Combination drill bit

A drill bit for drilling a hole in the ground, with cutting elements annularly cutting a core which, when it has reached a certain height, is continuously crushed by teeth on rolling cones. By combining these two processes, cutting and crushing, in this manner an improved drilling advancement is achieved as compared to separate use of the same processes. The cutting elements show relatively small variations as to radial positioning, which renders it possible to find a common approximately optimal rotational speed of said elements. The core is weak and may be drilled out relatively easily by the aid of crushing, as compared to drilling pure holes. This is due to the fact that the core geometry causes a more efficient growth of fractures for each tooth penetration, and that the core, due to annular cutting, is free from radial tensions from the surrounding rock formations. In order to increase the life of the PDC cutting element, the mechanical strength of said element is improved due to the fact that the edge of the element is rounded with a small visible radius.

BACKGROUND OF THE INVENTION 
The invention relates to a combination drill bit which is designed to drill 
holes by annular cutting and continuous core breaking. 
The new combination drill bit is designed to carry out a process for 
drilling by annular cutting and continuous core breaking. Experiments were 
carried out with jet beams cutting a core by annular cutting, which core 
is broken by a rock bit, cf. Maurer, W. C. Heilhecker, J. K. and Love, W. 
W., "High Pressure Drilling"--Journal of Petroleum Technology, July 1973. 
These experiments resulted in an increase of the drilling rate by 2-3 
times. The problem in utilizing a jet beam is that it requires a down-hole 
pump, which is able to produce the very high pressure necessary to enable 
the liquid beam to cut the formation. 
Previously, PDC (polycrystalline diamond compact) cutting elements and rock 
bits with teeth were combined, but then mainly with the intention to limit 
drilling advancement in soft formations in order to avoid clogging of the 
cutting elements, cf. U.S. Pat. No. 4,006,788. 
At present, mainly two kinds of drill bits are used, i.e. PDC drill bits 
and rock bits. PCD drill bits cut the formation with the aid of an edge 
comprised of a number of PCD cutting elements. Due to the fact that the 
cutting elements rotate at the same rotational speed about a common axis, 
cutting speed will vary from zero at the center, to a maximum outermost on 
the periphery of the drill bit. It is, thus, impossible to achieve an 
optimal cutting speed of all cutting elements at the same time. 
The cuttings formed when PDC cutting elements are used, often are very 
small, resulting in the fact that very limited geological information can 
be extracted from them. PDC-bits were constructed which cut a small core 
for use in geological analysis, cf. U.S. Pat. No. 4,440,247. Drilling 
operators reported that the their effect as regards acquiring larger 
pieces is quite low. 
The edge of a present PDC cutting element is 90.degree. and sharp. 
Consequently, it is comparatively weak and tends to chip. 
Rock bits break up the formation, by teeth which are mounted on the rock 
bits being urged towards the formation by so high a force that the 
formation will break under and around said teeth. Due to the relatively 
plane face of the hole bottom, crack propagation due to each tooth 
penetration is of relatively small effect as regards the volume to be 
drilled. If the volume to be broken is acquired in the shape of an 
unstabilized core, the efficiency of each tooth penetration will be 
considerably improved. 
Conventionally, the principle of annular cutting with continuous core 
breaking is not used, at present, for drilling holes. There are a number 
of patents based on this principle. According to one patent, diamonds 
baked into a matrix are used. This system provides for more grinding than 
cutting, requiring high rpm to achieve a satisfactory drilling 
advancement. The central rolling cones, which are used to break the core, 
then have to be run at too high rpm, cf. U.S. Pat. No. 3,055,443. 
According to another patent, edges of tungsten carbide are used, resulting 
in a very limited life of the drill bit due to insufficient resistance to 
abrasion of the edges. The last mentioned drill bit does not generate a 
cavity about the core before it is broken, i.e. the internal wall of the 
core drill bit has a stabilizing effect on the core, cf. U.S. Pat. No. 
