AU9537898A

AU9537898A - Drill bit

Info

Publication number: AU9537898A
Application number: AU95378/98A
Authority: AU
Inventors: Djurre Hans Zijsling
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1997-08-28
Filing date: 1998-08-28
Publication date: 1999-03-22
Anticipated expiration: 2018-08-28
Also published as: BR9811389A; WO1999011900A1; ID24483A; RU2206701C2; AU727657B2; EG21701A; GB2344842A; CN1098400C; GB0003248D0; AR016883A1; GB2344842B; NO20000938L; CN1268991A; NO318151B1; NO20000938D0; OA11327A; CA2300158A1

Description

WO99/11900 PCT/EP98/05703 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 5 bits are generally provided with abrasive cutting elements which are made of a material having a high wear resistance, diamond or tungsten carbide being for example commonly applied. The cutting action of these cutting elements mainly results from scraping of the elements 10 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 15 part of the overall wellbore costs there is a continuous need to reduce the drilling time, viz. to increase the drilling speed. Accordingly it is an object of the invention to provide a drill bit which allows an increased speed of 20 drilling of a borehole in an earth formation. 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 plurality of cutting elements arranged to cut a plurality of 25 substantially circular, radially spaced cuts into the bottom of the borehole, the cutting elements including at least one cutting/shearing element, each cutting/shearing element being arranged so as to cut into the bottom of the borehole at an angular interval behind another one of 30 said cutting elements, whereby a body of rock material is defined between the trajectories of the cuts created by WO99/11900 PCT/EP98/05703 -2 the cutting/shearing element and said another cutting element, the cutting/shearing element being provided with means for shearing-off said body of rock material in the direction of the cut created by said another cutting 5 element. The body of rock material can be sheared-off by the cutting/shearing element because 1) the other cutting element creates a first cut which forms a boundary for the body of rock material to be sheared-off and 2) 10 because the first cut provides a space into which the body of rock material is moved by the action of the cutting/shearing element. Thus, the drill bit according to the invention performs both a cutting action and a shearing action, allowing the drill bit to remove 15 significantly larger pieces of rock material from the borehole bottom than conventional drill bits. The speed of drilling is therefore significantly larger than for conventional drill bits. Suitably the rotary drill bit further comprises a bit 20 body and a plurality of roller cones rotatably connected to the bit body so as to substantially roll along the borehole bottom, the axes of rotation of the roller cones extending in different radial orientations, each roller cone being provided with at least one of said 25 cutting/shearing elements. Effective cutting and shearing-off of the body of rock material is achieved if each cutting/shearing element forms a disc cutter extending around the axis of rotation of the roller cone to which the cutting element 30 is provided. Preferably each roller cone is provided with a set of said disc cutters spaced along the axis of rotation of the roller cone, the sets of disc cutters of the different roller cones being displaced relative to each 35 other in radial direction of the drill bit.

