EP3555410A1

EP3555410A1 - Hybrid drill bit

Info

Publication number: EP3555410A1
Application number: EP17883635.9A
Authority: EP
Inventors: Greg West; Owen SCHICKER
Original assignee: Flexidrill Ltd
Current assignee: Flexidrill Ltd
Priority date: 2016-12-19
Filing date: 2017-12-19
Publication date: 2019-10-23
Also published as: AU2017381740A1; CL2019001666A1; EP3555410A4; US20200149349A1; CA3047509A1; US20210246731A1; WO2018116140A1; ZA201903585B

Abstract

A drill bit for coupling in use downhole drilling apparatus, the drill bit comprising: an annular coring drill bit that is rotatable to cut a formation bore face to create a core plug, a concentric drill bit that can be repeatedly axially moved (such as by oscillation (such as a vibration) and/or impact) to break the core plug.

Description

HYBRID DRILL BIT

FIELD OF THE INVENTION

The present invention relates to a drill bit for use in drilling apparatus. For example, it can be used in a reverse circulation drilling apparatus, and a reverse circulation drilling apparatus utilising such a drill bit. The drill bit can also be for use in a borehole drilling apparatus, and such a borehole drilling apparatus can utilise such a drill bit. Such a drill bit could be used in dry formations and also flooded environments.

BACKGROUND Reverse circulating (RC) drilling is used within the mineral exploration sector. The objective is to drill rapidly through formations (often very hard) to sample for various valuable minerals. There are two main types of RC drilling.

One is, air (pneumatic) reverse circulation drilling. This is the most common method, whereby large air compressors pump large volumes of compressed air down the outer cavity of dual walled drill rods. The compressed air: a) energises an air hammer for rapid penetration of hard formations to chip rocks, and b) pushes the rock chips (generated by the hammer striking a drill bit and crushing the rock) to surface through the centre cavity in the dual walled drill rods for analysis by geologists at surface.

While effective in dry competent formations, pneumatic hammers struggle as they encounter groundwater (a common occurrence) to the point that they cease to operate. In addition even in dry formations the volume of compressed air that needs to be pumped down deep holes becomes impractical. In addition the very large compressors required are extremely expensive to operate, as well as being potentially dangerous. As the depth of the borehole increases, the energy and pressure required to operate pneumatic hammers also increases. This increase becomes impractical, more expensive and dangerous at large depths due to the larger compressors posing significant safety concerns to operators such that the increased expense and safety concerns outweighs the benefits.

Alternatively, fluid reverse circulation hammer drilling is sometimes used. Fluid is used instead of air to energise fluid hammers and return rock chips to surface for analysis.

However to date, commercially available fluid hammers only work with very clean water, and are not able to survive with recycled drilling fluids. This makes them commercially impractical, except for niche applications where a ready supply of clean water is available, i.e., modifying water with drilling mud for bore control is not required. To date the only fluid driven hammer/oscillation tools that can reliably work with recycled drilling fluids (dirty fluids) in flooded environments, are those provided by the present applicant. Such fluid tools comprise: the magnetic hammer as described in WO2009/028964, the radial hammer described in WO2012/002827, and the vibratory apparatus described in WO2015/193799 or WO2012/161595. All these are incorporated herein by reference in their entirety. While these types of fluid tools can be used, they have lower impact energy (compared to pneumatic hammers) . This means that when they are used with conventional impact "hammer bits" the performance is less than desirable.

Drill bits need to cut a sufficiently large bore to enable uphole tools (hammer body drill rods etc.) to pass through the hole being drilled. As such, conventional drill bits rely on an outer row of bit inserts that are arranged at an angle (typically 35 °) to maintain the gauge or bore hole size diameter as drilling progresses. An example of such a prior art hammer bit 101 is shown in Figure 6. It comprises an outer row of inserts 100 that are placed on the outer perimeter 102 of the drill bit 101 at an angle. An inner row of bit inserts 103 is also provided. Drill bits like the hammer bit 101 in Figure 6 can come with a variety of inserts. For example, domed inserts are used for hard formations, and more aggressive conical/tapered inserts are typically be used for softer formations. With fluid driven hammer/oscillation apparatus such as the applicant's referenced above, there is the desire to have the ability to drill hard formations. However, such apparatus have less impact energy than pneumatic hammers. The use of aggressively shaped inserts in hard formations is desirable to assist drilling - however the angled outer rows of the conventional hammer bit fail prematurely in these applications. Tougher rounded inserts on the angled gauge rows could be used, however this results in unsatisfactory drilling performance when used with lower energy tools.

SUMMARY OF INVENTION

It is an object of the present invention to provide a drill bit for use in ground

engineering/exploration and/or reverse circulation drilling and/or a drilling apparatus comprising such a drill bit.

The present inventor has devised a drill bit that can be used with lower energy drilling fluid driven hammer/oscillation tools. An annular coring bit can cut the required outer gauge - without the need for either aggressively shaped angled inserts, (which break), or the use of conservatively shaped (e.g . domed) inserts which dramatically retard the speed of drilling. A concentric coring bit comprises an inner row of bit inserts that do not need to be placed at an angle. Instead the inserts run in the axial direction of the bore hole and can therefore be aggressively shaped to improve drilling performance. Even if aggressively shaped inserts are not used, improved drilling performance can be achieved over conventional hammer bits.

The drill bit can work in conjunction with fluid driven hammer/oscillation tools (such as those referenced above) with modest energy output and that can work in flooded environments and provide strong drilling performance, and good bit life.

In one aspect the present invention may comprise a drill bit for coupling in use downhole drilling apparatus, the drill bit comprising : an annular coring drill bit that is rotatable to cut a formation bore face to create a core plug, a concentric drill bit that can be repeatedly axially moved (such as by oscillation (such as a vibration) and/or impact) to break the core plug.

Optionally the concentric drill bit is axially setback from the annular coring bit to create a recess such that in use the core plug is unconfined by the surrounding formation.

Optionally the drill bit is for use in a reverse circulation drilling apparatus to create chip samples for return to surface by breaking the core plug.

Optionally the drill bit is for use in bore drilling apparatus.

Optionally in use when the concentric drill bit is repeatedly axially moved it contacts the unconfined core plug in the recess to break the core plug.

Optionally in use: the annular coring drill bit is or can be coupled to and rotated by a rotational drive of a drilling apparatus, and the concentric drill bit is or can be coupled to a repeatable force generating apparatus that can repeatedly axially move the concentric drill bit. Optionally the rotational drive is a drillstring casing of the drilling apparatus.

