore provide a suitable backup directional control mechanism, for example, in the event of position sensor failure. It will be appreciated, however, that the invention is not limited to using a position-based steering mechanism as a primary method and a pressure-based force-based mechanism as a secondary method. On the contrary, the a blade pressure-based method may also be used primarily with a position-based method being used secondarily (as a back-up), for example, in the event of a pressure transducer failure. 
     It will be appreciated that the present invention may also be used in combination with other hydraulic system and/or blade pressure control mechanisms. For example, such control mechanisms may include those depicted on  FIGS. 4 through 7  of co-pending, commonly invented, and commonly assigned, U.S. patent application Ser. No. 11/595,054 to Jones et al. (now U.S. Pat. No. 7,464,770), the specification of which is fully incorporated herein by reference. 
     With reference again to  FIG. 2 , electronics module  140  includes a digital programmable processor such as a microprocessor or a microcontroller and processor-readable or computer-readable programming code embodying logic, including instructions for controlling the function of the steering tool  100 . Substantially any suitable digital processor (or processors) may be utilized, for example, including an ADSP-2191M microprocessor, available from Analog Devices, Inc. 
     Electronics module  140  is disposed, for example, to execute pressure control methods  300 ,  350 ,  350 ′ and/or  400  described above. In the exemplary embodiments shown, module  140  is in electronic communication with pressure sensors  262 ,  272 A,  272 B,  272 C and position sensors  264 ,  274 A,  274 B,  274 C. Electronic module  140  may further include instructions to receive rotation and/or flow rate encoded commands from the surface and to cause the steering tool  100  to execute such commands upon receipt. Module  140  typically further includes at least one tri-axial arrangement of accelerometers as well as instructions for computing gravity tool face and borehole inclination (as is known to those of ordinary skill in the art). Such computations may be made using either software or hardware mechanisms (using analog or digital circuits). Electronic module  140  may also further include one or more sensors for measuring the rotation rate of the drill string (such as accelerometer deployments and/or Hall-Effect sensors) as well as instructions executing rotation rate computations. Exemplary sensor deployments and measurement methods are disclosed, for example, in commonly assigned U.S. Pat. No. 7,426,967 and co-pending, commonly assigned U.S. patent application Ser. Nos. 11/454,019 (U.S. Publication 2007/0289373). 
     Electronic module  140  typically includes other electronic components, such as a timer and electronic memory (e.g., volatile or non-volatile memory). The timer may include, for example, an incrementing counter, a decrementing time-out counter, or a real-time clock. Module  140  may further include a data storage device, various other sensors, other controllable components, a power supply, and the like. Electronic module  140  is typically (although not necessarily) disposed to communicate with other instruments in the drill string, such as telemetry systems that communicate with the surface and an LWD tool including various other formation sensors. Electronic communication with one or more LWD tools may be advantageous, for example, in geo-steering applications. One of ordinary skill in the art will readily recognize that the multiple functions performed by the electronic module  140  may be distributed among a number of devices. 
     It will also be understood that the aspects and features of the present invention may be embodied as logic that may be processed by, for example, a computer, a microprocessor, hardware, firmware, programmable circuitry, or any other processing device well known in the art. Similarly the logic may be embodied on software suitable to be executed by a processor, as is also well known in the art. The invention is not limited in this regard. The software, firmware, and/or processing device may be included, for example, on a downhole assembly in the form of a circuit board, on board a sensor sub, or MWD/LWD sub. Alternatively the processing system may be at the surface and configured to process data sent to the surface by sensor sets via a telemetry or data link system also well known in the art. One example of high-speed downhole telemetry systems is a wired drillstring, which allows high-speed two-way communications (1 Mbps available in 2008). Electronic information such as logic, software, or measured or processed data may be stored in memory (volatile or non-volatile), or on conventional electronic data storage devices such as are well known in the art. 
     Although the present invention and its advantages 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 invention as defined by the appended claims.