Summary
This paper describes a new drilling-riser concept and drilling methodology
for deepwater operations that will remove some of the well-control challenges
and limitations currently experienced when handling kicks and deep gas influxes
in deepwater regions, with the following results:
• Providing improved and more flexible well-control procedures.
• Reducing the potential of hydrate plug formation during well-control
operations.
• Allowing for drilling longer hole sections than normally considered feasible
when using conventional drilling methods, thus reducing the number of casing
strings required in the well.
• Allowing for improved drilling performance in depleted formations.
The main elements in the system are based on using a small, high-pressure
drilling riser [14-in. outer diameter (OD), 12.5-in. inner diameter (ID)] with
a split surface/subsea blowout preventer (BOP) and a subsea mud-lift pump
connected to the drilling riser and a separate mud-return line.
During drilling and well-control operations, the mud level in the riser is
maintained considerably below sea level to create a mud/air interface (i.e., a
“mud cap”) that can be continuously adjusted up or down by the mud-lift pumping
system. As a consequence, the bottomhole hydrostatic pressure will be
controlled. One of the main purposes of this system is to mitigate the
inherent problems with a conventional 21-in.-OD marine drilling riser during
well-control scenarios in deepwater operations. The system will compensate
for frictional pressures resulting from circulation and adjust the bottomhole
pressure (BHP) accordingly.
Introduction
Experiences from deepwater drilling operations in geopressured environments
such as the Gulf of Mexico (GOM) have shown that the upper layers of the
subsurface have low fracture strengths close to the hydrostatic pressure of
seawater. The resulting small margin between the pore pressure and the
formation strength typically requires four to six or more casing strings to be
set below the surface casing when drilling with a conventional marine riser
system.
When drilling in high-pressure/high-temperature (HP/HT) fields, or through
salt intrusions, small windows between pore pressure and formation strength can
be experienced. In some of these cases, after drilling only a short
interval, the incremental BHP caused by circulation [i.e., the equivalent
circulating density (ECD) effect] is high enough to require setting a casing
string to maintain adequate well-control margins.
Lost circulation is a problem experienced frequently when conventionally
drilling in deepwater areas, HP/HT areas, highly faulted and fractured
formations, and in depleted formations. The remedial process can be costly
(in both time and money).
When drilling with a conventional large riser system during a well-control
event, the kick is circulated out through the chokeline. This line has a
small diameter, and in deepwater wells, the friction in this line can be a
significant factor while circulating out a kick, even at low pump
rates. As a consequence, more than 75% of all deepwater kicks experience
formation ballooning, partial losses, and other downhole problems (Skalle et
al. 2002).
In severely depleted reservoirs, drilling and well-control operations are
often conducted within the small pressure region between formation fracture and
wellbore instability (collapse). The resulting challenge can restrict the
ability to drill underbalanced unless the BHPs can be controlled in a fast,
safe, and effective manner. The inherent limitations of conventional
well-control procedures can, as a consequence, cause severe lost-circulation or
hole-stability problems, which are extremely costly in deepwater
operations.
In deep water, the low mudline temperature and high pressure may lead to
hydrate formation, if gas is present. Hydrate plugs can cause delay in
operations and can cause severe well-control challenges (Barker and Gomez
1989).
In this paper, three different methods of pressure control will be
discussed. The first method is the conventional way of controlling
pressure in an open system with a high-pressure riser and a surface
BOP. The second method is the closed-loop method of managed-pressure
drilling (MPD) with a surface BOP, a rotating control device (RCD), and a
pressurized riser, and the third is the method referred to as the “controlled
mud cap” (CMC) with a split BOP between seabed and surface.
© 2006. Society of Petroleum Engineers
View full textPDF
(
641 KB
)
History
- Original manuscript received:
2 September 2004
- Revised manuscript received:
13 October 2005
- Manuscript approved:
10 November 2005
- Version of record:
20 June 2006
View Cart
