WO2002020936A2

WO2002020936A2 - Drill bit

Info

Publication number: WO2002020936A2
Application number: PCT/EP2001/010406
Authority: WO
Inventors: Frederik Damhof; Djurre Hans Zijsling
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Canada Limited
Priority date: 2000-09-08
Filing date: 2001-09-06
Publication date: 2002-03-14
Also published as: EG22664A; GB0304377D0; AU1222102A; US20040020693A1; BR0113687A; GB2384505B; GC0000369A; OA12369A; NO20031055L; WO2002020936A3; CA2421288A1; AR030606A1; AU2002212221B2; CN1452685A; NO20031055D0; CN1304719C; GB2384505A; MXPA03001924A

Abstract

The invention relates to a rotary drill bit for drilling a borehole in an earth formation, the drill bit comprising a first and a second disc cutter, each disc cutter being arranged to roll along the borehole bottom during rotation of the drill bit and thereby to cut respective first and second substantially circular, radially spaced cuts in the borehole bottom so that a body of rock material is defined between said cuts. The second disc cutter is arranged to cut into the bottom of the borehole at a selected rotational interval of the drill bit behind the first disc cutter and to shear-off said body of rock material in the direction of the first cut. Each disc cutter includes a set of mutually spaced cutting members arranged to create, during a turn of the drill bit, a corresponding set of cut sections of the cut created by the disc cutter, and wherein the cutting members are arranged so that the sets of cut sections created during subsequent drill bit turns are staggered relative to each other.

Description

DRILL BIT

The present invention relates to a rotary drill bit for drilling a borehole in an earth formation. In the prior art various types of drill bits are applied, for example roller cone bits or jet cutting bits. These drill bits are generally provided with abrasive cutting elements which are made of a material having a high wear resistance such as diamond or tungsten carbide. The cutting action of these cutting elements mainly results from scraping of the elements along the borehole bottom. The progress of the drill bit in the borehole (i.e. the speed of drilling) depends on many factors such as the amount of wear of the cutting elements, the hardness of the rock and the weight on bit. As the costs of drilling of a wellbore form a substantial part of the overall wellbore costs there is a continuous need to reduce the drilling time, viz. to increase the drilling speed.

WO 99/11900 discloses a rotary drill bit comprising a plurality of disc cutter formed on roller cones and arranged to cut a plurality of substantially circular, radially spaced cuts into the bottom of the borehole. The set of cutters of each roller cone is radially displaced relative to the set of cutters of another roller cone, so that for each pair of radially adjacent cutters a second one of the pair cuts into the borehole bottom at an angular interval behind the first one the pair. A body of rock material is defined between the respective cuts, which body is sheared off by the second cutter in the direction of the cut created by the first cutter.

It was found that the rate of penetration (ROP) into the earth formation of the known drill bit is relatively low in certain types of rock formation, particularly in hard rock formations, due to a tendency of shearing-off of the body of rock material in the direction of the upper end of the cut created by the first cutter. Accordingly there is a need for an improved drill bit having an enhanced ROP when drilling in such rock types. In accordance with the invention there is provided a rotary drill bit for drilling a borehole in an earth formation, the drill bit comprising a first and a second disc cutter, each disc cutter being arranged to roll along the borehole bottom during rotation of the drill bit and thereby to cut respective first and second substantially circular, radially spaced cuts in the borehole bottom so that a body of rock material is defined between said cuts, the second disc cutter being arranged to cut into the bottom of the borehole at a selected rotational interval of the drill bit behind the first disc cutter and to shear-off said body of rock material in the direction of the first cut, wherein each disc cutter includes a set of mutually spaced cutting members arranged to create, during a turn of the drill bit, a corresponding set of cut sections of the cut created by the disc cutter, and wherein the cutting members are arranged so that the sets of cut sections created during subsequent drill bit turns are staggered relative to each other.

