Abstract:
This invention relates to a controllable deflection tool. The controllable deflection tool is likely to have its greatest utility as part of a downhole assembly to steer a drill bit during drilling for oil and gas. There is provided a controllable deflection tool having a first end and a second end, the tool having: a conduit for a working fluid; a rotary element adapted for rotation within the tool; a deflection member; a vane motor configured to rotate the deflection member relative to the rotary element; and a valve for controlling the flow of working fluid to the vane motor. There is also provided a downhole steering assembly and a method of steering a downhole drilling assembly incorporating the controllable deflection tool.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to United Kingdom Patent Application No. GB1204386.5 filed on Mar. 13, 2012, the contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     This invention relates to a controllable deflection tool, a downhole steering assembly, and a method of use. The controllable deflection tool is likely to have its greatest utility as part of a downhole assembly to steer a drill bit during drilling for oil and gas, and the following description therefore refers primarily to such applications. The use of the controllable deflection housing in other applications is not thereby excluded. 
     BACKGROUND OF THE INVENTION 
     When drilling for oil and gas it is desirable to be able to steer the drill bit, i.e. to move the drill bit along a chosen path, so that the drill bit does not have to follow a path determined only by gravity and/or the drilling conditions. 
     One method for steering a drill bit is to utilise a steering component such as that described in our published European patent 1 024 245. That steering component allows the drill bit to be moved in any chosen direction, i.e. the direction (and degree) of curvature of the borehole can be determined during the drilling operation, and as a result of the measured drilling conditions at a particular borehole depth. 
     Another method of steering a drill bit is to use a deflection member. The deflection member is located close to the drill bit and has a fixed or adjustable deflection which will tend to steer the drill bit in a direction dependent upon the orientation of the deflection. The deflection member may for example be a bent housing, or it may cause the drive shaft or drill bit to deviate from the centre of the borehole being drilled. When it is desired to drill a linear (or more linear) section of borehole the deflection member is rotated so as to continuously change the orientation of the deflection and therefore to cancel out the tendency for the borehole to curve in one direction. Rotation of the deflection member may be effected by way of a downhole motor or by way of the drill string. 
     UK patent applications 2 435 060 and 2 440 024 both describe methods of steering a drill bit by way of a controllable deflection member, the deflection member comprising a bent housing. The bend is provided in the housing of a downhole motor which lies immediately behind the drill bit. The drill string is rotated and there is a rotatable connection between the drill string and the housing of the downhole motor. A clutch mechanism is provided within the rotatable connection, the clutch mechanism controlling the orientation of the housing and consequently the orientation of the bend. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a controllable deflection tool, i.e. to an apparatus which can control the orientation of the deflection member. As in the prior art controllable deflection members for steering a drill bit within a borehole, the deflection member can be controlled to operate in a first condition in which it rotates whereby to cancel out any tendency to deviate the borehole in a particular direction, and a second condition in which its rotation is controlled whereby to cause the borehole to deviate in a chosen direction. 
     The present invention provides a mechanically simple and robust apparatus which is expected to increase the applicability of downhole steering arrangements. 
     According to the invention there is provided a controllable deflection tool having a first end and a second end, the tool having: a conduit for a working fluid; a rotary element adapted for rotation within the tool; a deflection member; a vane motor configured to rotate the deflection member relative to the rotary element; and a valve for controlling the flow of working fluid to the vane motor. 
     Accordingly, by controlling the flow of fluid to the vane motor, the rotation of the deflection member relative to the rotary element can be controlled. The rotary element can be connected to the drill string for example, and can rotate with the drill string. Controlling the rotation of the deflection member relative to the rotary element thereby controls the rotation of the deflection member relative to the drill string. The deflection member can be made to rotate with the drill string, or to counter the rotation of the drill string and maintain a chosen orientation within the borehole. 
     It will be understood that a vane motor is a positive displacement motor, i.e. the rate of rotation is directly controlled by the rate of fluid flow through the motor. Also, a vane motor is mechanically simple and robust and can readily use drilling fluid. The inventors have therefore provided a controllable deflection tool, and can provide a downhole steering assembly, which is sufficiently mechanically simple, and is sufficiently robust, to be used in extremely harsh environments. 
