GB2181173A - Improvements in or relating to rotary drill bits - Google Patents

Abstract

A rotary drill bit comprises a bit body 10 having a leading face 11 and a gauge region 12, a number of blades 13 on the leading face of the bit body, and a number of cutting elements 16 mounted along each blade. A passage 23 in the bit body supplies drilling fluid to nozzles 21 in the leading face of the bit body for cooling and cleaning the cutting elements. Each nozzle 21 is so orientated, and the surface of the bit body in the region in front of each blade is so shaped, as to promote a vortex flow 30 of drilling fluid around said region, with part of the periphery of the vortex extending across the cutting elements on the blade, so that fluid in the periphery of the vortex passes across the cutting elements 16 mounted on each blade 13 before escaping through an exit channel 20 in the gauge region. <IMAGE>

Description

SPECIFICATION Improvements in or relating to rotary drill bits The invention relates to rotary drill bits for use in drilling deep holes in subsurface formations.

In particular, the invention relates to drill bits ofthe kind comprising a bit body providing a leading face, a gauge region,and an annularspace (known as the annulus) on the side of the gauge region remote from the leading face, a number of blades on the leading face, each blade having an outersurfacewhich,in use,facesthesurfaceoftheformation being drilled and afront surface facing in the direction of normal forward rotation ofthe bit, a number of cutting elements mounted along each blade, a delivery passage in the bit bodyforsupplying drilling fluid to a numberofopenings in the leading face ofthe bit, and at least one exit passagefor returning said drilling fluid pastthe gauge region to the annulus.Said openings are so located on the forward side of a blade asto promote a flow ofdrilling fluid acrossthefront surface of the blade and across the cutting elements mounted on the blade. The exit passage is usually in the form of an exit channel orjunkslotformed in the outer surface of the gauge region, but the invention also includes arrangements where otherformsof exit passage are employed.

A drill bit of this kind is described in U.S. Patent Specification No.4,499,958. In the drill bit described in thatspecification,there are provided four blades and two openings for drilling fluid on theforward side of each blade, the openings being atdifferent radial distances from the axis of rotation of the bit.

There are provided fourjunkslots each of which ex- tends, around the periphery of the bit, from the front surface of each blade to a location spaced a short distancefrom the rear ofthe next blade.Accordingly, almostthe whole of the space between adjacent blades is in direct communication with the associated junk slot. Such an arrangement provides substantial clearance and a large flow path for the escape, through thejunkslot, of cuttings produced by the cutting elements and it is believed that this is one of the reasons for the high penetration rates which drill bits ofthis kind may achieve in certain types offormation. However, a further important function ofthe drilling fluid is to cool the cutting elements and the formation on which the cutting elements are acting, and with the arrangement described in U.S.Specific- ation No. 4,499,958 such cooling may not be particularly effective in view of the fact that drilling fluid emerging from the nozzles in the bit body can flow directlytothejunkslotwithoutproviding sufficient high velocity turbulent flow of fluid overthe cutting elements and the formation to provide effective cooling.

The present invention provides an improvement to a drill bit of the kind first referred to, the improve mentcomprising the provision of a structure which controlstheflow of drilling fluid emerging from the openings in the bit body in such manner as to pro mote a controlled vortex offlow on the forward side of each blade to give a greater velocity gradienttothe flow over the cutting elements and thereby provide more effective cooling ofthe cutting elements and the formation on which they are acting.

It should be explained that in most conventional drill bits of the kind first referred to above, including the type described in U.S. Patent Specification No.

4,499,958, the flow of drilling fluid overthe surface of the bit body and cutting elements is complex, and it may be assumed that such flow will also involve the generation of one or many small vortices, but in a generally uncontrolled manner. The present invention, however, depends on the production of a controlled vortex, that is to say the configuration ofthe bit bodyand the location oftheopeningsthereinfor drilling fluid are such as inevitably to result in the generation of at least one majorvortex in the region adjacent the front surface ofthe blade, the peripheral region ofthe vortex passing across several or all of the cutting elements mounted on the blade.

