About PetroSKills






Bit WhirlA New Theory of PDC Bit Failure


Brett, J. Ford
Warren, Thomas M.
Behr, Suzanne M.
Amoco Production Co.

This paper presents the results of a study showing that the most harmful polycrystalline-diamond-compact (PDC) bit vibrations can be attributed to a phenomenon called "bit whirl." During whirl, the instantaneous center of rotation moves around the face of the bit, and the bit whirls backward around the hole. Cutters on a whirling bit can move sideways, backward, and much faster than those on a true rotating bit. The impact loads associated with this motion cause PDC cutters to chip, which, in turn, accelerates wear. Laboratory and field results show the detrimental effects of whirl on PDC bit rate of penetration (ROP) and life.

Limits of PDC Bit Application

a good percentage of total rotating hours occurs in slower drilling situations where the ROP advantages of PDC bits cannot be consistently achieved. The degree to which PDC bits have penetrated the bit market demonstrates this fact. Even in the face of an aggressive program to apply PDC bits to operations, only 4% of the bits Amoco Production Co. purchased in North America during 1987 were PDC. Even though PDC bits represented just 4% of the total number of bits Amoco purchased, they accounted for some 15% of the total footage drilled, which shows that most PDC bit use was confined to softer intervals. PDC bits clearly have had a dramatic impact on the soft, faster-drilling formations, but they have yet to replace three-cone bits in most slower drilling situations (i.e., under 25 ft/hr). Moreover (to the extent that Amoco's North American drilling activity is representative of the entire industry), Fig. 3 shows that 60 to 80% of all rotating hours occur at rates less than 25 ft/hr. Most of the footage is drilled at high ROP's, but most of the time is spent drilling relatively slowly. This statistic is similar for Amoco's New Orleans and Denver regions, which shows that even in widely different geologic provinces (i.e., deltaic to midcontinental), most of the drilling is done at slow ROP's. If the ROP advantages of PDC bits could be consistently applied to harder intervals, the number of hours spent drilling each year would be greatly reduced.

Sharp and Dull PDC Cutting Mechanisms

With clear water, the pressure is partially equalized and the penetration per revolution pressure is partially equalized and the penetration per revolution decreases, but not as much as with the mud.