GB2415208A

GB2415208A - Drag bit

Info

Publication number: GB2415208A
Application number: GB0413712A
Authority: GB
Inventors: Stale Vikra; Karl Gunnar Tjeltveit
Original assignee: Statoil ASA
Current assignee: Equinor ASA
Priority date: 2004-06-18
Filing date: 2004-06-18
Publication date: 2005-12-21
Anticipated expiration: 2024-06-18
Also published as: GB0413712D0; GB2415208B

Abstract

A rotary drag bit 10 comprising at least one exposed fluid channel 14 and at least one initially unexposed fluid channel 15, wherein the initially unexposed channel 15 is arranged to be exposed as the bit is worn down in use prior to the complete erosion of the exposed fluid channel 14. In this way, the life of the drag bit 10 can be extended, with a new cutting surface being exposed before the erosion of the first. A plurality of unexposed fluid channels 15 can be provided at different depths such that the drag bit 10 can be seen to have a self renewing cutting surface. The unexposed channels 15 may be partially blocked prior to becoming exposed. The fluid channels 14 may be connected to a fluid delivery passage which may be non-linear in shape. Also disclosed is a rotary drag bit 10 having fluid channels in fluid communication with a fluid delivery hole 16, the fluid delivery hole 16 being offset from the rotational axis of the drill.

Description

24 1 5208 Drag Bit This invention relates to rotary drill bits, and, in

particular, to drag bits such as those used for subterranean drilling.

A variety of types of drill bits are used in the oil production industry. Some bits, known as rock or roller bits, have rotating cutters on the drill tip. These are generally used for drilling into comparatively soft ground.

Rotary drill bits with no such moving elements are typically referred to as drag bits or fixed cutters. They are mainly used for subterranean drilling in areas of hard ground.

Drag bits generally consist of a metal core surrounded by a cutting structure. This structure is made from, for example, hard metal such as tungsten carbide and forms a cutting surface for grinding against the earth. The cutting surface usually comprises a plurality of exposed fluid channels which form a wear pattern of ridges between them.

These channels are usually between 5-20mm in depth. In use the drill bit is supplied with a constant supply of fluid at high pressure which provides cooling and lubrication as well as a means of transporting away the cuttings, thus keeping the cutting surface clean and more able to perform its function. The cutting surface is often impregnated with harder cutting elements such as natural or synthetic diamond or other ceramic material. This increases the bit's cutting ability, thus improving its rate of penetration (ROP) as well as increasing its life time.

In use the drill bit is attached to a motor either via a 'drill string' from surface, or to a motor included in the drill string. The string may be hundreds or thousands of feet in length and is therefore extremely heavy. The weight of the drill string is supported from above such that the unsupported weight of the drill string provides enough force to drive the drill into the ground. As the bit penetrates the ground, the drill string can be lengthened by attaching additional sections of drill pipe.

In use, the drilling fluid ('mud') is pumped down the centre of the drill string at high pressure to the drill bit where it exits via a central fluid delivery hole and flows radially through the fluid channels. The fluid then returns up the well bore carrying away the cuttings.

As the drill bit is used, the cutting structure is worn away, decreasing the size of the channels. This increases the pressure within the channels which tends to force the drill bit away from the surface it is cutting. To counteract this, the suspension of the bit is lessened, thus placing more weight on the bit.

In time, the cutting structure is worn away until the cutting surface is virtually flat. At this point the fluid exiting the fluid delivery hole forces the bit away from the cutting face of the bore hole to the extent that further cutting is prevented. The drill bit must then be removed from the borehole and replaced. This is a time consuming process and can take from several hours to more than two days to achieve. Therefore, it is highly beneficial to have a drill bit with a long effective life to reduce the number of replacements required. At present, depending on the hardness of the material, drag bits have operational lifespans ranging from five minutes to over four days.

One seemingly useful solution to this problem would be to increase the height of the ridges/blades, and consequently the depth of the channels, thereby increasing the time taken for them to completely wear down.

However, the depth of the channels is limited by the need to have sufficient fluid pressure in the channels to allow for effective transportation of cuttings. If the channels are too large the pressure drop is too great and there is a loss in washing action. In addition, an increase to the size of the channels weakens the bit and so makes it less able to withstand torque during operation and more likely to break.

According to the present invention there is provided a rotary drag bit comprising a plurality of cutting surfaces each defined by a plurality of fluid channels and having at least one fluid delivery hole, the surfaces being arranged to be exposed in sequence such that a subsequent cutting surface is exposed prior to the complete erosion of the current cutting surface.

