Drilling interbedded formations through alternating soft and hard layers is challenging due to torsional instability that could damage the PDC bit. The traditional PDC bit design approach to overcome this challenge is to use secondary elements that control Depth Of Cut (DOC) and limit torque fluctuations. While this approach can be effective, it limits ROP. This paper presents a unique concept that utilizes advanced cutting elements across the bit to control DOC while improving ROP.

The Design engineering focal group have developed a new PDC bit with an alternate cutter arrangement to drill transitional zones more efficiently. Although PDC bit designs with various configurations of cutters have been utilized, the need remains for an innovative solution to improve stability and ROP. The novel PDC bit has cutters that are strategically placed in the cutting structure with alternating passive and aggressive back rake angles to manage DOC and increase ROP. The strategic cutters arrangement enables the PDC bit to shear the formation effectively from different points of contact in a passive and aggressive manner.

Introducing the new concept in four PDC bits designs helped to achieve a step change in drilling performance in challenging applications that generate extensive torsional vibrations. The initial testing in 16-in vertical section showed 24% ROP improvement, achieving the highest ROP in a well-established field of more than 300 wells. Moreover, the rerun of the same PDC bit achieved the second highest ROP in the same field. Additional testing in lateral sections showed 6 1/8-in PDC bit increased ROP by 42% in offshore, and 5 7/8-in PDC bit drilled the longest footage in Ghawar field, achieving 41% ROP improvement and 14% increase in footage. Subsequent testing in 17-in vertical section showed similar performance with 22 % ROP improvement in south Ghawar.

The developed concept provides an advanced solution to the challenge of drilling with PDC bit through transitional formations. The performance studies demonstrate that PDC bit with alternating cutter back rake angles reduced torsional instability by 30% in interbedded formation, decreased torque fluctuations and delivered a smooth torque response. The new PDC bit also increased the durability and the dull condition by two wear scale. Overall, the innovative cutters configuration helped to enhance drilling efficiency and provided significant cost savings.

This paper demonstrates the value of the new PDC cutting structure layout that can deliver enhanced performance and dull condition in variety of applications where torsional vibrations leads to premature bits failure. The new concept can also be applied to existing designs to further improve performance in transitional applications and reduce drilling cost.

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