Technical field

The present invention relates to the drilling of boreholes.

Background

In the oil and gas exploration and production industry, boreholes in the Earth’s subsurface are typically drilled by means of a rotating drill string. Such a drill string is typically supported and rotated by equipment at surface, e.g. a drilling platform or rig, and has a drill bit at the end of the string for cutting into the subsurface rock.

The drill string comprises sections of drill pipe which are coupled in end-to-end relationship along the string, allowing a drilling fluid (also called mud in the following) to be circulated into the well along the drill string through the inside of drill pipes. The drill string may also include other tubular subs or elements which together with the drill pipe form a connected interior flow path through the drill string. The drilling fluid exits the drill string at the downhole end of the wellbore, at or near the drill bit, and is circulated back uphole along an outside of the drill string in an annulus between an outer surface of the drill string and the surrounding wall of the borehole. As the drill string advances into the subsurface, further tubulars e.g. stands of two or three sections of drill pipe, are added to the string.

The type of drilling fluid to be used can be selected depending upon requirements, and may be varied or may change in composition during the drilling process. The key functions of the drilling fluid are to carry and transport cuttings out of the well, to control pressure in the wellbore (e.g. for hindering influx of formation fluids), and to cool the drill bit and other critical components of the drill string (also referred to as the string in the following). Cuttings comprise rock particles or fragments removed from the formation below the drill bit during the drilling process.

Cuttings transport and hole cleaning can present challenges. Cuttings typically have a higher density than mud and will therefore tend to slip vertically when suspended in the mud. They also tend to accumulate and form a cuttings bed on the low side of an inclined section of the borehole when the mud is not able to keep them in suspension. Often the cuttings bed stabilizes around a certain height for a given flow rate and inclination. However, when the mud flow temporarily stops while further pipes are added to the string, the cuttings bed can often become unstable, especially in deviated sections of the borehole having an inclination between 30 and 65 degrees.

P29500PC00 As can be appreciated, conventional drill pipes are axisymmetric and almost perfectly straight in their unloaded state. However, over sufficient length scales and loading in use, the drill string has some flexibility that allows it to follow a curved borehole trajectory that is inclined and deviates from vertical.

Lateral string forces, which result mainly from inclination and gravitation but also from borehole curvature and axial force, can result in a conventional drill string residing more or less continuously on the “low” side of the borehole in an inclined borehole section, even when it is being rotated. Drill pipes, which have tool joints of larger diameter than the pipe body, will normally carry all the lateral contact forces, meaning that there is normally in that case no direct contact between the pipe body and the borehole.

The eccentric position of the pipe within the deviated borehole can result in significantly lower fluid speeds prevailing in a so-called “dead zone” below the pipe body as compared with the region of low resistance above the drill string pipe. Furthermore, this eccentricity of position also means that rotation-induced flow vorticity will be low. The vorticity, that is the screw component of the mud flow, is even further reduced if settled cuttings fill the lower gap so that the string is partially buried in the cuttings bed.

The result can be an avalanche of cuttings that can completely plug the annulus or cause the string to get stuck in the borehole.

At least one aim of the invention is to obviate or at least mitigate one or more drawbacks or challenges associated with prior art.

Summary of Invention

According to a first aspect of the invention, there is provided a drill string for drilling a borehole, the drill string being configured to be located in the borehole whereby an annulus is provided between the drill string and a wall of the borehole for conveying a drilling fluid entrained with cuttings along the drill string, the drill string having an axis of rotation, the drill string comprising at least one exciter for generating lateral motions of the drill string by rotation of the string and exciter together in the borehole for discouraging cuttings from settling in the annulus for reducing or hindering development of a cuttings bed.

In this way, lateral motions due to the exciter may facilitate agitating a cuttings bed or stimulate fluid in the wellbore in the region near and/or around the string. It may produce oscillations of

P29500PC00 the string in lengths uphole and downhole of the exciter, to provide agitation and stimulation of fluid along the string. This in turn may facilitate to release a struck string from a cuttings bed and/or urge the stuck string to moved off or away from a side of the borehole.