3,075,592. A third patent discloses utilizing cutting edges requiring 
channels/grooves in front of/behind the edges. The channels/grooves must 
be large enough to permit the pieces of broken core to pass to the outside 
of the drill bit. The core is broken by the aid of a toothed roller which 
has too much scraping effect due to its geometry. This will cause the 
teeth of the roller to be worn down far too rapidly. Nozzles are used to 
flush the toothed roller and to moisten the core so as to weaken it, cf. 
U.S. Pat. No. 2,034,073. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to utilize cutting edges of 
polycrystalline diamond and/or a ceramic material for annular cutting of a 
core which is then continuously broken or crushed. It is essential in this 
connection to achieve a core that may be readily crushed. At the same time 
the proportions of the core must be correct in view of the total volume 
which has to be removed in the actual case to drill the hole. This means 
that an unstabilized core showing correct height and diameter relative to 
the drill hole diameter should be achieved. Shear stresses inherent in the 
core can then be activated in an advantageous manner during crushing. Also 
annular cutting to provide said core is carried out with the aid of a tool 
and to an extent rendering the total drilling more efficient than 
conventional drilling. 
According to the invention, a combination drill bit as mentioned above is 
provided. 
It be important that the rock bit is dimensioned to cover the entire 
undercut end cavity cross-section, i.e. that the rock bit will also be 
efficient in the annular area which will be present in the cross-section 
between the internal wall of the end cavity and the cylindrical wall of 
the formed unstabilized core. Broken-off matter which is present in this 
area will be crushed by the rock bit and made to pass through the wall 
openings. The polycrystalline or ceramic cutting elements which are placed 
to form an annulus provide for excellent annular cutting in an efficient 
manner to form the core. 
The formed unstabilized core will break down under the influence of the 
crushing means and the core matter may in an advantageous manner pass out 
through relatively low wall openings. 
It is desirable to achieve good stabilization of the drill bit in the hole, 
and at the same time good transport of matter upwards, past the drill bit. 
This is achieved by the special design of the outside of the drill bit, 
with wide stabilizing wall portions alternating with channels for 
transport upwards of drilled matter. The channels are dimensioned to 
permit relatively large pieces to pass. The wall openings and the channels 
should be associated to permit pieces passing through the openings to pass 
on, via channels. 
Theoretically, a fracture in a material will appear at the point where 
shear stress is at a maximum, i.e. the fracture will start in a plane at 
45.degree. relative to maximum shear stress. In rock the internal friction 
of the material is essential to which angles of fracture the material will 
develop. The angle of fracture may be written as follows: 
EQU Angle of fracture=45.degree.-1/2 internal frictional angle. 
The internal frictional angle of rock will vary from almost zero to more 
than 60.degree.. Resulting angles of fracture are from almost 45.degree. 
to less than 15.degree.. When fractures are initiated, they will always 
develop along the path of least resistance. During continuous core 
breaking the fracture will generally not cross the center line of the 
core. Calculations on this basis show that the unstable core height should 
advantageously be between twice and 0.5 times the core diameter. The 
direction of maximum main stress is then assumed to be parallel with the 
direction of drilling. Experiments have shown that the lower one may be as 
low as 0.2, which is attributed to the shape of the core top during 
continuous crushing, as well as to variation of the direction of main 
stress. In view of energy considerations, the core should be as large as 
possible, but to ensure sufficient strength of the core drill bit the 
diameter of the core must be reduced relative to the hole diameter. 
Considering variations of cutting speed across the core drill bit, the 
core diameter should not be less than 0.4 times the drill hole diameter. 
For suitable annular cutting with continuous core crushing, the core 
diameter should, thus, be at least 0.4 times the drill hole diameter. It 
will then be possible to select an rpm value which is approximately 
optimal for all cutting elements. 
According to the invention, one or a plurality of high pressure nozzles is 
advantageously connected with jet channels directed into the end cavity. 