According to one aspect of the invention, in a rotary drill bit ofthe kind first referred to, at leastone of the aforesaid openings is located in a region adjacentthefrontsurfaceofa blade which region is bounded, when the bit is in use, partly by a surface portion of the bit body and partly by a surface portion oftheformation being drilled,theopening being so orientated with respect to the said surface portions asto promote a vortex flow drilling fluid around said region, with part of the peripheryofthevortex extending across the cutting elements on the blade, at least part of said surface portion of the bit body which defines the aforesaid region being smoothly and concavely curved in a mannertofacilitatethefor- mation of said vortex, said exit passage being so loc atedthatfluid may esca pe fro m the periphery of the vortex into said exit passage and thence to said annulus.

At least a portion ofthevortex flow over said sur- face portion ofthe formation may be in a direction outwardly away from the central axis of rotation of the drill bit, and in this case said exit passage may comprise an exit channel formed in the outer surface of the gauge region, which channel is located adjacentthefrontsurface ofthe bladewherebyfluid may escape directly into said exit channel from the periphery ofthe vortex after having passed outwardly across the cutting elements. Preferably, the partofsaid surface portion ofthe bit body which is adjacent said exit channel is inclined inwardly so as to face partly towa rds the long itudina I axis of rotation ofthe drill bit.

According to a second aspect of the invention, there is provided a rotary drill bit comprising a bit body providing a leading face, a gauge region, and an annulus on the side ofthe gauge region remote from the leading face, a number of blades on the leading face, each blade having an outer surface which, in use, faces the surface ofthe formation being drilled and afrontsurfacefacing inthedirec- tion of normal forward rotation ofthe bit, a number of cutting elements mounted along each blade, a de livery passageinthe bit bodyforsupplying drilling fluid to a number of openings in the leading face of the bit, and at least one exit channel formed inthe outer surface ofthe gauge region for returning said drilling fluid past the gauge regiontothe annulus, at least one of the aforesaid openings being located in a region adjacent the front surface of a blade which region is bounded, when the bit is in use, partly by a surface portion ofthe bit body and partly by a surface portionoftheformation being drilled,the opening being so orientated with respect to the said surface portions asto promote a vortexflow of drilling fluid around said region,with partofthe peripheryofthe vortex extending across the cutting elements on the blade, at least a portion of the vortex flow over said surface portion oftheformation being in a direction outwardly away from the central axis of rotation of the drill bit, wherebythe fluid in the vortex reaches the outer periphery of the drill bit immediately after having passed outwardly across the cutting el- ements, there being provided an exit channel adja centthefrontsurfaceofthebladesothatfluid may escape directly into said exit channel from the periphery ofthe vortex after having passed across the cutting elements, partofsaid surface portion ofthe bit body which is adjacent said exit channel being inclined inwardly so as to face partly towards the longitudinal axis of rotation of the drill bit.

The feature of part of the surface portion of the bit body, adjacent the exit channel, being inclined inwardly may be provided either in addition to oras an alternative to the smooth curvature of the surface portion of the bit body in the previously mentioned arrangements where fluid may escape directly into the exit channel from the periphery ofthe vortex.

That is to say, if the surface of the bit body is inclined as specified adjacent the exit channel, it becomes less necessary for the surface portion to be smoothly curved and the vortex may be adequately generated buy a more angular configuration of the surface.

Arrangements in accordance with both aspects of the invention provide two important main advantages. In order to provide efficient cooling and clean ing ofthe cutting elements in a drill bit it is desirable thatthe drilling fluid flows across the surfaces ofthe cutting elements at high velocity. This requires a high velocity gradient in the fluid adjacent the cutting elements, which in turn necessitates a very high velocity at a distance from the cutting elements and therefore a highvolumetricflowrate. In conventional drill bits, the drilling fluid normallyflows only once past each cutting element andthusthethermal cap acityofthefluid is not used efficiently in coolingthe elements.By recirculating a high proportion ofthe fluid in a controlled vortex however, in accordance with the invention, the recirculated portion ofthe fluid acts on the cutting elements more than once, and consequently performs its cooling function over a much longertotal effective flow path than isthe case with conventional bits. The cooling efficiency of the fluid may thus be considerably enhanced, per- mitting improved utilisation of the available flow rate of drilling fluid which, in any given drilling situation, is usually limited.

