WO2015111016A1

WO2015111016A1 - Drill bit for drilling a borehole

Info

Publication number: WO2015111016A1
Application number: PCT/IB2015/050574
Authority: WO
Inventors: Abdelhakim Hahati; Benoit Deschamps
Original assignee: Tercel Ip Limited
Priority date: 2014-01-24
Filing date: 2015-01-26
Publication date: 2015-07-30

Abstract

This invention relates to a drill bit (1) for drilling a borehole in earth formation, comprising a bit body (2) having a central axis (3), a bit face (4) including a plurality of blades (6) with a plurality of cutter elements (7), (8) having a substantially planar cutting face (9) and a cutting tip for cutting the formation, wherein said cutter elements (7), (8) comprise at least one rounded cutter element (7) having a rounded cutting tip (10), and at least one pointed cutter element (8) having a pointed cutting tip (11). Preferably said substantially planar cutting surface (9) is mounted on a substrate (12) and is a polycrystalline diamond table or a cutting layer made of superabrasive materials including thermally stable diamond or cubic boron nitride. The invention also relates to the use of such a drill bit for drilling a plastic shale formation.

Description

Drill bit for drilling a borehole,

Technical field

The present invention relates to a drill bit for drilling a borehole formation. In particular, the drill bit of the present invention present an interest for drilling plastic or sticky formation.

State of the art

Drilling a wellbore for exploitation or exploration of an oilfield is commonly performed with a drill bit connected to an assembly of pipe section and forming a "drill string". The drill string is hang and generally rotated from the surface of the wellbore. The drill string comprises a bottom hole assembly comprising the drill bit. The bottom hole assembly may comprise additional means for rotating the drill bit, such as a mud motor located near the drill bit.

Two types of drill bits are commonly used for drilling a borehole: the fixed cutter drill bits and the roller cone drill bits. Many of these drill bits have been disclosed in the art and most of them have been designed for drilling specific formations.

The first generation of fixed cutter drill bits comprised natural diamond or thermally stable polycrystalline diamond. These drill bits were manufactured by first placing diamonds into a mold, then the bit body was obtained by adding a carbide powder into the mold, heating the mold at high temperature for melting the carbide powder and cooling the mold for recovering the drill bit. The diamonds of the first generation of drill bits are embedded in a superficial layer of the tool body which retains mechanically the diamonds. Generally, a nozzle is provided at the center of the bit for allowing circulation of a drilling fluid within shallow water courses provided on the surface of the bit body, for cooling the drill bit and washing the diamonds. The flow rate of drilling fluid on the bit body surface is therefore particularly elevated and erosion is minimized by providing a bit body made of tungsten carbide. An example of such a drill bit of the first generation is disclosed in document EP0164297. This document discloses the use of prismatic thermally stable polycrystalline diamond cutters in combination with blunt thermally stable polycrystalline diamond cutters for removal of medium-soft to medium formations that behave in a brittle manner. Nowadays, fixed cutter drill bits of the second generation are manufactured differently. The bit body is first manufactured and is provided with a plurality of blades comprising pockets for inserting cutter elements. The bit body may comprise a plurality of nozzles located between the blades for projecting a flow of drilling fluid against the blades for cooling and cleaning purpose. The cutter elements generally comprise a substrate, typically a tungsten carbide support, on which is bonded a hard cutting layer made of polycrystalline diamond. These cutter elements are called polycrystalline diamond compact cutters and abbreviated "PDC" cutters. Instead of polycrystalline diamond compact, some cutter elements may comprise a cutting layer made of superabrasive materials such as thermally stable diamond or cubic boron nitride. The substrate is inserted and secured into the pockets of the blades, exposing the cutting layer towards at the surface of the blades.

Various fixed cutter drill bits are available on the market and may be selected in function of the type of formation that is to be drilled, more particularly in function of the hardness of the formation that will be encountered.