WO99/11900 PCT/EP98/05703 -3 The invention will now be described by way of example with reference to the accompanying drawings in which Fig. 1 schematically shows a side view of a first embodiment of the drill bit; 5 Fig. 2 shows the right half of the first embodiment shown in Fig. 1; Fig. 3 schematically shows a side view of a second embodiment of the drill bit; and Fig. 4 schematically shows a side view of a third 10 embodiment of the drill bit. In the Figures like reference numerals relate to like elements. The first embodiment shown in Figs.l and 2 of a drill bit 1 includes a bit body 3, a screw connector 5 for 15 connecting the bit 1 to a drill string, and two opposite roller cones 7, 9 rotatably connected to the bit body 3 whereby the axes of rotation 10, 11 of the roller cones 7, 9 are inclined in downward direction and intersect with axis of rotation 12 of the bit 1 at intersection 20 point 14. Alternatively, the roller cones can be offset in the sense that the axes of rotation of the roller cones do not intersect the axis of rotation of the drill bit 1. Roller cone 7 is provided with a set of cutting/shearing elements in the form of disc cutters 7a, 25 7b, 7c, 7d, 7e and roller cone 9 is provided with a set of cutting/shearing elements in the form of disc cutters 9a, 9b, 9c, 9d, 9e. Each disc cutter has a cutting edge extending perpendicular to the axis of rotation of roller cone 7, 9 to which the disc cutter pertains. The disc 30 cutters 7a-d and 9a-d are arranged so that the envelope of contact points of the disc cutters with the borehole bottom has a convex shape. The set of disc cutters 7b-e and the set of disc cutters 9b-e are arranged staggered relative each other 35 in radial direction of the borehole. Thus, when seen in WO99/11900 PCTIEP98/05703 -4 radial direction, the disc cutters 7b-e and the disc cutters 9b-e alternately contact the borehole bottom. To illustrate the staggered arrangement of the sets of disc cutters, the lower portions of the disc cutters 7b-e are 5 indicated in phantom between the lower portions of the disc cutters 9b-e. Thus, the disc cutters 7b-e assume the positions shown in phantom when the drill bit 1 has rotated 1800 from the orientation shown in Fig. 1. The radially outermost disc cutters 7a, 9a are 10 arranged at substantially equal radial positions of the drill bit and are provided with (outer) insert cutting elements 16 at the side facing the borehole wall so as to cut a radially outer portion of the borehole. Similar (inner) insert cutting elements 16 are provided at the 15 inner end part of each roller cone so as to drill out the rock material located near the axis of rotation 12 of the drill bit 1. Each disc cutter has two side surfaces 18, 20 defining a wedge-shaped cutting edge 17, as shown in 20 Fig. 1 only, the other disc cutters having similar side surfaces and cutting edge. Each side surface 18 extends perpendicular to the axis of rotation 10, 11 of the roller cone 7, 9 to which the disc cutter pertains. Thereby during drilling side surface 18 pushes against 25 the rock in the direction of the point of intersection 14. Side surface 20 extends parallel to the axis of rotation 12 of the drill bit 1 at the point of contact with the borehole bottom. In Fig. 3 is shown a second embodiment of a drill bit 30 31 according to the invention which is largely similar to the first embodiment, the difference being that the drill bit 31 has three roller cones 35, 37, 39, of which only roller cone 39 is shown here. The three roller cones are oriented at 1200 intervals and are similar to those of WO99/11900 PCT/EP98/05703 -5 the first embodiment described with reference to Figs. 1 and 2. For ease of understanding only three disc cutters and no stabilisation cutters are assumed for each of the three roller cones. It will be understood that in 5 practice any suitable number of disc cutters and stabilisation cutters can be provided at each roller cone. In the embodiment of Fig. 3, roller cone 35 has set of disc cutters 35a, 35b, 35c, roller cone 37 has set of disc cutters 37a, 37b, 37c, and roller cone 39 has set of 10 disc cutters 39a, 39b, 39c. The three sets of disc cutters are arranged staggered in radial direction of the borehole. Thus, when seen in radial direction, the disc cutters 35a-c, 37a-c and 19a-c alternately contact the borehole bottom. To illustrate the staggered arrangement 15 of the sets of disc cutters, the lower portions of the disc cutters 35a-c and 37a-c are indicated in phantom between the disc cutters 39a-c. The disc cutters 37a-c assume the positions shown in phantom when the drill bit 1 has rotated 1200 from the orientation shown in Fig. 1, 20 and the disc cutters 39a-c assume the positions shown in phantom when the drill bit 1 has rotated 240' from the orientation shown in Fig. 1. In Fig. 4 is shown a third embodiment of a drill bit 41 according to the invention which is largely similar to 25 the second embodiment, the main difference being that the disc cutters are arranged so that the envelope of points of contact of the disc cutters with the borehole bottom has a concave shape as opposed to the convex shape of the first and second embodiment. Furthermore, the side 30 surface 18 of each disc cutter is oriented so as to push against the rock material in radially outward direction. Side surface 20 of each cutter (indicated by dotted lines) extends parallel to the axis of rotation 412 of the drill bit 41. Three roller cones 45, 47, 49 are WO99/11900 PCTIEP98/05703 -6 provided, of which only roller cone 49 is shown. Four disc cutters and no stabilisation cutter are assumed for each of the three roller cones. Again, it