Reducing Risk and Cost in Diverse Well Construction Applications: EccentricDevice Drills Concentric Hole and Offers a Viable Alternative to Underreamers

摘要

Increasingly, operators seek to underream. Whether driven bycementing tolerances, ECD improvements, pore pressurefracture gradients, production increases, swelling shales/saltsor setting sand screens, underreaming is sought after at allstages of field development.To date, the industry has a common perception thatconcentricity can only be delivered through concentric cuttingmechanisms. This paper proves otherwise. The performance,risks and suitability of an eccentric underreaming device arecompared with concentric underreamers. Usage in over 100well sections is tabulated and reviewed to verify the devicedrills concentric hole in differing formations and applications.Pilot bit and underreamer cutter characteristics are matchedfor directional control, durability and hole quality. Specificattention is paid to N Sea & GOM run history covering usagewith push and point-the-bit rotary steerables. In exploratory,deepwater or complex well paths, the device is placed above a3-D rotary steerable and full logging suite. Here the problemof leaving pilot gauge rathole is also addressed. In enhancedRPM, motor directional applications it is placed as nearbit.Computational fluid dynamics, nozzle and PDC cutter layoutsare also discussed with regard to optimizing cuttingsevacuation, hole cleaning, BHA stability and ROP.In conclusion, 20 different well construction activities arepresented and used as a benchmark for the evaluation of therisks and suitability of the device as compared with concentricunderreamers.Some drilling engineers may be surprised by part of thispapers’ title - an eccentric device drills concentric hole andoffers a viable alternative to underreamers - as there is acommon perception that concentric hole can only be attainedthrough concentric cutting mechanisms. Before proving thisperception inaccurate, it is worth defining certain terms. Forthe purposes of this paper, the following terms mean ‘theopening of a well-bore after passing through a restriction’.1. Underreaming2. Hole enlargement3. Reaming while drilling4. Simultaneous underreaming5. Drilling with a bicentre/eccentric bit6. Hole-openingHowever, ‘Hole-openers’ as downhole tools are beyond thescope of this study due to their nature. They have fixed predetermineddiameters and are best suited for top-holeapplications as they can not pass through restrictions orroutinely underream several thousand feet or more’ (Ref 1 &18).The perception surrounding concentricity arises fromdifferences between Eccentric Underreamers (E-UR) andConcentric Underreamers (C-UR).In the case of E-UR, which are integral underreamers orevolutions from bicentre bits, the perception is based on thefollowing:1. Eccentric bits have an unbalanced and eccentriccutting action.2. An unbalanced cutting action is unlikely to result ingood drilling dynamics.3. Therefore, an eccentric cutting action is unlikely toprovide concentric hole.While in the case of C-UR, which have moving parts andevolved from arm-type underreamers, it is essentially:C-UR have a balanced and concentric cutting mechanism.1. A balanced cutting mechanism is likely to result ingood drilling dynamics.