Geologically heterogeneous (mixed) formations pose a significant challenge to drillers in terms of drilling performance. Shale and other soft and plastically behaving rocks favor the use of PDC bits, while hard and abrasive sandstone with hard cement stringers favors roller cone bits.

With recent advances in PDC cutter technology it is possible to combine the best features of both bit designs to drill formations that constantly fluctuate between hard and soft lithology, and often without notice. This has been achieved by using a basic PDC bit design and replacing the secondary blades on the periphery with rolling cutters equipped with tungsten carbide inserts. The resulting product is frequently referred to as a hybrid bit. A hybrid bit allows the maintenance of overall penetration rate (ROP) at much higher levels than that of a roller cone or PDC bit alone. Avoidance of the necessity of tripping for bits due to formation changes or for premature bit wear is clearly advantageous. Experience from the USA, where up to a 35% reduction in drilling time has been achieved in heterogeneous formations, prompted the use of this technology in Brazil.

A similar challenge exists in NE Brazil with geologically interbedded sandstone and shale sequences. The challenge was to drill the interval with improved ROP and with reduced vibration while simultaneously reducing the need for bit change related trips. Experience in Brazil to date shows that by maintaining a consistent ROP in interbedded formations, aided by better toolface control due to lower torque fluctuations, a 44% decrease in total drilling time has been achieved while run length more than doubled. Future evaluation of this new technology extends to Brazil offshore basins (Campos, Santos, and Espirito Santo) where Albian and Aptian carbonate reservoirs have different lithologies but the challenges of geological heterogeneity are similar.

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