The Cobia-2 Subsea Satellite Experience

ABSTRACT The Cobia-2 subsea completion was installed in Bass Strait by the Esso Australia Ltd. / BHP Petroleum Pty. Ltd. Joint Venture during 1978 and 1979. It produced oil from the Cobia reservoir from 1979 to 1983 when it was superseded by production from the Cobia-A platform and subsequently plugged and abandoned in 1984. This paper reviews the Cobia-2 experience and discusses its installation, production, servicing, abandonment, and postmortem analysis. The operating history demonstrated that the flexibility of the Cobia-2 system enabled safe and continued production under a variety of adverse conditions. Furthermore, the postmortem analysis concluded that existing subsea components and servicing techniques can be used to confidently develop marginal hydrocarbon reserves. INTRODUCTION To date, over 360 subsea completions have been installed in all major producing provinces around the world (Mohr, 1985). Cobia-2 was the first subsea completion to be installed in Australian waters, and also the only one to have been recovered. Developed by Esso Australia Ltd. (operator) and BHP Petroleum Pty. Ltd. as part of the Bass Strait producing system (Figure 1), the Cobia-2 subsea completion produced almost two million barrels of oil between 1979 and 1983. Although the project experienced some technical challenges during its various phases, from installation through operation and finally abandonment, we view the Cobia-2 venture as highly successful. This paper will discuss the valuable experience provided by Cobia-2 and how it has been incorporated into the design of future subsea completions planned for Bass Strait. COBIA-2 SUBSEA COMPLETION The Cobia-2 exploration well was drilled in 250 feet of water in May 1977 and found excellent quality reservoir sands. The well was suspended for future re-entry as a subsea completion to allow production prior to the installation of the Cobia-A platform. Completion equipment originally purchased for another, less successful target, became available and was installed at Cobia-2 over two periods between 1978 and 1979, with final flowline tie-ins occurring in May 1979. Production to the Mackerel platform, 2.8 miles away (Figure 2), began on June 30, 1979. Total installed cost for the project was US$1S.6M. The well was completed as a single zone, dual 3-1/2 inch completion, with full TFL (Through Flowline) capability for downhole servicing. Figure 3 shows the Cobia-2 downhole completion schematic. Each tubing string contained an Otis tubing-retrievable, surface-controlled subsurface safety valve (TR-SCSSV) which, if required, could be permanently locked open and was capable of accepting a pump-down ball valve. TFL circulation between the two tubing strings was provided by an H-member just above the single packer. Although never used, two sliding sleeve H-members higher in the completion provided gas lift injection points. The subsea tree was a 5000 psi Cameron 3"x3"x2" tree with dual master valves (lower manually actuated, upper hydraulically actuated), and was latched to a Cameron 18–3/4", 10,000 psi wellhead. All valves on the Christmas tree were failsafe-closed including the crossover valve. Figure 4 shows a schematic of the Cobia-2 subsea tree.