Formation acts as if drilling rig is on the seafloor
- Fig. 1 [16,087 bytes]
- Fig. 2 [58,040 bytes]
- Fig. 3 [24,981 bytes]
- Drilling progress with and without mudlift [24,483 bytes]
- 9,000' WD well with conventional technology [43,522 bytes]
- Same 9,000' WD well with riserless drilling technology [48,130 bytes]
- Kick detection in riserless sytems [37,911 bytes]
In ultra-deepwater and in parts of the world where currents are excessively high, oil and gas exploration faces major challenges. They include the inability to reach geologic objectives, inability to drill an adequate hole size for economic field development, highly limited fleet of rigs, excessive downtime in operations, and the need for more casing strings, mud volume and riser support than is economical or technically feasible.
With conventional marine riser drilling systems, these problems will grow as water depth increases. Reportedly, 40-60% of all deepwater wells face significant current problems. One solution is to eliminate the riser and return fluids and cuttings to the surface through a small diameter pipe separate from the drill pipe. Such a system would provide the following:
- Large bore completion
- Extend water-depth capability of smaller rigs
- Extend operating window in high-current areas
- Reduce the number of casing strings required.
The JIP allows companies to share the high cost of such development and also use the technology developed from the process. Participants get the first option to use the new equipment emerging and gain and understanding of how the technology can be best applied.
RationaleFloating drilling operations in deepwater necessitate using a 21-in. marine riser with a capacity of about 400 bbl for every 1,000 ft in length. In deepwater, the mud volume within the riser comprises most of the mud system, but does not benefit the drilling process.
In fact, this long, weighted mud column introduces hydrostatic pressures that make it necessary to use numerous casing points in certain locations. The many casings require a larger subsea wellhead, which requires a larger marine riser, which requires a larger drilling rig to support the riser and mud column.
This cycle intensifies as the water depth increases and must be altered for economic and technology reasons. A key way to do this is to reduce the hydrostatic pressure at the seafloor to near that of a column of seawater, thus tricking the well into believing the drilling rig is located on the sea floor. To achieve this feat, the annulus-type riser must be replaced at the wellhead by a mud return line.
Since the 1960s, various schemes for riserless drilling have been proposed. But even as late as the early 1990s, few wells were drilled in water depths beyond 3,000 ft, creating little impetus to develop technology to go any deeper. Today, however, the industry wants to move into water that is 10,000 ft and deeper.
In 1996, Conoco and Hydril jointly began pursuing the development of a system designed to address deepwater drilling problems in a different way. The two major components of the proposed solution were:
- Mudlift: A mudlift mechanical system, that simulates a seawater gradient from the seafloor to the rig, allows the mud gradient to be referenced to the seafloor rather than the surface. Pumps with a rating of 3,000-4,000 hp at the seabed would be needed to lift mud through the return line to the surface 7,000-10,000 ft above.
- Return line: A return line conducts mud and cuttings from the sea floor to the surface.
Conveniently, riserless drilling's closed system will permit under-balanced drilling, unlike conventional systems, which are open at the surface return level. This should be a convenience in older mature fields where formation pressures are known.
Circumventing problemsDeepwater drilling that employs a marine riserless system circumvents problems inherent in the technology now employed for deepwater drilling. The system is designed to work because of the strengths of the mudlift and return line.
The mudlift is the component of the system which simulates the dual gradient mud system. This offers the following advantages:
- Reach geologic objectives in virtually any water depth
- Use larger diameter production casing to obtain high flow rates
- Reduce the cost of a dry hole by at least $5 million
- Reduce casing setting requirements
- Provide contingency casing options which can allow drilling shallow hazards with mud, or offers additional protection against deeper drilling hazards
- Potentially extend the water depth limit of second and third generation rigs by eliminating large riser tensioner requirements and reducing deck load
- Achieve better well control
- Reduce mud volumes by filling 21-in. risers with seawater
- Maintain a riser margin even when disconnected from well
- Eliminate effects of choke and kill line friction pressure on well
- Reduce the effects of currents
- Potentially extend the water depth limit of second and third generation rigs
- Extend deepwater drilling capability beyond 10,000 ft for fourth generation drilling rigs and tanker-based drill ships
- Offer rapid disconnect and reconnect features
- Reduce weight and space requirements during mobilization
- Prolong the usability of older drillships and semisubmersibles
- Enable rapid deployment and recovery of the subsea drilling equipment
- Dramatically enhance development economics by increasing well productivity and decreasing well count.
- Substantially decrease the cost of development by reducing drilling time, the number of casing strings required, the volume of mud needed, and many other related drilling cost
- Increase the number of existing rigs capable of drilling deepwater prospects.
Phases II and IIIPhase II of the riserless drilling development JIP, which includes moving the critical components into the commercial viability stage, is scheduled to begin in early January of 1998. Goals for Phase II include the following:
- Designing and testing of prototype mudlift system components
- Analysis of critical riser and marine related activities
- Critical operational and well control procedures
- Validation of the design of the return line component.
Phase III of the JIP will revolve around system design and testing and is scheduling to get underway in mid-1999. Goals for Phase III include complete system integration, detailing operational and well control procedures, developing a riserless drilling well control school, and design and manufacture of a well control simulator.
At the conclusion of Phase II and Phase III, the JIP participants expect fabrication of a complete system, endorsement by regulatory authorities, and preparations for testing of the system on an offshore rig. The cost of Phase III is projected at $6-7 million. Only companies who participate in Phases II, III and IV will have first option privileges for the newly developed equipment.
Phase IV: ValidationProvided industry support continues, Phase IV will validate engineering design by testing a prototype system on an offshore drilling rig. After tests are completed successfully, the equipment will be made ready for commercial sale. The projected cost of Phase IV, if it is completed successfully, is $5-15 million.
Only those companies participating in JIP Phases II, III and IV will have the right to receive new equipment prior to being marketed to the industry. Each participating company will receive their first riserless drilling system before any other participant receives a second system.
All participants to Phase II will contribute an equal share of the cost of technology and equipment development. To fund Phase II, each participant will contribute $1.3 million. Those who did not participate in Phase I will contribute $150,000 as a late joining fee. However, they will receive the work developed during Phase I: Conceptual Phase. Should total contributions exceed the estimated budget of $12 million, then each participant will be refunded a proportionate share of the excess contributed, exclusive of any late joining fees.
Membership to participate in Phase II will be kept open until January 9, 1998. Late membership, with additional fees, is available through April 10, 1998. Afterwards, no new membership will be accepted.
Members of Phase I of the JIP include Amerada Hess, Amoco, ARCO, BP, Chevron, Conoco, Diamond Offshore, Elf Exploration Production, Exxon Production Research Company, Foramer, Global Marine Drilling Company, Hydril, Mobil, Ocean Drilling Program (international earth sciences program funded by the National Science Foundation and international partners), Petrobras, Reading and Bates Drilling Company, Saga Petroleum, Sedco Forex, Smedvig, Statoil, Transocean, and Texaco.
(Editor's Note: Individuals and companies interested in the JIP should contact Andy Efthim at Conoco; P.O. Box 2197; Houston, TX 77252-2197; Phone: 281-293-3314; Fax: 281-293-2158; E-mail: [email protected]) or Charlie Peterman at Hydril; P.O. Box 60458; Houston, TX 77205-0458; Phone: 281-985-3241: Fax: 281-985-3353; E-mail: [email protected])
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