| Popis: |
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 unconfmed compressive strength and angle of internal friction can be employed to indicate drillability as it pertains to optimized bit selection. |
