Formation Compressive Strength Estimates for Predicting Drillability and PDC Bit Selection

Abstract

Various indirect measuring techniques are presently employed for estimating formation strength, which in turn is correlated to drillability and polycrystalline diamond compact (PDC) bit selection. Formation mechanical properties derived from conventional open-hole logs in combination with a rigorous assessment of formation shear wave velocities can be used successfully to determine drillability as it pertains to bit selection. Formation drillability is best determined from unconfined compressive strength and the angle of internal friction assessment. Rock strength is found to correlate well with the overall measures of bit effectiveness. Further, the angle of internal friction, which is a subsidiary rock strength parameter, correlates with PDC bit wear rates. Compressive strength and drillability have been linked in the laboratory and observed in the field since the early 1960's. Compressive strengths, however, need to be qualified as a function of confinement stress. The concept that compressive strength increases with confinement stress is well-understood and can be easily explained with Mohr's failure criteria. With the Mohr's failure technique, it is important to understand that inherent rock strength properties (cohesion and angle of internal friction) must be known before compressive strengths can be estimated. This paper also presents case histories from Gulf of Mexico wells that illustrate how unconfined compressive strength and angle of internal friction can be employed to indicate drillability as it pertains to optimized bit selection. Introduction Aggressiveness and wear resistance are two fundamental properties that must be considered when selecting a bit for a specific application. For simplicities sake, these two bit properties, while not totally independent, may be considered separately. The aggressiveness of the bit is determined by the depth of cut it is designed to take. In roller cone bits, aggressiveness is determined by projection, pitch of the teeth, and cone offset. In PDC bits, aggressiveness is determined by the exposure of the cutters and the cutter angle (backrake). Wear resistance, on the other hand, is determined by the density of the cutters, especially those on and near the gauge. In roller cone bits, increasing wear resistance is accomplished by adding more gauge cutters; more durable shapes of cutters; applying diamond to the cutters that contact gauge; and modifying carbide grade (at the expense of making cutters more brittle) or increasing the number of carbide inserts on the shirttail. P. 569

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