Formations encountered while drilling oil and gas wells are not homogeneous. This requires drill bits to be capable of drilling the most difficult rocks in the interval rather than the "average" rock and can result in a significant penetration rate penalty for cases where a few hard streaks preclude the use of PDC bits. Bit manufacture are actively trying to improve materials and bit designs to provide more robust bits, but in many cases it appears that some type of drillstring vibration limits the use of PDC bits in these hard streaks. Extensive data from drilling tests conducted at Amoco's Catosa test facility indicate that PDC bits are damaged very rapidly when they eccounter certain hard rocks. The visula appearance of the damage suggests some type of vibration damage, but the normal cures for bit whirl and drillstring stick/slip do not seem to be very helpful. The analysis of nineteen wells where a downhole vibration sensor was run and PDC bits encountered severe damage indicates that torsional resonance of the drill collars could result in backward rotation of the bits.
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