Thus, the drill bit contains a plurality of cutting surfaces. Therefore, upon erosion of the first cutting surface, rather than the drilling operation being suspended while the drill bit is changed, drilling can continue uninterrupted due to the exposure of a second cutting surface. More surfaces are preferably provided so that, as that second surface is eroded, a third is exposed and so on.

In effect this invention allows an instantaneous change of drill bits at the cutting face, therefore negating the lengthy removal process, and increases the speed of the drilling operation.

Although embodiments of the invention can be envisaged where the cutting surfaces are discrete, it is preferred that they form a continuum, with each surface being identified by the exposure of a fresh set of fluid channels.

Thus, a cutting surface is defined by a plurality of exposed fluid channels. Therefore the exposure of a new channel during the use of the drill equates with the start of a new cutting surface, one which overlaps with the previous cutting surface.

An alternative way of viewing the invention is as the provision of a selfrenewing cutting surface, whose wear pattern changes as various fluid channels are exposed and Therefore viewed from a further aspect, the present invention provides a rotary drag bit comprising at least one exposed fluid channel and at least one initially unexposed fluid channel, wherein the initially unexposed fluid channel is arranged to be exposed as the bit is worn down in use and prior to the complete erosion of the exposed fluid channels.

The additional, unexposed fluid channel increases the lifetime of the drill bit because upon exposure it defines what is effectively a fresh cutting surface and allows the drilling to continue even after complete erosion of the originally exposed fluid channel.

Preferably the drill bit comprises a plurality of exposed fluid channels to create a conventional channel pattern defining ridges on the cutting surface. Also, it is preferable that the drill bit comprises a plurality of unexposed fluid channels arranged at differing depths within the drill bit. Thus, the fresh cutting surfaces may each (as they are formed) provide a conventional channel and ridge pattern. Also, by increasing the number of unexposed fluid channels, the life time of the drill bit can be further increased.

It is important that the initially unexposed fluid channel(s) are arranged such that new fluid channel(s) are exposed prior to the complete erosion of the previously exposed fluid channel(s). If all the exposed fluid channel(s) are allowed to completely erode without a new fluid channel being opened up, the fluid pressure will force the bit away from the cutting face. New channels being exposed prior to the complete erosion of the previous channels prevent this.

The unexposed fluid channels preferably form conduits (e.g. tubes) within the drill bit that, upon erosion of the drill bit, are opened up to form the new channels. These tubes preferably each form a flow path extending In a generally radial direction from a fluid delivery passage to the exterior of the bit.

If these tubes are left unblocked prior to exposure, fluid from the delivery channel will flow out of the bit though them. This effect can be utilised as a means of centrallsing the drill bit within the borehole and therefore the channels can be left open. However, this requires an increase in the overall fluid pressure into the drill to avoid an excessive drop in fluid pressure at the cutting face. Therefore, preferably, the non exposed channels are blocked until they are exposed at the cutting face to prevent the need to increase fluid pressure.

This can be achieved in a number of ways. Firstly the entire tube can be blocked with a soft or brittle material that will be quickly removed/ broken upon exposure to the cutting face. Alternatively 'pegs' can be used, which are inserted into the conduits to provide a more secure blockage. In this case, the tubes would preferably have one or more circumferential indent into which a part of the peg will expand in order to prevent the pressure of the fluid forcing it out of the tube. Alternatively the tubes can be tapered so that they narrow towards the circumference of the bit.

As discussed above, as the fluid channels are eroded, the fluid pressure acting on the face of the bit alters.

This results in the need to alter the suspension of the drill in order to ensure the drill bit is maintained in contact with the cutting face. When a drill bit according to the present invention is used, the fluid pressure will Increase as the exposed channels are worn away and decrease when an unexposed fluid channel is opened up. This can make control of the fluid pressure difficult. Therefore it is preferable that the unexposed fluid channels are arranged such that, during erosion of the bit the total flow area (TEA) remains constant. In this arrangement, as the exposed fluid channels are eroded, new fluid channels are exposed at an equal rate to ensure the fluid pressure remains constant.

The fluid delivery passage through the drag bit can be straight. However, it is preferable that this passage is non-linear to increase the strength of the drag bit. This can be achieved by having a "zigzagged" or a spiral passage.

This results in the fluid delivery hole altering its position as the drill bit is worn down, which helps to ensure that all parts of the cutting face are exposed to a grinding action.

Any suitable material, such as tungsten carbide, can be used to make the drag bit and, optionally, cutting elements such as tripaxis, ceramics or natural diamond can be embedded within the bit to provide harder cutting surfaces in the known manner.

The channels should preferably be shaped to provide an even fluid pressure, and hence an even washing (transportation) action across the cutting surface. This can be achieved by creating tapered channels, whose width increases towards the outer radius of the cutting surface.

