Abstract:
An apparatus for steerable rotary drilling of a borehole having a wall in the earth comprising, a mandrel, having a central opening there through for the passage of drilling fluids, said mandrel having a lower connection for operatively connecting to a drill bit structure, said mandrel having an upper connection for operatively connecting to a drill string above said apparatus, and the mandrel having an intermediate portion, an outer housing surrounding longitudinal extent of said mandrel intermediate portion, a differential displacement drive within the outer housing, one or more pairs of radially-extensible, opposed side-force exerting elements controlled by said differential displacement drive to provide for side force exertion against the said borehole wall, said drive including a pair of pistons for activating each pair of radially-extensible opposed side-force elements, one or more displacement transducers for said pair of pistons, control valves within said outer housing for fluid pressure control of said differential displacement drive, and sensing, control and power supply elements to control operation of said control valves, to steer drilling in a desired direction by selective operation of said side force exerting elements.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to controlling of the direction of drilling a borehole in the earth, for causing that borehole to traverse a desired path within the earth.  
         [0002]     Early apparatus and methods used for this purpose employed a device called a whipstock that was lowered into a borehole and oriented to the direction of desired borehole divergence from its initial path. This apparatus had a tapered portion that would force the drill bit to diverge in the oriented direction. Later apparatus and methods were developed that used a down-hole motor, driven by drilling-mud flow or other means. Such motors are typically mounted to the lower end of a bent subassembly such that the longitudinal axis of the motor, and the drilling bit at its lower end, are at a slight angle to the direction of the drill string above the bent subassembly. When it is desired to drill in a generally straight path, the motor may be not activated, if desired, and drill string is continuously rotated. When it is desired to cause the path of the borehole to diverge in a given direction, continuous rotation of the drill string is stopped. Then the drill string, bent subassembly, motor and bit are rotated to position the direction of bend in the bent subassembly in the desired direction of divergence, the upper part of the drill string is held in this position and the down-hole motor is started. This causes the borehole to diverge in the desired and selected direction. Down-hole motors are expensive and have a relatively short life while drilling.  
         [0003]     As an alternative to the use of a bent subassembly and a down-hole motor, various other apparatus and methods have been developed for steerable rotary drilling. Most, if not all of these, provide some means of providing a sideways-direction force relative to the lower end of the drill string to cause the path of the drill string to diverge from a straight path.  
         [0004]     Three early U.S. Pat. Nos. 4,394,881, 4,635,736 and 5,038,872, disclosed two spaced-apart centralizers that were mounted to a collar by a number of bladders or other flexible elements that were fluid-filled. Fluid passages connected upper bladders to lower bladders such that if an upper was compressed on the low side of the hole, a lower one would receive pressure on the high side of the hole to force the bit down. There were no sensor elements and no gain functions in the system.  
         [0005]     Two other rotary steering developments are disclosed in prior patents, referred to as a modulated bias unit, GB 2,259,316 and U.S. Pat. No. 5,520,255, and a control unit, GB 2,257,182, U.S. Pat. Nos. 5,265,682 and 5,695,015. This apparatus is generally described in a Schlumberger brochure, “PowerDrive, The New Direction in Rotary Drilling”.  
         [0006]     The modulated bias unit as generally described in the brochure, is firmly attached to the drill string and bit and has piston-like members that can be pushed out to provide side force. The control unit provides control of valving for these pistons that results in cycling the actuators in the modulated bias unit to keep the force acting in a desired spacial direction, as the drill string and bit rotate. The valving for the bias units is controlled by a shaft at the output of the control unit. The shaft is stabilized in space about the rotation axis, but is not however stabilized with respect to level. The attitude of stabilization provides the direction in which the bias unit will push. The control unit basically provides a mechanical control of the bias unit. For example, the Summary in U.S. Pat. No. 5,265,682 states, “The invention also provides a steerable rotary drilling system comprising a roll stabilized instrument assembly having an output control shaft the rotational orientation of which represents a desired direction of steering . . . ”. That patent does not disclose or include a “strapped-down” configuration of sensors. The Background of the Invention states, “With the drill collar rotating, the principle choice is between having the instrument package, including the sensors, fixed to the drill collar and rotating with it, or having the instrument package remain essentially stationary as the drill collar rotates around it (a so-called “roll-stabilized” system).  
