Torsional Resonance of Drill Collars with PDC Bits in Hard Rock

Abstract

Abstract Formations encountered while drilling oil and gas wells are not homogeneous. This requires drill hits 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 manufactures 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 Catoosa test facility indicate that PDC bits are damaged very rapidly when they encounter certain hard rocks. The visual 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. The observed vibration has been known to exist (in a theoretical sense), but is not recognized as a significant cause of PDC bit failure. The Catoosa drilling data shows conclusively that a wide range of PDC bits sustain this type of vibration when they drill hard rocks. Laboratory tests demonstrate that reverse rotation of PDC bits result in catastrophic Cutter failure. The evidence collected so far suggests that reverse rotation due to torsional vibration is probably the cause of the rapid damage in certain hard rocks, but this postulation has not been proven because the sensor used in the tests was not adequately sensitive to say positively that the bit rotates backwards. P. 625