Alternatively the channels can be constructed with a concave end towards the outer radius of the cutting surface. Other shapes are also possible.

Conventional drag bits have a single central fluid delivery hole so that fluid is distributed evenly across the cutting surface. However, this results in an area of the cutting face opposite the delivery hole not being subjected to a grinding action. As noted above, this problem may be overcome by means of a zigzagged or spiral fluid delivery passage. However,a straight but offset passage is also effective.

Therefore, it is preferable that the fluid delivery passage in the bit of the present invention is offset from the central axis so that every area of the cutting face is exposed to a cutting action.

This feature is considered to be inventive In its own right and therefore, according to another aspect the present invention, there is provided a rotary drag bit comprising a cutting surface with fluid channels provided therein, the channels being in fluid communication with a fluid delivery hole, wherein the fluid delivery hole is offset from the axis of the bit. The position of this hole may be arranged to vary as the bit is worn down.

This arrangement of the fluid delivery passage ensures that the entire cutting face is exposed to a grinding action during each rotation of the bit and hence increases the effectiveness of the drill bit.

Preferably the width of the passages is determined in order to compensate for the offset delivery hole so that an even transporting affect is achieved.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the following figures, in which: FIGs IA-C are schematic sectional views showing a S conventional drag bit in operation; FIG 2 shows a perspective view of a drag bit according to a first embodiment of the present invention; FIG 3 shows a plan view of a cutting surface according to the embodiment of Figure 2; FIG 4 shows aside view of a drag bit according to the embodiment of Figure 2; FIGs 5A- B show a drag bit according to a second embodiment in operation; FIGs 6A- C are sectional views of details of a number of embodiments of the invention show various methods of blocking fluid channels; and FIG 6D shows an alternatively shaped fluid channel.

FIG 1A shows a conventional drag bit 120 in operation.

The drag bit 120 is situated at the bottom of a borehole 100. The drag bit 120 is connected to the surface by drill string 125 where (via the conventional apparatus) it is ultimately powered by a motor either at surface or subsurface. The drill string also carries fluid 130 down to the drag bit 120 via fluid delivery passage 126. The cutting surface 121 of the drag bit is in contact with the bottom of the borehole 100, referred to as the cutting face 110. The cutting surface 121 comprises a plurality of fluid channels 123 which are connected to the fluid delivery passage 126. This allows fluid 130 to flow out over the cutting surface 121 to clean, cool and lubricate the surface. The fluid 130 then moves up the borehole 100, carrying the cuttings to the surface. This is not shown for clarity.

The weight of the drill string is used to force the drill bit 120 into the cutting face 110 of borehole 1()0.

Because of the great length of a typical drill sting, this force uncontrolled would be too great and so the dill bit is partially suspended, creating an upwards force S which partially counterbalances the gravitational force acting on the bit 120 and the drill string. The fluid 130 is pumped through the fluid delivery passage 126 at high pressure, which also applies a slight uplifting force P to the bit 120.

As the drag bit 120 is rotated, the cutting surface 121 grinds at the cutting face 110, gradually penetrating lS further in to the ground. As this occurs, the cutting surface is worn down, reducing the depth of the channels 123 (see FIG 1B). This increases the fluid pressure within the channels 123 and hence the upward force on the bit 120 is increased by amount x. To compensate for this the suspension of the bit 120 is reduced by an equivalent amount to ensure the cutting surface 121 is still in contact with the cutting face 110, i.e. the weight on bit is increased.

Eventually, the channels 123 are completely worn away, leaving a smooth cutting surface 121. When this occurs the pressure of the fluid 130 forces the cutting surface 121 away from the cutting face 110 and drilling can no longer continue. The drag bit 120 must then be removed from the borehole 100 and replaced, which can take up to several days.

FIG 2 shows a drag bit 10 according to an embodiment of the present invention. As with the conventional drag bit, drag bit 10 comprises a cutting surface 12 for contact with a cutting face. The cutting surface 12 can also be seen in FIG 3 and comprises a series of fluid channels 14 radiating from a fluid delivery hole 16. The channels 14 form ridges 18 in the cutting surface 12 which abrade the cutting face.

Fluid is delivered to the cutting face 12 via the delivery hole 16 and channels 14 in the same manner as a conventional bit.

Unlike the conventional bit, however, a drag bit in accordance with the preferred embodiment has a plurality of unexposed fluid channels. The unexposed fluid channels are arranged such that new channels are exposed prior to the total erosion of all the exposed fluid channels. In this way the situation described in FIG 1C does not occur when the initially exposed channels are worn away. Instead, the drag bit 10 is able to continue drilling the borehole. This greatly increases the operational life of the drag bit.

