Torque control device for a downhole drilling assembly

This invention relates to a torque control device for a downhole drilling assembly. The torque control device is adapted for connection to a drill bit, and has an outer sleeve and an inner shaft, the outer sleeve being movable longitudinally relative to the inner shaft. The torque control device also has a cylinder, a piston located within the cylinder, and a controller to control the volume of the cylinder. Changing the volume of the cylinder causes relative longitudinal movement between the outer sleeve and the inner shaft. The controller can receive inputs from a torque sensor and/or an accelerometer in order to determine if a threshold torque has been exceeded. Alternatively the controller can comprise a rotary valve which automatically responds to torque in the assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to United Kingdom Patent Application No. GB1203433.6 filed on Feb. 28, 2012, and United Kingdom Patent Application No. GB1211300.7 filed on Jun. 26, 2012, the contents of each one incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a torque control device for a downhole drilling assembly.

BACKGROUND OF THE INVENTION

When drilling for oil and gas, the downhole drill bit is connected to surface equipment by way of a drill string. The drill string is hollow whereby drilling fluid or mud can be pumped down the borehole, the mud acting to lubricate the drill bit and to carry drill cuttings back to the surface. The mud and entrained drill cuttings return to the surface along the outside of the drill string, the drill string being smaller than the diameter of the borehole.

In some drilling applications the drill string is rotated at the surface, with the rotation being communicated to the drill bit by the drill string. In other drilling applications a downhole motor such as a mud motor is provided, which uses the flowing mud to drive the drill bit to rotate. A downhole motor may be used with a rotating, or a non-rotating, drill string.

The surface equipment applies a downhole force upon the drill string, which force is communicated to the drill bit. In addition to the torque seeking to rotate the drill bit there is also a force acting to advance the bit into the rock at the leading end of the borehole, the latter force typically being referred to as “weight on bit”.

The drill operator will typically seek to maximise the weight on bit so that the drill advances as quickly as possible through the rock. However, there is a maximum limit for the weight on bit which depends upon the bit design and the drilling conditions. Exceeding the maximum weight on bit for the particular bit design and drilling conditions will increase the drag upon the drill bit and cause the drill bit to slow down or stall, i.e. the drill bit will rotate more slowly, or in extreme cases stop rotating altogether.

If the drill bit does rotate more slowly than the drill string, or than the output of the downhole motor, then the drill string will be caused to twist as torque output from the surface equipment (or downhole motor) increases in response to maintain the original rate of rotation. Eventually, torque at the drill bit will exceed the resistance to rotation and the drill bit will start to rotate again.

Such a phenomenon is known as “stick-slip” and is a major concern to drill operators. Firstly, the drill string may be damaged by the requirement to twist as the drill bit slows down or stops. Secondly, the drill bit will often rotate very rapidly, and uncontrolledly, as the torque in the twisted drill string is relieved. Periods of slow or non-rotation of the drill bit followed by rapid and uncontrolled rotation of the drill bit will often be repeated if they are not countered.

Drill operators seek to avoid stick-slip by reacting to reductions in the rate of rotation of the drill bit by reducing the weight on bit, so that the drill bit resumes its desired rate of rotation quickly without undue twisting of the drill string. A reduction in the rate of rotation of the drill bit can be detected directly by measuring the rate of rotation of the drill bit, or (more typically) by measuring the torque being applied to the drill bit, the torque increasing as the rate of rotation reduces.

The prior art includes torque control devices which can automatically reduce the weight on bit if the torque upon the drill bit exceeds a certain threshold. One prior art arrangement is described in WO 2004/090278 (Tomax). This document has an outer sleeve connected to the drill string and an inner shaft connected to the drill bit. The outer sleeve and the inner shaft are interconnected by a helical thread. A spring biases the inner shaft outwardly of the outer sleeve, into engagement with a fixed stop upon the outer sleeve. During normal drilling operations the inner shaft is driven to rotate by the sleeve, and in turn drives the drill bit to rotate at the same rate as the drill string. If the drill bit slows down or stops, however, the torque upon the drill bit increases sufficiently to drive the sleeve to rotate relative to the shaft, compressing the spring. The helical thread between the inner shaft and the outer sleeve means that rotation of the inner shaft relative to the outer sleeve causes the inner shaft to retract into the sleeve, thereby retracting the drill bit and reducing the weight on bit. As the weight on bit is reduced a point is reached where the drill bit can resume its rotation. The spring causes the inner sleeve to return to its extended position in engagement with the fixed stop, during which the drill bit rotates faster than the drill string.