By virtue of the feature that each disc cutter is formed of a set of discrete cutting members instead of the continuous disc cutter of the prior art, it is achieved that the instantaneous contact surface of the cutter with the borehole bottom is lower, leading to deeper penetration of the drill bit into the rock formation for a given weight on bit. Thus the cuts created by the cutters are deeper, and thereby shearing- off of the body of rock material tends to be more in the direction of the lower end of the first cut than in the direction of the upper end thereof as in the prior art.

It will be appreciated that, since the disc cutter is no longer continuous, the full circumferential cut has to be created in a plurality of drill bit turns. This is achieved by dimensioning the drill bit so that the cutting members cut during a subsequent drill bit turn in the rock material between the cut sections created by the cutting members in a previous turn. The invention will now be described by way of example with reference to the accompanying drawings in which

Fig. 1 schematically shows a bottom view of a drill bit according to the invention;

Fig. 2 schematically shows a partial side view of the drill bit of Fig. 1;

Fig. 3 schematically shows a bottom view of a roller cone of the drill bit of Fig. 1; and

Fig. 4 schematically shows a transverse cross section of a borehole bottom during various stages of drilling with the drill bit of Fig. 1.

In the Figures, like reference numerals relate to like elements.

In Fig. 1 is shown a bottom view of a drill bit 1 having a bit body 3 and three roller cones 4, 5, 6 arranged at 120° intervals with respect to the axis of rotation 7 of the drill bit. The roller cones are rotatably connected to the bit body 3. Roller cone 4 is provided with a set of disc cutters 4a, 4b, 4c, roller cone 5 is provided with a set of disc cutters 5a, 5b, 5c, and roller cone 6 is provided with a set of disc cutters βa, βb, 6c, each disc cutter 4a, 4b, 4c, 5a, 5b, 5c, βa, βb, 6c extending along the circumference of the respective roller cone in a plane substantially perpendicular to the axis of rotation of the roller cone. Each disc cutter 4a, 4b, 4c, 5a, 5b, 5c, βa, βb, 6c is arranged to cut a substantially circular cut in the borehole bottom during rolling of the respective roller cone 4, 5, β along the borehole bottom when the drill bit 1 is rotated.

Referring further to Fig. 2, there is shown a partial side view of the drill bit 1 and roller cone 4. The roller cones 5, β are not shown for reason of simplicity, however the portions of cutters 5b, 5c, βb, βc at the respective points of contact with the borehole bottom are shown in phantom. The radially outermost cutters 4a, 5a, βa are arranged so as to cut into the borehole bottom at equal radii relative to the axis 7, i.e. these cutters are arranged at equal axial positions on the respective roller cones 4, 5, 6. The inner cutters 4b, 4c, 5b, 5c, 6b, βc are arranged on the respective roller cones 4, 5, 6 in an axially (relative to the roller cone axis of rotation) staggered relationship. In other words, cutter 5b cuts into the borehole bottom at a radius in between the radii at which cutters 4a, 4b cut into the borehole bottom, cutter 5c cuts into the borehole bottom at a radius in between the radii at which cutters 4b, 4c cut into the borehole bottom, cutter βb cuts into the borehole bottom at a radius in between the radii at which cutters 4a, 5b cut into the borehole bottom, and cutter βc cuts into the borehole bottom at a radius in between the radii at which cutters 4b, 5c cut into the borehole bottom.

Each disc cutter has two side surfaces 10, 12, whereby in Fig. 2 only the respective side surfaces 10, 12 of cutter 4a, 4b, 4c are shown for reason of simplicity. Each pair ^'Of side surfaces 10, 12 define a wedge-shaped cutting edge 14. Furthermore, each side surface 12 extends perpendicular to the axis of rotation (indicated by reference numeral 16 for roller cone 4) of the respective roller cone 4, 5, 6 so that during drilling side surface 12 pushes against the rock in the direction of the rotation axis 16. Side surface 20 extends parallel to the axis of rotation 7 of the drill bit 1 at the point of contact with the borehole bottom. As shown in Figs. 2 and 3, each disc cutter 4a, 4b, 4c, 5a, 5b, 5c, 6a, βb, βc is provided by a set of mutually spaced recesses 20 whereby the remaining portions of the disc cutter define a corresponding set of mutually spaced cutting members 22. For each roller cone 4, 5, 6 the sets of cutting members 22 of adjacent cutters are arranged staggered relative to each other.

