Ship-based system reduces costs
Paul Alcock
UWG Group
There are thousands of temporarily suspended subsea exploration and appraisal wells around the world that are redundant and require permanent abandonment. These wells, if not properly abandoned and the wellheads rem-oved, are a potential long-term environmental risk and, in certain cases, pose a significant snagging hazard to fishermen. There is, there-fore, growing pressure on oil companies everywhere to reduce the number of tempor-arily suspended subsea wells.
The cost of making such wells permanently safe and removing the wellheads from the seabed is often prohibitive, however. Historically, this process has required the use of a drilling rig or a specialist well intervention vessel. Additionally, if local authorities or governing bodies do not dictate abandonment timescales, wells have remained "forgotten" for lengthy periods.
The UK government has been proactive over recent years by establishing a comprehensive database of all wells and owners and a program of abandonment that seeks to reduce suspended wells by approximately 50 per year. As such, with approximately 280 wells ready to be abandoned in UK continental shelf alone, the abandonment market is increasingly attractive.
The US Minerals Management Service in the Gulf of Mexico has been driving the industry to dispose of suspended assets within stringent timeframes. MMS is reducing the amount of explosive that can be discharged during the well abandonment process. Although environmental groups and lobbyists support this move, the onus has been on the operator to engage "greener" practices, despite greater costs.
In response to a demand from operators for a more cost-effective well abandonment method, UWG Group has devised a system as an alternative to traditional abandonment techniques – the Subsea Well Abandonment Tool (SWAT).
This tool has been used to abandon the majority of the subsea suspended wells removed from UK waters and can be used to permanently abandon many temporarily suspended wells from relatively inexpensive, medium-sized, dynamically positioned vessels with ROV support and a crane.
SWAT deployment
Abandonment operations are performed with a double barrier system in place, while the vessel moves freely on the surface. The vessel is connected to the SWAT system by a flexible umbilical, so is less likely to be delayed by weather conditions.
The system does not require divers, as all functions are remotely controlled from the vessel. Using either a crane or a small derrick, it is deployed over the side or through a moon pool. The elimination of divers improves safety during well abandonment operations, and as the system is deployable from a wider range of vessels, costs are competitive.
This innovation challenges conventional industry practices. Traditionally, operators have lacked any real incentive to optimize well abandonment, since block rental is often a fraction of the cost of well abandonment. With no prospect of reward or return on investment from the block, the industry-wide lethargy is understandable. However, with technology advancements that facilitate using pooled resources, multiple operators can join forces and share the financial burdens, allowing well abandonment campaigns to be optimized. This allows the whole venture to be less financially harrowing.
Abandonment method
The notion of "sharing" an abandonment campaign recently came to fruition for Talisman Energy (UK) Ltd. and EnCana (UK) Ltd., who both had wells to abandon in proximate blocks in the North Sea. One vessel, two crew, and two SWAT systems were mobilized to abandon eight suspended subsea wells, four category 2.1 (requiring one annulus and the well bore to be plugged) and four category 2.2 (requiring two annuli and the well bore to be plugged).
Typically for the category 2.1 wells, the system is deployed on its own umbilical into the wellbore, where it is positioned and pressure-tested using two packers. These packers provide double barrier isolation for well control.
The SWAT system is lowered on its own umbilical into the wellbore, where it is positioned and pressure-tested, using two packers for double barrier isolation.
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Perforating guns are positioned below and between the packers. The guns are fired via hydraulic control lines from the vessel, punching holes in the wellbore casings to create a circulation path for the cement that has to be pumped as part of the abandonment process.
A central circulating hose runs through both packers into the wellbore. The circulating hose has two hydraulically actuated isolation valves with the capacity to run 1.5 in. OD wireline tools through this line when the umbilical is disconnected. A return hose terminates between the packers and has a single hydraulically actuated isolation valve. The entire system is rated to 5,000 psi and is capable of handling hydrocarbons in a safe manner in the unlikely event that any are encountered.
Once the system is installed in a well and successfully pressure-tested, the lower perforating guns are fired to punch a series of holes in the well casing. With the "A" annulus breached, a check is then made to ensure that there is no unexpected pressure in the wellbore annulus behind the newly perforated casing. After the pressure check, the upper guns are fired to provide a circulating route from the vessel, into the wellbore, and back to the vessel. Once circulation is established, annular fluids are removed and cement is pumped into the casing and annulus to form the permanent barrier, thereby making the well safe.
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After the newly installed cement barrier is pressure-tested the SWAT system is recovered and the wellhead is explosively severed some 10 ft below the seabed and retrieved to the vessel. This completes the abandonment operation.
Conclusion
To tackle the significant costs associated with well abandonment, a few key factors should be addressed:
- More technological developments are needed to facilitate faster abandonment practices – operations that can be completed with single deployment into the well will significantly diminish the costs of vessel/rig time
- Universal interfaces should be more widely employed to avoid expensive, wellhead connectors
- Additional vessels should be made available as simple commodities. Healthy competition between owners should be actively encouraged to reduce costs
- Improved access to remote locations worldwide is needed. Rapid mobilization turn-around times, combined with multi-vessel capability will help
- Easier access to independently verified technical assessments of well risks and status is needed to ensure complete project comprehension prior to abandonment.
Wells and rigs no longer have to mutually co-exist. There are an encouraging number of alternative approaches to well abandonment coming into play, but the industry as a whole, needs to cohesively challenge inherited conventions and be more willing to adapt to these innovative technologies.
Author
Paul Alcock, UWG Group Sales & Marketing manager, may be reached at tel: +44 (0)1603 767 438 or email: [email protected].