Optionally the repeatable force generating apparatus is an impact apparatus or an oscillation apparatus configured to repeatedly axially move the concentric drill bit by impact or oscillation (such as a vibration).

Optionally: the impact apparatus is a hammer, such as a magnetic hammer, pneumatic hammer, fluid hammer or any suitable hammer means to provide impact force to the concentric bit to break the core plug, or the oscillation apparatus provides oscillatory force (such as a vibration force) to the concentric bit to break the core plug.

Optionally the annular coring bit comprises: a body with a hollow core and a bit face cutting structure arranged around the end of the body, the bit face cutting structure comprising :

an inner ring of spaced apart inner teeth providing inner gaps between adjacent inner teeth,

an outer ring of spaced apart outer teeth providing outer gaps between adjacent outer teeth, and the inner and outer teeth being slanted such that: one or more inner teeth overlap with one or more outer teeth to provide mutual bracing, and one or more inner gaps overlap with one or more outer gaps to expose pathway for fluid flow between the hollow core and the body exterior.

Optionally the inner and outer teeth are slanted such that as the cutting teeth wear, for at least some of the inner and/or outer gaps, fresh parts of one or more inner gaps overlap fresh parts one or more outer gaps to expose fresh pathway for fluid flow.

Optionally the body comprising the bit face is cast as a monobloc.

Optionally the teeth and gaps take the form of a helix.

Optionally one or more of the inner teeth and/or one or more of the outer teeth comprise one or more apertures for fluid flow. Optionally the body comprises one or more apertures for fluid flow.

Optionally the inner teeth slant in a first direction around the body, and the outer teeth slant is a second opposite direction around the body.

Optionally the cutting structure is a diamond impregnated matrix.

Optionally the annular coring bit comprises: a body with a hollow core, a bit face with a cutting structure around an end of the body, the cutting structure comprising : an inner ring of inner spaced apart teeth and an outer ring of outer spaced apart teeth, the inner and outer teeth being slanted such that: a) one or more inner teeth overlap one or more outer teeth to provide mutual bracing, and b) there is pathway for fluid flow where the spaces between inner and outer teeth overlap. Optionally the annular coring bit is a diamond impregnated bit.

Optionally the concentric drill bit has bit inserts for breaking the core plug. Optionally the bit inserts are ballistic bits or PDC bits.

Optionally the bit inserts are tapered to increase point loading and promote core plug breaking, and preferably are at right angles to the concentric d rill bit face.

Optionally the concentric drill bit is splined to or relative to the annula r coring bit such that it can move axially relative to the annular coring bit but can rotate with the annular coring bit.

In one aspect the present invention may comprise a drilling apparatus with a drill bit according to any preced ing claim and configured to rotate the annular coring bit and repeatedly axially move the concentric drill bit. Optionally the drilling apparatus may comprise : a drillstring casing coupled to and operable to rotate the a nnular coring bit, and a repeatable force generating apparatus coupled to and operable to repeatedly axially move the concentric drill bit.

Optionally the repeatable force generating apparatus is splined to the drillstring casing .

Optionally the repeatable force generating apparatus provides a vibration to the annular coring bit sufficient to enhance the performance of the coring bit.

Optionally the drill bit or drilling a pparatus has a recess height, a number of bit inserts on the concentric d rill bit and a force applied to the drill bit configured to generate a desired size of chip sample when brea king the core plug .

Optionally the drill bit or drilling a pparatus wherein brea king the core plug produces fractures in the formation to assist drilling .

It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) . The term "comprising" as used in this specification means "consisting at least in part of". Related terms such as "comprise" and "comprised" are to be interpreted in the same manner.

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

DETAILED DESCRIPTION OF DRAWINGS

Embodiments will now be described with reference to the following drawings, of which : Figures 1A, IB show in diagrammatic form an elevation cross-sectional view of a drill bit to use with a reverse circulation drilling apparatus, with and without a core sample in a recess.

Figure 1C shows in diagrammatic form an exploded perspective view of the drill bit.

Figures 2A, 2 B show perspective views of the drill bit coupled to a drillstring casing Figure 3 shows a diamond impregnated annular coring drill bit component.

Figure 4 shows in diagrammatic form a reverse circulation drilling apparatus comprising the drill bit.

Figure 5A shows in diagrammatic form a bore drilling apparatus comprising the drill bit.

Figure 5B shows a variation on the annular coring bit without fluid flow holes in the body for use with the bore drilling apparatus. Figure 5C shows a hybrid drill bit using the variation of the annular coring bit without fluid flow holes in the body.

Figure 6 (prior art) shows a conventional hammer drill bit typically used.

Figure 7 shows an annular coring bit according to a second embodiment.

Figure 8/8A shows the cutting structure of the annular coring bit according to the second embodiment.

Figure 9A-9F shows various stages of wear of the annular coring bit according to the second embodiment.

Figure 10 shows a hybrid drill bit using the annular coring bit according to the second embodiment. Figure 11 shows an alternative cutting structure for the annular coring bit of the second embodiment.

Figure 12 shows a variation on the annular coring bit for the second embodiment without fluid flow holes in the body for use with the bore drilling apparatus of Figure 5A. Figures 13A to 13F show various stages of wear of the annular coring bit according to the second embodiment variation of the annular coring bit without fluid flow holes in the body.

Figure 14 shows a hybrid drill bit using the second embodiment variation of the annular coring bit without fluid flow holes in the body.

DETAILED DESCRIPTION OF EMBODIMENT

Figures 1A, IB, 1C, 2A, 2B, 3 show a drill bit (optionally termed "hybrid drill bit") according to an embodiment. It is for incorporation in/coupling to a reverse circulation (RC) drilling apparatus (such as shown in Figure 4) for use in RC drilling field

applications. It can also be used in boring applications (such as shown in Figure 5A), with a slightly modified hybrid drill bit according to a variation of the embodiment, as shown in Figures 5B and 5C, as will be described later. The (hybrid) drill bit 10 (for all embodiments described) comprises an annular coring drill bit (outer component) 11 and an inner concentric drill bit (inner component) 12. While the outer component is a coring drill bit, in this context it is used in a hybrid drill bit for chip sampling or in boring applications.