The exciter can be a component of a drill string that may be configured to break the rotational symmetry of the string and force a substantial length of the nearby drill string to move laterally, i.e. with radial component of movement with respect to the borehole axis and/or axis of rotation, when the string is rotated. For example, the drill string may comprise a first drill string section coupled to a second drill string section through a drill string joint. The first drill string section may include the exciter, which may operate to generate lateral movements of the second drill string section, in response to the rotation of the string. Lateral motions produced due to the exciter may affect the mud flow in an annulus of the borehole so as to reduce the tendency of cuttings particles to settle and stay in a cuttings bed on a low side of an inclined borehole.

The term “exciter” as used herein is to be understood as referring to a passive string component. This component may force a limited part of the string to move laterally when the string is rotated, e.g. by contact against the wall of the borehole. Accordingly, the term “exciter” is to be further understood as excluding active components, such as a mud motor where an eccentric rotor, which is driven by the drilling fluid flow, causes the motor housing and the nearby string to vibrate laterally.

The exciter herein can be embodied in a wide number of ways. The exciter may for example comprise or be one or more of: a cam exciter; a bend exciter; an eccentric pipe body exciter; a helical pipe exciter; and/or any other suitable exciter.

With regard firstly to the cam exciter, the cam exciter may comprise a tubular sub having an outer surface which can act as a cam, i.e. an outer cam surface. The tubular sub may have threaded end section for screw connecting the tubular sub to a complementary threaded end section of an adjacent component of the drill string, e.g. drill pipe, for thereby forming a joint between the tubular sub and the adjacent component. The joint may be made up by screwing the threaded end sections together, one rotating relative to the other, about a thread axis. In examples of the cam exciter, the distance in a radial direction from the thread axis to the outer surface may vary or change in accordance with or depending upon the rotation angle of the cam exciter with respect to the axis of rotation of the drill string, and may have an extent in the radial direction, e.g. a maximum distance, that exceeds the joint radius, e.g. maximum joint radius, of one or more joints between adjacent drill pipes in the string and/or between the tubular sub and the adjacent component of the drill string.

P29500PC00 The cam exciter may for example be a bi-centre sub. The bi-centre sub may have an outer surface having a circular cam profile or contour with an offset centre (relative to the thread axis). Upon rotating the string, the bi-centre sub having a circular profile or contour may force an adjacent length of the drill string to perform a circular or nearly circular lateral motion. The amplitude of this motion is typically substantially equal to the offset of the outer surface centre. To this end, the outer surface of the cam exciter may act against and/or obtain leverage from the wall of the wellbore to move the drill string laterally, e.g. toward and away e.g. lift and lower, with respect to the wall of the wellbore in dependence upon the rotational position of the cam sub and the part of the surface facing the wall.

The cam exciter may have a non-circular outer cam profile or contour. The cam profile or contour can have multiple lobes. For example, the cam exciter may comprise a sub having an elliptical outer profile or contour providing a two-lobed cam for forcing the string to perform two lateral cycles of movement, e.g. toward or away from the borehole wall, per revolution of the drill string, the sub being rotatable around the centre of the elliptical profile.

The outer surface of the sub can optionally comprise helical ribs. Helical ribs may be useful for example to accommodate an outer cam profile and exciter having a greater cross-sectional extent in the borehole where otherwise the mud flow in the annulus between the outside of the string and the surrounding borehole wall may be severely restricted. The ribs may extend in a helical direction along the string, preferably in a left-handed helical direction. The left-handed helical direction may enhance the screw component of the mud flow and thereby its ability to keep the cuttings in suspension.

Although the cam exciter may comprise a sub, e.g. between two drill pipes, the cam exciter may alternatively be integrated in a joint part of a drill string component, e.g. a joint part for forming a joint between two drill pipes. To this end, the joint part, for instance the pin joint of a drill pipe, may have a variably protruding outer cam surface, dependent upon rotational position around the thread axis. The joint part may be machined or hard-faced asymmetrically to provide an outer cam surface with desired outer cam profile or contour.

