The Evolution of Wellbore Integrity Solutions (WIS) Well Abandonment Technology

This technical article highlights the portfolio of WIS technologies used in the Plug and Abandonment (P&A) process. 

Installed in 1977, the Dunlin Alpha was a four-legged platform, constructed on a Concrete Gravity Base Substructure with a steel box girder modular support frame. It was located in the UKCS license Block 211/23 in the Northern North Sea (East Shetland Basin).

The structure stood in 151m [495 ft] of water and was over 200m [656 ft] high from the seabed to the top of the drilling derrick. Production started in 1978, and over 522 million barrels of oil were produced over its lifetime. In 2015, production ended, and the decommissioning phase began. The platform topsides removal was concluded in June 2022. 

With 45 wells safely plugged and abandoned over a 5-year period, several technologies evolved or were developed to improve operational efficiency while preserving wellbore integrity during the abandonment process.

Scope of Work and Requirements

Establishment of multiple barriers inside 9 5/8 inch and 13 3/8 inch casing and rock-to-rock zonal isolation of the production reservoirs.

Efficient cut and recovery of 9 5/8 inch and 13 3/8 inch casings. 

Advanced section milling, including:

• ROP optimization, reliability in high angle wellbores with extended interval lengths, centralizers, and collars.
• Simultaneous underreaming to enable rock-to-rock isolation. 
• Options also included concurrent bridge plug conveyance and setting.

High capacity, downhole jacking system, spear, and expandable anchoring mechanism to mitigate risks and aid casing and conductor recovery operations. 

Technology Implementation

The use of ProMILL System technology dominated the campaign, delivering advanced milling and underreaming capabilities to enable “rock to rock” solutions. Over 12,000 feet of casing was section milled using the ProMILL System during this project.

• WavEdge and TruEdge cutting structures were developed to increase durability and improve ROP. This contributed to an improvement in milling performance over the life of the project. (Fig. 2) 
• Cutting structure development included fundamental research in materials science (Fig. 3), structure geometries (Fig. 4), dynamic simulation modeling, and full-scale laboratory testing (Fig. 5). 
• The ProMILL High Ratio Underreamer (HRU) with application-specific cutter arms ensured that the rock interface was prepared for the barrier plug.
• An optional bridge plug in the milling and underreaming BHA offered greater efficiencies.

Conductor recovery also presented unique challenges, such as: 

• Risk mitigation for known weak, corroded, or parted connectors. 
• Dimensional variations, differing casing weight per foot or internal conductor diameter restrictions at the connectors. 

A heavy-duty, 16-inch diameter Pipe Cutter was widely used to perform both 20-inch casing and 30-inch conductor cuts.

A High Defection Spear was successfully introduced to pass through diameter restrictions and pull parted and weak conductors with connectors from the bottom.

The PullMaster Jacking System was deployed in a unique configuration to recover the 20-inch X 30-inch conductors that could not be recovered using conventional techniques. This system was rated to greater than 1 million lbs. (Fig. 6)

An innovative Expandable Conductor Anchor was designed and delivered as a rapid response contingency item during the project. The expandable conductor anchor was designed to pass through the 20-inch casing in its retracted position, and expand to securely retain the 30-inch conductor. (Fig. 7)

For further information, reference this Performance Bulletin “A Customized PullMaster System with Large Scale Deflection Spear Applied in a North Sea Conductor Recovery Operations”

Best Practices

Over the 5-year duration of this project, the local team provided focused support to continually improve performance and efficiency for the services provided. Best practices incorporated included:

• Rigorous pre-job planning and risk assessments
• Pre-job data review and post-job analytics
• Brief/debrief of personnel involved
• Capturing and sharing lessons learned

Project Highlights:

Acknowledgments:

Wellbore Integrity Solutions wishes to thank Fairfield Energy for their permission to publish this technical article. Dunlin Alpha Platform photos are courtesy of Fairfield Energy. 

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