Referring to the embodiment of Figures 1A, IB, 1C, 2A, 2B, 3, the annular coring bit 11 is rotatable (by a RC drilling apparatus 40 in use - see Figure 4) to cut into a formation 5 to cut/create a core sample 6 (see Figure IB and 4). The annular coring drill bit 11 preferably is a castellated diamond impregnated bit 30 such as shown in Figure 3, comprising a body with an annular metallic matrix 31 with synthetic diamond inserts 32 embedded throughout the matrix or a surface set matrix, and a hollow interior. Fluid flow holes 50 are provided around the body. An alternative embodiment of the annular coring bit is also described later, and could be used instead of the castellated annular coring bit. In use, and referring to Figure 1A, when rotated and weight-on-bit is applied, the annular coring bit 11 grinds the formation/rock 5 away at the bore face 8 as the bit is rotated and pushed into the formation. Referring to Figure IB, this creates/forms the core sample 6 in the interior 33 of the annular core bit 11.

The hybrid drill bit 10 also comprises a concentric drill bit (inner component) 12 residing concentrically (coaxially) inside the interior portion 33 of the annular core bit 11. The concentric drill bit 12 (which could also be termed "coaxial drill bit") can be splined to the interior wall surface of the annular coring bit 11 such that it rotates with the annular coring bit, but can move axially relative to it. In another option, the concentric drill bit is not splined to the annular coring bit 11, such that the concentric drill bit can still move axially relative to the annular coring bit, but the annular coring bit can also rotate relative to the concentric drill bit 12. The concentric drill bit is seated on the inner wall of the annular core bit 11 such that the concentric drill bit is axially set back a distance "X" from the face 15 of the annular core bit 11. This creates a recess 13 between the annular core bit interior 33 and the face 16 of the concentric drill bit 12. As the annular core bit 11 rotates and drills into the formation, the core sample 6 is formed in the recess 13 to create a core plug (also termed "knub") 6 (see Figure IB). Thus, the annular coring bit 11 creates an isolated core of rock 6 that is unconfined by the pressure of the surrounding rock mass 5. This unconfined core plug 6 is much easier to break. The concentric drill bit 12 bit face 16 has bit inserts 17.

The concentric drill bit 12 can be repeatedly moved axially as shown by arrow "A" in Figure IB (by an RC drilling apparatus 40 in use) relative to and within the interior of the annular core bit 11/recess 13 and relative to the bore face 8/core plug 6. The axial movement "A" can be a reciprocating movement (oscillations, such as in the form of vibrations) or an impact movement, for example. The concentric core bit 12 can be repeatedly moved by an apparatus (see Figure 4) that provides impacts and/or oscillations, as will be described later. The concentric drill bit face 16 repeatedly axially moves as per arrow "A" into the recess 13 so that the bit inserts 17 hit the core plug 6 created by the annular core bit 11, and breaks (also termed "chipping") the core plug 6 into chip samples - e.g. by cutting, chipping, crushing (or the like) the core plug. The bit inserts 17 can be any suitable inserts for breaking the core plug 6, such as ballistic bits or PDC bits. The bits can be any suitable shape to facilitate core plug breaking.

Preferably, the bits are tapered to increase point loading to facilitate core plug breaking. The bit inserts 17 are preferably placed at a perpendicular angle to the rock face 8/bit face 16, and preferably not angled away from the bit face 16. Angled inserts are not needed to maintain borehole gauge, as this outcome is achieved by the annular coring bit 11. This allows the bit inserts 17 to be more aggressively shaped (such as tapered as described above) - which increases the point loading into the formation, to improve chipping performance. A suitable number of bit inserts 17 is provided to achieve the required chipping. As will be described in more detail later, the height "X" of the recess 13, the number of bit inserts 17 and the magnitude of the axial force B provided to the concentric drill bit 12 can be configured to achieve the desired chip size. The chip size will also be dependent on the nature of the formation, for example whether it is hard or soft.

An example of a concentric drill bit 12 is shown in more detail in the exploded form of the hybrid drill bit in Figure 1C. It differs from a conventional hammer drill bit such as in Figure 6 as it has been reconfigured to work in combination with the annular coring bit 11. It does not have angularly placed bits or an outer row of bit inserts, in contrast to the conventional hammer bit 101 shown in Figure 6. This is because the annular coring bit can create the required bore hole size, so angular insert bits and the outer row of bit inserts are not required. This now enables all concentric drill bit 12 inserts to be aggressively shaped with tapers or the like and can also allow for a reduced number of inserts to be used in contrast to the conventional hammer bit. The insert bits of the concentric drill bit can all be aggressively shaped, if desired, thus improving drilling speed. Even if aggressively shaped insert bits are not used, the creation of the

unconfined core plug means that less energy, power and/or force is required to chip the core plug. As such, force generators (e.g. hammers or oscillatory apparatus such as those referenced herein) with lower energy, power and/or force output can still have acceptable drilling speeds with the drill bit described herein.

Optionally, the concentric drill bit 12 can also be rotated (by the RC drilling apparatus 40 in use) either with or independently to the rotation of the annular core bit 11 to assist with breaking the core plug 6.

Optionally, the annular core bit 11 can be vibrated or otherwise perturbed to assist with drilling into the formation to create the core plug.

The concentric drill bit has return holes 18 for passage of drilling fluid 49 (such as drilling mud) and chip samples. In use, drilling fluid 49 from the RC drilling apparatus 40 passes down through the apparatus, and exits through holes 40, and returns through the return holes 18 carrying the chip samples to transport them back top hole for analysis. This will be explained later further with respect to Figure 4.

To be clear the cuttings return through holes 18 only if the annular coring bit 11 is being used in the R.C. configuration of Figure 4. If it is used for conventional hammer drilling (as described with respect to Figure 5A and 14 later) then the mud being pumped from surface exists the drill bit via holes 18 and carries cuttings to surface up the outside annulus as in the flow path shown in Figure 5A.

An alternative (second) embodiment of the annular coring bit 11' could be used in the hybrid drill bit instead. The second embodiment of the annular coring bit 11' and its assembly into a hybrid drill bit is shown in Figures 7 to 11. This embodiment is also for incorporation in/coupling to a reverse circulation (RC) drilling apparatus (such as shown in Figure 4) for use in RC drilling field applications. It can also be used in boring applications (such as shown in Figure 5A), with a slightly modified hybrid drill bit according to a variation of the embodiment, as shown in Figures 12 to 14, as will be described later.

The alternative annular coring bit 11' is of a monobloc construction and can be assembled with the concentric coring bit 12 as shown in Figure 10 to provide a hybrid drill bit 10'. This hybrid drill bit can be used in the same applications as the embodiment described above.