         [0007]     In U.S. Pat. No. 5,265,682, the use of roll sensors is discussed, as follows: “As previously mentioned, the roll sensors  27  carried by the carrier  12  may comprise a triad of mutually orthogonal linear accelerometers or magnetometers”, and, “In order to stabilize the servo loop there may also be mounted on the carrier  12  an angular accelerometer. The signal from such an accelerometer already has inherent phase advance and can be integrated to give an angular velocity signal which can be mixed with the signals from the roll sensors to provide an output which accurately defines the orientation of the carrier.” 
         [0008]     U.S. Pat. No. 5,695,015 has a similar statement about “stabilized” vs. “strapped-down”. In all of these control unit patents, the stabilization torque is obtained by vanes in the mud flow and brakes, either electrical or mechanical. Power generation is disclosed as being from the same vanes.  
         [0009]     U.S. Pat. No. 5,803,185, entitled “Steerable Rotary Drilling Systems and Method of Operating Such Systems”, appears to combine one of the earlier bias and control units with additional hardware such that the valving in the control unit can also be used to transmit data to the surface through pressure pulses.  
         [0010]     U.S. Pat. No. 5,842,149, entitled “Closed Loop Drilling System”, addresses steerable rotary drilling and other techniques. It shows and mentions “Directional Devices to Correct Drilling Direction”.  FIG. 3  shows apparatus adjacent to the bit that can push on the sides. Such apparatus does not appear to be described as stabilized in space. The shaft for the drill bit drive appears centralized, while control elements are described as being in a non-rotating part. For example, the patent states “An inclination device  266 , such as one or more magnetometers and gyroscopes, are preferably disposed on the non-rotating sleeve  262  for determining the inclination of the sleeve  262 ”.  
         [0011]     U.S. Pat. No. 5,979,570 discloses an apparatus for selectively controlling, from the surface of the earth, a drilling direction of an inclined wellbore. The apparatus comprises a hollow rotatable mandrel having a concentric longitudinal bore, a single inner eccentric sleeve rotatably coupled about the mandrel and having an eccentric longitudinal bore, an outer housing rotatably coupled around the single inner eccentric sleeve and having an eccentric longitudinal bore with a weighted side adapted to seek the low side of the wellbore, a plurality of stabilizer shoes and a drive means to selectively drive the single inner eccentric sleeve with respect to the outer housing. Since the offset required to provide the desired divergence from the initial wellbore direction is created by the weighted off-center element, this apparatus is only of use in an inclined borehole and is not useful in a vertical, or near-vertical wellbore. Also, the drive means must be activated at the surface of the earth before entry of the drill string into the borehole.  
         [0012]     U.S. Pat. Nos. 5,307,885, 5,353,884 and 5,875,859 disclose the use of one or more eccentric cylindrical members to provide for lateral displacement of a section of the drill pipe. Universal joints are used so that the direction of the bit with respect to the drill string axis of the bit can be changed by the eccentric members. The axial load on the drill bit is transferred around the segment having the universal joints through a fixed outer housing. International Application WO 01/04453 A1 discloses an approach very similar to those three patents, but the drill-pipe segment containing the universal joints is replaced by a flexible pipe section that can be directly bent by the eccentric cylindrical member. In these four patents, as well as with the previously-cited approaches using eccentric cylinders, the degree of lateral offset is controlled by differential rotation of the eccentric cylinders about the borehole axis.  
         [0013]     All of the above prior disclosures lack the unusual advantages in construction, operation and results of the present invention.  
       SUMMARY OF THE INVENTION  
       [0014]     An important object of the present invention is to provide a simpler and less-costly apparatus for steerable rotary drilling that overcomes shortcomings of prior art apparatus, and is useful in boreholes having any directional path, from vertical to horizontal and beyond, and enables its effective direction control force to be set while the drill string is within the borehole.  
         [0015]     Another object of the invention is to provide a “side force” type of apparatus for rotary steerable drilling of a borehole in the earth, wherein a controlled differential displacement is provided between opposed pairs of side force elements that push against the borehole sides as drilling progresses.  