     The method can further include the steps of In drilling applications the working fluid is preferably drilling fluid which is pumped from the surface to the drill bit connected to the second end of the controllable deflection tool. In the simplest embodiments of the invention the controllable deflection tool and the drill bit are connected to a rotatable drill string, the drill bit being driven to rotate by, and at the same rate as, the drill string. In such embodiments the rotary element can be a drive shaft for the drill bit, and the vane motor can be configured to rotate the deflection member relative to the drive shaft. 
     In more typical embodiments the drill string carries a downhole motor, the motor having a stator and a rotor. In typical fashion, the stator is connected to the drill string, and the rotor is connected to the drill bit. The controllable deflection tool will preferably be connected between the downhole motor and the drill bit. In such embodiments a rotatable shaft is preferably provided within the tool to communicate rotary motion from the rotor to the drill bit. It is preferred that the rotatable shaft is separate from the rotary element of the controllable deflection tool, the rotary element for example being connected to the stator and therefore being indirectly connected to the drill string. The rotary element therefore rotates with the drill string, and the vane motor is required to counter the rotation of the drill string rather than the (much faster) rotation of the rotor. 
     Preferably, the valve controls the flow of drilling fluid to the vane motor so that the vane motor is actuated by a quantity of drilling fluid extracted from the drilling fluid flowing along the conduit. Alternatively, the valve controls a hydraulic fluid which passes around a closed loop within the controllable deflection tool. The latter arrangement requires a pump, whereas the former arrangement can avoid the requirement for a pump by utilising the differential pressure of the drilling fluid inside and outside the controllable deflection tool. 
     In common with known vane motors, the vane motor of the present invention comprises an eccentric housing within which is located a body carrying a plurality of vanes, the body being rotatable relative to the eccentric housing. The vanes are movably mounted upon the body so that they remain in contact with the eccentric housing during rotation of the body. 
     The invention also provides a downhole steering assembly adapted for connection to a rotatable drill string, the assembly comprising a drill bit, a downhole motor and a controllable deflection tool located between the downhole motor and the drill bit, the downhole motor having a stator and a rotor, the controllable deflection assembly comprising a rotatable shaft for communicating rotary motion from the rotor to the drill bit, a conduit for the passage of working fluid to the drill bit, a vane motor configured to rotate the deflection tool relative to the stator, and a valve for controlling the flow of fluid to the vane motor. 
     Preferably the stator is connected to the drill string. Preferably also the stator is connected to the body of the vane motor. It is arranged that in use the rotor rotates in the same direction as the drill string, in known fashion. 
     When the valve is closed and fluid does not flow through the vane motor, the deflection tool rotates with the drill string and a linear (or more linear) section of borehole is drilled. When the valve is opened the vane motor can drive the deflection tool to rotate relative to the drill string in the opposed direction to the rotation of the drill string. The rate of counter-rotation of the deflection tool can be matched to the rate of rotation of the drill string so that the deflection tool maintains a constant orientation within the borehole, and the deflection tool causes the drill bit to deviate from a linear path in a chosen direction. 
     Ideally, the vane motor has four vanes, each of which is slidably located in a respective channel of the body. The channels are preferably all open to the conduit for working fluid, so that the pressure of the working (e.g. drilling) fluid acts to drive the vanes towards their extended positions. The vanes are therefore maintained in engagement with the eccentric housing by the pressure of the working fluid within the deflection tool. 
     There is also provided a method of steering a downhole drilling assembly, comprising the steps of: 
     {i} providing a downhole motor, a controllable deflection tool and a drill bit, and connecting the controllable deflection tool between the downhole motor and the drill bit, the controllable deflection tool comprising: 
     a rotatable shaft for communicating rotary motion from the downhole motor to the drill bit, 
     a conduit for the passage of working fluid from the downhole motor to the drill bit, 
     a vane motor configured to rotate the deflection tool relative to the drill string, and 
     a valve for controlling the flow of fluid to the vane motor; 
     {ii} determining a curved path for the drill bit; 
     {iii} operating the valve whereby to rotate the vane motor relative to the drill string; and 
     {iv} modulating the valve whereby to maintain a chosen orientation of the deflection tool. 
