WO2012051674A1

WO2012051674A1 - A drilling apparatus

Info

Publication number: WO2012051674A1
Application number: PCT/AU2011/001351
Authority: WO
Inventors: Brian Thomas Sanfead
Original assignee: Drilling Tools Australia Pty Ltd
Current assignee: Drilling Tools Australia Pty Ltd
Priority date: 2010-10-22
Filing date: 2011-10-24
Publication date: 2012-04-26
Anticipated expiration: 2013-04-22

Abstract

A drilling apparatus including, at a working end (10): a drive sub (20) having an inner surface; and a drill bit (30) engaged with the drive sub (20) to form a drill bit assembly (90) having a longitudinal axis (L). A space is disposed between the drive sub (20) and drill bit (30) to define a fluid flow path (50) for a working fluid of the drilling apparatus to flow through the space and past the drill bit (30) toward the working end (10) of the drilling apparatus, in direction either the same as, or at an acute angle, which should be significantly less than 90 degrees, to the longitudinal axis (L) of the drill bit assembly (90). Such construction minimises pressure losses in working fluid travelling through the drill bit assembly (90) with object of increasing drilling efficiency.

Description

A DRILLING APPARATUS

FIELD OF THE INVENTION

The present invention relates to a drilling apparatus for use in reverse circulation ("RC") drilling.

BACKGROUND TO THE INVENTION

Down-hole drilling for geological sampling is generally conducted using a pneumatically operated hammer drill comprising a drill string with a drill bit at its bottom end. Such drills use a conventional down-the hole ("DTH") mechanism, with live compressed air being supplied to operate a reciprocating piston near the base of the hole being drilled. Exhaust air is returned through a central passageway extending through the drill string to the surface, carrying with it drilled rock cuttings and debris which can be analysed in surface laboratories. This is a reverse circulation ("RC") drilling operation which is of great importance in mineral exploration.

The drill bit impacts with high frequency on a rock formation to fracture rock into cuttings that may be recovered for sampling as above described. During impacts of the drill bit on the rock formation, air is also discharged or exhausted from the drilling apparatus. Such exhaust air flow assists in recovery of cuttings, for sampling, by flushing the cutting face of the drill bit so that cuttings are directed into the central passageway of the drill string. It is desirable that the air flow have a direction substantially parallel to a drilling axis to maximize benefits.

An undesirable problem encountered in some prior art drilling apparatus, particularly when drilling in soft ground, is borehole scouring. Borehole scouring occurs when turbulence and expansion of air discharged or exhausted from the drilling apparatus is excessive or the angle at which compressed air is forced through the drilling apparatus causes scouring of the borehole wall. The problem can cause loss of seal of drilling apparatus and borehole with the result that cutting samples are lost up the borehole outward of the drilling string. This will cause loss of sampling accuracy.

Australian Patent No. 763940 proposes a drilling apparatus designed to completely avoid the borehole scouring problem. To this end, the drilling string has a gauge sleeve and drill bit designed to direct exhaust air flow substantially parallel to the drill axis to reduce the amount of borehole scouring. The drilling apparatus has air passages defined between a gauge sleeve and a chuck, that is a drive sub. These air passages have terminal portions extending parallel to the axis of the drill bit and substantially in register with channels provided on the outside of the drill bit and extending through the cutting face of the drill bit. These air passages are fed with exhaust air through cross-drilled ports in the wall of the drive sub, these ports forcing exhaust air to be directed in a direction perpendicular to a longitudinal axis of the drilling apparatus. As a result, exhaust air flow must be bent through ninety degrees before being directed parallel to the drill axis. This drilling apparatus design addresses the problem of borehole scouring by directing air flow with a predominant downward component into the borehole. However, the drilling apparatus compromises, through its cross-drilled port design, on the energy of the exhaust air flow deliverable to the cutting face of the drill bit and the drilling efficiency. That is, pressure loss occurs when the exhaust air flow is bent through ninety degrees.

