Subsea well intervention presents significant challenges because of the high rig spread rate and the global scarcity of vessels capable of handling such operations. In plug and abandonment (P&A) activities—where there is no production upside from the intervention—operators are interested in decommissioning wells in the most economical manner possible.  For an offshore field in Australia that ceased production in 2011, an operator mobilized a heavy well intervention vessel (HWIV) to complete a multiwell subsea decommissioning program.

The abandonment campaign, conducted in accordance with the Oil and Gas UK (OGUK) Well Decommissioning Guidelines 2018—now known as Offshore Energies UK (OEUK)—was divided into two phases. In phase 1, the wells were abandoned at the source in batches. Wireline (WL) was used to evaluate existing barrier conditions, and coiled tubing (CT) was employed to spot cement plugs. Phase 2 involved cutting and recovering the subsea wellheads. A downhole camera was deployed to drift and gauge the well before plug setting and tubing cutting. Dual-string barrier evaluation was conducted to help determination the ultimate cement volume and placement, thereby optimizing the process.

The seven-well campaign proved beneficial in learning and refining subsea P&A techniques, with lessons learned applied progressively from one well to the next. Operational efficiency improved by up to 55% from the first well. The HWIV—equipped with a compensated system capable of deploying CT and WL—offered the flexibility to quickly convey evaluation and intervention tools on WL, while simultaneously deploying CT for cement placement. The built-in intervention tension frame (ITF) and skidding system facilitated the simplified changeout of services, eliminating the need for working at heights and addressing safety concerns. Using the pump-and-pull method for cement placement— rather than cement retainer squeeze—reduced the planned cement runs by half. Cement recipes were fine-tuned throughout the operation to ensure adequate pumping time, preventing situations where cement might be left in the pipe.

The completion of the P&A campaign was marked by the successful decommissioning of the field, while fully complying with governing guidelines.