Drilling through a region of soft shale, usually at great depth is challenging. Document GB2317632 exposes the problems encountered while drilling in such formations. At low depth, fixed cutter drill bits are efficient for drilling shale. PDC cutters are able to shear the shale and generate formation chips that can be evacuated by the drilling fluid. It is believed that as the depth of the well increases, the behavior of shale becomes as a plastic like material such that the cutters are able to deform the formation, but have low efficiency for shearing the formation and generate cuttings. Another theory explains the decrease of rate of penetration in these formations by the fact that the nature of the shale formation and the design of the drilling tools are such that cuttings are well generated but their evacuation is difficult because the cuttings gets stick together on the bit face. These formations are generally called "plastic formation" and refer to formations which under pressures and in contact with hydraulics, tends to act like a sticky bubble-gum mass which generally balls and clogs the bit. The term "plastic shale" is referred to shale that contains a relatively large amount of water and that can squeeze into the wellbore. The same document presents some solutions that were in used in prior art while drilling through a plastic shale formation and discloses an original drill bit for drilling in such formations. The drill bit disclosed in that document GB2317632 comprises a bit body having a bit face and a plurality of blades, wherein one of the blades comprises a row including a plurality of spaced cutter elements having cutting faces with cutting tips for cutting the formation material and wherein the backrake angle of cutting elements is varied along the row of cutting elements. The same drill bit may comprise cutter elements wherein the edges of the cutting faces have two different curvatures. Those cutter elements are mounted on the blades such that the cutting tips are formed on the larger- radiused portion of the cutting edge.

Document US4883132 discloses another drill bit for drilling in shales or plastic formations. In this disclosed drill bit, adhesion of plastic rock formation fragment is avoided by providing a cutting structure wherein the cutting elements are shaped and disposed on blades such that there is no other surface between adjacent cutters. A cleaning arrangement is also provided for ameliorating the cleaning of the cutters. The blades are connected to the bit body and are arranged to form a cavity between the cutting structure and the bit body. Nozzles are provided on the bit body for projecting a cleaning fluid through the cavity towards the cutters. The cavity formed between the cutting structure and the bit body increases the risk of failure of the blades. It is also rather complicated to insert and secure cutters in blades of a drill bit such that no other surface between adjacent cutters is present. Despite certain advances made in the fixed cutter drill bits for drilling plastic like shale, there remains a need for a fixed cutter drill bit having an improved cutting element arrangement for increasing the rate of penetration of such drilling tools.

The aim of the invention is to present a solution for the abovementioned drawbacks and needs. This has been achieved by the following. Summary of the invention

According to a first aspect, the present invention relates to a drill bit for drilling a borehole in earth formation. The drill bit comprises a bit body having a central axis and a bit face including a plurality of blades, said blades comprising a plurality of cutter elements, having a substantially planar cutting face for cutting the formation. Said plurality of cutter elements further comprises at least one rounded cutter element having a rounded cutting tip, and at least one pointed cutter element having a pointed cutting tip.

Preferably, said substantially planar cutting surface of the cutting elements is mounted on a substrate and is a polycrystalline diamond table or a cutting layer made of superabrasive materials including thermally stable diamond or cubic boron nitride.

Preferably, in a rotated profile view of the plurality of cutting elements into a single plane, at least one of said rounded cutter elements and at least one of said pointed cutter elements are positioned at the same radial position with respect to the central axis of the bit body.

Preferably, at least one of said blades comprises an edge which is a front edge in the direction of drilling rotation of the bit, and at least one of said pointed cutter elements is positioned at or along said edge.

Preferably, at least one blade comprises a row of pointed cutting elements.

Preferably, at least one blade comprises a row of rounded cutting elements. Preferably, said row of pointed cutting elements is disposed along an edge of the blade which is a front edge in the drilling direction of rotation of the bit.

Preferably, said row of rounded cutting elements is a first row of cutting elements, seen in the direction of rotation of the bit, and said row of rounded cutting elements is a second row disposed behind said first row.

Preferably, at least one blade comprises a row of pointed cutting elements as well as a row of rounded cutting elements.. Preferably, at least one of the cutter elements has a positive back rake angle. Alternatively, at least one of the cutter elements has a negative back rake angle.