Referring first to Figure 7, the annular coring bit 11' of this embodiment is a monobloc structure comprising a body 70. The body has a hollow core 71 such that it is cylindrical in configuration. The body also comprises a bit face 72 with a cutting structure formed into and extending around the bit face end/rim of the monobloc body 70. The cutting structure is annular, as can be seen in Figure 7. For clarity, the cutting structure will be described in general terms first, with reference to diagrammatic Figure 8 that shows a portion of the annular cutting structure flattened out for clarity. Then, the cutting structure will be described in detail with reference to Figure 7. As can be seen in Figure 7, the cutting structure comprises a crown of teeth 73

(preferably a diamond impregnated matrix) arranged around the rim of the body. The crown of teeth comprises an inner ring 73A of inner teeth 74A, and an outer ring 73B of outer teeth 74B. (For simplicity, only one reference numeral will be used to denote all the inner teeth and only one reference numeral to denote the outer teeth). Referring to Figure 8, the inner teeth 74A are arranged around the bit face end of the body in a spaced apart manner, such that there are gaps 75A between adjacent inner teeth (a gap between each pair of adjacent teeth). Further, the inner teeth are slanted/angled around the bit face end, such that they lean/extend in a first (clockwise or anticlockwise) direction around the bit face end. Likewise, the outer teeth 74B are arranged around the bit face end of the body in a spaced apart manner, such that there are gaps 75B between adjacent outer teeth (a gap between each pair of adjacent teeth). Further, the outer teeth 74B are slanted/angled around the bit face 72 end, such that they lean/extend around the bit face end in a second opposite direction to the inner teeth 74A (that is, in a anticlockwise direction if the inner teeth extend in a clockwise direction; or a clockwise direction if the inner teeth extend in a anticlockwise direction) around the bit face end.

As such, the inner teeth and outer teeth overlap - see portions labelled 74A/74B. This means that they contact each other to at least some degree, be that just touching, partially or fully touching, or even to the degree that they cross each other entirely. Preferably, each inner tooth 74A overlaps (preferably to the degree that it crosses - see region 74A/74B), at least one corresponding outer tooth 74B; and/or each outer tooth 74B overlaps (preferably to the degree that it crosses - see region 74A/74B) at least one corresponding inner tooth 74A. It is not essential that all teeth cross at least one corresponding tooth, although this is preferable. In the embodiment of Figure 7, the inner teeth 74A are wider than the outer teeth 74B. This means that each inner tooth 74A overlaps two outer teeth 74B fully, including fully crossing one outer tooth and fully overlapping a second outer tooth, whereas each outer tooth overlaps only one inner tooth fully (to the extent of fully crossing) and only overlaps a second inner tooth to extent of just touching. It will be appreciated that this is only one embodiment, and others are possible, which will be described later with respect to Figure 11.

The arrangement continues the cylinder shape of the body 70 such that the crown of cutting teeth 73 itself is cylindrical in shape and has a hollow interior 71. The

overlapping of corresponding inner and outer teeth results in a configuration where the overlapping teeth mutually brace each other, making them more resistant to breakage during the drilling process. Also, the overlapping of corresponding inner and outer teeth results in overlapping gaps (see regions 75A/75B) between adjacent teeth 74A, 74b such that a fluid pathway is provided between the hollow core and the exterior of the body where the gaps overlap.

Referring to the diagrammatic Figure 8A dimensions of the teeth and gaps are shown in general terms, which could apply to any embodiment, and not just those described. Each outer tooth has a slanted angle Θ1, a length LI, a height HI, and a width Wl, and there is a corresponding slanted angle Θ2, width W2, height H2 and length L2 of the gap between teeth. The teeth could be spaced apart in an even, or uneven manner, and the width of each tooth Wl could be the same as or different to that of the gap width W2. Likewise, each inner tooth has a slanted angle Θ3, a length L3, a height H3, and a width W3, and there is a corresponding slanted angle Θ4, width W4, height H4 and length L4 of the gap between teeth. The inner teeth could be spaced apart in an even, or uneven manner, and the width of each tooth W3 could be the same as or different to that of the gap width W4. The dimensions of the inner and outer teeth could be the same or different, and the dimensions of the inner and outer gaps could be the same or different.

Referring now to Figure 7, this one possible embodiment of the cutting structure will be described in more detail. In this embodiment, there is an inner ring 73A of four inner teeth 74A, and an outer ring 73A of six outer teeth 74B. In the embodiment of Figure 7, the inner and outer teeth angle, height, and length are the same, but the width is different. The inner and outer ga ps have the same angle, height, length and width . The number of teeth is not essential, and any suitable number could be used . The inner teeth 74A are slanted/angled in a helical formation in an anticlockwise direction when viewed from the bit face end of the annular coring bit 11'; and the outer teeth 74B a re slanted in a helical formation in a clockwise direction when viewed from the bit face end . The gaps 75A, 75B between adjacent inner teeth and adjacent outer teeth also take a slanted helical formation. Each outer tooth 74B crosses one correspond ing inner tooth 74A and preferably further extends to at least some way across the gap 75A between that corresponding inner tooth and the adjacent inner tooth, such that the outer tooth overlaps two corresponding inner teeth, including j ust contacting the second inner tooth (see point A, a nd point B on Figure 8) . Also, the gap 75B between the outer tooth 74B and an adjacent outer tooth 74B overlaps the gap 75A between the two corresponding adjacent inner teeth 74A, to provide a fluid pathway (see region 75A/75B) . The remaining outer teeth 74B are a rranged in the same manner. Described from the alternative viewpoint of the inner teeth 74A, each inner tooth crosses one corresponding outer tooth 74B and preferably further extends to at least some way across the gap 75B between that corresponding outer tooth 74B and the adjacent outer tooth 74B, such that the inner tooth 74A overlaps two corresponding outer teeth 74B, including contacting the second outer tooth fully (see C) . Also, the gap 75A between the inner tooth 74A and an adjacent inner tooth 74A overlaps the gap 75B between the two corresponding adjacent outer teeth 74B, to provide a fluid pathway (see region 75A/75B) . The remaining inner teeth are arranged in the same manner.

Preferably, the crown of cutting teeth 73 is formed as part of the body as a monobloc, but it alternatively could be formed and attached in a separate process. While the inner teeth slant anticlockwise and the outer teeth slant clockwise, this is not essential - the alternative directions are possible.