The Tomax arrangement can include an oil damper, i.e. the spring and cooperating helical threaded components can lie within an oil reservoir which damps out the movement of the inner shaft relative to the outer sleeve, preventing uncontrolled rotation of the inner sleeve and therefore the drill bit.

A similar arrangement is described in U.S. Pat. No. 7,044,240 (McNeilly), and also in Tomax's later U.S. Pat. No. 7,654,344, which uses a helical spring rather than a helical thread to interconnect the outer sleeve and the inner shaft.

The prior art arrangements all rely upon compression springs, and it will be understood that the force provided by those springs must exceed the weight on bit. The design of the tools must therefore include a calculation for the maximum weight on bit which can be catered for, and once the spring rate has been determined it cannot be adjusted. When drilling for oil and gas, however, the rock type through which the drill must pass can vary significantly during a drilling operation, and if the spring force is set too low the tool may reduce the drilling torque even if the drill is not sticking, i.e. the drill operator cannot exceed the weight on bit determined by the spring force, even if the drilling conditions are more favourable than expected and the drill bit would not stick with a greater weight on bit. If, on the other hand, the spring force is set too high for the particular drilling conditions, the drill bit may undergo significant stick-slip without actuation of the torque control device.

SUMMARY OF THE INVENTION

The inventor has realised that an improved device is required for reducing the weight on bit and thereby reducing the torque upon a drill bit whereby to reduce or avoid the likelihood of stick-slip. One object of the invention is to provide a device which enables the torque at which the weight on bit is reduced to be adjusted downhole to match the drilling conditions.

According to the invention there is provided a torque control device for a downhole drilling assembly, the torque control device being adapted for connection to a drill bit, the torque control device including an outer sleeve and an inner shaft, the inner shaft being movable longitudinally relative to the outer sleeve, the inner shaft having a through-bore for carrying drilling fluid to the drill bit, the device having a piston and cylinder arrangement and a controller which controls the volume of the cylinder.

It is arranged that the relative position of the inner shaft relative to the outer sleeve (in the direction of the longitudinal axis of the torque control device) is determined by the volume of the cylinder, so that the controller controls the (longitudinal) movement of the inner shaft relative to the outer sleeve. The controller preferably has a memory in which is stored a threshold value, the controller causing the inner shaft to move relative to the outer sleeve when the threshold value is reached or exceeded. The controller can desirably be adjusted (preferably downhole) whereby the threshold value can be adjusted to match the drilling conditions.

The controller can be connected to a torque sensor adapted to measure the torque in a part of the downhole assembly, suitably the torque in a part of the downhole assembly connected to the drill bit. Alternatively, the controller can be connected to a sensor such as an accelerometer which measures the rate of rotation of the drill bit (or a part of the downhole assembly connected to the drill bit) whereby to detect reductions in the rate of rotation of the drill bit. The controller can in some embodiments receive and compare the inputs from two accelerometers, one accelerometer located close to the drill bit and the other accelerometer located remote from the drill bit. Sticking of the drill bit can be detected by changes in the relative outputs of the two accelerometers.

Preferably the inner shaft is connected to the drill string and the outer sleeve is connected to the drill bit, but it will be understood that the orientation of these components can be reversed without departing from the invention.

Desirably, the cylinder is connected to the through-bore whereby the cylinder will be filled with drilling fluid in use. The drilling fluid can therefore provide the hydraulic fluid for the piston and cylinder arrangement. In such an arrangement, the cylinder can also be open to the periphery of the downhole assembly, so that in use drilling fluid can flow out of the cylinder into the annulus surrounding the downhole assembly, and along which the drilling fluid returns to the surface. Such arrangements take advantage of the pressure differential which occurs between the drilling fluid within the through-bore (i.e. upstream of the drill bit) and in the annulus (i.e. downstream of the drill bit).

Preferably, the controller controls the position of an actuating valve whereby to control the flow of drilling fluid into the cylinder. It can be arranged that the port from the through-bore into the cylinder is of larger cross-section than the port in the periphery of the downhole assembly. This arrangement avoids the requirement for a separate actuating valve controlling the egress of drilling fluid from the cylinder, it being arranged that the larger entry port will act to increase the volume of the cylinder when the actuating valve is opened, and the (always open) exit port will allow the drilling fluid to drain out of the cylinder, so as to reduce the volume of the cylinder, when the actuating valve is closed.