Preferably, said bit face has a nose zone and at least some of said pointed cutter elements are positioned in said nose zone.

Preferably, said nose zone also comprises at least some of said rounded cutter elements.

Preferably, said bit face has a cone zone and at least one rounded cutter element which is positioned at an edge of at least one blade, said edge being a front edge seen in the direction of drilling rotation of the bit, and

a shock reducer insert,

are positioned in said cone zone.

Preferably, the or each shock reducer insert is provided behind a rounded cutter element which positioned on the same blade.

Preferably, said bit face has a gage zone and a gage cutter positioned at an edge of at least one blade, said edge being a front edge in the direction of drilling rotation of the bit, and

an elongated stabilizing insert,

are positioned in said gage zone, and the gage cutter is positioned in front of the stabilizing insert, seen in said direction of rotation.

Preferably, the drill bit according to the present invention comprises a space for forming a core, the space being situated in the centre of the bit face.

Preferably, the space for forming a core further comprises a means for destroying the core or a means for breaking the core. Preferably, the drill bit of the present invention further comprises a cavity for evacuating the destroyed core or the broken core, said cavity being connected to said space for forming the core, said cavity being comprised between two adjacent blades, said cavity extending along the bit face and being recessed with respect to the bit face.

The present invention also relates to the use of a drill bit of the present invention for drilling a plastic shale formation. Brief description of the drawings

In the following, a particular embodiment and some alternative embodiments of a drill bit according to the invention will be described in a detailed way, and effects and advantages of the invention will be further clarified. In this description, we will refer to the annexed drawings, wherein

Fig. 1 is a perspective view of a drill bit and cutting structure according to the present invention. Fig. 2 shows a plan view of the cutting face of the drill bit shown in Figure 1.

Fig. 3 shows some cutter elements of the bit shown in figures 1 in a rotated profile view on one side of the central axis of the bit.

Figure 4a, 4b, 4c and 4d show a cutter element of the drill bit according to the present invention, the cutter element having a pointed cutting tip and being represented respectively in a front view, in a perspective view, in a first lateral view and in a second lateral view.

Detailed description of possible embodiments of the invention

The present invention relates to a drill bit (1) for drilling a borehole in earth formation. The drill bit of the present invention can be used for drilling plastic or sticky formation, such as plastic shale. The invention however is not limited to drill bits for this field of application.

An embodiment of a drill bit (1) according to the present invention is represented on figure 1 in a perspective view, and on figure 2 in a bottom view. This drill bit (1) comprises a bit body (2) having a central axis (3) about which the bit (1) rotates while drilling in a direction represented by arrow (D). The drill bit (1) further comprises a bit face (4) including a plurality of blades (6) extending along the bit face from an area near the central axis (3) towards the periphery of the bit face. The bit face (4) futher comprises a set of nozzles (28) for cleaning the blades (6) and for cooling purposes. In a non limitative embodiment of the invention, the blades (6) extend radially along the bit face (4). The blades (6) comprise a top surface (30), also called "cutter supporting surface", comprising a plurality of pockets in which are inserted and secured cutting elements (7), (8). The cutting elements (7), (8) have a substantially planar cutting face (9) having an outermost cutting tip (10), (11) furthest from the cutter supporting surface (30) to which the cutting elements (7), (8) are mounted. In the present invention the substantially planar cutting face (9) of at least one cutting element (7) has a rounded cutting tip (10), and the substantially planar cutting face (9) of at least one other cutting element (8) has a pointed cutting tip (11). The cutting element (7) with the cutting face (9) having a rounded cutting tip (10) is referenced as "rounded cutter element (7)" and the cutting element (8) having a cutting face (9) with a pointed cutting tip (11) is referenced as "pointed cutter element (8)" or "scribe cutter (8)".

