Investigations of underbalanced drilling tools and technology offshore have shown remarkable progress and acceptance.
At the National Drilling Technical Conference sponsored by the American Association of Drilling Engineers (AADE) in March, Don Hannegan of Weatherford's Enabling Technology Group showed attendees how underbalanced drilling (UBD) tools and technologies are being used for a variety of applications offshore.
UBD from jackups, semisubmersibles
Eighty-four wells have been drilled underbalanced or near-balanced from jackup rigs, and some UBD wells have been drilled from barge-mounted rigs. These were in the Dutch North Sea, Lake Maracaibo and the intercoastal waterways and bays of the United States.
In December 2000, one near-balanced well was drilled at the Albacora field from the semisubmersible Petrobras 17 in 1,575-ft (480-m) waters in the Campos basin offshore Macae, Brazil. This joint industry project was sponsored by Petrobras, ExxonMobil, Shell, Amerada Hess, Impact ES and Weatherford to investigate safe UBD operations from a floating drilling rig. A 6.8-lb/gal nitrified drilling mud was used during this test, along with a new rotating control head mounted on top of the riser. Petrobras is especially interested in this technology as 70% of the country's reserves lie in waters deeper than 1,312 ft (400 m).
Shallowwater flows
Another potential use for UBD technology is to treat shallowwater flows as a well control issue, especially since they have been encountered 700 ft to 1,800 ft (214 m to 549 m) below the mud line.
Four schools of thought address shallowwater flow risks:
• hope and pray the well will not encounter an abnormally pressured aquifer at the chosen location;
• pay for expensive seismic data interpretation to identify risky locations and avoid drilling there;
• have plenty of heavy mud on standby; and
• use UBD technology to drill riserless to place the initial casing and subsea wellhead wherever desired.
Although the latter might be easier and cheaper than the other three conventional methods, the well-control approach has not yet been prototyped or field-tested.
Dual-density drilling
As water depth increases, the difference between pore pressure and fracture pressure becomes narrow, making it difficult to maintain wellbore pressure balance and hole stability.
To drill such wells conventionally, additional casing strings often are required, reducing borehole size, potential total depth and production rates.
Three groups are developing variations on subsea rotating control heads to provide an annular seal for dual-gradient drilling:
• DeepVision LLC - Baker Hughes, Transocean Sedco Forex, BP and Chevron;
• Subsea Mudlift - BP, Chevron, Texaco, Conoco, Diamond Offshore, Global Marine, Schlumberger and project leader Hydril; and
• Shell - working alone to investigate using electric submersible pumps for dual-gradient drilling.
Dual-gradient drilling, which uses seawater-filled risers and drilling mud below the mud line, is expected to save operators US $5 million to $12 million per deepwater well. Most of these savings will accrue from using fewer casing strings and the ability to use smaller drillships or semisubmersibles. Two of these groups plan to conduct field trials of the subsea pumps in the Gulf of Mexico in 2002, Hannegan said.
Acceptance
"Offshore applications using the tools and technology of underbalanced operations have apparently met with a more receptive response by the regulating community than some would have guessed," Hannegan said. The fact that onshore UBD jobs have had a better well-control track record than conventional overbalanced drilling may have something to do with it. Also, because there is inherently a greater risk, UBD operations are more thoroughly planned, with more safety case discussions and documentation of contingency plans. Surface equipment can be tested and qualified as fit-for-purpose before drilling commences.
"With so much of the world's remaining hydrocarbon resources progressively depleting and/or lying under deeper waters, the offshore use of the tools and technology of underbalanced operations is an unstoppable trend," Hannegan said. "Approximately one-fifth of the world's known offshore resources of hydrocarbons are economically undrillable with conventional drilling equipment and methods. Resist as one may, there is simply no other option than for the industry to tool up and learn how to safely drill offshore with lighter-than-conventional fluids and to enjoy greater ECD management while doing so."

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