Preferably, the base of each inner tooth 76 also has an aperture coinciding/overlapping with a portion of the gap between two corresponding outer teeth to provide additional pathway for fluid flow. Optionally, the base of each outer tooth could also have an aperture (not shown) coinciding/overla pping with a portion of the gap between two corresponding inner teeth to provide additional pathway for fluid flow.

Each outer tooth 74B braces at least the first corresponding inner tooth 74A (point A), and to some extent the second corresponding inner tooth (at contact point B) ; and likewise each inner tooth 74A braces at least the first corresponding outer tooth 74B and to some extent the second corresponding outer tooth 74B (fully contacted). This provides mutual bracing between the inner 74A and outer teeth 74B, to make the teeth

(structure) more resistant against breakage during drilling. The pathway for drilling fluid where the gaps between adjacent teeth exist are shown by the fluid flow arrows. The annular coring bit 11' can be assembled into a hybrid drill bit 10' such as shown in Figure 10, with the same benefits as previously described for the annular coring bit 11 of the first embodiment used in the hybrid drill bit.

Figure 7 shows the annular core bit 11 in its virgin state before it is used for drilling. As the bit is used for drilling, it wears such that the cutting structure wears down as shown in Figures 9A to 9F. The cutting structure described is configured such that during wear it maintains at least some degree of mutual bracing between inner 74A and outer teeth 74B, and also maintains pathway for fluid flow, or at least reduces the disappearance of pathway for fluid flow. This will be explained further with reference to Figures 9A to 9F which show different stages of wear. Referring to Figure 9B after 5 mm of wear, it can be seen that some of the outer teeth 74B and inner teeth 74A have worn away such that they detach from the second corresponding inner/outer tooth (previously contact point B) to expose fresh pathway. This opens up fresh pathway for fluid flow by, which previously was blocked. However, those teeth are still braced by the first corresponding inner/outer teeth. Referring to Figure 9C after 10 mm of wear, it can be seen that more of the outer 74B and inner teeth 74A have worn away. Some of the existing overlapping gap 75a/75B has diminished, thus reducing the pathway for fluid flow. However, it can be seen in other areas, e.g . point C, the wear of the teeth has opened up new overlap 75A/75B between inner/outer gaps providing fresh pathway for fluid flow which at least part way compensates for the loss of fluid path flow in other areas. As can also be seen, the existing teeth 74A, 74B are still mutually bracing each other.

Referring to Figure 9D after 15 mm of wear, it can be seen that the original overlapping of gaps to provide the original fluid pathway have now predominately disappeared, but the fresh pathway has opened up further thus maintaining some degree of pathway for fluid flow. There is still mutual bracing between inner and outer teeth. Referring to Figure 9E and 9F after 20 and 22 mm of wear, it can be seen that some degree of fluid pathway is still provided, even towards the latter stages of wear. There is sufficient pathway to allow for adequate fluid flow for drilling operations. This arrangement provides improvements over the castellated annular coring bit 11 of the first embodiment. With that embodiment, the castellated teeth are not braced, so are vulnerable to breakage during drilling. Furthermore, as the drill bit wears away, the teeth lessen in height, which lessens the height of the corresponding gap between adjacent teeth, and therefore lessens the fluid pathway. As the teeth wear, no fresh pathway is exposed for fluid flow. In contrast, the annular coring bit according to the second embodiment maintains the pathway for fluid flow, or at least reduces the rate at which it disappears to maintain adequate fluid flow for a longer period. That is, as wear occurs, the fresh pathway that is exposed (opened up) provides additional pathway for fluid to compensate for pathway that has disappeared to due to wear. The additional and/or new fluid flow pathway that is exposed could provide more fluid flow pathway than previously existed to provide for more fluid flow volume. Alternatively, the additional and/or new pathway could provide replacement fluid flow pathway, that is, it replaces the pathway that was lost with new path way that provides for the same volume of fluid flow.

Alternatively, the additional and/or new pathway could provide less pathway than before the wear, (that is, provides for less volume of fluid flow) but it at least provides some compensatory pathway, so the loss of fluid flow pathway volume is not as great as it would have been had fresh pathway not been exposed.

Various alternatives of the second embodiment could be envisaged by those skilled in the art. It is not essential for the inner ring of inner teeth and the outer ring of outer teeth to slant in opposite directions. An alternative, the inner teeth and outer teeth could slant in the same direction, but to different degrees (angles), such that they still overlap to provide bracing and overlapping of gaps between adjacent teeth to provide pathway for fluid flow. Instead of having an aperture 76 at the base of the inner teeth/outer teeth, the gap between adjacent teeth could be shaped such that the base of e.g. an inner gap overlaps the base of a e.g. corresponding outer gap (or the aperture could be made large enough that it joins the inner gap) to expose the additional pathway for fluid flow. The same shaped arrangement could be made for the outer gap also. Different numbers of teeth, and different degrees of overlap with different numbers of corresponding teeth are possible also. Different widths of gap and teeth are possible, as are even or uneven spacings. It will be appreciated that various configurations of tooth and gap overlap can provide the required bracing and/or fluid path way. For example, while the embodiment described has each inner/outer tooth overlapping two corresponding teeth, alternatives are possible. For example, each inner/outer tooth might only overlap a single corresponding outer/inner tooth, or may overlap more than 2 corresponding inner/outer teeth. The angle of the slant, the length of each tooth, and/or the gap with between teeth could be modified to achieve this. The term "overlap" can encompass mere contact, as well as partial or full contact, or complete crossing. As an example, Figure 11 shows a diagrammatic flattened cutting structure according to another embodiment. Here, the width of the inner and outer teeth are the same, and they both overlap the same number of corresponding teeth to the same degree.

Changes to the dimensions of the inner and outer teeth can be contemplated to achieve the desired level of bracing and/or fluid flow pathway and to optimise bracing and/or fluid flow pathway for different levels of wear, as the drilling application requires.

