Dual Barrier Well Control Recovery for a Southern North Sea Subsea Well
In October 2024, a dual-barrier well control recovery was successfully executed on a Southern North Sea subsea well following loss of containment during P&A operations. Independently verified downhole barriers restored well integrity without reliance on tree valves or a riser, enabling safe recovery of the subsea Xmas tree and continuation of the P&A programme.
Region
Year
Challenge
The operator required a controlled recovery solution following loss of containment and equipment integrity concerns during Plug & Abandonment (P&A) activities on a Southern North Sea (SNS) subsea well. The operation progressed in an abnormal well state, with no reliable tree barriers available and constraints on conventional intervention methods. A robust downhole barrier solution was required to re-establish well control and enable safe recovery of the subsea Xmas tree prior to continuation of the P&A programme.
Earlier operations resulted in a situation where well control could not be assured at tree level. The subsea riser was not available, digital slickline equipment remained in hole, and known issues with the subsea tree valves prevented confirmation of valve integrity. Camera inspection confirmed the swab valve was not fully closed, leaving no verified barriers in place. Due to the well control status, dive intervention on the tree valves was not acceptable. The objectives were:
- Establish two independent, verified downhole barriers
- Avoid reliance on tree valves or a riser
- Enable safe recovery of the subsea Xmas tree
- Allow continuation of the approved P&A programm
Solution
Through close collaboration between the operator, drilling contractor, and Interwell, a dual-barrier recovery solution was implemented using proven mechanical barrier technology and independent verification.
Interwell proposed the installation of two 435–550 Medium Expansion (ME) Retrievable Bridge Plugs (RBPs), each equipped with Pump Open Subs (POSs), set within the 5.5” tubing. With no riser available, the plugs were deployed on drill pipe and guided into the well using Remote Operated Vehicle (ROV)
support.
The POSs were configured with shear-out values above the planned test pressures to avoid unintended opening during pressure testing and ensure test results could be confidently attributed to the intended barrier.The first ME RBP was set at 348 ft (106 m) Measured Depth (MD) to establish the initial mechanical barrier. A temporary inflatable packer was then run above the plug to enable pressure testing. The inflatable packer was equipped with Interwell’s Iconic Digital Transponder (iDT) system, allowing pressure and temperature data to be transmitted acoustically during a 1000 psi (69 bar) pressure test. The inflatable packer was subsequently retrieved, with recorded data reviewed on surface to confirm barrier integrity.
The second ME RBP was installed at 317 ft MD (67 m) and incorporated an iDT system to support verification of the upper barrier. With limited fluid volume between the plugs, pressure and temperature data was recorded acoustically by the iDT and stored in the iDT Memory Receiver. Once recovered to surface,
the data was downloaded and reviewed to confirm successful verification of the upper barrier.
Value Created
- Two independently verified downhole barriers installed without reliance on tree valves or a riser
- Successful pressure testing and verification of both barriers
- Safe recovery of the subsea Xmas tree
- Installation of drilling BOPs and riser completed without issue
- P&A programme able to continue as planned
Key Capabilities
- Dual barrier verification
- Advanced acoustic communication
- Gas tight V0 barrier technology
- Proven reliability