Furthermore, the centrifugal action of the vortex tends to cause cuttings entrained in the drilling fluid to migrate to the outer periphery of the vortex, from where they pass to the exit passage and thence to the annulus. Consequently, the recirculated portion of the fluid in the vortex is the inner, relatively clean fluid.

In the case where the exit passage comprises an exit channel along the gauge region ofthe bit, the exit channel may be shaped so as to increase in cross-section as it extends along the gauge region.

Asmoothly rounded edge region may be provided between the exit channel and said surface portion of the bit body.

Also in the arrangements referred to above in which fluid escapes directly into the exit channel from the periphery of the vortex, the cutting elements are preferably so disposed that said surface portion oftheformation being drilled is smoothly and concavely curved immediately adjacent said exit channel, and said exit channel extends generallytangentiallyto said concavely curved surface portioh of the formation.

As an alternative to the exit passage being an exit channel along the gauge region into which fluid passes directly from the periphery ofthe vortex, the exit passage may extend through the bit body from an opening in said surface portion ofthe bit body.

In this case at least a portion ofthe vortex flow over said surface portion of the formation is preferably in a direction inwardly towards the central axis of rotation ofthe drill bit, and said opening in said surface portion of the bit body is so located that fluid may escape through said opening and into the escape passage after having passed inwardly across the cutting elements. This inward flow of drilling fluid may provide better cooling and cleaning of cutting elements nearerthe outer periphery of the bit. The opening through which drilling fluid is delivered is preferably so orientated with respect to said surface portions of the bit body and formation as to be directed towards the vicinity of the junction between said surface portions at the outer periphery of the bit.As a resultthejetofdrilling fluid from the opening will impingewith high velocity on the general area where the outermost cutting elements are located, these being the elements in respect of which adequate cooling and cleaning may be most critical.

In the last-mentioned arrangements the surface portion of the bit body is preferably smoothly and concavely curved immediately adjacent said opening into the exit passage, and said passage extends, at least initially, generallytangentiallyto said concavely curved portion of the bit body.

In any ofthe arrangements in accordancewiththe invention preferably substantially the whole ofthe surface portion ofthe bitbodywhich boundsthe aforesaid region in which the vortex is formed is smoothly and concavely curved. Similarly, the cutting elements are preferably so disposed that sub stantially the whole of the surface portion ofthefor- mation which bounds the aforesaid region in which the vortex is formed is also smoothly and concavely curved. Preferably also the surface portions of the bit body and theformation are of substantially equal and uniform curvature so that the region bounded thereby is substantially circular.

In any ofthe above arrangements a number of cutting elements may be mounted along the junction between said outer and front surfaces of each blade.

The number of cutting elements are preferably arran ged in a single row along each blade.

The region adjacentthefrontsurfaceoftheblade within which the vortex is promoted may be defined partly by a recessed portion of the leading face ofthe bit adjacent said surface. For example, said recessed portion may be bounded partly by a wall surface spaced forwardly away from said front surface ofthe blade and substantially parallel thereto.

The following is a more detailed description ofem- bodiments ofthe invention, by way of example, reference being made to the accompanying drawings in which: Figure lisa vertical section through a drill bit, Figure2 is an end view of the bit shown in Figure 1, and Figures3and4,5and6, and 7and8aresimilar views to Figures 1 and 2 of alternative forms of drill bit according to the invention.

Referring to Figures 1 and 2, there is shown a rotary drill bit for use in drilling deep holes in subsurface formations comprising a bit body 10 having a leading face 11 and a gauge region 12.