With regard secondly to the bend exciter, the bend exciter can break the rotational symmetry of the drill string in that it may comprise a pipe bend which may provide a sudden change in the direction of string at the location of the pipe bend. When placed in a rotating string the bend exciter may force the adjacent sections of the drill string, e.g. adjacent drill pipes, to perform lateral motions. In contrast to examples of the cam exciter, which may induce a

P29500PC00 maximum lateral motion near the cam, the lateral displacement amplitude obtained from use of the bend exciter may increase over a limited axial distance from the bend exciter, until reaching a maximum before fading away toward greater distances. The affection length and/or distance to maximum lateral deflection amplitude may depend on many factors, such as the magnitude of bend angle and the flexural stiffness of the pipe and drill string components.

With regard thirdly to the eccentric pipe body exciter, the eccentric pipe body exciter may comprise a drill pipe comprising a pipe body extending between first and second ends. The first and second ends may each have a threaded end section for mating with complementary threaded end sections of adjacent components of the drill string by screwing one end section to the other by rotation about a thread axis. The threaded end sections may be arranged coaxially. A substantial, e.g. major, portion of the pipe body between the ends may be eccentrically shifted relative to the axis through the joints.

Upon rotating the string, the off-centre shifted portion of the pipe body may perform a circular or nearly circular lateral motion with an amplitude substantially equal to the eccentric offset. This eccentric offset may preferably be substantially less than the radius difference between pipe body and joint, so that in this way, the risk of well bore contact and wear on the outer surface of the pipe body can be minimised. A portion of the pipe body near the upper (box) joint part, at least, may not be offset. This may be useful to allow the pipe body to be set in slips with the joints centred and may thus avoid unnecessary joint centralization during connection operations with the pipe set in slips.

The offset portion of the pipe body may comprise a mass imbalance, which when combined with a rotation frequency close to a lateral string resonance frequency, can enhance the dynamic deflection so that the centre body starts to contact the borehole wall. Thus, the eccentric pipe body may facilitate lateral movements toward and away from borehole wall. Such dynamic enhancement of the lateral motion may facilitate hole cleaning.

With regard fourthly to the helical pipe body exciter, the helical pipe body exciter may comprise a pipe body extending between first and second ends. The first and second ends may each have a threaded end section for mating with complementary threaded end sections of adjacent components of the drill string by screwing one end section to the other by rotation about a thread axis. The threaded end sections may be arranged coaxially. In contrast to the eccentric pipe body exciter, the eccentric part of the pipe body in the helical pipe body exciter is not straight but helical. The direction of the helix may preferably be left-handed for enhancing the screw component of the mud flow and thereby its ability to keep the cuttings in suspension.

P29500PC00 The helical part may have an integer number of pitches, in which case, the helical pipe body may have substantially no mass imbalance.

Different parts of the helical pipe body exciter move out of phase relative to other parts around the axis of rotation, and in this way may generate lateral movements of the drill string in sections of the drill string near the exciter.

In various examples, the exciter may comprise an exciter drill pipe which may be rotationally asymmetric about the axis of rotation. The exciter may comprise a drill pipe or drill string component which may have a pipe bore extending longitudinally through the drill pipe and which may be arranged eccentrically within an outer circumference of the pipe. The exciter may comprise an exciter drill pipe which may have a pipe bore comprising a centre arranged to travel in a circle about the axis of rotation in the wellbore. Thus, part of the exciter may intermittently be moved toward or away from a section of the surrounding wall of the wellbore upon rotation of the string to initiate the lateral motions of the drill string.

According to a second aspect of the invention, there is provided drill pipe for use in drilling a borehole, the drill pipe comprising an axis of rotation and at least one exciter for generating a lateral motion of the pipe by rotation of the drill pipe and exciter together in the borehole for discouraging cuttings from settling in an annulus of the borehole for reducing or hindering development of a cuttings bed.