     Locating the vane motor between the drill bit and the downhole motor reduces the torque which the vane motor is required to provide in order to control the rotation of the deflection tool. The torque of the vane motor must overcome firstly the friction in the internal bearings and rotating componentry, and secondly the friction due to engagement with the borehole. The vane motor is not required to counter the significantly larger torque induced into the drill string by the downhole motor, as is the case with the prior art arrangements of UK patent applications 2 435 060 and 2 440 024 for example. 
    
    
     
       BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  represents a downhole steering assembly incorporating the controllable deflection tool according to the present invention, in the condition for drilling a linear section of borehole; 
         FIG. 2  is as  FIG. 1  but in the condition for drilling a curved section of borehole; 
         FIG. 3  is a sectional view of a part of the downhole steering assembly of  FIGS. 1 and 2 ; and 
         FIG. 4  is a cross-section through the vane motor of the controllable deflection tool. 
     
    
    
     DETAILED DESCRIPTION 
     The downhole steering assembly  10  of  FIGS. 1 and 2  comprises a drill bit  12 , a controllable deflection tool  14 , a downhole motor  16 , and a stabilizer  18 . The assembly is connected to drill string  20  which continues to the Earth&#39;s surface. 
     In known fashion, a drilling fluid, often called drilling mud, is pumped down the drill string  20 , and through the downhole motor  16 . The controllable deflection tool  14  is configured to operate with a rotating drill string  20 , the drill string being rotated by surface equipment (not shown) in known fashion. The stator (typically the housing) of the downhole motor  16  rotates with the drill string  20 , as represented by the arrow  24 . The downhole motor  16  is a positive displacement motor which converts the passage of drilling fluid into rotation of a rotatable shaft  22  whereby the rotatable shaft  22  rotates in the same direction as the drill string  20 , but at a significantly faster rate. 
     The rotation of the shaft  22  is communicated to the drill bit  12  by way of the controllable deflection tool  14 . The rotation of the drill bit  12 , which is represented by the arrow  26 , is in the same direction as, and at the same rotational rate as, the shaft  22 . 
     The drilling fluid, having passed through the downhole motor  16 , continues through the controllable deflection tool  14  and exits adjacent to the drill bit  12 . The drilling fluid, and entrained drill cuttings, flow along the outside of the downhole assembly  10  and drill string  20  back to the surface, in known fashion. 
     The stabilizer  18  has a number of blades  30  which engage the borehole and serve to centralise the stabilizer  18 . The controllable deflection tool  14  has similar sets of blades  32 ,  34 , the latter comprising a near-bit stabilizer. 
     In the arrangement of  FIG. 1 , the controllable deflection tool  14  is driven to rotate with the drill string  20  as explained below, and is therefore rotating in the same direction as the shaft  22  and drill bit  12 , albeit at a slower rate, the rotation of the controllable deflection tool  14  being represented by the arrow  36 . The orientation of the deflection tool  14 , and in particular the direction of the deflection member or bend  40 , is therefore continuously changing, so that the downhole assembly  10  tends to drill a linear section of borehole. 
     In the arrangement of  FIG. 2  on the other hand, the controllable deflection tool  14  is rotating relative to the drill string  20  in the opposite direction to the drill string, and at the same rate. Accordingly, the orientation of the deflection tool  14  within the borehole is substantially maintained and the downhole assembly  10  tends to drill a curved section of borehole determined by the deflection member, i.e. determined by the angle and orientation of the bend  40 . 
     It will be understood that the present invention can therefore benefit from the reduced sliding friction and hence increased reach (and in particular increased lateral reach) of the borehole which a rotating drill string can provide. However, in alternative embodiments it could be that if desired the drill string does not rotate continuously. 