Desirably, a return spring is provided to bias the piston so as to reduce the volume of the cylinder. It is arranged that when the actuating valve is closed the biasing force of the return spring is sufficient to force drilling fluid out of the cylinder and into the surrounding annulus so as to reduce the volume of the cylinder and drive the inner shaft to move longitudinally relative to the outer sleeve.

In certain embodiments the threshold value of the controller can be adjusted during use. It is known to communicate from the surface to a downhole tool, and it is also know to communicate by way of the drilling fluid. In the “RipTide” drilling reamer of Weatherford, Inc. radio frequency identification (RFID) units are injected into the drilling fluid and sent downhole with the fluid. As the RFID units pass a controller of the reamer they are read and used to adjust the status of the reamer. A similar system can be used with the present invention, with the controller being adapted to react to messages sent downhole, for example by way of RFID units, whereby the threshold value for actuation of the device can be adjusted during use. It is therefore not necessary to trip the downhole assembly in order to adjust the threshold value, and if the drilling conditions become more (or less) favourable and a greater (or lesser) weight on bit can be accommodated without incurring stick-slip, the threshold value can be increased (or decreased) accordingly.

Certain embodiments of the present invention can avoid the requirement for sensors communicating torque and/or acceleration to the controller. In such embodiments the controller is in the form of a rotary valve, and admission of drilling fluid into the cylinder is controlled by the rotary valve which automatically moves to an open position (or to a more open position) when the torque within the downhole assembly exceeds a predetermined threshold.

Whilst in the simplest embodiments of the present invention the drilling fluid is caused to flow into and out of the cylinder in order to determine the volume of the cylinder, in other embodiments a closed hydraulic system is used. In those other embodiments the volume of the cylinder, and therefore the position of the inner shaft relative to the outer sleeve, is determined by a hydraulic fluid which is isolated from, and independent of, the drilling fluid. Such alternative embodiments are more mechanically complex, but avoid the possible problems associated with the use of drilling fluid as the hydraulic fluid. The electrical and hydraulic power for a closed hydraulic system can be provided by a downhole pump in known fashion.

The inventors have also realised that the device of the present invention can be used for other downhole applications where the torque transmitted to the drill bit requires adjustment. One such application is in drilling applications using an under-reamer for example. An under-reamer, such as the aforementioned “RipTide” drilling reamer of Weatherford, Inc., uses a reamer as well as a drill bit, the reamer following the drill bit and reaming out a larger diameter borehole along chosen lengths of the borehole. It is advantageous to balance the drilling torque provided by the drill string between the drill bit and the reamer so as to maximise the rate of advance of the downhole assembly. The present invention can be located in the downhole assembly between the reamer and the drill bit and can control the torque transmitted to the drill bit and thereby control the proportion of the drilling torque utilised by the drill bit and that used by the reamer.

DETAILED DESCRIPTION

The torque control device10of the present invention is part of a downhole assembly12which is adapted to drill a borehole14into the Earth16. The longitudinal axis A-A of the downhole assembly12(which corresponds to the longitudinal axis of the torque control device10) is shown horizontal inFIGS. 1 and 2, but the orientation is unimportant and the present invention can be used with the longitudinal axis at any chosen angle.

The downhole assembly12includes a female threaded connector20by which the assembly may be connected to a length of drill string (not shown) connected to the surface. Alternatively, the connector20can be connected to a downhole motor such as a mud motor, or to a downhole steering tool such as that of EP 1 024 245. It will be understood, however, that the tool can be located uphole of a steering tool if desired.

The connector20is connected to an inner shaft22, which has a through-bore24through which drilling fluid can flow to the drill bit26, in known fashion. In common with prior art downhole assemblies, the drilling fluid passes out through ports (not shown) in the drill bit26, and then returns to the surface by way of the annulus30surrounding the downhole assembly12and the drill string.

Though not shown in the drawings, it will be understood that the torque control device10will typically include a plurality of blades which engage the borehole14and serve to centralise the torque control device10within the borehole14. The downhole assembly may in practice also include a stabiliser located between the torque control device10and the drill bit26, and/or between the connector20and the drill string.