The embodiment described above has fluid flow holes in the body of the annular coring bit. Where the bit to used for coring rather than chipping, reverse flow of fluid is provided and the fluid flow holes are not required. Figure 4 shows the hybrid drill bit 10 incorporated into/coupled into a RC drilling apparatus 40. The arrangement of Figure 4 could alternatively incorporate/couple the hybrid drill bit 10' with the alternative embodiment of the annular coring bit 11'. For simplicity, but no loss of scope, the remainder of the description will refer to the first hybrid drill bit 10. The drilling apparatus 40 comprises a drillstring 41. The drillstring comprises an outer rotatable drillstring casing 42 (comprised of drill rods coupled together) with a hollow interior. A drill rig 43 with a top drive/ rotational drive is coupled top hole to the outer drillstring casing 42 to rotate the drillstring outer casing in use, such that the outer drillstring casing becomes a rotational drive. The hybrid drill bit 10 is coupled/incorporated into the drillstring outer casing 42. The annular coring bit component 11 is coupled to or embedded in the end of the outer drillstring casing 42. Preferably, the annular coring drill 11 bit is screwed into the end of the drillstring outer casing 42, such that the annular coring bit rotates at the same speed as the outer casing 42/drill rig 43 at surface. The concentric drill bit component 12 is seated in an axially setback manner on the inside of the annular coring drill bit 11, as previously described. Optionally, it is also coupled to the annular coring drill bit. For example, in one option the concentric drill bit 12 is splined to the interior wall surface of the annular coring bit 11 such that it rotates with the annular coring bit, but can move axially relative to it. In another option, the concentric drill bit 12 is not splined to the annular coring bit 11, such that the concentric drill bit can move axially relative to the annular coring bit whilst the annular coring bit rotates relative to the concentric drill bit 12. An inner drillstring casing 46 (comprised of drill rods coupled together) extends inside the outer drillstring casing 42 to the uphole side/back of the concentric drill bit 12. The outer 42 and inner 46 drillstring casings form a dual casing drill string. The inner drillstring casing 46 has a hollow interior 51. A repeatable force generator apparatus 47 is provided within the drillstring outer casing 42 and is arranged so that in use it can provide a repeatable axial force B to the concentric drill bit 12 to repeatedly axially move the concentric drill bit 12 axially/longitudinally A (with respect to the bore hole and relative to the formation/bore face) to break a core plug 6. That force could be a repeatable impact/impulse force to provide a repeatable impact axial movement A, or it could be an oscillating/vibration force to provide a vibration/oscillating/reciprocating axial movement A. The force generator 47 could be: a) an impact apparatus like a hammer, such as a magnetic hammer, pneumatic hammer, fluid hammer or any suitable hammer means to provide impact/impulse force to the concentric drill bit 12 to break the core plug 6; or it could be: b) a vibration/oscillatory apparatus to provide oscillatory force to the concentric drill bit 12 to break the core plug 6.

In one example, the repeatable force generator apparatus 47 is a magnetic hammer as previously referenced. The concentric drill bit can be screwed into the end of the repeatable force generator apparatus 47, such as for example the shuttle of the magnetic hammer as previously referenced. The shuttle is then splined further up hole to the inside diameter of the drillstring outer casing 42 such that the concentric drill bit and the annular coring drill bit rotate at the same speed (in the case where the concentric drill bit is splined to the annular coring bit). In an alternative, the shuttle is not splined to the drillstring outer casing 42 and the concentric drill bit moves independently of the drillstring outer casing rotation. It will be apparent to those skilled in the art that other types of force generator apparatus could be used and configured and coupled to the drillstring and hybrid drill bit in other manners.

Examples of impact and vibration/oscillatory apparatus include those described in

WO2009/028964 (magnetic hammer), WO2012/002827 (radial hammer)

WO2015/193799 or WO2012/161595 (vibratory/oscillatory apparatus). All these are incorporated herein by reference in their entirety.

The height of the recess, the number of bit inserts 17 and the magnitude of the axial force provided to the concentric drill bit can be configured to achieve the desired chip size. Varying these parameters varies the size of the chips. The chip size will also be dependent on the nature of the formation, for example whether it is hard or soft. By altering the setback X of the concentric hammer bit relative to the annular coring bit, the height of the recess and therefore the height/length of the resultant rock core (knub or core plug) can be controlled. The height/length of the core plug influences the durability of the core plug and therefore the ability to chip the core plug into chips. The longer the core plug, the weaker it becomes as a longer core of rock is unconfined/unsupported. This makes it easier to chip. Further, the size of the rock chips can be manipulated by the combination of this knub/core plug height and

(a) the number of bit inserts, and/or

(b) the force applied to the concentric drill bit/bit inserts and therefore the resultant point loading on the bit inserts.

In general terms:

• a longer core plug is weaker and easier to chip, and will result in larger chips,

• a large number of bit inserts will reduce the chip size,

• increasing the force applied to the concentric drill bit will increase the chip size. By way of an example, having a rock core/recess height of < 8mm with bit inserts that are pointed and used in hard terrain using a magnetic hammer as herein described has been shown to generate rock chips of 5 -10mm in size. Using this same piece of equipment but with the hammer bit set back further to allow a rock core of 8-12mm has generally produced a larger chip in the range of 12+mm. There are other variables that can influence chip sizing such as the shape of the bit inserts in the hammer bit, rotation speed, impact force, rock formation and when R.C. Drilling - the size of the return ports through the drill bit, which are also taken into consideration to ensure the chip samples can return through the ports without blocking the same. It has also been found that by chipping the core plug, small fractures can occur in the formation, thus facilitating the drilling/coring process as the formation has been weakened. In general, the size and number of fractures increase as the core plug becomes shorter. Therefore, by configuring the recess height/core plug height, fractures can be induced and drilling performance can be improved. In use, the drill rig 43 is operated to rotate the outer drillstring casing 42 to rotate the annular coring bit 11 so that it progresses into the formation and cuts a core plug 6 which moves into the interior/ recess 13 of the hybrid drill bit 10 (see Figure 4 but also Figure 1A). Preferably, the annular coring bit is rotated at relatively high speed (600+ RPM). The annular coring bit provides the required bore hole gauge, which avoids the need for angled inserts on the concentric drill bit, as is required in conventional hammer bits. The annular coring bit is also effective at maintaining the hole gauge (diameter) even in very hard formations. This helps prevent a drilling assembly being wedged and stuck down hole. Also in use, the repeatable force generator apparatus 47 repeatedly axially moves A the concentric coring bit 12, (e.g. through repeated impacts and/or vibrations or oscillations) so that the bit inserts 17 contact and break the core plug 6 into chip samples.

A drilling fluid path is provided in the apparatus comprising an annular drilling fluid path 48 that exists between the walls of the outer 42 and inner 46 drill casings. Drilling fluid

49 in use is pumped down through the annular drilling fluid path 48 out through channels

50 in the outer drillstring casing 42 into the bore hole 7 and to the formation/bore face 5. A sealing flange 54 can be provided in the annular region 56 between the outer drillstring casing 42 and the bore hole 7. This is to prevent the down hole fluid 49 exiting out the drill casing 42 and then flowing back up hole in the annular region 56 between the drillstring outer casing 42 and the bore hole 7. The fluid 49 travelling downhole will flow towards the bore face 5.