     In this embodiment the deflection member of the controllable deflection tool  14  comprises a bend  40 , but it will be understood that an alternative deflection member could be utilised, such as an offset stabilizer or an offset drive shaft (i.e. offset from the longitudinal axis of the tool), as desired. As explained in detail below, the deflection tool  14  is directly driven by a vane motor in a contrary direction of rotation to that of the drill string  20 . By precise control of the speed of contra-rotation the deflection tool  14  is caused to adopt a constant orientation with respect to the borehole. By maintaining a constant orientation whilst the bit is rotating and drilling proceeds, a curved section of borehole can be drilled and the trajectory of the borehole is changed. 
     As shown in  FIG. 3 , the downhole motor  16  (only part of which is shown) comprises a rotor  42  and a stator  44 . The stator  44  is connected to the drill string  20  and rotates with the drill string. The rotor  42  is connected to the shaft  22  by way of a constant velocity coupling  46 . The shaft  22  communicates the rotation of the rotor through the controllable deflection tool  14 , and is in turn connected by way of another constant velocity coupling  48  to the driveshaft  50  which is connected to the drill bit  12 . The constant velocity couplings  46 ,  48  ensure that the drill bit  12  rotates at the same rate as the rotor  42 , but permit the required pivoting movement between the respective parts of the downhole assembly  10 . 
     In known fashion, the flow of drilling fluid through the downhole motor  16  causes the rotor  42  to rotate relative to the stator  44 . As represented by the small arrows in  FIG. 3 , the drilling fluid flows past the constant velocity coupling  46 , along a conduit  54  which surrounds the shaft  22 , past the constant velocity coupling  48 , along the driveshaft  50  and exits at the drill bit  12 . The drilling fluid thereafter flows along the outside of the downhole assembly  10  and drill string  20  back to the surface. 
     The conduit  54  is defined in part by a sleeve  58  which surrounds the rotatable shaft  22 . The sleeve  58  is connected to the stator  44  and rotates with the stator (and therefore with the drill string  20 ). The sleeve  58  comprises the rotary element in this embodiment. The sleeve  58  is not shown in  FIGS. 1 and 2  for clarity, but it will be understood that in practical embodiments the shaft  22  is not visible between the downhole motor and the controllable deflection tool since it is hidden within the sleeve  58 . 
     The controllable deflection tool  14  includes a vane motor  52 . The vane motor  52  in this embodiment is driven by the drilling fluid. A port  56  is in communication with the conduit  54 , the flow of fluid through the port  56  being controlled by a valve  60 . As shown in  FIGS. 3 and 4 , when the valve  60  is open, drilling fluid can pass along fluid conduit  62  and enter the chamber  64  between the body  66  and the eccentric housing  68 . 
     The apparatus can be run into the wellbore in the primary configuration. 
     The method can include the step of automatically returning to the primary configuration after a predetermined period of time. 
     The drilling fluid leaves the chamber  64  through the outlet port  72  and returns to the surface with the drilling fluid which has passed the drill bit. 
     The body  66  is connected to the stator  44  of the downhole motor  16  by way of the rotary element or sleeve  58 . The body  66  of the vane motor  52  is therefore directly driven to rotate with the stator  44  and therefore with the drill string  20 . 
     When viewed from the uphole end as in  FIG. 4 , the drill string  20  and consequently the sleeve  58  and body  66 , typically rotate clockwise. The vane motor  52  and thus the deflection tool  14  are therefore configured to counter the rotation of the drill string  20  by rotating the eccentric housing  68  counter-clockwise relative to the sleeve  58 . 
     The energy required to introduce drilling fluid into the vane motor  52  is provided by the differential between the pressure within the conduit  54  of the deflection tool  14  and the pressure outside the deflection tool (i.e. between the deflection tool  14  and the borehole). This differential pressure is approximately equal to the pressure drop across the drill bit  12 , and is typically several million Pascals (several hundred pounds per square inch). 
     The body  66  carries four vanes  70  and can rotate relative to the eccentric housing  68 , the vanes remaining in contact with the eccentric housing  68  as they rotate within the eccentric housing. The vanes  70  are movable relative to the body  66 , each vane  70  being slidably located within a respective channel  74 . A set of ports  76  through the sleeve  58  deliver drilling fluid into each of the channels  74 , the pressure of the drilling fluid acting to extend the vanes  70  into contact with the eccentric housing  68 . 