According to a third aspect of the invention, there is provided a drill string sub configured to be incorporated in a drill string for drilling a borehole, the drill string sub comprising an exciter for generating lateral motion of a drill string by rotation of the drill string and exciter together in the borehole for discouraging cuttings in an annulus of the borehole from settling for reducing or hindering development of a cuttings bed.

According to a fourth aspect of the invention, there is provided a drill string exciter for generating lateral motion of the drill string by rotation of the drill string and the exciter in the borehole for discouraging cuttings in an annulus of the borehole from settling for reducing or hindering development of a cuttings bed.

According to a fifth aspect of the invention, there is provided a method of drilling a borehole, the method comprising: providing a drill string in accordance with the first aspect of the invention in the borehole, an annulus being provided between the drill string and a wall of the

P29500PC00 borehole to convey a drilling fluid entrained with cuttings along the drill string; rotating the drill string, thereby rotating the exciter; and by way of the rotation, generating lateral motions of the drill string to discourage cuttings from settling in the annulus to reduce or hinder development of a cuttings bed.

The method may further comprise generating oscillations that may produce resonant oscillation of the drill string.

Typically, the drill string is formed from pre-prepared stands each comprising multiple elements including at least one exciter. Typically, the drill string comprises sections of drill pipe. Typically, the drill string is arranged to be rotated by rotation equipment of a drilling rig or platform. The exciter in various embodiments may comprise standard straight drill pipe adapted to provide or support an exciter or connected to up-hole and downhole standard straight drill pipe through one or more drill pipe bends. The drill pipe may be configured to transmit drilling fluid along an inside of the drill pipe toward a drill bit on an end of the drill string.

Any of the aspects of the invention may have one or more further features as set out in relation to any other aspect of the invention, wherever described herein.

Various embodiments using the exciter can be advantageous in that the perfectness of the drill string rotation may be broken or interrupted, and by rotation of the drill string lateral motions may advantageously be generated so as to stir an existing cuttings bed and help keep the cuttings particles suspended in the drilling mud. There are several mechanisms which may facilitate this: First, if the string in use is in contact with or partly buried within a cuttings bed, lateral motions may directly disturb it and help to fluidize it. Secondly, a reduction of the dead zone effect can be obtained, e.g. the string may be temporarily lifted off the low side of the well bore, so that the fluid speed and shear force on the cuttings bed interface may increase and kick the particles back into suspension. Thirdly, a local circular and lateral motion may be produced which may increase the mud flow vorticity and thereby inhibit cuttings bed formation. Thus, when rotating the string in use, a circular whirl like motion of the drill string may be produced which may help to inhibit cuttings bed formation.

Embodiments of the invention may be further advantageous in various ways as will be apparent from throughout herein.

Drawings and description

P29500PC00 There will now be described, by way of example only, embodiments of the invention with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a drill string including an exciter in use in a wellbore;

Figure 2 is a cross-sectional representation of a wellbore with the drill string arranged against the low side of the wellbore;

Figure 3A is a side-on representation of a cam exciter in the form of a cam sub;

Figure 3B is a sectional representation of the cam sub of Figure 3A along AA’;

Figure 4 is a sectional representation of a cam exciter in the form of a pipe joint exciter;

Figure 5 is a side-on representation of a bend exciter;

Figure 6 is a side-on representation of an eccentric pipe exciter; and

Figure 7 is a side-on representation of a helical pipe exciter.

With reference to Figure 1 , a borehole 1 is being drilled using a drill string 10 comprising drill string elements, including drill pipe 12, connected together end to end. The drill string 10 is located in the borehole 1. At a far end, the drill string 10 is provided with a drill bit 14. To perform drilling, the drill string 10 in the borehole 1 has a longitudinal axis of rotation 17 and is rotated by rotary equipment 18 of a drilling rig 19 at the surface 3. Drilling fluid in this case drilling mud, under normal operation while drilling progresses and the drill string 10 is advanced in the subsurface 5, is circulated into the wellbore 1 through the inside 13 of the drill pipe 12, exiting from the drill pipe 12 through an exit at or near the drill bit 14. The drilling fluid passes back uphole through an annular region 15 between an outer surface of the drill string 10 and the surrounding wall 16 of the wellbore. Cuttings produced under drilling, e.g. as the bit 14 cuts into the rock formation, are carried along with the drilling fluid toward the surface 3 and out of the wellbore.