The leading face of the bit body is integrallyformed with blades 13. As best seen in Figure 2 there are provided four such blades spaced substantially equally around the central axis of rotation of the bit. However, any other suitable number of blades may be employed, including only a single bade. Each blade has an outer surface 14which, in use ofthe bit, faces the surface ofthe formation being drilled, and a front surface 1 Sfacing in the direction of normal forward rotation of the bit. In the arrangements shown the front surface 15 of each blade lies in a substantially flat plane, but the invention includes within its scope arrangements where the front surface is concavely or convexly curved.

A number of cutting elements 16 are mounted on each blade along the junction between the outer face 14andthefrontface 15. Each cutting element may be, as shown, in the form of a circular preform comprising a front hard facing layer of polycrystalline diamond orothersuperhard material bonded to a less hard backing layer, such as tungsten carbide.

Each cutting elementmay be bonded to a post 17, for example oftungsten carbide, received in a socket in the blade 13. The junction between the outer surface 14 and the rear surface 9 of each blade may bechamfered as shown, for example at 45", to a line joust rearward of the post 17.

In known manner the gauge region 12 of the bit may be formed with wear or abrasion elements 18 which may, for example, comprise tungsten carbide studs impregnated with natural diamond particles and received in sockets in the gauge portion.

The cutting elements shown are only one example of the type of cutting elements which may be employed in a drill bit according to the invention, and it will be appreciated that the invention is applicable to drill bits incorporating any otherform of suitable cutting element. For example, there may extend along each blade only a few large cutting elements, or even a single elongate cutting element providing only a single continuous cutting edge.

The gauge portion comprises four axially extending kickers orwear pads 19 in which the abrasion elements 18 are mounted, exit channels 20 being dis posed between adjacent kickers 19. Each kicker may be formed with a slot 20a, the purpose of which is, in known manner, to allow drilling fluid to flowfreely past the drill bit as it is introduced into the bore hole or withdrawn from the bore hole.

On the forward side of each blade l3the bit body is formed with a recess 27. A nozzle 21 is moun- ted in a socket in the bit body at the bottom of each recess 27. Each nozzle communicates through a passage 22 with a central delivery passage 23. Drilling fluid supplied under pressure through the central passage 23 emergesfrom the nozzles 21 forthepur- pose of cleaning and cooling the cutting elements as well as cooling the formation.

In Figure 1,the surface oftheformation being drilled is indicated in dotted lines at 26. On the side of the gauge region remote from the leading face 11 of the bit, the bit body is of reduced diameter so as to provide an annular space 1, known as the annulus, between the bit body and the surrounding formation surface 26. The exit channel 20 provides communication between the leading face ofthe bit and the annulus.

A region 29 adjacentthefrontsurface 15 of each blade is bounded partly by a portion of the surface 26 oftheformationand partlybythesmoothlyandcon- cavely curved bottom surface 28 ofthe recess 27. The wall 25 ofthe recessfurthestfrom thefrontsurface 15 ofthe associated blade is substantially flat and parallel to said front surface. (In the case, mentioned earlier, where the front surface of the blade is curved, the spaced wall bounding the recess may be sim ill rely curved so as to be substantially parallel to the blade at all points along its length.) The associated exit channel 20 extends for the whole width ofthe recess 27, as shown in Figure 2.

As best seen in Figure 1, each nozzle 21 is so located in the region 29 and is so orientated (in this case substantially parallel to the central axis of rotation of the drill bit) that drilling fluid emerging under pressure from the nozzle impinges on the surface ofthe formation and is deflected in a curved path outwardly away from the axis of the bit into a vortex.The vortex will lie in a plant generally parallel to the front surface of the blade, the direction of flow being indicated generally by arrows 30. (The expression "generally parallel" isto be understood, in this context,to include arrangements where the vortex is curved to follow the general contourofa curved surface 15.) Asthefluidattheperipheryofthevortexflowsoutwardly across the leading face of the blade and across the preformed cutting elements mounted thereon, it cools the cutting elements and the adjacent formation and also carries away from each cutting element the cuttings which it has removed from the formation. Such cuttings are swept around the region 29 by the vortex and, duetothe centrifugal or cyclone effect thereof, are maintained in the outer peripheral region ofthevortex.

