To meet the world’s increasing demand for energy, petroleum-producing
companies must search for oil and gas in increasingly hostile environments. One
area showing great promise is in the deepwater areas of the US Gulf of Mexico.
This is evidenced by increased lease sales and drilling activity that have
occurred there within the recent years. As drilling moves into deeper waters,
new technologies must be developed for safe and successful operations.
Around 1996, four projects were initiated to develop dual-gradient drilling
(DGD) technology for use in water depths greater than 5,000 ft. The four
projects are Shell Oil Co.’s project (Gonzalez 2000), the SubSea MudLift
Drilling Joint Industry Project (Smith 2000) (SMD), the Deep Vision project
(Sjoberg 2000), and Maurer Technology’s Hollow Glass Spheres project (Maurer
2000). Several publications have discussed the advantages that DGD technology
has over conventional deepwater drilling in ultradeepwaters (Gault 1996;
Schubert 1999; Smith et al. 1999).
Although the advantages of the dual-gradient projects are well documented,
there has been little published on one of the major concerns expressed by all
four projects. That is, how does well control differ for Dual-Gradient Drilling
as compared to conventional riser drilling (Schubert 1999; Juvkam-Wold and
Schubert 2000; Weddle and Schubert 2000)? This paper reports on a comparison of
the well-control aspects of DGD to those of conventional riser drilling. It is
based on the work that the authors performed as part of the SMD project.
DGD is an unconventional method of drilling in which a relatively small
diameter return line is used to circulate drill fluids and cuttings from the
sea floor to the rig’s surface mud system (Fig. 1) (Schubert 2001).
During DGD, the rig’s marine riser is kept full of seawater. A rotating
diverter, which is similar to a rotating control head, separates the wellbore
and its contained fluids from the seawater in the marine riser. During wellkill
operations, the return line is utilized as the choke line in conventional riser
© 2006. Society of Petroleum Engineers
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- Original manuscript received:
24 June 2005
- Revised manuscript received:
5 April 2006
- Manuscript approved:
10 April 2006
- Version of record:
20 December 2006