Balancing aggressiveness, stability and durability bit behaviours is the principal engineering challenge when designing an application-specific PDC bit. The application this paper addresses is a directionally drilled interbedded abrasive formation that generates dysfunctional drilling torque. This paper demonstrates the engineering process of designing a bit that balances the three major bit behaviours, thereby optimizing drilling efficiency to meet the goals of successfully drilling the required interval while improving rates of penetration.
West Kuwait's 12.25" section primarily involves interbedded abrasive sandstone and hard shale sublayers that induce torque fluctuations. The combined result is impact and abrasion wear on cutters causing mid-run reduction in rates of penetration, prematurely terminating the run. The section involves a J-type directional profile drilled using an adjustable-bend mud motor or a rotary steerable system. It starts off drilling vertically till the kick off point followed by building up to the tangent section and holding angle to section end. The high torque and vibrations faced in this application make steering difficult for any directional drive type; further compounding the challenge to achieve higher rates of penetration.
Building a solution to match the application involved:
Reviewing bit dulls and studying locations of extensive wear on the bit body
Comparing current design performance against success metrics in the target application
Bit design requirements for specific rotary steerable systems and mud motors to build a universal design that works with all major drive types
Consideration of different torsional stability technologies and their appropriateness for our application
Field trials across different drive types
The engineering process concluded with a seven bladed PDC bit with aggressive cutting structure that successfully completes the interval while improving rates of penetration by over 50%. Trials were implemented across several wells involving the major drive types to verify the new design's effectiveness versus previous designs. Most trials proved the bit was more efficient in directing mechanical specific energy into the formation rather than squandering drilling energy on damaging vibrations. This resulted in rates of penetration higher than the field average through better bit stability and smoother steerability on all directional drive types used in the application.
This paper highlights the engineering process and the technology invested in developing a PDC bit for drilling directional runs through mixed abrasive and impact prone formations.