This paper describes the optimization of a hybrid bit which is a polycrystalline diamond compact (PDC) bit with rolling cone cutters to improve dynamics and durability. When drilling the hard & interbedded formations with conglomerates in the Tarim Basin in Northwest China, PDC bits break down with broken & chipped cutters in the cone, nose & shoulder sections of the profile. In addition severe torsional vibrations lead to twist-off's and stick-slip which damages the bit and drilling system. Even worse, because of the torsional vibration, less Weight on Bit (WOB) is being applied, which results in lower Rate of Penetration (ROP), less footage and more trips. Roller-cone bits generally drill faster than PDC bits, but bearing life is short in the hard and abrasive conglomerates. Due to success in similar application elsewhere in the world, a hybrid bit was proposed and tried to drill the challenging formations in the Tarim Basin. In the first run the entire conglomerate interval was drilled at higher ROP, with lower torsional oscillations and saving 16 rig days. Although this was a success there was an opportunity for further improvement by strengthening the PDC cutting structure to avoid premature cutter damage mainly in the central, cone section of the profile. An upgraded hybrid bit with split, overlapping blades and more durable Dual Chamfer cutters was deployed in the same interval. Respective performance of PDC, Roller Cone, Hybrid bit and Optimized Hybrid bit used in the same formation are compared in terms of footage, ROP, Mechanical Specific Efficiency (MSE) and dull condition. By using Optimized Hybrid bit, the customer saved multiple bit runs & drilled eight times farther with the ROP 92% faster than conventional PDC bit run in the same well.
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