Riserless Finds a Role in Offshore Well Plugging

With an increasing number of non-performing and shut-in subsea wells in mature areas of the Gulf of Mexico, operators are facing new regulatory and economic pressure to permanently plug and abandon old and inactive wells.

Offshore operators in the Gulf, Wood MacKenzie says, are expected to spend more than $1 billion annually on well decommissioning. Properly plugging and abandoning wells with primary and secondary barriers towards both the reservoir and flow zones, plus a surface plug as an environmental barrier and equipment removal, can cost $500,000 for shallow wells and $10 million or more for deepwater wells.

Operators continue to face economic challenges. Their ability to defer decommissioning old production facilities that are already past their design life and wells that are no longer economic is waning. Offshore service providers are watching for increased operator activity in well P&As even as they find new efficiencies in well intervention services.

Segments Drive Efficiency

The economics of offshore well abandonment can benefit from a segmented approach to the project. In a paper published in the Journal of Petroleum Science and Engineering, developing an operational P&A sequence divided into four segments provides the operator with highlights on simpler tasks that do not require a drilling rig.

In addition to a review of plugging materials and plug integrity issues, the paper notes that P&A operations are “now an environmental issue as much as about cost-efficiency.”

For a typical subsea well with a vertical Christmas tree, dividing the P&A project into four segments provides cost savings and improved efficiency.

1. Prep work: Retrieve hanger plugs from tubing, kill well, install deep-set mechanical plug, perforated tubing, circulate well clean.
2. Abandon reservoir: Rig-up blow-out-preventer, pull tubing hanger and tubing, install primary barrier at top of reservoir zone, install a secondary barrier to withstand future anticipated pressures.
3. Intermediate abandonment: Remove casing strings if needed, install primary and secondary barriers to potential flow zones in overburden, install surface plug as an environmental barrier.
4. Remove wellhead and conductor: Cut conductor and casing strings below the seabed to avoid potential contact with marine activity, retrieve casing strings, conductor and wellhead.

During a well’s productive life, the operator considers a variety of options from market conditions to subsea well intervention.

A segmented approach to P&A operations provides opportunities to conduct some of the less complicated tasks without calling in a drilling rig. For example, the operations in segments 1 and 3 can be performed using a riserless well intervention vessel rather than a drilling rig.