In order to prolong the life of cutting elements the mechanical strength of 
the edge may advantageously be improved by rounding the edge with a small 
visible radius.

DETAILED DESCRIPTION 
In FIGS. 1 and 2, a common drill bit 11 with rolling cones 3 is shown. 
Additionally, PDC cutting elements 4 are shown, the axially and radially 
outer edge of each of which is provided with a visible radius, as shown in 
more detail in FIG. 3. 
Cutting elements 4 are attached to a cylinder 1 and act against the annular 
drilling hole face 15, see FIG. 4. Rolling cones 3 with teeth 5 act, in 
use, against the top 14 of the cut-out core 13 to crush that top. Rolling 
cones 3 form part of a common rock bit 11. As shown in FIG. 1, rock bit 11 
is secured in a drill bit fastening means 2 which is, in turn, connected 
with cylinder 1 with the aid of a threaded portion 19. 
The drill bit rotates about central axis 17 and, at the same time, rolling 
cones 3 rotate about their own axis 16. Consequently, movement between 
rolling cones 3 and the base, which is core face 14 in this case, may be 
pure rolling movement. The pieces from the crushed portion of core 13 are 
transported with drilling fluid to the outside of the core drill bit 
through holes 6 in its wall. Above rolling cones 3 and at the end of the 
core drill bit, at the root of core 13 being drilled, nozzles 7 for drill 
mud open. The core drill bit and the rock bit are, as mentioned, connected 
by the aid of a drill bit fastening means 2, which is here also utilized 
for distribution of drilling fluid to nozzles 7. 
Connection of the drill bit and remaining drilling equipment is achieved 
with threaded portion 8. Numeral 9 indicates channels for transport of 
drilled matter by the aid of the drilling fluid. Plugs 10 of a hard 
material will prevent reduction of diameter (in operation). 
It will appear from FIG. 1 that end cavity 18 is undercut relative to the 
core diameter. A free annular space is, thus, achieved about the core to 
make core 13 unstabilized, which is essential in connection with 
subsequent crushing and removal of core material. By following the 
principles of the invention, a weak core is achieved, which core may be 
quite readily removed with the aid of crushing, as compared to drilling of 
conventional holes. As mentioned, this is due to the fact that the core 
geometry provides more efficient growth of fractures and that the core, 
due to annular cutting, will be free of radial tensions from surrounding 
rock. Overall, improved drilling advancement is achieved, as compared to 
the annular cutting and core breaking processes being used separately. 
FIG. 5 shows an advantageous design of wall openings 6. The tangent line to 
the rear wall of wall opening 6 in each point, apart from a rounding at 
the inlet, is rotated against the operational direction of rotation of the 
drill bit by an angle .alpha. relative to the drill bit sector line 
through the same point, as seen from the inlet of opening 6 towards its 
outlet, with .alpha.=.gtoreq.0.degree. and .ltoreq.90.degree.. By the rear 
wall of the opening is meant the side of the opening which is the last to 
pass a fixed sector line when the drill bit is rotated in an operative 
direction. By sector line is meant a straight line extending normally from 
the axis of rotation of the drill bit. By inlet to opening 6 is meant the 
side from which drilled out matter flows in through opening 6. In other 
words, the elements 6 are channels which, while opening generally radially 
through the drill bit body, have respective longitudinal axes which are 
slanted with respect to radians of the drill bit body, so as to dispose 
radially inner inlet ends of these channels angularly ahead of respective 
radially outer outlet ends thereof, by an angular amount in the range of 
.gtoreq.0.degree. to .ltoreq.90.degree.. 
As shown in FIG. 5, the polycrystalline cutting elements 10 are mounted in 
sockets extending perpendicular to the longitudinal axis of the 
combination drill bit, and their radially outer cutting surfaces are 
disposed so as to be tangent to the radially outer surface of the drill 
bit.