Australian Patent No. 656724 proposes a drill bit assembly with the drill bit having a number of outermost channels extending parallel to the longitudinal axis of the bit down the outer side of the bit in the head portion and directing air towards a periphery of the cutting face of the bit. An outermost cover acts as a shroud for these channels assisting the direction of air towards the cutting face of the bit. In this drill bit assembly, also, air flow must be directed through a space extending perpendicular to the longitudinal axis of the bit at least once as it travels downward past the drill bit. Like the proposal of Australian Patent No. 763940, the air flow must be moved through 90 degrees. Unlike the proposal of Australian Patent No. 763940, the airflow from this drill bit assembly has a significant outwardly directed airflow component said - in Australian Patent No. 763940 - to cause a tendency to borehole scouring. Again, there is a loss of energy in the air flow due to pressure losses caused by its movement through 90 degrees during passage past the drill bit and drilling efficiency could be improved if such pressure losses were reduced.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a drilling apparatus which delivers greater efficiency through delivering more energy for rock fracture than in previous drill apparatus. With this object in view, the present invention provides a drilling apparatus comprising, at a working end:

a drive sub having an inner surface; and

a drill bit engaged with the drive sub to form a drill bit assembly having a longitudinal axis, a space being disposed between the inner surface of the drive sub and drill bit to define a fluid flow path for a working fluid of the drilling apparatus to flow through the space past the drill bit toward the working end of the drilling apparatus

wherein said fluid flow path extends, along its length, past the drill bit and drive sub in direction either the same as, or at an acute angle to, the longitudinal axis of the drill bit assembly. The fluid is the working fluid for the drilling apparatus and is typically compressed air supplied from a suitably sized air compressor.

An embodiment provides a drilling apparatus being a drill bit assembly having a longitudinal axis and comprising a drill bit engaged with a drive sub having an inner surface, a space being disposed between the inner surface of the drive sub and drill bit to define a fluid flow path for a working fluid of the drilling apparatus to flow through the space past the drill bit toward the working end of the drilling apparatus wherein said fluid flow path extends, along its length, past the drill bit and drive sub in direction either the same as, or at an acute angle to, the longitudinal axis of the drill bit assembly.

Typically, the space will comprise a plurality of working fluid channels disposed about the drill bit and between the drill bit and the inner surface of the drive sub.

The drilling apparatus therefore has a working fluid flow path which does not require working fluid to be directed through ports or other passageways directed perpendicular to the longitudinal axis of the drilling apparatus. In this way, pressure losses - which would result from a requirement to move working fluid through 90 degrees on passage through the drilling apparatus - can be avoided and the working fluid pressure can be maintained and drilling efficiency achieved in a manner not possible with the previously mentioned proposals.

The drive sub may have first and second portions, each having inner surfaces of different inner diameter. The inner surface of the first portion of the drive sub, located towards a front end of the drilling apparatus in use, has greater inner diameter than the inner surface of the second rear portion. Spline portions are formed on the inner surface of both drive sub portions, working fluid channels being formed between the spline portions. The working fluid channels are comprised within the space disposed between drive sub and drill bit. The spline portions for the first drive sub portion may have different shape than the spline portions of the second drive sub portion and geometry of the spline portions is selected to maximize flow area for working fluid to pass through the working fluid channels. The spline portions are therefore advantageously chamfered or partly triangulated in shape, the preferred shape being a truncated triangular shape projecting - as at steep an acute angle as possible without compromising spline portion strength - from an inner diameter of the drive sub and terminating in a substantially flat apex surface. The working fluid channels have number and cross-sectional area selected to maximize air flow while minimizing risk of mechanical failure, due to insufficient strength, during drilling operations.

The drill bit has a head portion and a shank. The shank of the drill bit has a portion, which may be an intermediate portion located rearward from the head portion, formed with longitudinally extending spline portions correspondent, and interlocking, with complementary spline portions of the drive sub to form the drill bit assembly. In such drill bit assembly, rotation of the drill bit relative to the drive sub is prevented. Only sliding movement permitting reciprocation of the drill bit is permitted. The drill bit has channels extending longitudinally between the spline portions of the drill bit. Spline portions of the drive sub, preferably those spline portions for the second or rear drive sub portion, may be fully accommodated within the longitudinally extending drill bit channels advantageously formed in the intermediate portion of the shank of the drill bit. Space is left for provision of working fluid channels. In such case, a portion of the fluid flow path may extend through those working fluid channels between the complementary interlocking spline portions of the drill bit and spline portions of the drive sub. The working fluid channels extend generally parallel to the longitudinal axis of the drill bit assembly.