will be understood that in practice any suitable number of disc 5 cutters and stabilisation cutters can be provided at each roller cone. Roller cone 45 has set of disc cutters 45a d, roller cone 47 has set of disc cutters 47a-d and roller cone 49 has set of disc cutters 49a-d. Similarly to the second embodiment, the three sets of disc cutters 10 are arranged staggered in radial direction of the borehole. To explain normal operation of the first embodiment reference is made to Fig. 2 showing the right half of Fig. 1. For the purpose of explaining normal operation of 15 the drill bit 1 only three disc cutters 9c, 7c, 9d need to be considered, as the other disc cutters operate in a similar way. During rotation of the drill bit in the borehole, the cutters 9c, 7c, 9d create adjacent cuts 22, 24, 26 in the borehole bottom. In Fig. 2 these cuts are 20 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-j indicate the envelope of the depth-levels of the disc cutters as a function of the rotation angle of 25 the drill bit. Thus, when starting from 00 bit rotation at borehole depth level f, line g indicates the level at 1800 bit rotation, line h the level at 3600 bit rotation, line i the level at 540' bit rotation and line j the level at 7200 bit rotation. 30 When the bit has rotated 1800 cutters 9c, 9d create cuts 22, 26 to depth level g (1800 bit rotation). As cutter 9c creates cut 22 in the borehole bottom, side surface 18 of cutter 9c pushes against a body of rock material 28 in the direction of the bit axis 12, which WO99/11900 PCT/EP98/05703 -7 body of rock material 28 is located between cuts 22 and 26. However, shearing-off of this body of rock material is hampered by the presence of disc cutter 9d in cut 26. As the drill bit rotates to 3600, disc cutter 7c creates 5 cut 24 located between cuts 22 and 24. The disc cutters are now at depth level h, therefore cut 24 extends to level h. During cutting of cut 24 side surface 18 of disc cutter 7c pushes against a portion 28a of the body of rock material 28, which portion 28a is located between 10 cuts 24 and 26. Now the presence of cut 26 allows the portion 28a of rock material to be sheared-off along line sl extending between cut 24 at level h and cut 26 at level g. As the drill bit 1 rotates to 5400, cuts 22, 26 are 15 deepened by disc cutters 9c, 9d to depth level i. Side surface 18 of disc cutter 9c thereby shears off portion 28b of body of rock material 28 by virtue of the absence of previously removed rock portion 28a. Shearing-off of this portion 28b occurs along line s2 extending between 20 cut 22 at level i and cut 24 at level h. It will be clear that the cross-sectional shape of the borehole bottom at an arbitrary point in time during drilling is governed by the stepwise shearing-off of rock material portions along lines sl, s2 etc. Therefore each line f-g 25 in Fig. 2 only resembles the envelope of the disc cutters rather than the instantaneous shape of the borehole bottom. As drilling proceeds rock portions 28c and 28d are sheared-off along respective lines s3 and s4, in a 30 similar manner as described with reference to rock portions 28a and 28b. Cutting and shearing-off of the rock material by the other disc cutters of the drill bit proceeds in a similar way. Thus, the rock material at the borehole bottom is removed by a combination of cutting WO99/11900 PCTIEP98/05703 and shearing, allowing larger rock particles to be removed from the borehole bottom than by cutting alone (as is the case for conventional drill bits). The radially outer portion of the borehole is drilled 5 out by the outer insert cutting elements 16 at the radially outer disc cutters 7a, 9a and the centre portion of the borehole is drilled out by the inner insert cutting elements 16 at the inner part of each roller cone. The outer insert cutting elements 16 are subjected 10 to cutting forces from the rock. The resulting cutting force component in a plane perpendicular to the longitudinal axis of the bit acting on each insert element 16 is pointing inwards from the bore hole wall, and the magnitude of the resulting force increases with 15 the amount of radial penetration of the insert element 16 into the bore hole wall. The total resulting force in said plane is the vector sum of the resulting cutting forces acting on all outer insert elements. When the bit is subjected to a displacement in a direction 20 perpendicular to the centre line of the borehole, this vector sum will change in magnitude and have a component which counteracts the displacement. Thus, if the drill bit penetrates further into the borehole wall in one radial direction than in any other radial direction, the 25 deviation in the vector sum of the cutting force components in said plane counter-acts such penetration and thereby radially stabilises the drill bit during drilling. Similarly, the inner insert elements 16 are subjected to outwardly directed cutting forces from the 30 rock at the centre portion of borehole bottom, and thereby also contribute to the radial stability of the drill during drilling. Normal operation of the second embodiment is similar to normal operation of the first embodiment, albeit that 35 the rock material is sheared-off each 1200 rotational WO 99/11900 PCT/EP98/05703 - 9 interval of the drill bit instead of each 180' interval as in the first embodiment. Normal operation of the third embodiment is similar to normal operation of the second embodiment, albeit that 5 now the rock material us sheared-off in radially outward direction of the borehole as a consequence of the concave shape of the envelope of points of contact of the disc cutters with the borehole bottom.

Claims

1. A rotary drill bit for drilling a borehole in an earth formation, the drill bit comprising a plurality of cutting elements arranged to cut a plurality of substantially circular, radially spaced cuts into the 5 bottom of the borehole, the cutting elements including at least one cutting/shearing element, each cutting/shearing element being arranged so as to cut into the bottom of the borehole at an angular interval behind another one of said cutting elements, whereby a body of rock material is 10 defined between the trajectories of the cuts created by the cutting/shearing element and said another cutting element, the cutting/shearing element being provided with means for shearing-off said body of rock material in the direction of the cut created by said another cutting 15 element.

2. The rotary drill bit of claim 1, comprising a plurality of said cutting/shearing elements.

3. The rotary drill bit of claim 2, further comprising a bit body and a plurality of roller cones rotatably 20 connected to the bit body so as to substantially roll along the borehole bottom, the axes of rotation of the roller cones being oriented in different radial orientations, each roller cone being provided with at least one of said cutting/shearing elements. 25

4. The rotary drill bit of claim 3, wherein each cutting element forms a disc cutter extending around the axis of rotation of the roller cone to which the cutting element is provided.

5. The rotary drill bit of claim 4, wherein each disc 30 cutter has a wedge-shaped cross-section defining two side surfaces of the cutter, at least one of the side surfaces WO99/11900 PCTIEP98/05703 - 11 being arranged to push against said body of rock material so as to shear-off said body of rock material thereby forming the means for shearing-off the body of rock material. 5

6. The rotary drill bit of claim 5, wherein the other of said side surfaces extends parallel to the longitudinal axis of the drill bit.

7. The rotary drill bit of any one of claims 3-6, wherein each roller cone is provided with a set of said 10 disc cutters spaced along the longitudinal axis of the roller cone, and wherein the different sets of disc cutters pertaining to the different roller cones are arranged staggered relative to each other along the longitudinal axes of the roller cones. 15

8. The rotary drill bit of claim 7, comprising a first roller cone having a first set of disc cutters and a second roller cone having a second set of disc cutters.

9. The rotary drill bit of claim 8, further comprising a third roller cone having a third set of disc cutters. 20

10. The rotary drill bit of any one of claims 3-9, wherein each roller cone is provided with a plurality of cutting inserts arranged to cut an outer radial section of the borehole.

11. The rotary drill bit of claim 10, wherein each roller 25 is provided with an outer disc cutter, the outer disc cutters of the roller cones being arranged at substantially equal radial position of the drill bit, each outer disc cutter having an outer surface facing the borehole wall and being provided with said plurality of 30 cutting inserts.

12. The rotary drill bit of any one of claims 3-11, wherein each roller cone is provided with a plurality of inner cutting inserts arranged at the inner part of the roller cone so as to cut a central section of the 35 borehole. WO99/11900 PCT/EP98/05703 - 12

13. The rotary drill bit of any one of claims 2-12, wherein the envelope of the cutting/shearing elements at the borehole bottom has a convex shape, and wherein each means for shearing-off the body of rock material is 5 arranged to shear-off said body of rock material in radially inward direction.

14. The rotary drill bit of any one of claims 2-12, wherein the envelope of the cutting/shearing elements at the borehole bottom has a concave shape, and wherein each 10 means for shearing-off the body of rock material is arranged to shear-off said body of rock material in radially outward direction.

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