Over the past 5 years, it has been easy to observe progress in rotary steerables. Only keener observers, however, will have noted advances in underreamers.

With more than seven suppliers providing straightforward or sophisticated underreamers*, drilling engineers are being offered unprecedented choice. And with nearly 30 different applications it's difficult to know which to use where.

In the North Sea, for instance, underreamers are used to enhance equivalent circulating density (ECD), improve cementing or casing tolerances and to help set sand screens or gravel packs. In the Gulf of Mexico and West Africa, they are used to counter swelling shales or salts. In places where state-of-the-art bottomhole assemblies (BHAs) are run or where stuckpipe potential is high, they are used to make tripping easier. Worldwide, underreamers are used to facilitate deepwater well plans that have seven or eight casing strings.

Companies such as Security DBS and Tri-Max quietly revolutionized underreaming by introducing the concept of concentric uniform loading on three equidistantly placed cutter blocks.** Using sturdy cutter platforms and PDC inserts, Security DBS's Near Bit Reamer and Tri-Max's Expand While Drilling (EWD) greatly reduced underreamer downtime by providing unparalleled robustness, enhanced cutting surface areas and improved hole quality.

Both tools have proved themselves in rotary and slide applications and are easy to configure as complex hydraulic activation calculations are unnecessary. The EWD requires low pressure drop - 100 psi for a 12 1/4-in. to 14 3/4-in. tool at 800 gpm, 10.7 ppg and is routinely used to underream salt sections. An example of this is a 9,516-ft (2,902-m) salt section underreamed for ConocoPhillips in the Gulf of Mexico.
Certain underreamers are simply too heavy to be rotated by straight motors, let alone by directional motors whose offset bend will generate excessive torque, load up cutters and stall out motors. Due to the EWD's short length, 28-in. for a 6-in. to 7 1/2-in. tool, it is commonly run below directional motors. ExxonMobil in Green Canyon, Gulf of Mexico used the tool below a motor with a 1.5° bend. A 3,385-ft (1,032-m) section was drilled from a 48.5° sidetrack to a depth of 17,964 ft (5,479 m). Casing was run without incident and rate of penetration (ROP) averaged 65 ft (19 m) per hour.

Other bit companies have now developed second generation underreamers. These companies can supply integrated drilling and underreaming packages without risking quality standards by offering third party tools. With the trend for integrated services strengthening, it is hard for bit companies to take responsibility (and pay penalties) for a third party's tool failures or downtime. See June's column - "Bundled or bungled services?"

What's neat about second generation tools - Hughes Christensen's GaugePro and Smith's Rhino Reamer - is that both products are substantially advanced underreamers. Both are hydraulically expandable concentric underreamers. Both are also truly bidirectional underreamers in that cutter coverage (CC) is identical irrespective of underreaming direction. The "updrill" cutter used to underream upwards offers the same coverage as the "downdrill" cutter used to undeream downwards or horizontally. Here we should distinguish the term "backreaming" from bidirectional underreaming. Granted, "backreaming" can be done by underreamers with minimal CC but it is also done by fixed blade stabilizers and roller reamers. However, these tools do not have CC comparable with bidirectional underreamers. The distinction is important because bidirectional underreaming is beneficial in swelling shales or radial salt closure as the greater CC translates to greater time for well construction.

In soft or medium formations, hydraulic or pressure driven underreamers are ideal as they will remain fully activated irrespective of drilling weight. In soft formations, bits tend to drill off which result in lower drilling weight-on-bit. In these circumstances, a weight set tool may not receive sufficient weight for it to compress or fully activate. Performance data indicates that hydraulic tools are generating a track record in hard-to-drill applications, an area which was thought to be the preserve of weight set underreamers.

As hydraulic underreamers are unaffected by the BHA weight below them, they can be placed above complete logging and directional suites. Once drilling fluid parameters and pressure drops below the underreamer are known the tools can easily be configured for use above logging or rotary steerables. This keeps characterization close to the bit and ensures formation conditions are characterized sooner rather than later. Tools can also be configured to establish minimum updrill flow rates. This ensures activation pressure differentials are not reached and that updrilling is optional, and not obligatory, at a given flow rate.

Choices are growing for drilling engineers as Tri-Max launch the TriBitReamer. A 6-in. to 7 1/2-in. tool measures just 17-in. and has been successful in the field.
For more information, visit www.hugheschris.com, www.securitydbs.com, www.siismithservices.com
and www.trimax-inds.com.

* Excludes ream while drillling bits and retractable/expandable bits
**As opposed to hole openers with a fixed pre-determined diameter best suited to top-hole applications. Hole openers in this sense do not pass through a restriction and routinely underream sections exceeding 8,000 ft (2,440 m).


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