The cutting elements (7), (8) comprise an elongated substantially cylindrical substrate (12) or a partially cylindrical substrate (12) on which is mounted a hard cutting layer material (13) such as polycrystalline diamond, preferably leached polycrystalline diamond, or alternatively other superabrasive materials such as thermally stable diamond or cubic boron nitride. The substrate (12) comprises tungsten carbide. The cutting elements (7), (8) are received and secured within pockets formed on the surface of the blades (6). A portion of the cutting elements (7), (8) comprising the cutting tip (10), (11) protrudes from the cutter supporting surface (30). An embodiment of a pointed cutter (8) is represented in a front view in a plane XY on figure 4a, which shows the substantially planar cutting surface (9) of the pointed cutter (8). The cutting face (9) comprises a curved segment A, a first straight segment B, a second straight segment C, and a pointed tip (11) formed between the first segment B and the second segment C. The pointed tip (11) and the curved segment A comprise a chamfer (27). A first extremity of the curved segment A is connected to the first straight segment B, and a second extremity of the curved segment is connected to the second straight segment C.

The pointed cutter (8) comprises a substantially cylindrical or partially cylindrical substrate (12) as presented in figure 4b in a perspective view, and in figure 4c along an axis N perpendicular to the plane XY of the front view of the figure 4A. A first ruled surface (31a), preferably a tapered flat surface, extends along the layer material (13) and at least a portion of the substrate (12) from the first straight segment B of the cutting surface (9) with an angle a with respect to the normal N to the cutting surface (9) as presented in figure 4c. A second ruled surface (31b), preferably a tapered flat surface, extends along the layer material (13) and at least a portion of the substrate (12) from the second straight segment C of the cutting surface (9) with an angle (β) with respect to the normal N of the cutting surface (9) as presented in figure 4d. Preferably, both angles (a) and (β) have the same value and are preferably comprised between 1° and 45°, preferably between 5° and 25°, more preferably between 5° and 10°.

Both ruled surfaces (31a), (31b) extend from the straight segments B, C to the periphery (32) of a section (33) of the substrate (12) parallel to the cutting face (9). More preferably, the section (33) is nearby or at the base (34) of the substrate, opposite to the cutting surface (9). The shape of the pointed cutter (8) as presented herein above has the advantage of providing a substrate (12) having a wide surface of contact with the inner surface of the pocket of the blades. This contact surface has minimized sharp areas, allowing a better retention of the cutting elements within the pocket and allowing higher constraints to be applied on these pointed cutters (8). The pointed cutters (8) have a portion protruding from the cutter supporting surface (30), the portion comprising the pointed cutting tip (11) and the first and second ruled surfaces (31a), (31b). The shape of the pointed cutters (8) and their disposition on the blade with preferably a positive back rake angle provides a better evacuation of formation cuttings. Alternatively it may be preferred to have the pointed cutters (8) arranged with a negative back rake angle. Preferably, the pointed cutters are manufactured from cylindrical chamfered PDC cutters which are cut by electrical discharge machining (EDM) for forming the ruled surfaces (31a), (31b). Electric discharge machining allows cutting any workpiece of hard materials such as tungsten carbide, according to a desired shape. The material is eroded from the workpiece by means of electrical discharges that creates sparks. A preferred EDM method for forming the ruled surface (31a), (31b) of the pointed cutter (8) is the wire electric discharge machining which uses a wire electrode submitted to electric voltage and producing sparks with the workpiece, i.e. the cylindrical chamfered PDC cutter, that erodes the workpiece which is moved through the wire electrode for obtaining the desired shape.

It is well understood for the man skilled in the art that other embodiments of pointed cutter elements may be used.

Because of their shape, the pointed cutter elements (8) penetrate the formation more easily than the rounded cutter elements (7). The pointed cutter elements (8) scrap the formation forming a groove having a V shape, while the rounded cutter elements (7) scrap the formation forming a groove having a U shape. The rounded cutter elements (7) have a larger cutting surface which removes more formation than the pointed cutter elements (8). However, the effort required on the rounded cutter elements (7) for removing formation is higher than the effort required on the pointed cutter elements (8) for the same formation. Combination of pointed cutter elements (8) with rounded cutter elements (7) provides the advantage of reducing the effort on the rounded cutter elements (7) for drilling a formation.

