Recent experience in an Occidental Petroleum Corp. (Oxy) well off Albania shows how custom design and manufacturing of bits allow drillers can optimize each application.
RBI-Gearhart, Fort Worth, Tex., provided the single-cone bit for the Shpiragu-1 well. Gearhart says that while a robust design and stringent manufacturing technique can deliver rock bits with excellent performance and durability, finding the right combination of design features and matching them with the formation to be drilled, the mud program, and well plan can be problematic because each well is different.
RBI-Gearhart designed the bit with Oxy and Novatek Inc., Provo, Utah, the PDC cutter manufacturer, within a 5-day time frame.
Designing the bit
RBI-Gearhart says it uses a two-phase process, as follows, for determining the right bit.
1.The customer's drilling engineers and personnel work interactively with RBI-Gearhart engineers to identify the objective or base-line performance expected and all drilling parameters to use until an agreement is reached concerning the bit requirements and applicable special features.
2.The second phase involves the design engineers working with manufacturing experts to identify the required processes that result in the close tolerances necessary to ensure durability and consistency of operation throughout the bit's life. At the same time, every process is reviewed to minimize manufacturing time. The close cooperation with the customer's drilling engineers and personnel is maintained throughout the process.
RBI-Gearhart says, "The progress of each custom-designed bit is communicated so that any last minute information from the customer's drilling personnel can be evaluated against the design and revisions made."
Oxy ran a custom-designed 61/2-in. B67 DDLDRGDSP bit (photo on the left) during the 59th bit run in the Shpiragu-1 well (Fig. 1).After reviewing the drilling data that were provided by Oxy's drilling personnel, RBI-Gearhart found that the Eliminator one-cone bit would be the best fit for this project based on its knowledge of rock bit technology and a review of offset records from the area that indicated that the application required a fully diamond enhanced bit.
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RBI-Gearhart selected a 61/2-in. B67 DDLDRGDSP bit for the 59th bit run in the Shpiragu-1 well (Fig. 1). The formation was an Upper Cretaceous carbonate with interbedded chert, a 30,000-50,000 compressive strength formation.
RBI-Gearhart says that it developed this series of bits for slimhole applications and that this series has the following potential advantages over other small diameter three-cone and fixed cutter bits:
- Same proven technology as its journal-bearing motor bits.
- A cone-seal speed of about one-half that of a cone on a three-cone bit.
- A significantly larger cone than on a three-cone bit.
- A bearing capacity significantly greater than a three-cone bit.
- Larger and tougher inserts.
- A drilling torque significantly less than fixed cutter bits so that the life and performance of small diameter motors should be improved.
- Standard diamond-enhanced inserts on nose rows that are subject to heavy wear.
- Good response in applications requiring steering.
Drilling results
Mike Oldenburg, project drilling manager for Oxy, said the previous slimhole three-cone bits in the well were pulled based on the recommended cone revolutions for this product's size range.
In Shpiragu 1, the single-cone 61/2-in. B67 drilled from 5,180 m to 5,231 m, or 51 m, which was nearly three times the average length drilled by three-cone bits, at a slightly higher penetration rate, according to Oldenburg.
He explained that the average cone revolutions for the slimhole three-cone bits were 246,000 compared to 712,000 cone revolutions for the one-cone bit after the 51-m run. He added that after the run, the driller noted that the bit was fully in gauge and the bearing was still effective (Fig. 2).
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Oldenburg estimated that the single run saved $153,953 on the Shpiragu-1 well, which more than exceeded expectations (Fig. 3).