         [0016]     Elements of apparatus for steerable rotary drilling of a borehole in the earth comprise:  
         [0017]     a) a central portion or mandrel, having a central opening therethrough for the passage of drilling fluids,  
         [0018]     b) that central portion having a lower connection suitable for connecting to a drill bit,  
         [0019]     c) that central portion also having an upper connection suitable for connecting to a drill string, or other components, above the apparatus,  
         [0020]     d) an outer housing surrounding a longitudinal part of the central portion or mandrel,  
         [0021]     e) the outer housing having a rotary joint at its upper end for connection to the central portion and having a rotary joint for connection to the central portion so as to permit continuous rotation of the central portion about its longitudinal axis,  
         [0022]     f) one or more pairs of radially-extensible, opposed, side-force exerting elements controlled by a differential displacement drive mechanism within the outer housing to provide a side force exerted against the borehole wall,  
         [0023]     g) a pair of pistons associated with each pair of radially-extensible opposed side-force elements,  
         [0024]     h) one or more displacement transducers associated with each of said pair of pistons,  
         [0025]     i) control valves within the outer housing for control of the differential displacement drive mechanism and  
         [0026]     j) sensing, control and power supply elements to actuate the control valves so as to steer drilling in any desired direction.  
         [0027]     Another object is to provide radially extensible elements configured to be automatically activated whenever there is pressure interior to said mandrel provided by said drilling fluid. Typically there are two pairs of such elements.  
         [0028]     A further object is to provide sensing elements in the form of magnetometer, accelerometer, and/or gyroscopic elements.  
         [0029]     An added object is to provide apparatus for directionally steering a rotary drilling bit in a borehole, comprising  
         [0030]     a) mandrel structure in a drill string above the bit,  
         [0031]     b) multiple side force exerting elements carried by the mandrel,  
         [0032]     c) and means for controllably and selectively exerting hydraulic pressure acting to control lateral displacement of said elements for engagement with the borehole wall,  
         [0033]     d) said means including directional control instrumentation sensitive to displacement or positioning of said elements relative to the borehole, including at least one of the following: 
        i) a gyroscope     ii) an accelerometer     iii) a magnetometer.        
 
         [0037]     Such means may advantageous include position transducers carried by said side force exerting elements, and circuitry responsive to outputs of said transducers to control solenoid operated valves that in turn control application of borehole fluid pressure to actuators operatively connected to said side force exerting elements.  
         [0038]     These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: 
     
    
     DRAWING DESCRIPTION  
       [0039]      FIG. 1  shows a borehole in cross-section containing a steerable rotary drilling mechanism and also showing a typical desired path change for such a borehole;  
         [0040]      FIG. 2  shows cross-sections A, B and C of a prior art device using eccentric cylinders for directional control;  
         [0041]      FIG. 2   a  shows a longitudinal cross-section of another prior art mechanism having a modulated bias unit;  
         [0042]      FIG. 3  is a longitudinal cross-section of a steerable rotary drilling mechanism of the present invention;  
         [0043]      FIG. 4  is a schematic diagram of hydraulic control circuits of the present invention;  
         [0044]      FIG. 4   a  is a circuit schematic; and  
         [0045]      FIG. 5  shows a block diagram of related measurement, control and power supply equipment used with the steerable rotary drilling mechanism of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0046]      FIG. 1  shows diagrammatically a typical rotary drilling installation of a kind in which the present invention may be used. The bottom hole assembly includes a drill bit  1  and is connected to the lower end of drill string  2  which is rotatably driven from the surface by a rotary table  3  on a drilling platform  4 . The rotary table is driven by a drive motor  5 . Raising and lowering of the drill string, and application of weight-on-bit, is under the control of draw works indicated diagrammatically at  6 .  
         [0047]     The bottom hole assembly includes a bearing section  8  for attachment to the drill string  2  that permits rotary motion between the drill string  2  and the steerable section  9 . The outer surface of the steerable section  9  may be held in a fixed non-rotational direction or it may be allowed to rotate slowly as the drill string penetrates into the earth. Internal to the steerable section, a rotary element connects the drill string  2  to the drill bit  1 . Radially-extensible side-force exertion elements  45  are provided at the lower end of the steerable section  9 , that engage the bore wall and provide the side force acting on the bit enabling drilling to progress in any desired direction. The direction in space of the side force is typically controlled by elements within the steerable section  9 .  