A pointed cutter (8) can be disposed at a radial position between the radial position of two rounded cutter elements (7) or at the same radial position. High rates of penetration of the drilling tool have been observed when pointed cutter elements (8) are disposed in front of rounded cutter elements (7) at the same radial position. With such an arrangement, the pointed cutter elements (8) enter in contact with the formation first, and since the contact surface of the pointed cutter element contacting the formation is small, the friction between the contact surface and the formation is reduced and less effort is required to remove formation and make a first V shaped groove. When a rounded cutter element (7) is located behind a pointed cutter element (8) in the sense of rotation (D) of the bit (1), and at the same radial position, the rounded cutter element (7) runs over the V groove, and removes the remaining formation that was not removed during the first passage of the pointed cutter element (8). The rounded cutter element (7) removes therefore a concave volume of formation which requires less effort to remove than a full volume.

A rotated profile view of the plurality of cutting elements into a single plane is presented in figure 3, wherein at least one of the rounded cutter elements (7) is at the same radial position as at least one of the pointed cutter elements (8). Preferably, each of the pointed cutter elements (8) is at the same radial position as the rounded cutter elements (7). More preferably, each of the pointed cutter elements (8) is positioned in front of a rounded cutter element (7), seen in the direction of drilling rotation (D) of the bit (1).

Preferably, there is an intermediate distance of at least 1 mm between each cutting face (9) of a rounded cutter element (7) and the back of a pointed cutter (8) positioned in front of it.

In the embodiment shown on figure 1, each of the blades (6) comprises a row of pointed cutter elements (8) as well as a row of rounded cutter elements (7). On each blade (6), the row of pointed cutter elements (8) is positioned in front of the row of rounded cutter elements (7), seen in the direction of the drilling rotation (D) of the bit (1).

The row of pointed cutting elements (8) is positioned at an edge (14) of the blade (6), which is a front edge seen in the direction of rotation (D) of the drill bit (1) while drilling, and the row of rounded cutting elements (7) is positioned behind the row of pointed cutting elements (8).

On a first blade (6a) - see figure 2 - , a row of pointed cutter elements (8) and a row of rounded cutter elements (7) are disposed along a portion of the blade (6) such that the tips (11) of the pointed cutter elements (8) are at the same radial position as the tips (10) of the rounded cutter elements (7). This radial position will be referred to as 'the first radial position'. On a second blade (6b), a row of pointed cutter elements (8) and a row of rounded cutter elements (7) are disposed along a portion of the second blade (6b) such that the tips (11) of the pointed cutter elements (8) are at the same radial position as the tips (10) of the rounded cutter elements (7). This radial position will be referred to as 'the second radial position' Said second radial position on the second blade (6b) is shifted with respect to the first radial position on the first blade (6a).

At least one of said blades (6) comprises an edge (14) which is a front edge, seen in the direction of drilling rotation (D) of the bit (1). On each blade (6), the row of pointed cutter elements (8) is disposed along a portion of this front edge (14).

Preferably, at least one of the cutter elements (7), (8) has a positive back rake angle. The back rake angle is the angle between a cutting face of a cutter element and the normal to the formation to be drilled. When the cutter element has a positive back rake angle the angle between its cutting face and the surface of the formation to be drilled is more than 90°. When the cutter element has a negative back rake angle, the angle between its cutting face and the surface of the formation to be drilled is less than 90°. More preferably, all the cutting elements (7), (8) have a positive back rake angle. A positive back rake angle provides a bit which is more aggressive. Alternatively, it may be preferred to have at least one cutter element or all the cutter elements with a negative back rake angle to minimize vibrations and high torque while rotating the bit. When a negative back rake angle is chosen, it is advantageously chosen with a low back rake angle, for example between 1° and 25°, to provide a more aggressive bit.

