Increased use of self-supporting risers offers a novel integrated approach to drilling, coiled tubing (CT) intervention and production for deepwater satellite wells. Features of the new approach include improved safety and environmental protection; simplicity and high reliability; and reduced cost for intervention and support for conventional or CT drilling, production and abandonment. The reduced cost and better availability of intervention can improve the economics for many marginal reservoirs.

Variations of self-supporting risers have been used for many years. Placid installed one for production in Green Canyon in the 1980s. More recently self-supporting risers have been installed for production off West Africa and in the Gulf of Mexico (GoM) and used for drilling offshore China.

A prototype modular exploration and production system riser was installed on a simulated wellhead in 1,000-m (3,280-ft) water depth in the GoM in 2006. It was recovered for analysis after five years, during which it rode out two named storms and an eddy current. Forensic analysis showed no significant signs of wear, fatigue or other degradation.

In the past, CT intervention in deepwater satellite wells typically required a mobile offshore drilling unit (MODU), with the associated problems of cost, scheduling and logistics. Using a self-supporting riser engaged to the wellhead or tree can avoid the need for a MODU in favor of a lower cost, more easily scheduled CT intervention vessel and can provide improved safety and environmental protection.

Moving into deepwater

In their progression to deepwater, operators have probably passed over more oil than they have produced because the economics have favored larger reservoirs. The financial risk of satellite wells, in part due to the difficulty of downhole intervention, has deterred operators from developing marginal reservoirs.

Reduced cost of reentry for workover can make marginal reservoirs commercially attractive by reducing the risk of having to abandon a well before it nears its full potential. In the future, the option to install a self-supporting riser on a deepwater satellite well for downhole CT intervention at any point in the life of the well can significantly improve the commercial practicality of deepwater satellite reservoirs.

This approach to CT intervention offers many advantages. Rig time is not required for riser installation or removal. Lightweight components make the riser suitable for installation and recovery by a light construction vessel or a specially outfitted workboat.

Other advantages include simplified logistics and avoidance of at-sea vessel-to-vessel transfer of a completion and workover riser between a supply boat and a rig. Reentry can be done by a MODU or by a lower cost or more readily available CT vessel. For simple downhole tasks, it may be practical to operate the CT spread from a specially equipped workboat.

Seabed reservoir isolation

Provisions for reservoir isolation at the seafloor would normally be an integral part of the self-supporting riser mounted on a wellhead or tree. In addition to providing another level of environmental protection, this allows the CT contractor to safely use a conventional CT BOP and injector at the surface while fully conforming to regulatory requirements and practices, including those that resulted from the Deepwater Horizon incident.

The recent frequency of 100-year storms has led to more stringent design requirements for surface facilities, making them more expensive. Surface facilities may be widely separated, particularly in deepwater. If the distance to a host is prohibitive, using a self-supporting riser can make it practical to produce marginal reservoirs and take the wells through their entire life cycle without a host platform or large FPSO vessel. Conventional surface facilities will remain the standard for development of large deepwater reservoirs. The self-supporting riser offers a safe, low-cost alternative for drilling, production, workover and abandonment of isolated deepwater wells.

Self-supporting riser

A single permanently installed self-supporting riser can meet all riser needs for the life of a marginal reservoir. The cycle begins with exploratory drilling. If no oil is found, the riser can be recovered and reused in another location just as drilling risers are. If indications are promising, the same riser can be used for well testing, including through CT production tubing.

If the test is satisfactory, the self-supporting riser and the well test vessel can remain for early production while a surface facility is designed and built. If the well test indicates that the reservoir does not justify a dedicated surface facility, the riser and well test vessel can be used to deplete the reservoir.

Either way, the same self-supporting riser can remain in place for CT intervention, including replacing electric submersible pumps (ESPs), sidetracking, extending the well depth, cleanout or other workover and abandonment—all without the cost and complication of scheduling a MODU. There is a further benefit of reducing risk by having an additional layer of reservoir isolation capability and fewer personnel offshore.

Reservoir isolation capability at the bottom of the self-supporting riser allows both drilling and CT contractors to use surface BOPs. It can also allow using a lower cost near-surface tree and significantly simplified control system for isolated wells.

Drilling, completion, well testing, production

When a single self-supporting riser is left installed from the beginning of the life cycle of a deepwater well, a shallow-water semisubmersible or ship-shape rig can be moored over it, use the riser for drilling and completion and leave it in place. For well testing, a relatively small vessel with a test tree can engage the top of the self-supporting riser and produce through a dynamic flexible pipe, as shown in the illustration.

Depending on the results of the well test, it may be appropriate to remain onsite for early production of a promising reservoir or even until a marginal reservoir has been depleted.

A 15,000-bbl/d surface production facility can be installed on a converted 80-m (262-ft) vessel that need not be suitable for extreme weather. If a weather window is closing, the production vessel can simply shut in at the seafloor, drop the flexible production pipe and control umbilical and motor away. The self-supporting riser is not affected by surface conditions unless attached to a surface vessel.

For CT operations, motion isolation is required between the riser and the intervention vessel to prevent coupling of vessel motion into the riser. In the well test or production mode, a “lazy-S” flexible pipe sufficiently isolates the riser structure from the vessel unless the vessel makes excessive excursions, in which case the flexible pipe can be quickly disconnected from the vessel and dropped into the water.

Producing to a 15,000 bbl/d dynamically positioned FPSO unit with easy disconnection and reconnection allows many maintenance tasks for surface production equipment to be done at a shore facility by motoring to a dock. The option to do shore-side maintenance of surface production equipment is particularly attractive when production must be shut in for either downhole intervention or maintenance of surface equipment. Dockside maintenance of surface production equipment significantly improves the logistics and results in fewer personnel offshore for reduced risk.

Intervening or producing through a self-supporting riser with seafloor reservoir isolation makes it practical to use an intervention vessel or early generation MODU for downhole work, ESP replacement or abandonment, as shown by the vessel directly above the well in the illustration.

Quick disconnect capability with the riser left standing in place also greatly reduces the time for a dynamically positioned CT vessel to respond to a closing weather window—simply pull out of the hole, shut in at the seafloor, disconnect and motor away. The self-supporting riser is isolated from surface conditions and, when used with a surface ship that is free to depart, offers an approach that is not exposed to extreme surface conditions.

Acknowledgment:

Engineering development of self-supporting riser use for both CT intervention and well testing was funded in part by the Research Partnership to Secure Energy for America (RPSEA) through a contract with the U.S. Department of Energy.

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