Investigation Leads to Safety Recovery Around Failed Tree Seal

After a barrier was breached in an offshore gas well, the well was made safe by closing a primary barrier. An investigation then revealed the root cause of the failure and led to a solution that safely brought the well back online.

Source: Paper SPE 210725

Safe production from oil and gas wells always entails a two-barrier policy. In an offshore gas well, the primary barrier was breached while the secondary barrier was holding. Well fluids were observed in one of the tree ports during routine maintenance operations. The well was quickly made safe by closing a primary barrier, the downhole surface-controlled subsea safety valve, and installation of a secondary barrier in the form of plugs downhole.

To bring production back, the root cause was investigated, and it was determined that the straight-bore metal-to-metal seal, which seals the tubing at the interface of the tree and the tubing hanger, was compromised.

After a successful installation of a new seal, pressure integrity could not still be ensured because it was further determined that there was pitting corrosion, observed on the tubing hanger, that did not allow the seal to successfully seal the pressure across the tubing hanger neck.

Restoration of the well would depend on the repair of the tubing hanger, which would have required a complete workover, an approach that was deemed prohibitively expensive.

A novel approach was implemented to machine out an additional groove on the tree body to provide an additional elastomer seal as a secondary barrier as opposed to the traditional metal-to-metal seal.

This paper demonstrates the approach and successful restoration of the safety and well production. The approach can be widely used in establishing safety barriers in similar situations.

SPE members can download the complete paper from SPE’s Health, Safety, Environment, and Sustainability Technical Discipline page for free until 14 June.

Find paper SPE 210725 on OnePetro here.