       PRIOR ART  
       [0048]      FIG. 2  shows three cross-section views, normal to the borehole axis, of typical prior art deflection mechanisms that tend to bend the drill string to provide lateral deflection of the drill string with respect to an outer housing. Apparatus of this type is generally referred to as “point the bit” types since the axis of rotation of the bit is changed from the axis of rotation of the driving drill string. An outer cylindrical housing  20  contains two eccentric cylinders, the outer eccentric cylinder  21  and the inner eccentric cylinder  22 . Interior to the inner eccentric cylinder  22  is the drill string pipe  23 . The center of the outer cylindrical housing is at  24 . In the left-hand cross-section A, the eccentric cylinders  21  and  22  are positioned with their eccentricities opposite each other so that the drill string pipe  23  is centered on the center of the outer cylindrical housing at  24 . In the center cross-section B, the eccentricities of the eccentric cylinders are aligned and the drill string pipe  23  is displaced as shown below the center of the outer housing at  24 . This orientation of the offset may be rotated around the borehole axis to cause deflection in any desired direction. Further, as shown in the right-hand cross-section C, the magnitude and direction of the offset may be set to any desired magnitude and direction by combination of the angular positions of the two eccentric cylinders.  
         [0049]      FIG. 2   a , adapted from U.S. Pat. No. 5,803,185, shows another type of apparatus that is generally referred to as a “side-force” type, since a side force is generated just above the bit to force the bit in the desired direction. The axis of rotation of the bit remains colinear with the axis of rotation of the driving drill string. The bottom hole assembly includes a modulated bias unit  25  to which the drill bit is connected and a roll stabilized control unit (not shown) which controls operation of the bias unit  25  in accordance with an on-board computer program, and/or in accordance with signals transmitted to the control unit from the surface. The bias unit  25  can be controlled to apply a lateral bias to the drill bit in a desired direction so as to control the direction of drilling.  
         [0050]     Referring to  FIG. 2   a , the bias unit  25  comprises an elongate main body structure provided at its upper end with a threaded pin  26  for connecting the unit to a drill collar, incorporating the roll stabilized control unit, which is in turn connected to the lower end of the drill string. The lower end  27  of the body structure is formed with a socket to receive the threaded pin of the drill bit. Provided around the periphery of the bias unit, towards its lower end, are three equally spaced hydraulic actuators  28 . Each hydraulic actuator  28  is supplied with drilling fluid under pressure through a respective passage  29  under the control of a rotatable disc control valve  30 -located in a cavity  31  in the body structure of the bias unit. Drilling fluid delivered under pressure downwardly through the interior of the drill string, in the normal manner, passes into a central passage  32  in the upper part of the bias unit, through a filter  33  consisting of closely spaced longitudinal wires, and through an inlet  34  into the upper end of a vertical multiple choke unit  35  through which the drilling fluid is delivered downwardly at an appropriate pressure to the cavity  31 . The disc control valve  30  is controlled by an axial shaft  36  which is connected by a coupling  37  to the output shaft of the roll stabilized control unit.  
       PRESENT INVENTION  
       [0051]      FIG. 3  shows a longitudinal cross-section of a steerable rotary drilling mechanism that provides lateral force applied at the bottom hole assembly to cause drilling to diverge or proceed in a desired direction. A housing  30  contains elements of the steerable assembly. Interior to the housing is a mandrel  31  with extends longitudinally through the assembly. At the upper end  31   a  of the mandrel, means  110  are provided for operative connection to a rotary drill string. Interior to the mandrel, mud or other drilling fluids  32  may flow unrestricted toward a drill bit attached to the bit box  47 , seen in  FIG. 1 . An upper thrust bearing  33  and associated thrust load spring  34  provide axial and radial support between the housing  30  and the mandrel. Another axial bearing  46  is provided at the lower end  111  of the mandrel just above the bit box. Interior to the mandrel, filter screens  35  provide filtered drilling fluid supplied from mandrel bore  31   a  to a rotary hydraulic fluid joint and clean fluid reservoir  36  for control of the apparatus. These items provide a path for clean drilling fluids from the bore of the mandrel  31  to the housing  30  Screens  35  are exposed at  35   a  to drilling fluid in the mandrel, and ducts  112  pass clean fluid to  36 .  