Advantageously, the fluid flow path extends either along (i.e coaxially) or parallel to the longitudinal axis of the drill bit assembly for most of its length. However, the fluid flow path does not extend parallel to the longitudinal axis of the drill bit assembly for its entire length. Provision is made for an outwardly divergent deflection in direction of the fluid flow path at one or more transition(s), this deflection being necessary for delivering working fluid to a periphery of the cutting face of the drill bit but in a manner that reduces loss of working fluid pressure during transit of working fluid to the cutting face of the drilling apparatus. Conveniently, the fluid flow path may have a rear portion and a front terminal portion. At a transition between the rear portion and front terminal portion, the fluid flow path diverges at an acute angle, preferably significantly less than 90 degrees, from the longitudinal axis of the drill bit assembly. The acute angle may be between 20 and 45 degrees, more advantageously being between 20 and 30 degrees to the longitudinal axis of the drill bit assembly. Therefore, during use of the drilling apparatus, working fluid flowing through the fluid flow path is caused to be deflected from its generally longitudinal direction in the transition. Forward of the transition, working fluid is again caused to flow in a longitudinal direction parallel to, and outwardly disposed of, the longitudinal axis of the drill bit extending through the rear portion of the fluid flow path. Forward of the transition, working fluid may flow through channels formed between spline portions of the drive sub which bear against a forward portion of the shank of the drill bit.

Channels of the drill bit and drive sub may be provided with portions to assist flow of working fluid through the fluid flow path, particularly at or proximate the transition. These portions may take the form of angled or curved portions. For example, longitudinally extending channels of the intermediate portion of the drill bit may include curved portions or scallops to assist flow of compressed air, that is working fluid for the drilling apparatus, through the transition. These curved portions may be formed at a front terminal surface of each channel, the portions curving outwardly with gentle slope towards an inner surface of the drive sub and borehole wall in a direction towards the periphery of the working end of the drilling apparatus. The front terminal surface therefore acts as a guide surface assisting deflection of working fluid flow at an acute angle (significantly less than 90 degrees) to the longitudinal axis of the drill bit assembly. The drive sub may be provided with a correspondent guide surface of relatively sharp geometry also assisting to deflect working fluid flow from a longitudinal direction to an outwardly divergent direction. The drive sub guide surface may also be provided at a transition between its first and second or rear portions.

The drill bit is also typically provided with working fluid channels in its head portion. A portion of a surface of each such channel located in the head of the drill bit forward of the first fluid flow path, and at the rear of each head portion channel, may also act as a further transition having a guide surface assisting deflection of fluid outward toward a periphery of the cutting face of the drill bit. To that end, the guide surface may angle or curve outwardly again at desired acute angle to a longitudinal axis of the drill bit. The angle is selected to minimize issues of borehole scouring.

The drilling apparatus may provide hole sealing without use of a separate gauge sleeve component. However, this is not preferred and a gauge sleeve is conveniently included in the drill bit assembly being mounted outwardly of the drive sub. Conveniently, a hardened but sacrificial gauge sleeve having similar hardness and wear resistance to the drill bit is adopted. Alternatively, the gauge sleeve may be armoured and wear resistant being made of a material that has substantially lesser wear rate than the wear rate of the drill bit. The gauge sleeve conveniently terminates substantially flush with a front end of the drive sub.

The drilling apparatus is suited for reverse circulation drilling for use in mineral exploration. The drilling apparatus may be of retention or non-retention type.

BRIEF DESCRIPTION OF THE DRAWINGS

The drilling apparatus may be more fully understood from the following description made with reference to the accompanying drawings in which:

Figure 1 is a side section of an assembly of drive sub, gauge sleeve and drill bit in a drilling apparatus in accordance with one embodiment of the invention.

