 |
United Kingdom |
|
Advances in well abandonment
Greg McKenna, WellCut Decommissioning Services, Acteon Group
WellCut Decommissioning Services abandoned 11 wells in the North Sea for five different clients in a single continuous campaign during September and October 2004. To comply with health, safety and environmental legislation, the work on each well needed to be completed within 72 hours. This objective was achieved in every case. In fact, the time taken to abandon a well was reduced from the previous norm, with each well being fully pressure tested and sealed at the first attempt.
 |
One of 11 well casings and annuli, cut below the mudline.
|
|
The success of the campaign can be linked to enhancements to the company’s suspended-well abandonment tool (SWAT), improved operating and project management systems, and the use of a new, purposely equipped well-intervention and service vessel, the Island Frontier.
Two of the wells abandoned belonged to Kerr-McGee. Jim Manson, a senior drilling engineer with the company, said: “We saw the cost advantages of a multiple-operator approach using the new SWAT technology, and we appreciated that WellCut had worked hard to develop terms and conditions that reflected its experience of multi-client campaigns. Kerr-McGee has a long history of operation in the North Sea and, although we have reduced our inventory, we still have wells that need to be decommissioned. Companies that are not flexible enough to work in this kind of multi-operator campaign will need to produce their own contracts and this will involve greater expense.”
Technology and management improvements. One of the principal features of the new system, first introduced in 1996, is that it can be deployed from a dynamically positioned Class 2 well-service vessel, rather than an expensive drilling rig. This allows the system to be quickly and easily moved between wells without the need for anchor handling and its associated costs.
Once the SWAT tool is installed on the wellhead, the vessel pays out the tool’s umbilical to form a lazy S, which permits operations to continue even under extreme weather conditions. If necessary, pressure-control equipment on the tool enables the vessel to unlatch the umbilical and move away while the well remains under control. The tool perforates and cleans the well casings before plugging the well and all the annuli with cement. The wellhead is then severed 10 ft below the seabed, using either explosives or abrasive cutting technology.
At the start of 2004, a decision was taken to carry out a systematic review of the tool. This led to the redesign of several components and the incorporation of new technology (see figure) to speed up operations and enhance the tool’s capabilities. A new, lighter, drop sub-assembly was developed with a hydraulic release to improve reliability, and the bottomhole assembly (BHA) was ceramic-coated to increase the number of times the tool could be used in a single campaign. The BHA was also fitted with upgraded perforating guns that used specially designed shaped charges to improve the tool’s performance.
The cement flowrate was increased to 2 bbl/min, from 0.5 bbl/min, which enhanced the cement flow characteristics, improved the cement bond and reduced the chance of channeling. In connection with the cementing operation, fluid dynamics modeling was used to ensure that the barriers across the annuli will meet the requirements set out in the UK’s UKOOA guidelines. These state that cement barriers should extend across all the casing annuli and should be at least 100 ft in length – pressure testing is required to a minimum of 500 psi and to 0.1 psi/ft over the fracture gradient at the plug base.
Other improvements to the system include new hydraulic pumps with faster response times that give more accurate inflation control of packers. The umbilical has been extended to 275 m and its pressure capability raised. Finally, adding of a new control cabin with an improved layout has resulted in greater operational efficiency and ease of maintenance.
Improvements were not limited to the technology. A senior team analyzed the needs and concerns of the various clients and developed a fresh, multi-client project management system for the campaign. Furthermore, careful consideration was given to selection of the offshore team and full provision made for the five different client representatives. The effort put into planning the work paid off: the same crew worked throughout the campaign, which was completed without a safety or environmental incident.
Looking to the future. UKOOA is committed to reducing the number of suspended, open-water wellheads in the UK sector of the North Sea to fewer than 100 by the end of 2006. This will mean abandoning 140 existing suspended wells, plus any suspended between now and 2006. Looking to the bigger picture, there are now about 4,000 subsea wells worldwide. Therefore, wells that eventually need to be abandoned will escalate with the advent of technology that enables exploitation of small independent reservoirs tied back to existing infrastructure.
One of the challenges for the well-abandonment industry is to bring operators together and work out agreements that satisfy their different contractual and indemnity arrangements, operating practices and business cultures. WellCut’s recent multi-client campaign demonstrates that such issues can be overcome with the right approach.
Legislation is changing, and with new rules come fresh challenges. For example, there is a requirement to set deeper barriers that will maintain their integrity while resisting formation pressure. However, most suspended wells already have shallow plugs that hinder this process. Available technology dictates that this plug be drilled out from a rig, which has high associated costs. This subsea coiled-tubing technology will enable drilling shallow plugs and setting deep plugs from DP vessels. A tool with this capability would probably reduce the cost of such a well abandonment by 30 to 40%. 
|