For a pointed cutter element (8) positioned on a blade (6) at the same radial position as a rounded cutter element (7) of the same blade (6) or of another blade (6), the tip (11) of the pointed cutter element (8) is positioned at the same height as the tip (10) of the rounded cutter element (7) which is at the same radial position. This allows efficient penetration of both pointed (8) and rounded cutter elements (7) in the formation, allows a better distribution of the load on the tips (10), (11) of the cutters (7), (8), and prevents premature wear of the tips (11) of the pointed cutter elements (8) when compared to the wear of the tips (10) of the rounded cutter elements (7). Figure 3 shows a rotated profile view of the bit face on one side of the central axis (3) according to an embodiment of the present invention. The bit face (4) of the drill bit (1) is divided into a plurality of zones: a cone zone (15), extending from the central axis (3) of the bit (1) to the nose zone (16);

a nose zone (16), which is the lowermost zone of the drill bit (1) when the bit face (4) of the drill bit (1) is oriented downwards;

a shoulder zone (17), extending from the nose zone (16) towards a gage zone (18) and;

- a gage zone (18) which is the uppermost zone of the cutting structure when the bit face (4) is oriented downwards, the gage zone (18) being preferably parallel to the central axis (3) of the bit (1).

This general configuration is well known in the art, The pointed cutter element (8) are located at least in the nose zone (16). In the embodiment shown in figures 1 to 3, a row of pointed cutter elements (8) is disposed along the edge (14) of the blades (6), in both the nose zone (16) and the shoulder zone (17).

In said cone zone (15), a rounded cutter element (7) is positioned at the edge (14) of at least one blade (6) in the direction of drilling rotation (D) of the bit (1). Preferably, all the blades (6) comprise a rounded cutter element (7) positioned at the edge (14) of the blade (6) in the cone zone (15). Generally, cutter elements disposed in the cone zone (15) penetrate the formation more easily than cutter elements disposed in other zones of the bit face (4). A shock reducer (19), for example a dome shaped insert, is provided behind or nearby the rounded cutter elements (7) located at the edge (14) of the blade (6) in the cone zone (15), for ensuring that the rounded cutter elements (7) do not penetrate to easily the formation and for avoiding resulting sticking of the drill bit.

Preferably, said gage zone (18), comprises an active gage zone (18a) which comprises a cutter element called "gage cutter" or "gage trimmer" (20) that preferably comprises a cutting face (9) having a straight cutting edge (21). Preferably, the gage cutter (20) is positioned at the front edge (14) of the blade (6), seen in the direction of rotation (D) of the bit (1), and in front of an elongated stabilizing insert (22). This gage zone (18) further comprises a passive gage zone (18b) located up to said active gage zone (18a) when the bit face (4) is oriented downwards. The passive gage zone (18b) comprises a plurality of stabilizing inserts (29), for example cylindrical tungsten carbide inserts or diamond impregnated insert, inserted substantially perpendicular with respect to the central axis (3) of the bit (1) and inserted such that they do not protrude from the passive gage surface.

The drill bit according to the present invention comprises a space for forming a core (24), the space being situated in the centre of the bit face (4). This embodiment provides minimization of lateral vibration while drilling due to the stabilizing effect of the central core. The space for forming a core (24) further also comprises a means (26) for destroying the core or a means for breaking the core.

By the term "means for destroying the core" it is understood a means that: continuously crushes and fracture the core in small pieces, such as a stinger positioned on the bit face inside the space for forming the core, preferably coaxial with the central axis of the drill bit;

continuously abrades the core, such as abrasive material disposed on the bit face inside the space for forming the core.

By the term "means for breaking the core", it is understood a means that periodically breaks the core without destroying the core is small pieces, such as for example a surface connected to the bit face (4) or to a blade (6), the surface being at the bottom of the space for forming the core and disposed obliquely with respect to the axis of formation of the core such that when the core encounters this surface, the surface provides a lateral effort that breaks the base of the formed core. The drill bit of the present invention further comprises a cavity (25) for evacuating the destroyed core or the broken core, said cavity being connected to said space for forming the core (24), said cavity (25) being comprised or located between two adjacent blades (6), said cavity extending along the bit face (4) and being recessed with respect to the bit face (4).