       FIG. 4  shows a small clearance between the vanes  70  and their respective channels  74 , and also between the vanes  70  and the eccentric housing  68 , but that is only for the purpose of clarity and it will be understood that the vanes are in sliding and sealing contact with their channels, and in sliding and sealing contact with the eccentric housing  68 . 
     The sleeve  58  and body  66  are supported by thrust bearings  78  and radial bearings  80  which facilitate rotation of the sleeve  58  and body  66  within the deflection tool  14  and in addition transfer drilling loads from the deflection tool  14  to the downhole motor  16 . Similarly, thrust bearings  82  and radial bearings  84  transfer drilling loads from the drill bit  12  to the deflection tool  14 . 
     When the valve  60  is closed the vane motor  52  is hydraulically locked against rotation relative to the sleeve  58 . The eccentric housing  68  is driven to rotate with the body  66  and since the eccentric housing  68  is connected to the housing  28  of the controllable deflection tool  14 , the housing  28  rotates at the same rate as the drill string  20 . This is the situation represented in  FIG. 1 . 
     To change the trajectory of the borehole a signal (in this embodiment a coded pressure pulse within the drilling fluid) is communicated from the surface, specifying the required orientation of the deflection member or bend  40 . This signal is detected by a pressure sensor  86  and decoded in the control module  88 . 
     A control signal is communicated to the valve actuator  90 , whereupon the valve  60  is gradually opened, causing drilling fluid to flow into the chamber  64  of the vane motor  52 . The body  66  and vanes  70  continue to rotate with the sleeve  58  and drill string  20 , and fluid flowing into the chamber  64  causes the rate of rotation of the eccentric housing  68  (and thereby the rate of rotation of the deflection tool housing  28  and the deflection member  40 ) to reduce. 
     With sufficient fluid flow through the vane motor  52 , the vanes  70  and body  66  are driven by the fluid to rotate relative to the eccentric housing  68  at the same rate as they are being driven by the drill string  20  relative to the borehole, at which point the eccentric housing  68  stops rotating relative to the borehole (and similarly the tool housing  28  stops rotating relative to the borehole, with the flowing fluid effectively driving the vane motor  52  to rotate in the opposite direction to the drill string). A sensor module  92  detects that the counter-rotation of the deflection tool  14  matches the rotation of the drill string  20 . The valve  60  is thereafter modulated until the required orientation of the deflection tool  14  is achieved and maintained. This is the situation represented in  FIG. 2 . 
     Confirmation of the orientation of the deflection tool  14  and measurements of the borehole trajectory are sent to the surface by way of a pulser module  94  which introduces a coded pressure signal into the drilling fluid by venting drilling fluid through a pulser valve  96 . 
     In this embodiment electrical power for the valve actuator  90 , control module  88 , sensor module  92 , pulser module  94  and pulser valve  96  is supplied by a battery module  98 . However, in alternative embodiments an electrical generator, powered either by drilling fluid flow or from rotation of the driveshaft  50  or rotatable shaft  22 , could be used instead of, or in addition to, the battery module. 
     If it is desired not to use the drilling fluid to power the vane motor  52 , a pump (such as a separate vane pump for example) could be driven by the driveshaft  50  or shaft  22  to provide a closed loop supply of hydraulic fluid to the vane motor  52 . 
     It will be understood that the controllable deflection tool  14  could be used with a rotating drill string without a downhole motor. In such embodiments the drill bit rotates at the same rate as the drill string and there is no requirement for a separate rotatable shaft. One such embodiment could differ from the arrangement shown in  FIG. 3  by omitting the shaft  22  and continuing the rotary element or sleeve  58  through the tool  14 , the sleeve being connected to the constant velocity coupling  48  and thereby to the drive shaft  50 . The vane motor  52  could operate in the same way in order to rotate the tool housing  28  and deflection member  40  relative to the sleeve  58 . 
     It will be understood that the use of pulse signals in the drilling fluid is only one means of communicating from and to the surface, and alternatively other known means of communicating with downhole tools could be used if desired.