Aquila was discovered in 1982 following a concerted drilling campaign by Agip in the deepwater southern Adriatic. The reservoir lies in the carbonatic Mesozoic formation in the Umbro-Marchigiano sedimentary basin, and contains around 20MM bbl of 36deg API oil.
These small reserves, combined with the field's remoteness in 850 metres of water, 45 km offshore Brindisi, made the project marginal in the extreme in the mid-1980s. Agip, however, felt it was worth pursuing, along with other deepwater prospects in other countries, through a new in-house R & D programme that focused on subsea production, riser design, multiphase pumping and TLPs.
Agip returned in 1993 to drill the Aquila 2 well, followed by two extended production tests. An integrated engineering team took their time analyzing the test results and wrestling with mooring and riser scenarios. However, a further positive appraisal well in 1995 confirmed their view that an FPSO was the way forward.
Aquila is a 32-month development, due onstream by the end of this year. Two of the completed subsea wells will be connected to the permanently moored FPSO, which will be capable of storing around a month's production, with onboard capacity for 550,000 bbl. Oil will be offloaded periodically to shuttle tankers (not dedicated). The converted vessel, the FPSO Firenze, was itself formerly a tanker operating on the Nilde Field in the Sicily Channel.
Weather conditions in its new location, the Otranto Channel, are classified as "severe". However, the turret is designed to allow free and full 360deg rotation for weathervaning, and the swivel design should ensure operational continuity of production and service lines and electrohydraulic umbilicals.
Saipem and SBM are joint contractors for the conversion, which is under way in Sardinia. They will also manage the FPSO out on the field for a minimum of five and a half years. And they are leasing not just the vessel to Agip, but also the riser system. This comprises flexible 6-in. production lines and 2.5-in. service lines, plus control umbilicals.
Topsides process plant includes gravity separators and a stabilizer column to maximize oil recovery. Produced water will be processed via hydrocyclones and flotation tanks to limit oil content to 10ppm prior to overboard disposal. Associated gas will be used mainly for well reinjection, to enhance oil recovery, and for power generation of the vessel. Remaining gas will be flared.
Reservoir studies showed that Aquila oil production must be boosted in order to maximize flow rate and oil recovery over the anticipated 10-year field life. For this reason, two horizontal displacement sections of 200 metres and 750m were drilled on the Aquila 2 and 3 wells.
To keep workover interventions in this remote location to a minimum, single point gas lift injection was chosen. Inside the tubing, injection rates will be determined by a SCRAMS (surface controlled reservoir analysis and management system) device integrated in the completion string. This device will control the gas-lift multi-orifice valves as well as partializing the production string to aid acidizing, should this prove necessary later on.
Gas lift will be controlled at intervals via three dedicated orifices in the valves. The SCRAMs will also be equipped with a remotely controlled device to allow flow diversion either in production from, or injection to, the heel or toe of the Aquila 3 well's horizontal section (through a 2 7/8-in. concentric string).
Aquila's wells will be completed with horizontal subsea xmas trees and a conventional drilling riser, rather than a more costly workover riser. Control will be through a multiplexed electro-hydraulic subsea pod that will also supervise remotely the downhole SCRAMS. This control system will respond to pre-programmed emergency shutdown sequences.
Installation of subsea production components will be diverless and guidelineless, based on Agip's SAF technique which was pioneered on its Luna Field 40 well. SAF works in waters down to 1,000 metres, employing the `Master Vehicle' ROV for the installation and subsequent maintenance operations. Trees and flowlines will be connected via the SAF jumper configuration, which allows independent retrieval of both items. All seals will be metal-to-metal.
Materials used throughout the subsea production system will be compatible with produced and injected fluids. Chemicals will be injected during start-up and shut-down to prevent hydrate formation. There will also be continuous chemical injection downhole to avoid asphaltene build-up.
Among the safety aspects which had to be addressed on the ship were the high levels of H2S (12,000ppm) in the well fuel gas used for the engine plant. Care had to be taken to avoid sour gas leaks from pipes in the living quarter area. A dropped-object study is currently in progress to look at the impact of damage to subsea wells caused, for instance, by errant mooring system anchors.
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