SPE Member

Introduction

"Bit whirl", the backwards whirling motion of polycrystalline diamond compact (PDC) drill bits, has been identified as a significant contributory factor in the premature failure of PDC bits in the field. Two SPE papers have discussed bit PDC bits in the field. Two SPE papers have discussed bit whirl and the testing of various potential solutions, and one described the field testing of PDC bits modified to produce a "low friction gauge". This paper describes the design and subsequent testing in the laboratory and then in the field of a PDC bit designed from scratch to resist backwards whirling motion by incorporating various design features including the "low-friction gauge" concept described by Warren et al (described below as an "Antiwhirl bit"). It details the importance of the magnitude of the lateral-component of the resultant cutting force on the bit and the angular extent of the bearing pads that react with this force, and describes and explains the directional behaviour of Antiwhirl bits in the field.

ANTIWHIRL BIT DESIGN
General

The 8 1/12" (215.9mm) diameter Antiwhirl PDC bit that is the subject of this paper is shown in figures 1 and 2.

The cutting forces arising from every PDC cutter on the bit are calculated using a mathematical model, which has itself been validated against laboratory tests. They are then summed vertorially to give the expected weight and torque requirements of the bit, together with the magnitude and direction of the lateral component of the resultant cutting force ("out of balance force"). The actual cutter positions on each bit, measured using a coordinate measuring machine, are input to this model to ensure that the performance of the bit is not compromised by any unfavorable build-up of manufacturing tolerance. In each of the laboratory and field tests described below, this measurement confirmed that the bit used had no cutters significantly out of position. The PDC cutters are arranged so that the out off balance force is directed towards two large unaggressive (or "low friction") bring pads. The absence of any aggressive cutting elements in the region of these pads should ensure that the bit does not start to whirl provided that the resultant force is directed towards them. This idea has been used in gun drilling for many years but was only recently introduced to rock drilling by Warren et al.

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