This paper details the improvements to drilling performance and torsional response of fixed cutter bits when changing from a conventional 19-mm cutter diameter configuration to 25-mm cutter diameters for similar blade counts in two different hole sizes. Key performance metrics include rate of penetration (ROP), rerun-ability, torsional response, and ability to maintain tool-face control during directional drilling.
A high-performance drilling application was selected with several existing offset wells using a 12¼-in., five-bladed, 19-mm (519) drill bit design, and a concept bit developed using 25-mm diameter cutters while maintaining comparable ancillary features. This was tested in the same field on both vertical and S-shape sections using the same bent-housing motor assembly and drilling performance compared to the existing offsets. A 17½-in. hole size application that experiences high drillstring vibration was also selected, and a 25-mm cutter diameter drill bit was designed with comparable ancillary features to replace a six-bladed, 19-mm (619) drill bit. This was tested in the same field with the drilling performance, and vibration propensity was assessed.
Initial testing in the 12¼-in. section showed extremely promising initial results, breaking the field ROP record in a well-established field of more than 3,000 wells. The rerun of the same bit without repair placed fourth in the field in terms of ROP records. Additional testing in the vertical and s-shape sections showed the new 25-mm cutter diameter design consistently exceeding the ROP performance of the 519 drill bit design while achieving directional targets without any reported drilling concerns. Subsequent trials with other operators saw similar performance improvement with multiple instances of breaking field ROP records.
The first trial of the new 17½-in. hole size design with 25-mm diameter cutters had 34% average higher ROP than the offset average ROP, achieving the field ROP record. An overall 70% improvement during trials was seen in ROP versus the existing 619 drill bit design. The daily drilling reports and client feedback reported a significantly reduced level of drillstring vibration versus offset wells.
This paper demonstrates the potential for a paradigm shift in drilling response and overall ROP by using 25-mm diameter cutters on fixed cutter bits. When correctly modeled, designed, and selected for specific applications, they benefit operators by reducing the time it takes to drill the section, improving repairability, reducing the time that an openhole is left exposed, and reducing drilling costs.