Each roller cone 4, 5, 6 is dimensioned so that the cutting members 22 of each disc cutter of the roller cone create, during a turn of the drill bit 1, a set of cut sections (not shown) in the borehole bottom and that the sets of cut sections created during subsequent drill bit turns are rotationally displaced relative to each other. In other words, during a subsequent turn of the drill bit 1, the cutting members 22 of the disc cutter cut into the borehole bottom at locations not cut before by the cutting members 22 before. This implies that the complete circular cut is created by the disc cutter in a plurality of drill bit turns. In case the cutting members 22 are regularly spaced along circumference of the respective disc cutter, as is the case in the embodiment of the Figures, the desired dimensioning can be achieved if it is avoided that during a subsequent drill bit turn the cutting members 22 cut exactly in the same cuts as during previous turns. One way of achieving this is by avoiding the condition that L = i . T in which

L = circumferential length of the cut, i = integer number 1, 2, 3, ... etc., T = pitch of the cutting members. With T = π.d/n and L = π.D d being the rolling diameter of the disc cutter, n being number of cutting members of the disc cutter, D being the diameter of the cut created by the cutter, it follows that the drill bit is properly dimensioned if D/d = (i + f)/n.

Preferably 0.3 < f < 0.7. If, for example, f is about 0.5 the full cut is created in about two drill bit turns. Suitably D and d are selected so that 1.5 < D/d < 2. In Fig. 3 is shown a bottom view of roller cone 4 with indicated values of the angular extend of the cutting members (angular extend 32°) and the recesses (angular extend 28°) . Also shown is the angular overlap of 2° of the cutting members of disc cutter 4a with the cutting members of disc cutter 4b. A similar angular overlap exists between the cutting members of the respective cutters 4b, 4c. Furthermore the roller cones 4, 5, β are provided with conventional cutting inserts 26 at side surfaces 10 of the outermost cutters 4a, 4b, 4c and at the inner parts of the roller cones.

During normal operation the drill bit 1 is rotated in the borehole, as a result of which the roller cones 4, 5, 6 roll and cut along the borehole bottom. The cutting action is described hereinafter for cutters 4b, 5b, βb only, as the action of the remaining cutters is similar thereto. Referring further to Fig. 4, the cutters 4b, 5b, 5c create respective adjacent cuts 32, 34, 36 in the borehole bottom. These cuts are indicated as straight lines, however in practice the cuts will have a shape corresponding more or less to the section of the cutters penetrating the borehole bottom. Lines f, g, h, i, j indicate the respective envelopes of the lower ends of the disc cutters 4b, 5b, 5c as a function of the rotation angle of the drill bit. Thus, starting from 0° bit rotation at borehole depth level f, line g indicates the level at 120° bit rotation, line h the level at 240° bit rotation, line i the level at 360° bit rotation and line j the level at 480° bit rotation.

At rotation angle 120° cutter 4b has created cut 36 to level g. At rotation angle 240° cutter 5b has created cut 34 to level h. A body of rock material 38a is thereby defined between cut 36 at level h and cut 34 at level g, which body of rock material 38a is sheared by side surface 12 of cutter 5b along line si extending between the respective cuts 36, 34.

At rotation angle 360° cutter 6b has created cut 32 to level i, thereby defining a body of rock material 38b between cut 34 at level g and cut 32 at level i, which body of rock material 38b is sheared by side surface 12 of cutter βb along line s2 extending between the respective cuts 34, 32.

