Field Testing of Low-Friction Gauge PDC Bits

20416

Sinor, L.A.
Brett, J.F.
Warren, T.M.
Behr, S.M.

Summary
This paper presents the results of developmental work applied to modify "off-the-shelf" polycrystalline-diamond-compact (PDC) bit designs to incorporate low-friction-gauge technology. The low-friction gauge limits harmful bit-whirl tendencies by directing the radial bit imbalance to a gauge pad that makes smooth sliding contact with the wellbore wall, Field testing indicates that use of a low-friction gauge improves bit life and rate of penetration (ROP). penetration (ROP). Introduction

Computer modeling, laboratory testing, and field drilling have shown that bit whirl contributes significantly to PDC bit failures in the field. I Bit whirl occurs when the instantaneous center of rotation is not constant but moves around the bit face as the bit rotates. This type of motion causes high-impact loading and chips the cutters. It is detrimental especially in harder drilling situations where the rock is competent enough to withstand significant impact loading. PDC bit runs that encounter hard streaks often result in catastrophic bit failure. Previous papers 1,2 discuss the theory and existence of bit whirl and testing of various potential solutions to eliminate it. One solution, the use of a low-friction gauge, was the most promising for prevention of bit whirl and extension of bit life. promising for prevention of bit whirl and extension of bit life. The low-friction concept provided that the cutter imbalance force be directed to an area devoid of cutters that included a smooth pad that made sliding contact with the borehole wall. This interrupted the movement of the center of rotation necessary to sustain whirl. This paper presents the results of work conducted to evaluate low-friction-bit performance under field drilling conditions.

Catoosa Field Testing

The only clue to the failure was a real-time strip-chart recording of the rotary torque that showed an instantaneous 2,200-ft-lbf increase in torque. The ROP was 260 ft/hr for several feet before the torque spike; then ROP reduced slightly. The bit most likely encountered a hard nodule that damaged the cutters. If the bit was damaged by cutter loading and not vibrations, it should have been able to drill with minimal reduction in ROP as long as its antiwhirl stability had not diminished. The bit was rerun to see whether the damaged PDC cutters would self-sharpen as drilling progressed. progressed. After being rerun, the bit drilled successfully to 1,260 ft and was pulled for cutter inspection. Photographs showed small wear flats on the previously damaged cutters and diamond "lips," which proved that the cutters had in fact self-sharpened. These lips are not present on whirling bits. PDC bits without these lips are essentially dull and will not drill hard intervals effectively. The bit was run back into the hole and drilled to 1,445 fl, where it was pulled because wear on the low-friction-gauge pad was causing the bit to hang up. The bit had minimal wear out to the gauge, which is remarkable considering that the bit was set lightly and had only three cutters cutting gauge. To ensure that superior performance of Bit B was not a fluke, Bit A was repaired by turning the chipped cutters 180' and replacing those that were severely damaged. Bit A then was modified to be low-friction and was run on a subsequent hole. Drilling began just below the Pink limestone and continued to 1,730 ft deep before stopping because of gauge wear. Fig. 3 compares the ROP and footage drilled during the three bit runs. The drilling depths (1,445 and 1,730 ft) of the two low-friction bits broke all previous PDC-bit-depth records at Catoosa.