The drill bit26is connected (in the embodiment ofFIGS. 1 and 2directly, but in other embodiments indirectly) to an outer sleeve32which surrounds a part of the inner shaft22. At least one set of splines34interconnect the inner shaft22and the outer sleeve32, so that the inner shaft22can slide longitudinally relative to the outer sleeve32, but cannot rotate relative to the outer sleeve. The number and disposition of the splines will depend upon the torque which is to be transmitted from the inner shaft22to the outer sleeve32.

During normal drilling operations, in the absence of stick-slip, the torque control device10is in the condition shown inFIG. 1. Rotation of the drill string (and/or downhole motor) is communicated to the connector20and, by way of the inner shaft22and splines34, to the outer sleeve32and the drill bit26.

The through-bore24has a port36which opens into a valve chamber within the body of a piston40, the piston40comprising an enlargement of the inner shaft22. An actuating valve42is located within the valve chamber of the piston40, the actuating valve42being controlled by a controller44. The actuating valve42controls the passage of drilling fluid from the through-bore24, through the port36and into a cylinder46. The cylinder46has another port50which is open to the periphery of the device10, and therefore to the annulus30surrounding the downhole assembly12.

It will be understood that the pressure of the drilling fluid within the through-bore24is substantially higher than the pressure of the drilling fluid within the annulus30, the difference in pressure being caused primarily by the pressure drop across the drill bit26. It is arranged that the entry port36is of significantly larger area than the exit port50, so that when the actuating valve42is opened drilling fluid flows into the cylinder46from the through-bore24at a faster rate than fluid can flow out of the cylinder46through the port50.

If the weight on bit is too great for the particular drilling conditions, the rotation of the drill bit26will slow relative to the rotation of the connector20. In the present embodiment this is detected by a strain gauge52located upon the shaft22. It will be understood that the strain gauge52is sufficiently sensitive to detect very small angular twisting movements of the inner shaft22, as caused by small angular deviations of the drill bit26relative to the connector20, which are indicative of the drill bit slowing and the possible onset of stick-slip. The strain gauge52detects the strain in the inner shaft22and communicates this to the controller44. The communication is preferably by wires (not shown), but the form of data transmission is not critical to the invention.

The controller44has a memory in which is stored a high threshold strain value, and against which the strain measured by the strain gauge52is continuously or repeatedly compared. If the comparison is not continuous, it is sufficiently frequent so as quickly to identify unacceptable increases in the measured strain. The high threshold strain value may be determined by calculation or experiment. If the measured strain exceeds the high threshold strain value the controller opens the actuating valve42and permits drilling fluid to flow into the cylinder46.

As shown inFIG. 2, when the actuating valve42is opened, drilling fluid flows into the cylinder46through the entry port36. Since the flow rate through the entry port36and past the valve42into the cylinder46is greater than the flow rate out of the cylinder through the exit port50, the volume of the cylinder46is thereby caused to increase. The piston40is fixed to the inner shaft22and does not move relative to the inner shaft22. Instead, as the volume of the cylinder46increases the outer sleeve32moves to the right as drawn. This rightwards movement is represented inFIG. 2by the drill bit26being lifted from the bottom of the borehole14; in practice the actual movement may be very small, but the force with which the drill bit26engages the end of the borehole (i.e. the weight on bit) can be reduced significantly.

During this retracting movement of the outer sleeve32, the connector20continues to rotate, and at some point the torque upon the drill bit26will exceed the frictional resistance to rotational movement and the drill bit will resume rotation (and will unwind any twist which has been imparted into the drill string).

As the drill bit26resumes is rotation, the strain upon the inner shaft22will reduce, and will be detected by the strain gauge52. The memory of the controller44also stores a low threshold strain value, the low threshold strain value being a chosen amount lower than the high threshold strain value so as to avoid “hunting”. When the low threshold strain value is passed the controller44closes the actuating valve42.

In other embodiments the controller44stores only a single threshold strain value, the controller opening the valve42when the measured strain rises above that value, and closing the valve42when the measured strain falls below that value.

The controller44can if desired close the actuating valve42to an intermediate position at which the rate of drilling fluid flowing into the cylinder46closely matches the rate of fluid flowing out of the cylinder, and it may be arranged to maintain the intermediate position for a predetermined period of time, perhaps a few seconds, so that the device dwells in that operational position (with the volume of the cylinder46remaining substantially constant).

