Development of a Whirl-Resistant Bit


Warren, Thomas M.
Brett, J. Ford
Sinor, L. Allen

The detrimental effects of impact loading on PDC bits have long been recognized, but most previous discussions of PDC bit wear have concentrated primarily on thermal effects. Amoco Production Co.'s field tests have shown that cutter failure, especially early in the life of a bit, is more likely to be caused by impact damage than by thermal effects. Impact damage is sometimes difficult to observe because it often precedes and is destroyed by the subsequent thermally accelerated wear that is frequently evident when dull bits are pulled. A reduction in the frequency of broken and chipped cutters, which accelerate cutter wear, would allow longer bit runs, faster rates of penetration (ROP's), and possibly cheaper bits because fewer diamond cutters would be needed. Brett et al. describe bit whirl and show that it is the predominant cause of impact loading. Whirl is defined as a predominant cause of impact loading. Whirl is defined as a condition where the instantaneous center of rotation moves about the bit face as the bit rotates. This type of loading chips cutters and accelerates cutter damage and wear for PDC bits. The objective of the research presented here was to extend use of PDC bits into rocks that are too "ratty" (i.e., inhomogeneous) for acceptable performance from current PDC designs. Most of the field testing was conducted at the Catoosa test facility near Tulsa. Warren and Canson describe this test rig, and Winters et al describe the site's geology.

Bit Stability

bit balance can determine the force pushing the bit away from a constant point of rotation, but it cannot tell whether the bit will have a tendency to return to or move farther away from that point when it is displaced. This limitation of the static analysis results from the assumption that the cutter forces are constant for a full revolution of the bit. The restoring force necessary for a stable bit design can potentially result from forces that act on the drill collar above the potentially result from forces that act on the drill collar above the bit, from features that are built into the cutting structure of the bit, or from stabilizer pads on the bit. No matter how the restoring force is created, a relatively large force for a small displacement is required to prevent whirl.

Stabilization Above the Bit Face

laterally. This provides a damping that may reduce the effects of bit whirl at higher inclinations. As Brett et al. discuss, high rotational speeds increase the tendency for a bit to whirl, resulting in much larger side forces and displacements. In most situations, these create a greater tendency for a bit to whirl when it is run on a downhole motor than when it is rotated by the drillstring. There is also a greater tendency for the motor stabilizers to hang up on ledges caused by intermittent bit whirl. In cases where a motor is run and the drillstring is rotated, an opportunity exists to uncouple the cutting of the final hole diameter from the bit. This can be accomplished by use of a slightly undersize bit and by stabilization of the motor with a radial stabilizer preceded by an axial cutting section to cut the final borehole wall, as shown in Fig. 3. Because the motor is rotated more slowly than the bit, the tendency to whirl is reduced. Consequently, the stabilizers have a much better chance of making tight contact with the wellbore wall.



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