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’.UnderreamingHole enlargementReaming while drillingSimultaneous underreamingDrilling with a bicentre/eccentric bitHole-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:Eccentric bits have an unbalanced and eccentric cutting action.An unbalanced cutting action is unlikely to result in good drilling dynamics.Therefore, an eccentric cutting action is unlikely to provide concentric hole.

New sponge coring service and technology have been introduced to the oil and gas industry. Admittedly sponge coring has been an industry offering in some form for over twenty years but until now it has been plagued by chronic problems. The objective of sponge coring was to determine separate in-situ oil and water saturations of the formation materials. The problem with conventional coring has always been that the fluids would be expelled and lost from the core by the expanding gas while bringing it to the surface. The long-time solution was to surround the core with a special oil-absorptive (oleophilic) sponge material that would capture the expelled oil and hold it in place for laboratory analysis. The challenge was to fit the sponge tightly enough around the core to prevent fluid migration and mud contamination in the sponge-core clearance annulus, and yet avoid core jamming and sponge damage. This new, more accurate sponge liner coring service is now in place, showing excellent results. A balance seems to have been achieved between smooth core entry and a properly fitting, pre-saturated sponge with virtually no fluid migration. This new service cuts and provides oil-absorptive sponge-encased 3½-in. diameter core in 30 ft lengths with a maximum downhole temperature and pressure of 195°F (90° C) and 15,000 psi (1,034 bar) respectively. Special vacuum pump service equipment and sealing system are utilized to pre-saturate the sponge liner with brine. In late 2012 a major operator utilized this new and previously unproven system to core nearly 300 ft of sponge core in New Mexico, USA. The coring program used a special low-invasion coring fluid with a low spurt loss and a staged trip-out-of-the-hole schedule to minimize gas expansion/oil movement. The precision core bit that cut a tight-clearance core provided exceptional results with an average rate of penetration (ROP) of 10.4 ft/hr, with 97% core recovery and observable oil saturation in the sponge, indicating the system worked as designed. This case study will be described in detail within this paper.

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Because underbalanced drilling creates a natural tendency for fluid to flow from the formation into the borehole, successful underbalanced drilling depends on appropriate selection of circulating fluid. Under these conditions, use of conventional mud systems often results in lost circulation, formation damage, high mud costs and a need for expensive completions. Use of compressible fluids, on the other hand, can inhibit or eliminate many of the problems associated with drilling in environments in which formation damage is likely.Use of a compressible fluid in the circulating system, referred to as air drilling, lowers the downhole fluid pressure, allowing drilling into formations where loss of circulation and damage to productive formations are problems of major concern.Other advantages to air drilling include increased penetration rates, improved drill bit performance, and contamination-free drill solids for ready detection of hydrocarbons. (References and illustrations at end ofpaper.) 135Reduced pressure air drilling techniques include not only gas continuous phase methods and use of dry gas and gas mist, but also gas internal systems with stable foams and aerated fluids.This discussion of problems related to underbalanced drilling addresses types of drilling equipment used, and provides an overview of experience gained from both successful and unsuccessful wells.Underbalanced Drilling... SPE 27351Essentially, there are three ways to affect the drag in the well: