Abandonment of 19 Subsea Wells in the Jabiru/Challis Fields

Abstract

Abstract This paper describes the pre-planning and execution of Australia's largest subsea well abandonment campaign to date. The Jabiru/Challis Fields are located in the Timor Sea 640km from Darwin, Australia. The fields were discovered in 1983/84 with first production in 1986 and reached the end of their economic life in 2010. The fields consisted of 13 subsea producers and 6 suspended/partially abandoned wells making it a 19 subsea well abandonment programme. A semi-submersible rig was selected over a jack-up primarily based on water depth and subsea hardware handling capability considerations. Abandonment solely using a construction vessel was discounted due to the age and condition of the wells and associated risks of using coiled tubing. The condition of the wells required detailed barrier management. The main challenge was to maintain adequate barriers throughout the abandonment programme when some of the standard barriers did not exist to begin with. A number of the barriers that were in place during the well construction were no longer present. The Drilling Contractor was fully engaged to ensure single barrier operations were acceptable. Drawing on the experience gained in the North Sea, the Oil & Gas UK Guidelines for the Suspension and Abandonment of Wells were adopted. With numerous known well issues and more potential unknown issues, coupled with significant logistical challenges, contingency planning was vital. By the end of the campaign, problems encountered included corroded casing, corroded/parted/collapsed tubing, iron sulphide scale, naturally occurring radioactive material (NORM), H2S, non-functioning tubing retrievable surface controlled subsurface safety valves (TRSCSSVs) and blocked perforations. Significant use of construction vessels was made pre and post rig activities. A programme change during the campaign resulted in the storage of subsea hardware assemblies on the seabed for later retrieval by a construction vessel. This was a safer and lower cost option than recovery to the rig. This paper is of use to engineers and managers commencing the planning of a subsea well abandonment campaign. It describes actual experience in a relatively uncommon operation in the industry and includes learnings which can be applied to future development planning.