This paper presents the work performed by Conoco Indonesia Inc. in developing and testing a unique re-entry system for wireline work on two seafloor wells in 295 feet of water at Kepiting Field in the Natuna Sea of Indonesia. This system does not require the use of a floating drillship for conventional wireline work in a seafloor well.
The main components are:
A 240 foot buoyantly tensioned stool seafloor riser A surface buoy which connects temporarily to the seafloor riser and also supports an above water work platform for the wireline lubricator assembly A set of strategically located permanent workboat moorings A workboat arranged to serve as the wireline unit's operating platform and to provide all support services.
This paper discusses the components, installation and performance of this unique wireline re-entry system.
The Kepiting Field is located about 5 miles Southwest of Conoco's Udang Field. (See Figure 1). The area is out of the typhoon track and has a relatively mild environmental regime. The 100 year wave is 38 feet (hs=19 feet). The Kepiting reservoir was discovered in June, 1982 at a depth of 5900 feet. The oil sand has 43 feet of not pay and tested at rates of 5240 and 6010 BOPD in two separate zones. Not withstanding the mild weather system and encouraging reservoir test results, there were three prominent obstacles to economic production:
Water depth is 295 feet Mapped recoverable reserves are insufficient to economically support the investment in a platform based drilling and production system Crude oil has a 100 Fahrenheit pourpoint and carries erosive sand
Studies of various alternative development schemes were evaluated. These included subsea completions tied back to a Floating Production Storage and Offloading vessel (FPSO), a steel piled wellhead tripod with an FPSO tethered to the same, or a limited production facilities platform with an FPSO anchored on a single point mooring. Because the small reserves dictated minimum capital investment, it was found that the seafloor development with an FPSO was the most economically attractive. The subsea system uses multiple seafloor trees linked to the FPSO by flexible risers which provide well control, production flowline and gas lift flowline. This system was developed and is currently operating two seafloor wells at the Kepiting Field.
In the studies of the above mentioned development schemes, it was recognized that inexpensive well maintenance was needed for efficient reservoir management and would present a challenge to the field developers. A review of existing subsea systems disclosed that none had successfully and inexpensively solved the problem of wireline re-entry using conventional equipment and techniques in this waterdepth.
Artificial lift was required and the two zones at Kepiting needed to be produced independently, either through dual completions, or by using multizone-single selective completions similar to those used to produce the reserves in the nearby Udang Field. Multizone-single selective seafloor completions, equipped with gas lift valves for artificial lifting, were selected for Kepiting wells. Figure 2 illustrates the downhole equipment arrangement and indicates the various downhole operations required to produce such wells. Wireline works include:
Pulling and checking the subsurface safety valve. Setting a plug to close the lower zone and opening a side door sleeve to the upper zone.. Changing out gaslift valves.. Bailing sand, if needed.
An estimated minimum of 3 wireline re-entries are required during the well life. Several more trips may be required if problems develop with the safety valve or gaslift valves. For the two well completion at Kepiting, this amounts to a minimum of 6 wireline operations.