The future of exploration and production (E&P) operations in the Gulf of Mexico (GoM) is focused on deep water, that is, >5,000 ft [1,524 m] water depth. As the industry pushes the envelope to reach deeper reservoirs, subsea wells have become an important part of the offshore production. Although subsea completions may offer significant economic and environmental advantages over seafloor-founded structures, they require floating drilling or well intervention vessels to enable wellbore access should a workover operation be required to maintain production at an economic level or even to plug and abandon (P&A) the wells once they become uneconomical to produce. Interventions and P&A operations are very costly and may involve multiple services. The P&A of subsea wells represents no financial upside to the operators, other than the possible recycling of the subsea trees. Consequently, reducing costs and avoiding future environmental liability are the prerequisites for successful subsea decommissioning.
Riserless subsea P&A operations are often completed by setting the plugs with drillpipe after the production tubing is removed with the rig. As part of the operational improvement process, a coiled tubing (CT) service company evaluated the option of performing this task using the riserless CT technique from a multiservice vessel (MSV) after the zonal and casing isolation packer had been successfully set and tested. This technique would allow the operator to save time to complete these operations, thus reducing the cost.
Between 2010 and mid-2015, the CT service company has successfully completed 18 riserless subsea interventions at well depths ranging from 4,900 ft to 21,000 ft [1,494 m to 6,400 m] and water depths from 4,600 ft to 7,000 ft [1,402 m to 2,133 m]. Riserless wellbore access with CT has proved to be an efficient P&A method that has helped reduce operating time by up to 40% as compared to riserless workover rig-based operations.
Detailed engineering and planning as well as continuous improvements have allowed implementing the CT riserless technique in accordance with the Bureau of Safety and Environmental Enforcement regulations.