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.

What is claimed is:  
     
         1 . A rotary drag-type drill bit in which a relationship of torque to rotary bit speed is such that torque generally increases with bit speed.  
     
     
         2 . method of designing a rotary drag-type drill bit by ascertaining the depth of cut/rotary bit speed relationship for a plurality of different drill bit designs and selecting from those designs a design where a depth of cut/rotary bit speed relationship is such that depth of cut generally increases with bit speed.  
     
     
         3 . A rotary drag-type drill bit in which the relationship between the overall cutting efficiency of the bit and the rotary speed of the bit is such that the overall cutting efficiency falls with increasing rotary speed.  
     
     
         4 . A rotary drag-type drill bit comprising a bit body having an axis of rotation, a plurality of blades upstanding from the bit body, a plurality of primary cutters and a plurality of secondary cutters, the primary and secondary cutters being mounted upon the blades, wherein the plurality of secondary cutters are mounted at a lower cutting height than the plurality of primary cutters.  
     
     
         5 . A drill bit as claimed in    claim 4   , wherein the secondary cutters are mounted at a cutting height spaced from that of the primary cutters by a distance falling within the range of about 5% to about 10% of a radius of one of the primary cutters.  
     
     
         6 . A drill bit as claimed in    claim 4   , wherein the secondary cutters are orientated to have a higher cutting efficiency than the primary cutters.  
     
     
         7 . A drill bit as claimed in    claim 6   , wherein the secondary cutters have a back rake angle lower than the primary cutters.  
     
     
         8 . A drill bit as claimed in    claim 6   , wherein the secondary cutters have a side rake angle lower than the primary cutters.  
     
     
         9 . A drill bit as claimed in    claim 4   , wherein the secondary cutters are shaped to have a higher cutting efficiency than the primary cutters.  
     
     
         10 . A drill bit as claimed in    claim 9   , wherein a radius of the secondary cutters is smaller than that of the primary cutters.  
     
     
         11 . A rotary drag-type drill bit comprising a bit body having an axis of rotation, a plurality of blades upstanding from the bit body, and a plurality of cutters carried by the blades, wherein the drill bit has a cutting efficiency which varies with the depth of cutting.  
     
     
         12 . A drill bit as claimed in    claim 11   , wherein the cutters consist of a plurality of primary cutters mounted at a first cutting height and a plurality of secondary cutters mounted at a second, lower cutting height, the secondary cutters having a higher cutting efficiency than the primary cutters.  
     
     
         13 . A drill bit as claimed in    claim 12   , wherein the secondary cutters are mounted at a cutting height spaced from that of the primary cutters by a distance falling within the range of about 5% to about 10% of a radius of one of the primary cutters.  
     
     
         14 . A drill bit as claimed in    claim 12   , wherein the secondary cutters are orientated to have a higher cutting efficiency than the primary cutters.  
     
     
         15 . A drill bit as claimed in    claim 14   , wherein the secondary cutters have a back rake angle lower than the primary cutters.  
     
     
         16 . A drill bit as claimed in    claim 14   , wherein the secondary cutters have a side rake angle lower than the primary cutters.  
     
     
         17 . A drill bit as claimed in    claim 12   , wherein the secondary cutters are shaped to have a higher cutting efficiency than the primary cutters.  
     
     
         18 . A drill bit as claimed in    claim 17   , wherein a radius of the secondary cutters is smaller than that of the primary cutters.