Increasingly, operators seek to underream. Whether driven by cementing tolerances, ECD improvements, pore pressure fracture gradients, production increases, swelling shales/salts or setting sand screens, underreaming is sought after at all stages of field development.

To date, the industry has a common perception that concentricity can only be delivered through concentric cutting mechanisms. This paper proves otherwise. The performance, risks and suitability of an eccentric underreaming device are compared with concentric underreamers. Usage in over 100 well sections is tabulated and reviewed to verify the device drills concentric hole in differing formations and applications.

Pilot bit and underreamer cutter characteristics are matched for directional control, durability and hole quality. Specific attention is paid to N Sea & GOM run history covering usage with push and point-the-bit rotary steerables. In exploratory, deepwater or complex well paths, the device is placed above a 3-D rotary steerable and full logging suite. Here the problem of leaving pilot gauge rathole is also addressed. In enhanced RPM, motor directional applications it is placed as nearbit.

Computational fluid dynamics, nozzle and PDC cutter layouts are also discussed with regard to optimizing cuttings evacuation, hole cleaning, BHA stability and ROP.

In conclusion, 20 different well construction activities are presented and used as a benchmark for the evaluation of the risks and suitability of the device as compared with concentric underreamers.

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