Rotary drag-type drill bits and methods of designing such bits

Abstract

The invention provides a rotary drag-type drill bit in which the relationship of torque to rotary bit speed is such that torque generally increases with bit speed. Such a bit has a reduced tendency to exhibit the phenomenon known as "stick-slip" while drilling, when compared with bits having a different torque / speed characteristic, as a result of the positive damping effect of the torque / speed relationship. One method of designing such a drill bit comprises ascertaining the torque / rotary bit speed relationship, or other correlated relationships, for a number of different drill bit designs and then selecting from those designs a design having the desired type of relationship. Examples of drill bit structures specifically designed to achieve the desired torque / speed relationship are also disclosed.

Description

[0001] This is a continuation-in-part of U.S. patent application Ser. No. 09 / 210,518, filed Dec. 11, 1998, by Malcolm Taylor, et al., entitled "Rotary Drag-Type Drill Bits and Methods of Designing Such Bits" now pending.[0002] 1. Field of the Invention[0003] The invention relates to the design of rotary drag-type drill bits for use in drilling holes in subsurface formations.[0004] 2. Description of the Related Art[0005] As is well known, drag-type drill bits for boring earth formations comprise a bit body having a shank for connection to a drill string and a plurality of fixed cutters mounted on the bit body. A passage in the bit body supplies drilling fluid to nozzles in the surface of the bit for cleaning and cooling the cutters. In one common form of bit, a bit body has a leading face which comprises a number of circumferentially spaced blades extending outwardly away from the central axis of rotation of the bit, cutters being mounted along each blade. In polycrystalline diamond ...

AI Technical Summary

Benefits of technology

[0020] In using these methods, the design of a bit exhibiting a low tendency to stick-slip, in accordance with the invention, is passive in that a bit design having the required characteristics is selected from a plurality of alternative bit designs which have been designed using other parameters. However, the present invention also allows drill bits or associated downhole components to be actively designed specifically to achieve the desired characteristics which have been found to reduce the tendency to stick-slip.

[0024] Alternatively, or additionally the device may be adapted to increase the depth of cut of cutters mounted on the drill bit with increasing rotary bit speed.

[0025] In an alternative arrangement, the rotary bit speed responsive device may comprise means to increase the effective cutting diameter of the drill bit with increasing rotary bit speed. Such increase in effective cutting diameter will also increase the bit torque.

[0031] In use, as the rotary speed of the drill bit falls due to the stick-slip behavior of the bit, the depth of cut will tend to increase. The increase in cutting depth will result in the proportion of cutting undertaken by the relatively efficient cutters increasing, improving the overall efficiency of cutting. This change in the overall efficiency of the drill bit will tend to dampen slowing of the drill bit. If the bit speed increases, then the relatively efficient cutter will perform less cutting, reducing the overall efficiency and damping the increase in bit speed.

[0032] It will be appreciated that such damping will tend to reduce stick-slip behavior.

Problems solved by technology

It is well known that rotary drag-type drill bits may under certain conditions, particularly at low rotary speeds, and high weight-on-bit, be subject to torsional vibration as a result of a phenomenon commonly referred to as "stick-slip".

In stick-slip the situation arises where the bottomhole assembly is rotating more slowly than the upper end of the drill string due, for example, to frictional torque acting on the bottomhole assembly, with the result that the drill string begins to wind up.

This transfer of torsional energy from the drill string to the bottomhole assembly can occur periodically giving rise to torsional vibrations.

Such torsional vibration is undesirable since it can lead to rapid wear of PDC bits, particularly in harder formations, due to damage to the cutters as a result of impact loads caused by the torsional vibration.

The effect of such reverse rotation on a PDC cutter may be to impose unusual loads on the cutter which tend to cause spalling or delamination, i.e. separation of part or all of the polycrystalline diamond facing from the tungsten carbide substrate.

This change in the overall efficiency of the drill bit will tend to dampen slowing of the drill bit.

If the bit speed increases, then the relatively efficient cutter will perform less cutting, reducing the overall efficiency and damping the increase in bit speed.

Images