Minimal well number dictated by deepwater development economics
A small number of wellbores, primarily of the extended reach type, must be highly productive in order for many of the prospective fields West of Shetlands to be economically successful. The reservoir sands are relatively thin and extend over a wide area, but more critically, lie not far below the seabed in water depths of 300-800 meters.
Besides thin reservoirs, other challenges facing fields such as Foinaven west of the Shetland Islands include an 85-meter thickness between the gas-oil contact and the oil-water contact, adverse mobility of reservoir fluids, soft and friable sands, and a hydrostatic reservoir gradient.
Reservoir, technology, and economic challenges facing development of the West of Shetland frontier area were cited by Graham Prentice, Foinavaen Development Engineer for BP in a presentation to a drilling conference held in Aberdeen in November. He cited the following economic challenges:
- Reduce well costs: Well costs must be brought below $10 million/well. One possible technical means to lower well costs is multi-lateral completions, where production from two or more lateral sections are drained into a central wellbore.
- Lower subsea costs: Operating costs for subsea wells also must be reduced. Two solutions are better reliability of equipment and new technology for quick and efficient workovers.
- Rig availability: Having a semisubmersible rig available at predictable day rates for well intervention and workover later in the life of the field is critical to operating cost control. What will rig rates be 5-8 years from today?
- The extended reach development well schematic employed by BP for Foinavin showing the challenges and solutions.
In order to meet the first two economic goals, Prentice cited a number of needs that could be fulfilled by technological development in the future:
- Drilling without a riser at inclinations above 45
- Establish drilling departures at a depth of 11,000 ft and over at the top reservoir.
- Ensure that coiled tubing drilling is able to access to full depth.
- Be able to suspend wells with little if any formation damage.
- Drill and complete multi-lateral wells at any depth
- Conduct wireline and coiled tubing operations without a drilling unit.
Last year, BP actually utilized a number of new technologies and methods in achieving results in the Foinaven development drilling program.
A 17 1/2-in. hole to the desired depth at a directional angle of 23 was drilled without a riser. Later, a horizontal distance of 1,882 meters of 8 1/2-in. hole was drilled along an interval. A geosteering process was used to maintain position within the interval.
ReferenceGraham Prentice, Extended Reach Wells and their Application West of Shetland, 8th Annual Drilling Conference, Aberdeen, Scotland, 22-23 November, 1994.
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