THE COBIA 2 SUBSEA COMPLETION

Abstract

Subsea completions are recognised worldwide as a cost effective means of developing marginal reservoirs, accelerating production and draining reservoir extremities which cannot be reached from conventional platforms. To date, more than 280 subsea completions have been installed around the world. Cobia 2, the first subsea completion in Australian waters, commenced production in Bass Strait in June 1979. It continued to produce until April 1983, when it was shut-in following the commencement of production from the Cobia platform. In its four years of operation, the well produced over 280 megalitres (1.78 million barrels) of crude oil, with peak well rates reaching as high as 750 kilolitres per day in the latter stages of its producing life. Overall, Cobia 2 has been a technical and commercial success.

The need for regular pumpdown or TFL ('through flowlines') wax-cutting operations in the flowlines to maintain high levels of production generated a great deal of confidence in the use of TFL techniques for routine and non-standard subsea well servicing. In an industry 'first', TFL methods were developed to lock open and seal a leaking subsurface safety valve and, within it, set a special insert subsurface safety valve. This work allowed the well to be returned to production in a situation where a conventional workover of the well was not feasible.

Other well-servicing techniques developed during the Cobia 2 project involved the novel use of a coiled tubing unit to retrieve TFL tools which had become stuck in the flowlines during the wax-cutting operations.

The highly successful operation of Cobia 2 has proved the viability of this type of completion for marginal field development in Australia.