As the vortex reaches the exit chanel 20 it meets an edge region 31 where the curved concave surface 28 of the bit body meets the bottom wall 32 of the exit channel 20.

Adjacenttheedge region,the surface portion 28 of the bit body may be inclined inwardly at about 45 so asto face partly towards the longitudinal axis of the bit, as shown. Other angles of inclination may be used, butinorderto promotethevortexthe angle is preferably always inclined inwardly. The edge region has the effect of dividing the flow in the vortex so that an outer portion, carrying a higher proportion of cuttings, exits through the exit channel 20 whereas the cleaner inner portion is re-circulated in the vortex around the curved concave portion 28 of the bit body.

In the arrangement shown the edge portion 31 is sharply angled but the edge may be rounded as indicated in dotted I ines at 31 a.

The exit channel 20 may increase in area, as shown, as it extends away from the region 29.

It is preferred to provide only one nozzle on the forward side of each blade since the single nozzle can be of comparatively large aperture diameter with less risk of blockage than if two or more nozzles were used,sincesuch nozzles would haveto beofsmaller aperture. However, the invention includes within its scope arrangements in which morethan one nozzle are used. Also the number and configuration ofthe blades may be altered without departing from the scope ofthe invention, as may be the type and arrangement of the cutting elements on the blades.

Figures 3 and 4 show a modified arrangement in which, in orderto enhancethegeneration of avortex in the region 129, the configuration ofthe blades 113 andthe location ofthe cutting elements 116 is such that the surface 126 of the formation, cut by the cutting elements on the blade, forms a smooth continuation of the concave surface 128 of the recess 127. To enhance the generation of a vortex the concave surface 128 ofthe bit body and the surrounding surface 126 oftheformation togetherapproximateto a circle.

The profile oftheformation surface immediately adjacenttheexitchannel 120 is thus smoothly and concavely curved and the exit channel 120 extends tangentially thereto.

In orderto minimise leakage of drilling fluid from the recess 127 across the leading face 111 ofthe bit body, an upstanding fence 109 is formed along the upper edge of the wall of the recess 127 opposite the blade 113, the fence extending also around the end of the recess containing the nozzle and joining the blade 1 13.Thefence 109 engagesthesurface ofthe formation being drilled to minimise leakage of drilling fluid from ortho the recess 127 and across the leading face of the bit, thereby assisting in maintain ingthe vortex and ensuring that mostofthe drilling fluid escaping from the recess 127 passes directly to the corresponding exit channel 120. In orderto provide a more effective seal againsttheformation,the fence 109 could comprise a separate element of flexible material secured to the bit body.

As will be seen from Figure 3 there will be small leakage paths for fluid between the blade 113 and the surface ofthe formation 126, between the cutting elements 116, butthis leakage is small and the leakage spacewill become smaller as the cutters 116 become worn.

As previously explained, an object ofthe invention is to ensure that drilling fluid escapes to the exit channel 20 or 120 from the periphery of the vortex so that the proportion of fluid escaping to the exit channel carries with it the high proportion of cuttings which are entrained in the peripheral region ofthe vortex due to centrifugal force, the proportion of drilling fluid which is recirculated in the vortex entraining a much smaller proportion of cuttings. In orderto enhance the vortex flow, it may be desirableto allow a proportion ofthe comparatively clean drilling fluid to escape from the central region of the vortex. Accordingly, there may optionally be provided forthis purpose a circular exit passage leading from the central region ofthe vortex.Such passages are, for example, indicated at 207 in Figures 5 and 6, the drill bit shown in those figures being otherwise similarto that of Figures 3 and 4. As best seen in Figure6 each exit passage 207 extends from an outlet 206 in the wall ofthe recess 227 adjacent the blade 213 and ex tends through the main body of the bitto open into an exit channel 205 in the gauge region, the exitchan nels 205 being'located around the gauge region alternately with the exit channels 220 with which the recesses 227 communicate directly.

