Summary
In deepwater drilling, a conventional large-diameter riser requires drilling
vessels with large weight and space capacities, large mud volume to circulate
through a riser, and many casing points because of narrow gaps between pore and
fracture pressures. A large number of casing points also require a larger
wellhead and a larger marine riser. These problems are inter-related and
intensify as the water depth increases. Although there are some successes to
set a new drilling record on water depth, it is impractical to extrapolate
current technology with a marine riser to 10,000 ft water depth.
Subsea mudlift drilling (SMD) is a term used to describe an unconventional
technique using a relatively small diameter pipe as a mud-return line from the
seafloor instead of a large-diameter marine riser. The scheme also balances
internal and external pressures at the seafloor by reducing the internal
pressure to make a dual pressure gradient possible. It has potential advantages
of cost and time savings and rig upgrades for deepwater applications.
Kick detection and well control will not be hurdles for field applications
of SMD. If the circulation rate is less than the maximum free-fall rate, there
will be a time delay in kick detection. In this case, the circulation rate may
need to be increased or a drillstring valve that provides a positive
surface-pump pressure should be used. In case of transient U-tubing or fill up,
other operations should be avoided for easy kick detection and well control,
unless there is an accurate prediction and monitoring tool available.
Introduction
As proven petroleum reserves decline through continued production, the
industry extends its search into ultra-deepwater and marginal areas that
present significant economic risks and technical hurdles. With new technologies
and hardware developed in the last few years, the industry has reached its
record water depth of 10,011 ft for drilling (Discoverer). However, production
is still limited to 6,500 ft water depth.
One of the basic and most challenging problems in deepwater operations is
the use of a marine riser. A marine riser has been used to provide a connection
between the drilling vessel and the wellhead. This serves as a guide for the
drillpipe into the hole and as a mud return path to the vessel. It also
supports choke and kill lines. Floating drilling operations in deep water
presently involve the use of a 21-in. outer diameter (OD) marine riser.
As water depth increases, it is impractical to extrapolate current
technologies with a marine riser to 10,000 ft water depth because of the
problems listed in Table 1, which are also mentioned in several references
(Gault 1996; Choe and Juvkam-Wold 1997a, 1997b; Peterman 1998; Choe and
Juvkam-Wold 1998; Choe 1998; Schubert et al. 2003). These problems are
interrelated and intensify as water depth increases. The high operational cost
and narrow gap between fracture and pore pressures can prevent rig crews from
reaching the target depth with the required hole size. These problems also make
completion and production difficult in ultra-deepwater.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
18 May 2004
- Meeting paper published:
2 March 2004
- Revised manuscript received:
9 January 2007
- Manuscript approved:
29 July 2007
- Version of record:
20 December 2007
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