Development of a Whirl-Resistant Bit
- Thomas M. Warren (Amoco Production Co.) | J. Ford Brett (Amoco Production Co.) | L. Allen Sinor (Amoco Production Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- December 1990
- Document Type
- Journal Paper
- 267 - 275
- 1990. Society of Petroleum Engineers
- 5.3.4 Integration of geomechanics in models, 1.10 Drilling Equipment, 1.6.1 Drilling Operation Management, 1.12.6 Drilling Data Management and Standards, 1.6.6 Directional Drilling, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.6 Drilling Operations, 1.5 Drill Bits, 1.5.1 Bit Design
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Summary. Bit whirl is a major cause of early failure and reduced performance of polycrystalline-diamond-compact (PDC) bits. Attempts to performance of polycrystalline-diamond-compact (PDC) bits. Attempts to control bit whirl by stabilizing the drillstring have been unsuccessful, but a "low-friction" bit design has been discovered that substantially eliminates whirl. The low-friction design is based on placing the cutters so that the net imbalance force from the cutters is directed toward a smooth pad that slides along the wellbore wall.
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.
Several papers discuss the desirability of producing a bit with a balanced cutting structure. A balanced design is one in which the cutting forces acting on the cutters can be resolved into an axial force or weight on bit (WOB), moment about the bit centerline (bit torque), and a near-zero radial force called the bit imbalance force. Because the magnitude of the imbalance force is almost directly proportional to the WOB, the imbalance force is normally referred to as a percentage of the WOB. Reasonably good analytical tools are available to evaluate the balance of a particular bit design. These tools provide a static evaluation based on the assumption that the loads on all cutters are constant for a full revolution of the bit. A general trend in the industry is to apply these analytical tools to design bits with lower degrees of imbalance. Our measurements of numerous bits indicate that a highly balanced commercial bit might be 2% imbalanced; however, 10% imbalanced is more typical, and values greater than 15% are not unusual. The imbalance of a particular bit design may vary considerably from bit to bit as a result of manufacturing tolerances. Although some evidence indicates that improvements in bit balance will improve performance, low static imbalance alone is not sufficient to prevent whirl. Figs. 1 and 2 show the bottomhole pattern and the spectrum data for a bit with only 2% imbalance. There pattern and the spectrum data for a bit with only 2% imbalance. There is no doubt that this bit whirled. Other similar tests confirm that a low static balance will not prevent whirl. In the case of bit whirl, the instantaneous center of rotation continues to move around the bit face. A dynamically stable bit must have its center of rotation at a fixed point on the bit that is a node of stability. Any perturbation from this point must be resisted by a restoring force that moves the bit back to its original position. The static analysis of 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
In most rotating machinery, the problem of vibration and whirl is minimized by ensuring that the rotating member is properly aligned, balanced, and adequately confined with tight-fitting, properly spaced, low-friction bearings. A similar solution to bit whirl could exist for drilling assemblies, where stabilizers are used as the bearings. If the stabilizers fit tightly in the borehole, then the stiffness of the drill collar is generally large enough to generate restoring forces. Unfortunately, this often is not the case because the hole is enlarged and/or because slightly undergauge stabilizers are used. One major difference between most rotating systems and a drillstring is that the drillstring moves axially along a path cut by a member on the rotating system. If for any reason the hole becomes slightly overgauge, the bearings (stabilizers) fit loosely in the hole and the string is unconstrained for small displacements. Once the bit is even slightly unconstrained, it begins to whirl and the hole becomes more overgauge. Once started, the process is self-perpetuating. Stabilization mechanisms above the bit must fit tightly in the hole to prevent whirl and must not hinder the axial progression of the string. These two requirements are somewhat contradictory. The bottom line is that the conventional bit and stabilizer system cannot reliably prevent whirl. Any perturbation at the bit that causes even the slightest overgauge hole will reduce the stabilizing benefit and the hole may be further enlarged as the bit progresses. The very nature of the system where the bit and stabilizers rotate on a common shaft dictates that the stabilizers cannot prevent bit whirl. In a vertical hole, little force is required to displace the drillstring laterally if it is not confined by the stabilizers or forces on the bit. In a directional hole, a force determined by the drill-collar weight, stiffness, and borehole inclination is required to move the string 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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