In the arrangements described above, the recess 27, 127 or 227 in front of each blade leads directly, at the periphery ofthe bit, into the associated exit channel 20,120 or 220, and thence to the annulus.

Figures 7 and 8 show a modified,two-bladed, arrangementwhere such exit channels are not provided, and communication between each recess 327 and the annulus 301 is by means of an opening 303 in the curved bottom wall 328 of the recess 327, which opening communicates with the annulus301 through an exit passage 302 which extendsthrough the bit body itself.

In this arrangement each nozzle 321 is so located that the vortex rotates clockwise as viewed in Figure 7, so that the portion of the vortex flow which is over the formation and cutting elements is in a direction inwardly towards the central axis of rotation ofthe drill bit. The nozzle is also directed generallytan gentially to the curved surface 328 ofthe bit body so asto impinge on theformation 326 in thevicinityof the junction between the bit body surface 328 andthe formation. This ensures that the outermost cutting elements 316 of the bit are subjected to the high velocity jet of drilling fluid immediately after it emerges from the nozzle 321.

The exit opening 303 is so located in the surface portion 328 ofthe bit body that fluid at the periphery of the vortex may escape through the opening 303, into the passage 302, and thence into the annulus 301, after having passed inwardly across the cutting elements 316. To facilitate such escapethe passage 302 extends tangentially from the surface portion 328 of the bit body. The nozzle 321 also extends tangentially to the surface portion to facilitate the generation of the vortex.

Fluid carrying the bulk ofthe cuttings thus escapes from the periphery of the vortex through the opening 303 leaving the relatively clean drilling fluid to be recirculated in the vortex.

Claims (19)

1. A rotary drill bit comprising a bit body provid ing a leading face, a gauge region, and an annulus on the side of the gauge region remote from the leading face, a number of blades on the leading face, each blade having an outersurfacewhich, in use,facesthe surface oftheformation being drilled and a frontsur- face facing in the direction of normal forward rotation ofthe bit, a numberofcutting elements mounted along each blade, a delivery passage in the bit body forsupplying drilling fluid two a number of openings in the leading face ofthe bit, and at least one exit passage for returning said drilling fluid past the gauge region to the annulus, at least one of the aforesaid openings being located in a region adjacentthefront surface of a blade which region is bounded, when the bit is in use, partly by a surface portion ofthe bit body and partly by a surface portion oftheformation being drilled, the opening being so orientated with respect to the said surface portions as to promote a vortex flow of drilling fluid around said region, with part of the periphery ofthe vortex extending across the cutting elements on the blade, at least part of said surface portion ofthe bit bodywhich definestheaforesaid region being smoothly and concavely curved in a manner to facilitate the formation of said vortex, said exit passage being so located that fluid mayes capefrom the periphery of the vortex into said exit passage and thence to said annulus.

2. A rotary drill bit according to Claim 1,wherein at least a portion of the vortex flow over said surface portion oftheformation is in a direction outwardly away from the central axis of rotation ofthe drill bit, and wherein said exit passage comprises an exit channel formed in the outersurface ofthe gauge region,which channel is located adjacent the front surface ofthe blade whereby fluid may escape directly into said exit channel from the periphery ofthe vortex after having passed outwardly across the cutting elements.

3. A rotary drill bit according to Claim 2, wherein the partofsaid surface portion ofthe bit body which is adjacent said exit channel is inclined inwardly so as to face partly towards the longitudinal axis of rotation of the drill bit.