Figure 2 is a detail view at B of the assembly shown in Figure 1 .

Figure 3 is a front view of the assembly shown in Figures 1 and 2.

Figure 4 is a detail view at A of the assembly shown in Figure 1 (which is the side section taken along section line A-A of Figure 3) and Figures 2 and 3.

Figure 5 is a top view of the assembly shown in Figures 1 to 4.

Figure 6 is a side view of the assembly shown in Figures 1 to 5. Figure 7 is a transverse section view of the assembly shown in Figures 1 to 6 taken along section line C-C of Figure 6.

Figure 8 is a detail view at D of the assembly shown in Figure 7.

Figure 9 is a side section view of a drive sub of the assembly shown in Figures 1 to 8.

Figure 10 is a front view of the drive sub of Figure 9.

Figure 1 1 is a detail view at N of the drive sub of Figure 10.

Figure 12 is a side view of a drill bit of the assembly shown in Figures 1 to

8.

Figure 13 is a side section view of the drill bit of Figure 12.

Figure 14 is a detail view at K of the drill bit of Figures 12 and 13.

Figure 15 is an orthogonal view of an alternative drill bit to be used in accordance with an alternative embodiment of the drilling apparatus of the present invention.

Figure 16 is a detail view of the front end of the drill bit shown in Figure 15.

Figure 17 is a plan section view of the drill bit shown in Figures 15 and 16. Figure 18 is a detail from Figure 17 showing an air channel transitioning between shank and head of the drill bit.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to Figures 1 to 14, there is shown portion of a working or front end 10 of a drilling apparatus being a percussive hammer used in reverse circulation mode for mineral exploration, cuttings being recovered for resource evaluation. The percussive hammer operates using compressed air as a working fluid, the compressed air being supplied from a suitably selected air compressor. At the working or front end 10 of the drilling apparatus is located a drill bit assembly 90 comprised of a drive sub 20, a drill bit 30 and gauge sleeve 40.

Drive sub 20 has first and second portions 21 and 22, each having different inner diameter. The first portion 21 of the drive sub 20, located towards the working or front end 10 of the drilling apparatus in use, has greater inner diameter than the second rear portion 22 of drive sub 20. Spline portions 27, with mesic shape having a flattened apex or upper surface 27A and steeply angled sides, are formed in and project from the inner diameter of the first drive sub portion 21 . Channels 26 are formed between spline portions 27. Spline portions 28 are formed in and project from the second drive sub portion 22. Spline portions 28 slightly differ in shape from spline portions 27 in having a truncated triangular shape with a flattened apex or upper surface 28A and more steeply or acutely angled sides than spline portions 28. This geometry allows achievement of a greater cross-sectional area for working fluid channels as described below and this enables optimized compressed air flow rates and faster drilling rates.

Spline portions 27 and 28 extend longitudinally along the length of the first and second drive sub portions 21 , 22 respectively. Second rear portion 22 of drive sub 20 has a threaded surface to enable connection with an outer sleeve (not shown) of the drilling apparatus.

Drill bit 30 has, viewed rearwardly along its length, a head portion 30a and a shank 30c with an intermediate portion 30b and a cylindrical portion 30d. Cylindrical portion 30d is relatively short in length in comparison to the length of the intermediate portion 30b of shank 30c. Head portion 30a includes a cutting face 31 1 . The surface of intermediate portion 30b is formed with spline portions 31 correspondent with spline portions 28 of drive sub 20. The surface of cylindrical portion 30d is generally smooth, acting as a bearing surface for spline portions 27 of first drive sub portion 21 . As drill bit head 30a, provided with round headed cutting inserts 35, of tungsten carbide or like material, distributed over the surface of the cutting face 31 1 , impacts the bottom of a hole (not shown), cuttings are formed and returned for surface sampling through passages 32 and 33 diverging from return passageway 36. The drill bit 30 impacts the bottom of the hole in response to a piston (not shown) cyclically striking the top strike face 38 of the drill bit 30 in response to cyclic variation of compressed air pressure above and below the drill bit 30.