The drilling fluid 49 returns back past the broken core plug 6 carrying with it chip samples up through the return holes 18 in the concentric drill bit 12 and back up through the inner hollow path 51 (which also forms part of the drilling fluid path 48) of the inner drillstring casing 46. The chip laden fluid returns back up hole for analysis. The annular path 48, channel 50, bore 7, return holes 18 and interior 51 of the inner drill casing all form part of the drilling fluid path.

The combination of the rotation R of the annular coring bit 11 and the repeated axial movement A of the concentric drill bit 12 improves chipping performance. The core plug 6 generated by the annular coring bit 11 through rotation forms an unconfined core plug 6, which is weaker and more prone to breaking. This makes it easier for the concentric drill bit 12 under repeated axial force to move, contact and break (via the bit inserts 17) the core plug 6 into chip samples. In addition, optionally there could be a vibration apparatus or other apparatus to perturb the annular coring drill bit 11. Optionally, the force output from the repeatable force generation apparatus 47 could be indirectly communicated to the annular coring drill bit 11 to provide a perturbation. For example, in the case of a diamond impregnated annular coring bit, such indirect vibration that the diamond impregnated bit does experience is enough to significantly speed up the annular coring bit progress into the formation, but not sufficient to prematurely damage the annular coring bit.

In addition, optionally the concentric drill bit 12 could be rotated to expedite breaking of the core plug, e.g. through being splined to the drillstring outer casing 42 and/or the force generator apparatus being splined to the drillstring outer casing.

Figure 4 is just one example of a reverse circulation drilling apparatus that the drill bit could be used with. It will be appreciated that the hybrid drill bit could be used in any reverse circulation drilling operation with any suitable reverse circulation drilling apparatus that can provide rotation to the annular coring bit and repeatable force to the concentric coring bit to repeatedly move the concentric coring bit.

Advantages

The drill bit described can be used with vibrator /impact devices with modest energy output (such as those previously referenced), that preferably operate with recycled / modified and dirty fluids, such that strong performance can be obtained in tandem with long bit life.

In reverse circulation drilling, the drill bit described herein allows for use of a lower energy fluid driven drilling apparatus, that otherwise would not be able to obtain the drilling performance using conventional hammer bits.

The two components working in tandem have several unique advantages; · the annular coring bit provides the required bore hole diameter, without the need for angled bits (which cannot be aggressively shaped), while still providing a bit that can provide hammering. This allows all bit inserts on the concentric drill bit to optionally be aggressively shaped to improve drilling speed.

• A conventional annular coring bit on its own would in a conventional set up drill at a slow rate of penetration, however testing has shown that with a coaxially positioned hammer / vibratory device, there is sufficient indirect vibration interacting with the coring bit to dramatically speed up the rate of advancement of this component - but not so much that the coring bit is destroyed.

• The annular coring bit also provides an unconfined core plug which is easier to break even with lower power, energy and/or force hammer/oscillatory apparatus.

The resultant short knub of rock core that the force generator/concentric drill bit has to chip has a significantly weakened structure that can be more easily chipped / crushed using a lower forced hammer/vibratory device (even with non- aggressive insert bits).

• The concentric drill bit is very efficient at crushing the inner core of rock than a conventional full faced bit would be as a full face bit relies on surface speed to grind the rock, but is very poor at cutting the inner diameter parts of the core, due to there being a smaller diameter and therefore much less surface speed.

• Continuous chipping can be carried out which is advantageous from a drilling efficiency point of view. In traditional core drilling, chipping is carried out until a depth where it can no longer operate using an RC drilling apparatus such as the air hammer system described in the background. Then that is uninstalled and is replaced with a separate coring rig and drilling apparatus, at which point coring continues. The downtime and additional costs associated with having two systems is highly undesirable. In contrast a RC drilling apparatus/drill bit as described herein can be used for chipping, and can proceed at a satisfactory speed rate. The same rig and drilling apparatus can also be used for the coring operation - cutting out the downtime and additional costs associated with having the two systems.

Testing has shown the drill bit as described herein is advantageous in earth / rock drilling, particularly earth / rock drilling flooded environments. The result of such an improvement leads to better health and safety for the operators of such a system with less environmental impact. Reduced environmental impacts of the present invention stem from not having to use air hammers that require large

compressors that create particulates of dust and oil hazards that pose health and safety issues to the operators and the dust and oil droplets settle on equipment and the surrounding environment. Existing fluid driven drilling apparatus require clean non- recycled water and as such are only suitable for niche applications where there is abundant clean water. Using a lower power, energy and/or force fluid driven drilling apparatus with a drill bit as described herein that can use drilling fluid that can be monitored and recycled in a closed loop system is much more preferable from an environmental, health and safety point of view. Additionally as the drilling fluid can be monitored, the bore hole well integrity can be monitored via the use of drilling additives being added into the drilling mud.

In relation to using the annular coring bit 11' according to the second embodiment, these advantages can be experienced. When using a conventional castellated coring bit in conj unction with a hammering apparatus (either conventional flush drilling - or reverse circulation), the diamond impregnated castellated coring bit is subject to a n increased level of vibration, which while assisting the bit to cut the formation being drilled, can cause premature structura l failure of the drill bit. As a consequence a variety of varying structurally designed diamond impregnated bits have been tested, which have aided the structural integrity of the impregnated bits - while stronger some suffer from fluid galleries being compromised as the cutting structure wea rs, causing fluid pressure spikes which are on occasions interpreted by the driller as the drill bit having expired . The result can be a time consuming pulling of the drill string out of the hole - with the expectation that the drill bit needs to be cha nged - only to find the drill bit is fine - but that the tra nsition of one fluid opening closing and the next opening has not yet happened . This does not happen with the annular coring bit 11' according to the second embodiment due to the refreshing flow paths, while also maintaining structural integrity of the bit. Referring to Figure 5A, the drill bit described according to either embodiment could also be used in boring drilling apparatus applications, but with a minor variation to the annular coring bit 11, 11'. That is, a va riation of the a nnular coring bit according to the first embodiment with the fluid flow holes 50 removed (see Figure 5B) could be assembled into a hybrid drill bit (see figure 5C) and used in the boring drilling apparatus of Figure 5A. Alternatively, a variation of the annular coring bit according to the second embodiment with fluid flow holes 50 removed (See Figure 12, 13A to 13F) could be assembled into a hybrid drill bit (see Figure 14) and used in the boring drilling apparatus of Figure 5A. In other respects, the va riations of the first and second embodiments are the same as previously described . The Figure 5A apparatus is a similar arrangement to Figure 4, except that there is no sealing flange 54 and the fluid flow is reversed . Drilling fluid is passed down through the inner drillstring casing 51 and out through the junk slots 18, to the bore face 8. Chip samples carried by the fluid can travel back uphole in the annular path 56 between the drill string outer casing 42 and bore hole wall 7. The kerf of the annular coring bit can be larger in diameter than the drill string diameter to encourage the drilling fluid to flow around the d rill bit a nd up the annular path 56.