4. A rotary drill bit comprising a bit body providing a leading face, a gauge region, and an annuluson the side ofthe gauge region remote from the leading face, a numberofblades on the leading face, each blade having an outersurfacewhich, in use, faces the surface oftheformation being drilled and a frontsurface facing in the direction of normal forward rotation ofthe bit, a number of cutting elements mounted along each blade, a delivery passage in the bit body for supplying drilling fluid to a numberofopenings in the leading face ofthe bit, and at least one exit channel formed in the outersurface ofthe gauge re gion for returning said drilling fluid past the gauge region to the annulus, at least one of the aforesaid openings being located in a region adjacentthefront surface of a blade which region is bounded, when the bit is in use, partly by a surface portion of the bit body and partly bya surface portion oftheformation being drilled, the opening being so orientated with respect to the said surface portions as to promote a vortex flow of drilling fluid around said region, with part of the periphery ofthe vortex extending across the cut ting elements on the blade, at least a portion ofthe vortex flow over said surface portion oftheformation being in a direction outwardly away from the central axis of rotation ofthe drill bit, wherebythefluid in the vortex reaches the outer periphery of the drill bitim- mediately after having passed outwardly across the cutting elements, there being provided an exit channel adjacent the front surface ofthe blade so that fluid may escape directly into said exit channel from the periphery of the vortex after having passedac- ross the cutting elements, part of said surface portion ofthe bit body which is adjacent said exit channel being inclined inwardly so astoface partly towards the longitudinal axis of rotation of the drill bit.

5. A rotary drill bit according to any of Claims 2to 4, wherein the exit channel is shaped so as to increase in cross-section as it extends along the gauge region of the drill bit.

6. A rotary drill bit according to anyofClaims2to 5, wherein a smoothly rounded edge region is provided between the exit channel and said surface portion of the bit body.

7. A rotary drill bit according to any of Claims 2to 6, wherein the cutting elements are so disposed that said surface portion of the formation being drilled is smoothly and concavely curved immediately adjacent said exit channel, and said exit channel extends generally tangentially to said concavely curved surface portion oftheformation.

8. A rotary drill bit according to Claim 1,wherein said exit passage extends through the bit body from an opening in said surface portion ofthe bit body, whereby fluid may escape through said passage and intotheannulusfromtheperipheryofthevortex.

9. A rotary drill bit according to Claim 8,wherein at least a portion ofthe vortex flow over said surface portion oftheformation is in a direction inwardly towards the central axis of rotation of the drill bit, and wherein said opening in said surface portion ofthe bit body is so locatedthatfluid mayescapethrough said opening and into the exit passage after having passed inwardly across the cutting elements.

10. A rotary drill bit according to Claim 9, wherein said opening through which drilling fluid is delivered is so orientated with respect to said surface portions ofthe bit body and formation asto bedirectedtow ardsthe vicinity of the junction between said surface portions at the outer periphery ofthe bit.

11. A rotary drill bit according to any of Claims 8 to 10, wherein said surface portion ofthe bit body is smoothly and concavely curved immediately adjacent said opening into the exit passage, and said passage extends, at least initially, generally tangentially to said concavely curved portion of the bit body.

12. A rotary drill bit according to any ofthe preceding claims,whereinsubstantiallythewholeofthe surface portion ofthe bitbodywhich bounds the aforesaid region in which the vortex is formed is smoothly and concavely curved.

13. A rotary drill bit according to any ofthe preceding claims, wherein the cutting elements are so disposed that substantially the whole ofthe surface portion ofthe formation which bounds the aforesaid region in which the vortex is formed is smoothly and concavely curved.

14. Arotarydrill bitaccordingtoClaim 12and Claim 13,wherein said surface portions ofthe bit body and the formation are of substantially equal and uniform curvature so that the region bounded thereby is substantially circular.

15. A rotary drill bit according to any of Claims 1 to 14, wherein a number of cutting elements are mounted along the junction between said outer and front surfaces of each blade.

16. A rotary drill bit according to any of Claims 1 to 15, wherein said number of cutting elements are arranged in a single row along each blade.

17. A rotary drill bit according to any of Claims 1 to 16, wherein the region adjacent the front surface of the blade within which the vortex is promoted is defined partly by a recessed portion ofthe leading face ofthe bit adjacent said surface.

18. Arotarydrill bit according to Claim 17, wherein said recessed portion is bounded partly buy a wall surface spaced forwardly away from said front surface of the blade and substantially parallel thereto.

19. A rotarydrill bitsubstantiallyas hereinbefore described with reference to the accompanying drawings.