COMMENT

Leonard Le Blanc

Drill where?

Ten years ago, the suggestion that drilling operations might someday have to be carried out on the seafloor brought a laugh and knowing look. Today, no one laughs. Producers and drilling contractors are confronting serious challenges in ultra-deepwater.

The proposed solutions for surface-to-seabed drilling linkage and maintaining a drilling fluid circuit are either robust, meaning big iron, large connections, and more weight, or complex, with fluid routing and treatment, lift power, and pipe stripping. Meanwhile, the costs mount.

Producer asset management committees charged with chasing deepwater reserves are beginning to look disparagingly at the proposed solutions. If oil and gas reservoirs in ultra-deepwater were not so large, there would be a great temptation to chuck it all. Despite dire appearances, the industry has been in this position before - twice.

In the 1950s, oil-prone formations were turning up on seismic sections, beyond the water depth range of jackup drilling units and submersibles. Moving the BOP stack to the seafloor and using a riser for fluid returns was almost too revolutionary.
As late as the mid-1980s, subsea wellheads were considered the last choice in development solutions. Not until producing water depths exceeded saturation diving capabilities did the industry get serious about re-designing seafloor equipment for intervention by remotely operated vehicles.

In a circumspect fashion, the drilling transition to the seafloor is already underway. In order to solve one of the most critical problems in ultra-deepwater - the narrowing margin between the fracture gradient and formation pore pressure - riserless drilling and other solutions to lighten the weight of the fluid column in the riser are being examined. Some equipment designers would like to close the drilling fluid loop on the seafloor and separate out gases and unwanted solids.

The more radical step would be moving drill pipe make up, break out, and rotation to the seafloor. Drillstring breakout under water brings up environmental issues - not a minor hurdle. There are alternatives, however, that might be workable:

To preserve the jointed drill pipe process, a method could be developed to rotate pipe on the seafloor using rotating injectors or hydraulic rams on a turntable, thereby eliminating the kelly joint. Such a step would keep pipe racking, make up, and break out at the surface, and the drillstring could then be fed to the seafloor rotary as needed.
Conventional methods could be used to drill and case down to surface casing depth, eliminating most of the geotechnical risks. Afterwards, a coiled tubing assembly, reeled on the surface or seabed, with seafloor injectors, could be used to drill to formation and complete the wells.

In either case, flexible lines from the surface would be used to convey, hydraulic fluid, power cables, and monitoring and control electronics to the drill floor. Drillers manning control panels at the surface could monitor automated seafloor processes. Large work ROVs would move equipment and consumables to the seafloor as needed.

There are no readily apparent solutions in ultra-deepwater - just costly or risky ones. For example, pumping drilling fluid back to the surface from the seabed, or closing the drilling fluid loop at the seabed, would require the presence of 2,000-4,000 hp mud pumps on the seafloor. What's next? Maybe it's time to look harder at a complete re-design of the drilling process for ultra-deepwater.

Anyone long for dry land simplicity - and much smaller reserves?

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