In a further embodiment, the fluid delivery hole 16 is offset from the centre of the cutting surface 12 to ensure that all areas of the cutting face are exposed to the grinding/cutting action of the bit. In order to increase the strength of the bit 10 fluid discharge passage 26 is zigzagged so that the position of the discharge hole is staggered. This is best seen in FIG 4 and FIGs 5A and B. Alternatively the passage can be helical.

As can also be seen from these figures, in order to provide an even fluid distribution with the offset delivery hole 16, the channels have differing widths. An even fluid distribution across the cutting surface can be achieved using a variety of channel shapes, including that shown in FIG 6D.

FIGs 5A and B show the drag bit 10 in operation within a borehole 100 with cutting face 110. Cutting surface 12 is in contact with the cutting face 110. Fluid 130 is delivered to the cutting surface 12 by the fluid delivery passage 26 and dispersed along channels 14. During use, the channels 16a gradually wear away. While channels 14 are removed, channels 15a are exposed. Fluid can then flow through channels 15a, preventing the drag bit 10 from lifting away from cutting face 110 and cutting continues.

Prior to the total erosion of fluid channels 15a, fluid channels 15b are exposed, thus allowing the drill bit to continue drilling for much longer than the conventional drill bit.

Prior to being exposed, channels 15 can be blocked to prevent fluid 130 from flowing through these channels and creating a pressure drop at the cutting face.

Methods of blocking channels 15 can be seen in FIGs 6A C. FIG 6A shows a channel filled with soft impermeable material 152 that will erode quickly upon exposure to the cutting face. This material does not need to extend throughout the length of the channel, as long as it provides an adequate blockage to the fluid 130.

A more secure method is shown in FIG 6B. Here, channel contains circumferential indents 151. A peg 152 having circumferential projections is fitted in the channels with the projections located in these indents to block the flow of fluid through the channel. The projections may be formed by forcing the pegs into the channels and allowing them to expand radially. The pegs 152 are made of a material that will erode quickly or shatter upon exposure to the cutting face.

FIG 6C shows a tapered channel 15 in which a solid plug 153 of material is housed. The diameter of the plug is greater than the diameter of the channel at the outer wall of the drill bit 10.

Finally FIG ED shows a concave/convex tapered channel 1.. This shape of channel can be used to ensure an even fluid pressure across the cutting surface.

To manufacture the hit as described above, a metal, e.g. an alloy, core is provided in the conventional manner.

The plug is cast, again in a manner like the prior art, with an important exception. This is that the network of channels must be created. This can be achieved by means of the known lost-wax process.

A mou]d of the channels are made from a material having a lower melting point than the material being used to form the cutting structure. Powdered material is placed around the mould and is heated, usually under pressure to create the drill bit head. During the baking of the drill bit the mould melts. The mould could also be removed by various other means, e.g. removal with acid or by machining.

Another method of producing the different wear layers is to form them separately and join them together to create a single unit.

Claims

Claims: 1. A rotary drag bit comprising at least one exposed fluid channel

and at least one initially unexposed fluid channel wherein the initially unexposed fluid channel is arranged to be exposed as the bit is worn down in use and prior to the complete erosion of the exposed fluid channels.
2. A rotary drag bit as claimed in claim 1, further comprising a plurality of exposed fluid channels.
3. A rotary drag bit as claimed in claims 1 or 2, further comprising a plurality of initially unexposed fluid channels arranged at differing depths within the bit.
4. A rotary drag bit as claimed in claim 1, 2 or 3 wherein the unexposed fluid channel(s) are at least partially blocked.
5. A rotary drag bit as claimed in claim 4 wherein the unexposed fluid channels comprise circumferential indents and are blocked by pegs having portions that engage within said indents.
6. A rotary drag bit as claimed in claim 4 or 5 wherein the non-exposed fluid channels are fully blocked.
7. A rotary drag bit as claimed in any preceding claim wherein the exposed and unexposed fluid channels are connected to a fluid delivery passage, the fluid delivery passage being non-linear in shape.
8. A rotary drag bit as claimed in claim 7 wherein the fluid delivery passage is offset from the rotational axis.
9. A rotary drag bit as claimed in claim 7 or 8 wherein the fluid delivery passage is zigzagged or helical.
10. A rotary drag bit as claimed in any preceding claim wherein the fluid channels are shaped such that the fluid pressure across the surface of the drill bit is uniform.
11. A rotary drag bit as claimed in any preceding claim wherein the drag bit is impregnated with cutting elements.
12. A rotary drag bit comprising a cutting face, the cutting face with a plurality of fluid channels provided therein and a fluid delivery hole, the channels being in fluid communication with the fluid delivery hole, wherein the fluid delivery hole is offset from the rotational axis of the drill.