Claims

1. Drill bit (1) for drilling a borehole in earth formation and comprising :

a bit body (2) having a central axis (3) ;

a bit face (4) including a plurality of blades (6),

a plurality of cutter elements (7),(8) provided on said blades (6), said cutter elements having a substantially planar cutting face (9) and a cutting tip for cutting the formation, characterized in that said plurality of cutter elements (7), (8) comprises at least one rounded cutter element (7) having a rounded cutting tip (10), and at least one pointed cutter element (8) having a pointed cutting tip (11).

2. Drill bit according to claim 1 characterized in that said substantially planar cutting surface (9) of said cutter elements (7),(8) is mounted on a substrate (12) and is a polycrystalline diamond table or a cutting layer made of superabrasive materials including thermally stable diamond or cubic boron nitride.

3. Drill bit according to claim 1 or 2 characterized in that, in a rotated profile view of the plurality of cutting elements (7), (8) into a single plane, at least one of said rounded cutter elements (7) and at least one of said pointed cutter elements (8) are positioned at the same radial position with respect to the central axis (3) of the bit body (2).

4. Drill bit according to any of the preceding claims characterized in that at least one of said blades (6) comprises an edge (14) which is a front edge seen in the direction of drilling rotation (D) of the bit (1), and that at least one of said pointed cutter elements (8) is positioned at or along said edge (14).

5. Drill bit according to any of the preceding claims characterized in that at least one blade (6) comprises a row of pointed cutting elements (8).

6. Drill bit according to any of the preceding claims characterized in that at least one blade (6) comprises a row of rounded cutting elements (7).

7. Drill bit according to any of the preceding claims characterized in that at least one blade (6) comprises a row of pointed cutting elements (8) as well as a row of rounded cutting elements

(7) .

8. Drill bit according to claim 5 or 7 characterized in that said row of pointed cutting elements

(8) is disposed along an edge (14) of the blade (6), which is a front edge seen in the direction of drilling rotation (D) of the bit (1).

9. Drill bit according to claim 7 or 8 characterized in that said row of pointed cutting elements (8) is a first row of cutting elements (7), seen in the direction of drilling rotation (D) of the bit (1), and that said row of rounded cutting elements (7) is a second row disposed behind said first row.

10. Drill bit according to any of the preceding claims characterized in that at least one of the cutter elements (7), (8) has a positive backrake angle.

11. Drill bit according to any of the preceding claims characterized in that at least one of the cutter elements (7), (8) has a negative back rake angle.

12. Drill bit according to any of the preceding claims characterized in that said bit face (4) has a nose zone (16), and that at least some of said pointed cutter elements (8) are positioned in said nose zone (16).

13. Drill bit according to claim 12 characterized in that said nose zone (16) also comprises at least some of said rounded cutter elements (7).

14. Drill bit according to any of the preceding claims characterized in that said bit face (4) has a cone zone (15) and that

at least one rounded cutter element (7) which is positioned at an edge (14) of at least one blade (6), said edge (14) being a front edge seen in the direction of drilling rotation (D) of the bit (1), and

a shock reducer insert (19).

are positioned in said cone zone (15).

15. Drill bit according to any of the preceding claims characterized in that said bit face has a gage zone (18) and that a gage cutter (20) positioned at an edge (14) of at least one blade (6), said edge (14) being a front edge seen in the direction of drilling rotation (D) of the bit (1), and an elongated stabilizing insert (22), are positioned in said gage zone (18), and that the gage cutter (20) is positioned in front of the stabilizing insert (22), seen in said direction of drilling rotation (D).

16. Drill bit according to any of the preceding claims characterized in that it comprises a space (24) for forming a core, situated in the centre of the bit face (4).

17. Drill bit according to claim 16 characterized in that the space for forming a core (24) further comprises a means (26) for destroying or for breaking the core.

18. Drill bit according to claim 17 characterized in that it comprises a cavity (25) for evacuating the destroyed core or the broken core, said cavity (25) being connected to said space (24) for forming the core,

being comprised between two adjacent blades (6a), (6b),

extending along the bit face (4) and

being recessed with respect to the bit face (4).

19. Use of a drill bit according to any of the preceding claims for drilling a plastic shale formation.