At rotation angle 480° cutter 5b has created cut 34 to level j, thereby defining a body of rock material 38c between cut 36 at level i and cut 34 at level j, which body of rock material 38c is sheared off by side surface 12 of cutter 5b along line s3 extending between the respective cuts 36, 34.

Similarly, after a further rotational angle of 120° a body of rock material 38d is defined between respective cuts 34 at level j and 32 at a still deeper level, which body of rock material 38d is sheared off by side surface 12 of cutter βb along line s4 extending between the respective cuts 36, 34. The outer cutters 4a, 5a, βa cut and shear off rock material located between the cut (not shown) created by these cutters and cut 32 created by cutter 5b, in a manner similarly as described above. The cutting action of cutters 4c, 5c, βc is similar to the cutting action of respective cutters 4b, 5b, βb.

The discrete cutting sections created by the cutting members 22 extend deeper into the borehole bottom than cuts which would have been created by continuous disc cutters (i.e. cutters without recesses) at equal weight on bit. Therefore the bodies of rock material, defined between adjacent cuts, have a larger size in the direction of penetration of the drill bit, so that the mere superficial cutting action during drilling in hard formations is avoided and thereby a more efficient drilling process is achieved.

Claims

C L I M S

1. A rotary drill bit for drilling a borehole in an earth formation, the drill bit comprising a first and a second disc cutter, each disc cutter being arranged to roll along the borehole bottom during rotation of the drill bit and thereby to cut respective first and second substantially circular, radially spaced cuts in the borehole bottom so that a body of rock material is defined between said cuts, the second disc cutter being arranged to cut into the bottom of the borehole at a selected rotational interval of the drill bit behind the first disc cutter and to shear-off said body of rock material in the direction of the first cut, wherein each disc cutter includes a set of mutually spaced cutting members arranged to create, during a turn of the drill bit, a corresponding set of cut sections of the cut created by the disc cutter, and wherein the cutting members are arranged so that the sets of cut sections created during subsequent drill bit turns are staggered relative to each other. 2. The rotary drill bit of claim 1, wherein the cutting members are substantially regularly spaced along the cutter, and wherein the drill bit is dimensioned so that

D/d = (i + f)/n in which D = diameter of the cut created by the cutter; n = number of cutting members of the cutter; d = rolling diameter of the cutter; i = integer number 1, 2, 3, ... etc.; 0 < f < 1. 3. The drill bit of claim 2, wherein 0.3 < f < 0.7

4. The drill bit of claim 2 or 3, wherein f is about 0.5.

5. The drill bit of any one of claims 2-4, wherein 1.5 < D/d < 2. β. The drill bit of any one of claims 1-5, wherein the drill bit includes a bit body and a plurality of roller cones rotatably connected to the bit body so as to substantially roll along the borehole bottom during rotation of the bit in the borehole and being oriented in different radial orientations, a first one of the roller cones being provided with the first disc cutter and a second one of the roller cones being provided with the second disc cutter.

7. The drill bit of claim β, wherein each roller cone is provided with a plurality of said disc cutters spaced along the axial direction of the roller cone, and wherein the sets of cutting members of adjacent disc cutters of the roller cone are arranged in a staggered relationship.

8. The drill bit of any one of claims 1-7, wherein for each disc cutter, each cutting member of the disc cutter extends along a section angle α of the disc cutter and each spacing between adjacent cutting members of the disc cutter extends along a section angle β of the disc cutter, and wherein α = β + δ and 0° < δ < 4°. 9. The drill bit of claim 8, wherein 1° < δ < 3°, and preferably δ is about 2°.

10. The drill bit of any one of claims 1-9, wherein each set of cutting members is defined by a set of mutually spaced recesses formed in the disc cutter to which the set of cutting members pertains.

11. The drill bit substantially as described hereinbefore with reference to the drawings .

D28/TS6257FF