When the actuating valve42is closed the compression spring54acts to drive the drilling fluid out of the cylinder46, through the exit port50, so that the tool returns to the condition ofFIG. 1. Desirably, the exit port50is sufficiently small so that it takes several seconds (e.g. 2-3 seconds) for the device to move from the condition ofFIG. 2to the condition ofFIG. 1, it being preferred that the weight on bit be gradually increased back to its desired level rather than suddenly increased.

The drill operator at the surface will be aware that the torque control device10has been actuated by virtue of the reduction in pressure of the drilling fluid caused by the opening of the actuating valve42. The drill operator will typically react by reducing the weight on bit at the surface so as to avoid the onset of stick-slip. The operator can check that the device10does not undergo repeated actuation, and if so can steadily increase the weight on bit back to the desired level.

Since the actuation of the torque control device10is not dependent upon the force exerted by a spring, the drill operator can set the maximum weight on bit for the drilling conditions. The spring54can therefore be made sufficiently strong to exceed the maximum weight on bit which the surface equipment can impart (so that the spring54can drive the tool from the condition ofFIG. 2to the condition ofFIG. 1when the actuating valve42is closed, regardless of the actual weight on bit. It is not necessary to set the spring force dependent upon the likelihood of stick-slip as in the Tomax and other prior art arrangements.

The drill operator can also adjust the high and low threshold strain values for the actuating valve downhole, without needing to trip the downhole assembly. Specifically, the drill operator at the surface can communicate with the tool10, and in particular with the controller44, whilst the tool10is downhole. Such communication may be effected by any of the known means for communicating with downhole tools, for example by wire, radio waves, mud pulsing, or RFID units injected into the drilling fluid. Thus, if it is determined that the threshold for actuating the valve42is set too low, so that the valve is actuated at strain levels which would not result in damaging stick-slip, the high threshold strain value may be increased without tripping the tool. The drill operator can also switch the torque control device10on and off remotely, it perhaps being desirable to switch the torque control device off in certain situations so as to save power.

An alternative embodiment of torque control device110is shown inFIG. 4. Though not shown inFIG. 4, the downhole assembly112will also include a drill bit (perhaps similar to the drill bit26of the embodiment ofFIGS. 1 and 2) which is secured by way of a male threaded connector56. Alternatively, a mud motor for example may be located between the drill bit and the torque control device110.

The connector120is connected to an upper shaft60, which has a through-bore124by which drilling fluid can flow to the drill bit (not shown), in known fashion.

The connector56is connected to an outer sleeve132which surrounds a lower shaft122and part of the upper shaft60. At least one set of splines134interconnects the lower shaft122and the outer sleeve132, so that the lower shaft122can slide longitudinally relative to the outer sleeve132, but cannot rotate relative to the outer sleeve. As with the embodiment ofFIGS. 1 and 2, the number and disposition of the splines will depend upon the torque which is to be transmitted from the lower shaft122to the outer sleeve132.

The upper shaft60is separate from the lower shaft122,FIG. 4showing an exaggerated gap62between the facing ends of these shafts. The upper shaft60has an enlarged end which forms a piston140as described below. A part of the piston140surrounds the end of the lower shaft122, and a set of axial bearings64interconnect the piston140and the lower shaft122. The axial bearings64permit relative rotation between the piston140and the lower shaft122, but resist relative longitudinal movement. It is therefore arranged that the piston140is fixed upon the upper shaft60, and can rotate relative to the lower shaft122.

The through-bore124within the lower shaft122has a port136which lies within the region of the lower shaft122which is surrounded by the piston140. The piston has a conduit66which can be aligned with the port136whereby drilling fluid can pass from the through-bore124into a cylinder146.

The cylinder146has an exhaust conduit150which in this embodiment passes through the piston140, and opens into a spring chamber68. An exhaust port70is provided for the spring chamber68, the exhaust port70being open to the periphery of the downhole assembly112.

It is arranged that the port136and conduit66are of larger cross-sectional area than the exhaust conduit150, so that when the conduit66is fully aligned with the port136drilling fluid flows into the cylinder146from the through-bore124at a faster rate than fluid can flow out of the cylinder146through the conduit150.

A spring72is located within the spring chamber68. One end of the spring72is located in a piston spring pocket74and the other end of the spring is located in a sleeve spring pocket76. The spring72acts primarily as a torsion spring, and seeks to rotate the piston140relative to the sleeve132. Since the sleeve132is non-rotatably connected to the lower shaft122by way of the splines134, the spring72also acts to rotate the piston140relative to the lower shaft122. It is arranged that the spring72is biased to move the conduit66out of alignment with the port136.