Claims

1. A drill bit for coupling in use downhole drilling apparatus, the drill bit comprising : an annular coring drill bit that is rotatable to cut a formation bore face to create a core plug,

a concentric drill bit that can be repeatedly axially moved (such as by oscillation (such as a vibration) and/or impact) to break the core plug.

2. A drill bit according to claim 1 wherein the concentric drill bit is axially setback from the annular coring bit to create a recess such that in use the core plug is unconfined by the surrounding formation.

3. A drill bit according to any preceding claim wherein the drill bit is for use in a reverse circulation drilling apparatus to create chip samples for return to surface by breaking the core plug.

4. A drill bit according to any preceding claim wherein the drill bit is for use in bore drilling apparatus.

5. A drill bit according to any preceding claim wherein in use when the concentric drill bit is repeatedly axially moved it contacts the unconfined core plug in the recess to break the core plug.

6. A drill bit according to any preceding claim wherein in use:

the annular coring drill bit is or can be coupled to and rotated by a rotational drive of a drilling apparatus, and

the concentric drill bit is or can be coupled to a repeatable force generating apparatus that can repeatedly axially move the concentric drill bit.

7. A drill bit according to claim 4 wherein the rotational drive is a drillstring casing of the drilling apparatus.

8. A drill bit according to claim 4 or 5 wherein the repeatable force generating apparatus is an impact apparatus or an oscillation apparatus configured to repeatedly axially move the concentric drill bit by impact or oscillation (such as a vibration).

9. A drill bit according to claim 6 wherein :

a. the impact apparatus is a hammer, such as a magnetic hammer, pneumatic hammer, fluid hammer or any suitable hammer means to provide impact force to the concentric bit to break the core plug, or b. the oscillation apparatus provides oscillatory force (such as a vibration force) to the concentric bit to break the core plug.

10. A drill bit according to any preceding claim wherein the annular coring bit comprises: a body with a hollow core and a bit face cutting structure arranged around the end of the body, the bit face cutting structure comprising : an inner ring of spaced apart inner teeth providing inner gaps between adjacent inner teeth, an outer ring of spaced apart outer teeth providing outer gaps between adjacent outer teeth, and the inner and outer teeth being slanted such that: one or more inner teeth overlap with one or more outer teeth to provide mutual bracing, and one or more inner gaps overlap with one or more outer gaps to expose pathway for fluid flow between the hollow core and the body exterior.

11. A drill bit according to claim 10 wherein the inner and outer teeth are slanted such that as the cutting teeth wear, for at least some of the inner and/or outer gaps, fresh parts of one or more inner gaps overlap fresh parts one or more outer gaps to expose fresh pathway for fluid flow.

12. A drill bit according to claim 10 or 11 wherein the body comprising the bit face is cast as a monobloc.

13. A drill bit according to any one of claims 11 to 12 wherein the teeth and gaps take the form of a helix.

14. A drill bit according to any one of claims 11 to 13 wherein one or more of the inner teeth and/or one or more of the outer teeth comprise one or more apertures for fluid flow.

15. A drill bit according to any one of claims 11 to 14 wherein the body comprises one or more apertures for fluid flow.

16. A drill bit according to any one of claims 11 to 15 wherein the inner teeth slant in a first direction around the body, and the outer teeth slant is a second opposite direction around the body.

17. A drill bit according to any one of claims 11 to 16 wherein the cutting structure is a diamond impregnated matrix.

18. A drill bit according to any one of claims 1 to 9 wherein the annular coring bit comprises: a body with a hollow core, a bit face with a cutting structure around an end of the body, the cutting structure comprising : a n inner ring of inner spaced apart teeth and an outer ring of outer spaced apa rt teeth, the inner and outer teeth being slanted such that: a) one or more inner teeth overlap one or more outer teeth to provide mutual bracing, and b) there is pathway for fluid flow where the spaces between inner and outer teeth overlap.

19. A drill bit according to any preceding claim wherein the annular coring bit is a dia mond impregnated bit.

20. A drill bit according to any preceding claim wherein the concentric drill bit has bit inserts for breaking the core plug .

21. A drill bit according to claim 20 wherein the bit inserts are ballistic bits or PDC bits.

22. A drill bit according to any preceding claim wherein the bit inserts a re tapered to increase point loading and promote core plug breaking, and preferably are at right angles to the concentric drill bit face.

23. A drill bit according to any preceding claim wherein the concentric drill bit is splined to or relative to the a nnular coring bit such that it can move axially relative to the annular coring bit but can rotate with the annular coring bit.

24. A drilling apparatus with a drill bit according to any preceding claim and configured to rotate the annular coring bit and repeatedly axially move the concentric drill bit.

25. A drilling appa ratus according to claim 24 comprising :

a drillstring casing coupled to and operable to rotate the annular coring bit, and a repeatable force generating apparatus coupled to a nd operable to repeatedly axially move the concentric drill bit.

26. A drilling apparatus according to claim 25 wherein the repeatable force generating apparatus is splined to the drillstring casing.

27. A drilling apparatus according to claim 25 or 26 wherein the repeatable force generating apparatus provides a vibration to the annular coring bit sufficient to enhance the performance of the coring bit.

28. A drill bit according to any one of claims 2 to 23 or a drilling apparatus according to any one of claims 24 to 26 wherein the recess height, the number of bit inserts on the concentric drill bit and the force applied to the drill bit can be configured to generate a desired size of chip sample when breaking the core plug.

29. A drill bit or drilling apparatus according to any preceding claim wherein breaking the core plug produces fractures in the formation to assist drilling.