Gauge sleeve 40 is a hardened but sacrificial gauge sleeve having similar hardness and wear resistance to the drill bit 30, for example as described in Australian Patent No. 638571 , the contents of which are hereby incorporated herein by reference. Alternatively, the gauge sleeve may be armoured and wear resistant including a tungsten based material which wears at substantially lesser rate than the wear rate of drill bit 30. The tungsten based material is formed from a tungsten containing powder processed by plasma transferred arc (PTA) processing. Gauge sleeve 40 is fixed to drive sub 20 by a flange portion 42 bevelled at its top surface.

Drill bit 30 is engaged with the drive sub 20 by interlocking of complementary spline portions 31 and 28 such that rotation of drill bit 30 relative to drive sub 20 is prevented. Sliding movement to enable reciprocation of the drill but 30 relative to drive sub 20 is permitted. The assembly 90 of drive sub 20 and drill bit 30 is such as to allow a space to be disposed or left between the drive sub 20 and drill bit 30. This space defines a fluid flow path 50 for the working fluid, compressed air, to flow - in a direction A - past the drill bit 30 and drive sub 20 toward the working or front end 10 of the drilling apparatus.

Referring particularly to Figures 1 and 2, fluid flow path 50 extends, along its length, past the inner surface 25 of drive sub 20 and past the shank 30c of the drill bit 30 in a direction the same as (including parallel) and an acute angle to a longitudinal axis L of the assembly 90. For most of its length, fluid flow path 50 extends parallel or coaxially to the longitudinal axis L of the assembly 90.

However, the fluid flow path 50 does not extend parallel to longitudinal axis L of assembly 90 for its entire length. The fluid flow path 50 has a rear portion 52 and a front terminal portion 55. Fluid flow path 50 is coaxial with the longitudinal axis L in rear portion 52. Fluid flow path 50 is parallel to longitudinal axis L in at least a substantial part of front terminal portion 55. At the first transition E between the rear portion 52 and front terminal portion of the fluid flow path 50, the fluid flow path 50 outwardly diverges at an acute angle, significantly less than 90 degrees (and more in the range 20 to 30 degrees) to longitudinal axis L of assembly 90. Therefore, during use of the drilling apparatus, compressed air flowing through the fluid flow path 50 is caused to be deflected from its generally longitudinal direction in transition E. Forward of transition E, compressed air is again caused to flow in a direction parallel to , and outward of, longitudinal axis L of assembly 90. The fluid flow path 50 allows for both a freer flow of compressed air toward the working or front end 10 of the drilling apparatus and direction of compressed air toward the periphery of the cutting face 135 of the drill bit 30. There is no requirement for compressed air to be directed through ports or passageways extending perpendicular to longitudinal axis L of drill bit assembly 90. As a result, a higher working compressed air pressure and higher drill bit 30 impact energy for rock fracture may be achieved. More efficient cutting and drilling results with perhaps 20% or more efficiency gain over conventional reverse circulation drilling apparatus being achieved for the drilling apparatus described here.

The fluid flow path 50 is formed, proceeding in rearward direction, by:

1 ) working fluid or compressed air channels 26 formed between spline portions 27 of the first portion 21 of drive sub 20 and the outer surface 30e of cylindrical portion 30d of drill bit shank 30c; and

2) working fluid or compressed air channels 28a formed between the spline portions 28 of the second portion 22 of drive sub 20 and the spline portions 31 of intermediate portion 30b of drill bit 30. Channels 26 form the fluid flow path in the front terminal portion 55 of fluid flow path 50. Channels 28a form the fluid flow path in the rear portion 52 of the fluid flow path 50. Eight each of air channels 26 and 28a are provided though as many channels as desirable may be provided subject to manufacturing cost, maintaining strength of drill bit 30 and delivering optimal compressed air pressure to the cutting face 31 1 of drill bit 30 from the perspective of drilling rates. Cross sectional area of air channels 26 and 28a is also selected to ensure desired compressed air flow behaviour through the fluid flow path 50 subject to the cost, strength and operational constraints indicated above. The fluid flow path 50 comprises the fluid flow paths through each of air channels 26 and 28a.