To facilitate in the drilling operations, the drill string 10 includes an exciter 100. This may be useful particularly in or after a pause in drilling e.g. in a situation where drill pipe has become struck, as may occur where the drill string 10 is advanced into a deviated section of the wellbore 1 , where due to gravity the drill pipe lies toward the low side of the hole.

Figure 2 shows the position of a section of drill pipe of the drill string 10 on the low side of the borehole 1. The drill pipe includes a drill pipe body 51 extending between joint parts 54 at respective ends of the pipe. The outer diameter of the joint part 54 is greater than that of the drill pipe body 51. Thus, the drill pipe and string 10 rests on joint parts 54 against borehole wall. The pipe body 51 between the joint parts 54 is therefore supported with some clearance

P29500PC00 between the wall and an outer surface of the pipe body, as determined by the amount that the joint parts extend radially beyond the outer surface of the pipe body. A dead zone 6 is formed on the low side of the borehole between the pipe body 51 and the borehole wall, where cuttings may accumulate, and drill fluid may not circulate effectively.

Upon rotating the drill string 10 however, the exciter 100 operates to impart a lateral motion to the drill pipe. This in turn can facilitate to urge the drill pipe away from the low side and agitate I stimulate flow and circulation of the drilling fluid.

The exciter 100 may in practice be embodied in a wide range of ways, and selected examples will now be described in further detail with reference additionally to Figures 2 to 5, representing just some of the possible embodiments or classes of embodiments for the exciter.

In Figure 3A and 3B, a cam exciter generally depicted in the form of a bi-centre, tubular cam sub 200 which has an outer surface 208 that acts as a cam. The cam sub 200 has tubular body 201 . A linear bore 202 extends through the body 201 . The centre axis 217 extends end to end through the bore 202 and coincides with the axis of rotation 17 of the drill string.

The body 201 extends between first and second ends 204, 205. The first end 204 has a threaded box section connector 204c for connecting the cam sub 200 to a complementary threaded pin section of an adjacent element, e.g. drill pipe, of the drill string 10. The second end 205 has a threaded pin section connector 205c for connecting the cam sub 200 to a complementary threaded box section connector of an adjacent element, e.g. drill pipe of the drill string 10. The threads are made up to form a joint by rotation one relative to the other about a thread axis 218, coincident with the centre axis 217 of the bore.

The outer surface 208 varies in radius R (from the centre axis 217) with angular position around the axis 217, 218. Furthermore, the outer surface 208 defines a circumferential profile or contour C that is eccentric with respect to the pipe bore 202, and in this case is circular with a centre 211 which is offset from the centre axis 217.

In the radial direction D, the reach and thickness of the wall material between the pipe bore and the outer surface extremity is greater than in directions perpendicular or opposite to the direction D.

By virtue of the eccentricity, the sub 200 also has mass imbalance. When the drill string 10 is rotated, the outer surface 208 rotates asymmetrically and forces the string to oscillate laterally.

P29500PC00 When the string is rotated, the drill string in the region neighbouring the cam sub 200 performs a circular and lateral motion synchronous to the rotation of the drill string around its own axis 17, 217, 218. The radius of this circular motion equals the radial offset distance between the two centres, i.e. the centre 211 of the outer cam surface and that of the axes 217, 218.

It can be appreciated that the outer surface 208 may engage against the wall of the borehole so that as the drill string 10 rotates the cam surface 208 cyclically obtains leverage from the wall of the borehole for producing the lateral motions, e.g. a rise and fall toward or away from the wall, in sections of the string near and/or adjacent to the sub 200 in accordance with the variations in cam radius R.

In some variants, the outer cam surface 208 of the cam exciter comprises hard-facing or tungsten carbide material for minimizing abrasive wear, e.g. due to contact or interactions between the outer cam surface 208 and the borehole wall.