Thus, in normal operation the conduit66is out of alignment (or at least out of full alignment) with the port136, whereby drilling fluid either cannot flow into the cylinder146at all, or at most flows into the cylinder146at a rate below that at which it flows out along the conduit150. The volume of the cylinder146is therefore minimised, and the sleeve132is extended (to the left as drawn) to its farthest extent relative to the upper shaft60and piston140.

If the weight on bit exceeds the maximum for the drilling conditions, the rate of rotation of the drill bit will reduce. The drill bit is connected to the sleeve132so that the rate of rotation of the sleeve, and thereby the lower shaft122, also reduce. The drill string and therefore the upper shaft60, however, continue to rotate, so that there is relative rotation between the piston140and the lower shaft122. The conduit66and the port136will thereby be forced into greater alignment, against the torsional bias of the spring72, and perhaps into full alignment as shown inFIG. 4. When so aligned, the flow rate of drilling fluid into the cylinder146will exceed the flow rate of fluid out of the cylinder146, so that the volume of the cylinder146increases and the sleeve132is forced towards the right as viewed, automatically reducing the weight on bit.

As the weight on bit is reduced the rate of rotation of the drill bit increases and the torque within the downhole assembly110is reduced. The spring72can then rotate the conduit66and port136out of alignment and the drilling fluid bleeds out of the cylinder146.

It will therefore be understood that the port136and conduit66act as a rotary valve to automatically control the volume of the cylinder146by allowing drilling fluid (or more drilling fluid) into the cylinder when the rate of rotation of the drill bit drops below that of the drill string.

The spring72can determine a threshold value for the torque which will be required to open the rotary valve. It will be understood that the piston140needs to rotate through only a few tens of degrees in order to move a totally misaligned conduit66and port136into full alignment, and the range of relative rotation may be limited by stops (not shown). The torque control device110can be assembled with the spring72under a chosen pretension, i.e. the spring72can in normal conditions bias the piston140against a rotational stop.

Whilst the primary function of the spring72is to control the rotary valve66,136, it also acts as a compression spring and assists the movement of the sleeve132(and therefore the drill bit) to the left as drawn as the drilling fluid drains from the cylinder146. However, unlike the prior art arrangements, the compression force of the spring72does not provide the upper limit for the weight on bit.

In the embodiment shown inFIG. 4the relative rotation of the piston140and the lower shaft122is directly dependent upon the torque applied to the drill bit by the drill string. In a further modification, a detent mechanism can be provided between the piston140and the lower shaft122, the detent mechanism allowing relative rotation only when a predetermined threshold torque has been exceeded. With such a modification, the opening movement of the rotary valve would be less progressive than the embodiment ofFIG. 4.

It will be understood that a small gap is shown between the inner shaft22and the outer sleeve32inFIGS. 1 and 2, and similarly between the inner shafts60and122and the outer sleeve132inFIG. 4, for the purposes of clarity. In practice, these components would be in sliding engagement, with suitable seals for the cylinder46,146etc.

FIG. 3represents schematically another useful application of the torque control device10,110. In this application, the torque control device10,110is located between the drill bit26and a reaming tool60. In known fashion, the reaming tool60includes cutting blades62which can be refracted into the body of the tool60when not required (during passage through a borehole casing for example) and then actuated to their extended condition as shown at a chosen location downhole. When the cutting blades62are extended, the drill bit26and the reaming tool60are both engaging respective sections of rock. To maximise the rate of advance of the downhole assembly it is desirable to impart a proportion of the torque provided by the drill string to the drill bit26and another proportion of the torque to the reaming tool60, the actual proportions depending on the drilling conditions and the cross-sectional area of rock being removed by the respective components. The tool10,110can be used to reduce the torque being imparted to the drill bit26, and thereby to increase the torque being imparted to the reaming tool60, the respective proportions being determined by the threshold strain value set for the actuating valve42in the embodiment ofFIGS. 1 and 2, or that set for the rotary valve66,136in the embodiment ofFIG. 4. If the threshold strain value is set correctly, the efficiency of the downhole assembly will be increased, i.e. both the drill bit26and the reamer blades62will be driven against the respective rock faces with an appropriate force and the advance of the downhole assembly will be maximised.

The torque control device10,110is expected to have its greatest utility when used with PDC drill bits, but the invention can be used with other types of drill bit if desired.