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

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 s

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