Technical Paper: Reducing Risk and Cost in Diverse Well Construction Applications: Eccentric Device Drills Concentric Hole and Offers a Viable Alternative to Underreamers

Society: SPE
Paper Number: 92623
Presentation Date: 2005
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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. Some drilling engineers may be surprised by part of this papers' title - an eccentric device drills concentric hole and offers a viable alternative to underreamers - as there is a common perception that concentric hole can only be attained through concentric cutting mechanisms. Before proving this perception inaccurate, it is worth defining certain terms. For the purposes of this paper, the following terms mean 'the opening of a well-bore after passing through a restriction'.

  1. Underreaming
  2. Hole enlargement
  3. Reaming while drilling
  4. Simultaneous underreaming
  5. Drilling with a bicentre/eccentric bit
  6. Hole-opening

However, 'Hole-openers' as downhole tools are beyond the scope of this study due to their nature. They have fixed pre-determined diameters and are best suited for top-hole applications as they can not pass through restrictions or routinely underream several thousand feet or more (Ref 1 & 18). The perception surrounding concentricity arises from differences between Eccentric Underreamers (E-UR) and Concentric Underreamers (C-UR).

In the case of E-UR, which are integral underreamers or evolutions from bicentre bits, the perception is based on the following:

  1. Eccentric bits have an unbalanced and eccentric cutting action.
  2. An unbalanced cutting action is unlikely to result in good drilling dynamics.
  3. Therefore, an eccentric cutting action is unlikely to provide concentric hole.

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