In another variant, the cam exciter has an outer cam surface that is elliptical. The centre of the elliptical profile may or may not coincide with the centre axis 217 or thread axis 218. The elliptical outer cam surface profile variant is an example of a cam exciter comprising two lobes, whereby upon rotating the string two rise and fall cycles may be generated by the rotatably travelling elliptical contour against the borehole wall. In other variants, other numbers of lobes may be possible, e.g. the cam excite in other variants can for example have one or three lobes, providing corresponding number of bumps or impulses to the string as the cam exciter is rotated with the drill string.

With reference to Figure 4, it can be noted that the cam concept can also be implemented as a modification of the joint parts of an ordinary drill pipe. Accordingly, in Figure 4, the cam exciter is the form of a bi-centre cam joint exciter 600 comprising a pipe joint part 654 which has an outer cam surface 608. The joint part 654 is threaded for screw connection with a complementary section of an adjacent component of the string and defines a thread axis 618, around which the joint part 654 is turned to make up the joint. The thread axis 618 is coincident with centre axis 617 of the bore 602 through the joint part 654. The outer cam surface 608 has a circular profile or contour with a centre 611 which is eccentrically offset from the centre axis 617, 618. Upon rotation of the string therefore, like the example of Figures 3A and 3B, in accordance with the variation of radial distance of outer cam surface 608 as it is turned, the joint exciter 600 induces lateral motions in adjacent or neighbouring sections of the drill string. The outer cam surface 608 can similarly be machined to produce the asymmetric contour or

P29500PC00 profile and the surface 608 may be hard-faced to withstand wear. Maximum effect can be obtained if the joints of every second pipe has a similar cam profile with the same orientation.

Bi-centre pipe joints on respective ends of a pipe section can have approximately the same action as the eccentric pipe discussed below.

In Figure 5, an example of the exciter in the form of a pipe bend sub 300 is depicted. The pipe bend sub 300 has a has first and second pipe body portions 301a, 301b comprising respective pipe bores 302a, 302b which are connected through a change in bore direction at point P. For connection to adjacent elements, e.g. drill pipes, of the string, the one pipe bore 302a has a threaded box section connector 304c at one end 304, and the other pipe bore 302b has a threaded pin section connector 305c at the other end 305 of the sub. The change in direction, as indicated by bend angle H, results in the second pipe body portion 301b veering away from the pipe axis 317 of the first pipe body portion 301a. In this way, the second pipe body portion 301b of the bend sub may exert an intermittent lateral force component on the drill string as the sub rotates about axis 317 so as to generate lateral motions of the string.

In some examples, the bend can be a smooth or abrupt change in centre axis direction of the (unloaded) drill string. Short, single bend subs can be applied to generate lateral motion of a rotating sting. However, unless the bend angle is so large that the pipe bend sub 300 temporarily loses contact with the well bore, single bends may generate relatively small lateral motions. The deflection is zero at the pipe bend sub and increases to a maximum near the centre of the pipe body of the adjacent drill pipe before the deflection may fade away toward the neighbouring joints.

In some variants, a bend sub 300 such as depicted in Figure 5 is coupled to another bend sub end to form a bend couple with equal but opposite bend angle shifts. The bend couple can then for example couple between a first straight drill pipe and a second straight drill pipe which extends longitudinally in parallel but is offset or shifted laterally with respect to the first drill pipe. In this way, the bend sub 300 can also be used to provide a “shift exciter”. If implemented in a relatively short sub, such a shift sub will have a similar action as a mono-lobed (bi-centre) cam. However, a difference is that the lateral motion of a shift sub is not purely circular. Instead the elevation from the lowest position is more like the single-sided rectified function 8r ■ max (cos(Ht) , 0), where 8r is the axis separation, n is the drill string rotation speed and t is the time variable. This function is a periodic function with a plurality of higher harmonics. Another difference as compared with the bi-centre cam is that the two sides of the offset sub is excited with opposite phases, meaning that that when one side is elevated the other is not.