The drill bit 30 has working fluid or air channels 33 extending longitudinally along intermediate portion 30b of shank 30c. Spline portions 28 of drive sub 20 are accommodated within the channels 33 though leaving the air channels 28a as above described. Splines 31 of drill bit 30 are accommodated within the longitudinally extending channels 29 of drive sub 20, channels 29 being formed between spline portions 28. Channels 27a and 28a have curved portions, or scallops, to assist flow of compressed air through the fluid flow path 50. Channels 33 each have a front terminal surface 34 curving outwardly with gentle slope towards the inner surface 25 of drive sub 20 and borehole wall in a direction towards the periphery of the working or front end 10 of the drilling apparatus. Front terminal surface 34 acts as a guide surface assisting deflection of compressed air flow, at the acute angle to longitudinal axis L of assembly 90 through the transition E. Drive sub 20 has a correspondent surface 124 of relatively sharp edged geometry in comparison to front terminal surface 34, the sharp edged geometry also assisting to deflect compressed air flow from its longitudinal direction to an outward direction.

The drill bit 30 is also typically provided with air channels 37 in its head portion 30a, these air channels 37 forming a further fluid flow path 80. Eight air channels 37 are provided. A surface 93 located at a rear end 39 of each channel 37 and located forward of the first fluid flow path 50 diverges at acute angle relative to longitudinal axis L of assembly 90. Surface 93 acts as a guide surface assisting deflection of compressed air outward in direction D toward the periphery of the cutting face 135 of the drill bit 30, this forming a second transition providing for further deflection of compressed air flow to the deflection of compressed air flow through first transition E. The acute angle of deflection , and direction D, is selected to minimize the problem of borehole scouring, particularly prominent in soft formations.

Referring now to Figs 15 to 18, there is shown an alternative drill bit 130 for use in the drilling apparatus. The drill bit 130 has similar elements to drill bit 30 so these elements are numbered in the same way except for addition of the prefix "1 ".

Drill bit 130 has a head portion 130a and a shank 130c with an intermediate portion 130b. However, in the periphery of lower shank portion 130d - in contrast to cylindrical portion 30d of drill bit 30 - there is formed a number of air channels 133a. Channels 133a are extensions of channels 133, through which compressed air flows in direction A. Between channels 133a are left segments with curved surface 170 which act as bearing surfaces for a complementary surfaces of drive sub 20. The channels 133a may have uniform depth and increase the flow area for compressed air to flow towards cutting face 131 1 of drill bit 130. This increased flow area allows higher compressed air flow rates to cutting face 131 1 and improved drilling performance through increased drilling rates.

Guide surfaces 193 formed in air channels 130, diverging at an acute angle from the longitudinal axis of drill bit 130, assist deflection of compressed air outward in direction D toward the periphery of the cutting face 31 1 of the drill bit 130.

Modifications and variations to the drilling apparatus of the invention will be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present disclosure.

Claims

1 . A drilling apparatus including, at a working end: a drive sub having an inner surface; and a drill bit engaged with the drive sub to form a drill bit assembly having a longitudinal axis, a space being disposed between the inner surface of the drive sub and drill bit to define a fluid flow path for a working fluid of the drilling apparatus to flow through the space and past the drill bit toward the working end of the drilling apparatus, in direction either the same as, or at an acute angle significantly less than 90 degrees to, the longitudinal axis of the drill bit assembly.

2. A drilling apparatus being a drill bit assembly having a longitudinal axis and comprising a drill bit engaged with a drive sub having an inner surface, a space being disposed between the inner surface of the drive sub and drill bit to define a fluid flow path for a working fluid of the drilling apparatus to flow through the space and past the drill bit toward the working end of the drilling apparatus wherein said fluid flow path extends, along its length, past the drill bit and drive sub in direction either the same as, or at an acute angle to, the longitudinal axis of the drill bit assembly.

