Deepwater Decommissioning: What O&G Industry Could Learn from the Pioneer Subsea Temporary Well Suspension Project in Mauritania.

Abstract

Fifteen (15) subsea wells in Mauritania, consisting of 9 oil producer wells, 5 water injector wells and 1 gas injection well, were temporary suspended in preparation for plug and abandonment at a later phase. This paper describes the overview, challenges faced and lessons learnt captured during the planning and execution of pioneer deepwater decommissioning project, particularly on the subsea temporary well suspension (TWS) campaign in Mauritania. Lack of wells production history became one of the biggest challenge during the planning for the temporary well suspension and Plug and abandonment (P&A) campaign. The remoteness of the field locations contributed to logistics difficulties. Sourcing of H2S rated equipment and drillpipe riser, integration between Xmas tree and Intervention riser system due to incompatible connector took long lead time. Barrier management was a key consideration during the planning phase for known well integrity issues, and potential lack of barrier establishment that could be encountered during operations. The difference in water depth also impacted on rig selection. Problems encountered during the execution of the temporary well suspension campaign included H2S, annulus & tubing communication problem, high annulus pressure, non-functioning tubing retrievable surface-controlled safety valve, unexpected waxy sludge in well, blocked perforations, hydrate, brine incompatibility, difficulty in accessing well due to over-trawlable structure and dependency on FPSO for pressure monitoring prior to tree cap removal due to barrier establishment problems. The temporary well suspension campaign was executed with abandonment in mind.