         [0052]     Space  37  for an electronics and power section is provided in the housing, and a hydraulic control system  38  is provided for the control of the apparatus. Numerals  37   a  and  38   a  designates these elements in  37  and  38 . Two pistons or rams  39 ,  40  at opposite sides o the mandrel axis are controlled by the hydraulic control system  38 . Two or more such pairs may be provided for complete 360° azimuth directional control of steering. Note that in  FIG. 3  the elements are shown in a fully-retracted position, prior to the application of any pressure from the drilling fluid. A pair of radially-opposed side-force elements or pads  44 ,  45 , later referred to as Pad  1  and Pad  3  respectively, are forced radially outwardly by inclined surfaces, on cam members  41 ,  42  as those members are controllably pushed axially by the pistons  39 ,  40  as commanded by the control system. These side-force exerting elements engage the nominal borehole wall indicated at  48 . Pads  1 ,  2 ,  3  and  4  may be provided at 0°, 90°, 180° and 270° azimuth positions relative to the mandrel axis. When the same hydraulic pressure is applied to the two pistons  39  and  40 , both side-force elements or pads  44  and  45  are radially extended symmetrically to engage the borehole wall. When the hydraulic control system provides different pressures in the two opposed pistons, the pads are differentially displaced, to effect drilling at a controlled angle or angles.  
         [0053]     It is an important feature of the invention that this differential displacement is accurately controlled. One or more linear displacement transducers are typically provided to sense the linear position of each piston or pad. These transducers may be of suitable type and are shown schematically at  115  and  116 , and at  117  and  118 . They may sense either the axial displacement of the pistons or the radial displacement of the pads. From any of these measurements, the actual pad positions with respect to the housing may be obtained, as by instrumentation at  37   a.    
         [0054]      FIG. 3  also shows interengaged cam surfaces  125  and  126 , and  127  and  128  on the piston driven actuators  129  and  130 , and on the pads, to effect outward driving of the pads. Piston cylinders appear at  39   a  and at  40   a.    
         [0055]      FIG. 4  shows a schematic diagram of one version of the hydraulic control system. A source of filtered fluid at internal drill string pressure is shown at  58 . This internal pressure is designated P 1 . A source of filtered fluid at the borehole annulus pressure outside of the housing  30  is shown at  63 . This external annulus pressure is designate Pa. When the source of drilling fluid pressure, generally mud pumps is not operating, the internal Pressure P 1  and the external annulus pressure Pa will be equal. When such pumps are operating, there will be a substantial pressure drop across the bit resulting from the mud flow through the bit. Thus the internal pressure P 1  may typically be on the order of 300 to 600 p.s.i. higher than the external annulus pressure.  
         [0056]     The charge/discharge valve  50  is spring loaded to expose channels  53 ,  54  (note high pressure from filtered source  58  is provided each channel and the upper piston  51 ) from internal pressure P 1  to each of the pistons  51  and  51   a . (Note channel  53  is connected to port  57  as is channel  54  to port  56 ). Other pairs of pistons not shown are similarly connected and nominally equally spaced to the pair shown. When the mud pumps are operated, the pressure P 1  at  58  increases and is applied directly to the input channels to the valve controlled pistons. The pressure P 1  is also applied to the upper surface of piston  51 , forcing that piston downward and thus closing off the channel  53 . The rate at which this happens is controlled by the bleed rate valve  51   a  which is connected from channel  52  to the port  64  on the external annulus pressure Pa source  63 . This valve may be adjusted to the desired timing for each application circumstance. When the pumps are shut down and P 1  is no longer greater than Pa, the spring-loaded chamber  50   b  in the charge/discharge valve  50  will slowly fill and once again open each piston to the Pa pressure. This relieves the charge of pressure P 1  to the pistons allowing the pistons to relax to the retracted position.  