P29500PC00 In Figure 6, an exciter in the form of an eccentric pipe body exciter 400 is depicted. The eccentric pipe body exciter 400 has a pipe body 401 extending between first and second ends

404, 405. The first and second ends each have a threaded end sections 404c, 405c for mating with complementary threaded end sections of adjacent components of the drill string by screwing one end section to the other. The threaded end sections may be arranged coaxially. An intermediate portion 401 b of the pipe body is eccentrically shifted relative to the axis 418 through the joints. The axes 418 coincide with the axis of rotation 17 of the drill string 10 in the borehole. The pipe body 401 includes sections 401a, 401c that are coaxially aligned the intermediate section 401b.

To avoid excessive wear in the intermediate portion 401 b the linear eccentricity (offset F between centres) is less than the radial clearance G between outer surface of adjacent pipe body portions 401 a, 401 b of the pipe body portion and outer surface of the pipe joint parts 404,

405.

The length of the body portion 401a from the upper joint part 404 to the nearest bend couple 413 is long enough to span the slips grip zone on the rig 19 so that the upper joint 404 is easily centralized when the pipe 400 is set in slips. The eccentric pipe body portion 401 b represents a mass imbalance that can enhance the deflection dynamically, especially when the rotation frequency is close to the lateral eigenfrequency.

Two bend couples can alternatively be used for making an eccentric pipe body which can functions similarly to that of Figure 6.

The small static deflections through bend couples 413, 414 between respective pipe body portions 401a, 401 b and the intermediate portion 401b can be counteracted by using slightly higher bend amplitudes of the two bends farthest away from the joints 404, 405, as compared with the other two. Unless a small correction bend is applied in the middle of the pipe this asymmetry represents a small net bend of the pipe in its unloaded state.

In Figure 7, an exciter in a helical pipe exciter 500 is depicted. The exciter 500 has a helical pipe body 501 and pipe bore 502 following a helical trajectory from one end 504 to another end 505 of the pipe 500.

Spiralled or helically formed pipes such as in Figure 7 can have approximately the same effect as the shifted pipe body 400 discussed above. When the pipe 500 is rotated upon rotation of

P29500PC00 the drill string, the spiralled part of the pipe will perform a circular motion which can enhance the flow vorticity and improve the hole cleaning. The pipe 500 is rotatable about a thread axis 518 coincident with the drill string axis 17 through end joints 504, 505. Left-hand helical screw direction of the pipe body 501 is preferable because it amplifies the vorticity of the mud flow. The helical pipe 500 may create some vorticity and reduce the dead zone problems even in sliding drilling when the string is not rotated but slid into the well. A difference from the variant of Figure 6 is that the static mass imbalance is lower, especially when the spiralled part covers an integer number of pitches. It can mean that the potential dynamic enhancement effects may be smaller for the spiralled pipe form than for the straight eccentric form of Figure 6. In the same way as for the eccentric shifted pipe exciter of Figure 4, the slips grip zone is provided for below the upper (box) end joint 504 by a straight coaxial pipe body portion 501 b having a length to span the necessary slips grip zone.

All of the exciters mentioned herein may be advantageous, in addition to their primary task of improving hole cleaning and/or cuttings transport, in having an ability to increase the vorticity of the drilling fluid. This represents an increased string torque resistance which disfavours or hinders torsional stick-slip oscillations which are driven by non-linear bit torque and well bore friction and are characterized by large variations of the drive torque and the downhole rotation speed. Even though stick-slip oscillations in other circumstances can be advantageous for hole cleaning, they are generally so detrimental to the drilling process and downhole tools that they are highly unwanted.

Because one exciter has a limited affection length (i.e. the length experiencing lateral motions) on both sides of it, a plurality of exciters will often be needed in the drill string in practice. The spacing between neighbouring exciters, at least when it comes to cam or bend exciters, can typically comprise two or three drill pipe lengths. A limited number of exciters may desirably be put in the parts of the string which reside in zones of the wellbore which are at a high risk of cuttings avalanches and/or cuttings bed formation.

P29500PC00