Bit Whirl: A New Theory of PDC Bit Failure

Abstract

Abstract This paper presents the results or a study investigating PDC bit vibrations. The study shows that the most harmful vibrations can be attributed to a phenomena called "bit whirl". During whirl the instantaneous center of rotation moves around the face of the bit, and the bit backwards whirls around the hole. Cutters on a whirling bit can move sideways, backwards, 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, causes accelerated wear. Lab and field results showing the detrimental effects of whirl on PDC bit penetration rate and life are included. LIMITS OF PDC BIT APPLICATION Since their introduction in the early 1970s, Polycrystalline Diamond Compact (PDC) bits have almost completely replaced tricone bits in relatively soft, and in nonabrasive formations. They have sometimes replaced tricone bits in harder or slower drilling intervals - if the section is uniform. It is fair to say, however, that drillers do not normally consider selecting a PDC bit when drilling in harder formations or even in soft formations with infrequent hard streaks. The problem with running PDC bits in hard formations is not the penetration rates. Adequate penetration rates are possible, at least for short periods of time. The problem is that the bit life is too short. Figure 1 shows that a sharp PDC bit can achieve penetration rates several times that of tricone bits in Carthage marble. Carthage marble has an unconfined compressive strength of approximately 16k psi and is stronger than rocks usually drilled with PDC bits. Yet, a new PDC bit will typically drill two to three times faster than the best tricone effort.