         [0057]     A dual valve  59 , 60  is activated by a solenoid or other means for thrust control of piston # 1   39  and relief of piston # 3   40 . Similarly, thrust control of piston # 3   40  and relief of piston # 1   39  is provided by dual valve  61 , 62 . A similar arrangement is provided for each additional pair of pistons of radially opposed pistons in the apparatus. As shown in the figure, channels  54  and  56  would connect to a second pair of pistons.  
         [0058]     When drilling is to begin, the pumps turn on to provide drilling fluid pressure, the pistons  51  and  51   a  are charged to pressure P 1  and the charge/discharge valves  50  and  50   a  slowly compress shutting off the charge/discharge ports of each pad piston  39  and  40 . As pressure builds up on the pistons,  51  and  51   a  connecting rods or actuators from the pistons activate the radially-extensible elements or pads outward to engage the borehole wall  48  of  FIG. 3 .  
         [0059]     Assume for example that the apparatus is in a horizontal hole as seen in  FIG. 3 , and that pad # 3   45  is on the low side of the hole and all of the cantilevered weight of the bottom hole assembly is resting on pad # 3 . Clearly, pads # 1 , # 2  (not shown) and # 4  (not shown) with no weight on them will expand to full gauge of the borehole. Assume that it is intended to drill straight ahead. This requires that the radial extension of all pads be the same and that the bit is centered in the borehole.  FIG. 4   a  shows position transducers on each of the pistons to provide signals as to the actual position of each piston and therefore equivalently for each pad. With respect to the opposing pistons shown, these signals are subtracted to provide an error signal that opens valves  61 , 62  so as to force pad # 1  to retract and pad # 3  to extend. When they reach equivalent positions, the error signal is reduced and the drill bit is centered in the borehole parallel to the axes of the pair of pistons. See  FIG. 4   a . Similarly, but not shown, a second pair of pads # 2  and # 4  would equalize their extension. The transducers may comprise one of the following: gyroscope, magnetometer, and accelerometer.  
         [0060]     If it is desired to build up the angle of the borehole, a command signal at  131  is sent to the control system, for example to solenoids, that will operate valves  61 , 62  so as to cause hydraulic piston activation to extend pad # 3  to a greater amount and retract pad # 1  by an equal amount. This places the drill bit above the centerline of the borehole and thus causes the direction of the hole to move upward. Similarly, if it is desired to drop the angle of the borehole, the opposite actions would be commanded. The same procedure can be used with a second pair of pads to cause the borehole direction to move left or right. In all of these actions, the opposed pads of each pair maintain their average radial position and individually have a differential displacement. This controlled action results in the pads continually engaging the borehole wall and stabilizing the orientation of the bit in the borehole for most efficient drilling.  
         [0061]      FIG. 5  shows a block diagram of related measurement, control and power supply equipment typical of such elements used with the present invention. The main blocks are a hydraulic control box  38 , a command box  86 , a sensor box  85 , a power supply  84  and a primary power source  83 . Connections  71  to  78  represent hydraulic lines to each end of four piston cylinders. Connections  89  to  92  represent displacement signals from four pistons or pads. Inputs  87  and  88  represent inputs of the internal drilling fluid pressure P 1  and the annulus drilling fluid pressure Pa. Sensors for these pressures may be of any suitable type. The command box  86  accepts inputs  79  from other equipment to provide either discrete directional commands or a general desired pathway for the borehole. Based on other inputs  81  from the sensor box and power  95  from the power supply, the command box sends by line  80  commands for the positioning of each of the pistons to the hydraulic control box which uses such commands to carry out the operations described above. The sensor box  85  contains all of the sensors that may be desired or needed to control the apparatus. Such sensors may include one or more accelerometers, one or more magnetometers, one or more gyroscopes, various logging sensors and/or various drilling-condition sensors. The power supply box provide any needed regulation, secondary power conversions and distribution of secondary of electrical power. The primary power supply may be batteries or a generator powered by the drilling fluid flow.  
         [0062]     It will be clear to those skilled in the art, that pairs of radially-extensible side force elements or pads can be replaced by any suitable odd number of such elements. For example, three such elements may be used and equivalent commands for pairs of elements can then be resolved into the three directions of operations of such elements.