3. A drilling apparatus as claimed in claim 1 or 2, wherein said inner surface of said drive sub includes longitudinally extending spline portions and said working fluid channels are formed between the spline portions.

4. A drilling apparatus as claimed in claim 3, wherein the spline portions for a first portion of the drive sub have different shape than the spline portions of a second portion of the drive sub and geometry of the spline portions maximizes flow area for working fluid to pass through the working fluid channels.

5. A drilling apparatus as claimed in claim 3 or 4, wherein the spline portions are chamfered or partly triangulated in shape, a truncated triangular shape projecting from an inner surface of the drive sub and terminating in a substantially flat apex surface.

6. A drilling apparatus as claimed in any one of claims 2 to 5, wherein the drill bit includes a head portion and a shank, the shank includes an intermediate portion located rearward from the head portion, formed with longitudinally extending spline portions correspondent, and interlocking, with complementary spline portions of the drive sub to form the drill bit assembly.

7. A drilling apparatus as claimed in claim 6, wherein the rotation of the drill bit relative to the drive sub is prevented.

8. A drilling apparatus as claimed in claims 6 or 7, wherein a portion of the fluid flow path extends through the working fluid channel(s) between the complementary interlocking spline portions of the drill bit and spline portions of the drive sub.

9. A drilling apparatus as claimed in any one of the preceding claims, wherein the fluid flow path does not extend parallel to the longitudinal axis of the drill bit assembly for its entire length.

10. A drilling apparatus as claimed in claim 9, wherein the fluid flow path deflects and diverges outwardly at one or more transition(s), such that the working fluid is delivered to a periphery of a cutting face of the drill bit.

1 1 . A drilling apparatus as claimed in claim 9 or 10, wherein the fluid flow path includes a rear portion and a front terminal portion and at a transition between the rear portion and the front terminal portion, the fluid flow path diverges at an acute angle from the longitudinal axis of the drill bit assembly.

12. A drilling apparatus as claimed in any one of the preceding claims, wherein the acute angle is between 20 and 45 degrees to the longitudinal axis of the drill bit assembly.

13. A drilling apparatus as claimed in claim 12, wherein the acute angle is between 20 and 30 degrees to the longitudinal axis of the drill bit assembly.

14. A drilling apparatus as claimed in claim 10 or 1 1 wherein, in use of the drilling apparatus, the working fluid flowing through the fluid flow path is caused to be deflected from its generally longitudinal direction in a transition.

15. A drilling apparatus as claimed in claim 10 or 1 1 , wherein forward of a transition, working fluid is caused to flow in a longitudinal direction parallel to, and outwardly disposed of, the longitudinal axis of the drill bit assembly.

16. A drilling apparatus as claimed in claim 15, wherein forward of the transition, working fluid flows through channels formed between spline portions of the drive sub which bear against a forward portion of the shank of the drill bit.

17. A drilling apparatus as claimed in claim 16, wherein the channels of the drill bit and drive sub are provided with portions to assist flow of working fluid through the fluid flow path, particularly at or proximate the transition.

18. A drilling apparatus as claimed in claim 17, wherein the portions to assist flow of working fluid through the fluid flow path are angled or curved.

19. A drilling apparatus as claimed in any one of the preceding claims, wherein the drill bit is provided with working fluid channels in its head portion and a rear portion of a surface of each said working fluid channel acts as a further transition in said fluid flow path having a guide surface assisting deflection of fluid outward toward a periphery of the cutting face of the drill bit.

20. A drilling apparatus as claimed in claim 19, wherein the guide surface angles or curves outwardly at an acute angle to a longitudinal axis of the drill bit.

21 . A drilling apparatus as claimed in any one of the preceding claims, wherein a gauge sleeve is included in the drill bit assembly, mounted outwardly of the drive sub.

22. A drilling apparatus as claimed in any one of the preceding claims being a reverse circulation drilling apparatus for use in mineral exploration.

23. A drilling apparatus as claimed in any one of the preceding claims, wherein the drilling apparatus is of retention or non-retention type.