In deepwater Angolan block 17, Total has brought onstream two FPSO-based production hubs and plans two more. According to an article in the company’s latest house magazineEnergies, it also plans to extend this model to ultra-deep block 32, where it has at least five commercial finds.
However, each combination brings its own technical problems. In the case of the next project on the line-up, Pazflor, embracing the Perpetua, Zinia, Acacia and Hortensia fields, the tieback distances are greater, at up to 60-km. Additionally, Perpetua and Zinia’s Miocene crudes are even heavier than Dalia’s and are also “non-eruptive.” So, the planned subsea wells will all have to be in service from startup.
The preferred solution involves subsea separation of gas and liquids, at a depth of around 1,000 m, which would also be reliable enough to function for long periods without intervention. The proposed technique is intended to eliminate the risk of hydrate formation and also to limit the energy needed to lift the fluids to the FPSO.
The Institut Francais du Petrole has formed a joint venture with Total to test the proposed technique at IFP’s research center in Solaize, outside Lyon. According to IFP’s Claude Mabile, work on this study started early in 2006, leading to construction of a Gas-Oil Water Separation Platform (GOWSP). This is a closed loop facility that can operate with gas, crude oil or water.
The GOWSP has overall dimensions of 35 x 25 x 5 m, and can test flow rates of up to 1,000 cu m/hr for gas, and 15 cu m/hr for both oil and water, at pressure of up to 55 bar, and with over a temperature range of 40-80°C. The facility incorporates the following main components:
- A 6 m long, 1.5 m dia. storage tank holding maximum volumes of 10.8 MMcm; oil/water dosing pumps, each driven by 25 kW of electrical power, provide flow rates of up to 12 cu m/hr
- Liquid heat exchangers occupying an area of 7.7 sq km, providing heat capacity of up to 47 kW
- A compressor for gas circulation, with gas flow rates varying from 300-1,100 cu m/hr
- A gas-cooling exchanger providing 19 kW of cooling capacity
- One 2 m long, 70 cm ID, horizontal three-phase separator, again operating at 55 bar over a 40-80°C temperature range, capable of handling volumes of 1.15 cu m
- One 3 m high, 50 cm ID vertical separator, with the same pressure and temperature values, but handling volumes up to 0.40 cu m.
Inside each separator are pressure and temperature sensors, multiple-level sampling devices, an oil level sampling instrument, and two inter-phase gauges. There are also flowlines up- and downstream of the separators which are instrumented with further transducers, mass flowmeters, an oil viscometer and an oil gammameter, and various other flow sampling devices.
The dual aim of the trials to date has been to test both the performance of the proposed separators and the various system internals.
“Conventional three-phase separators can cause problems,” Mabile explains, “and if they don’t perform as they should, that can affect resultant oil quality. And if that is the case, you must perform post-treatment.
“Some of the subsea separator designs put forward to date have maybe been bigger, heavier, and more expensive than they should be. Recently, we have been testing a prototype vertical separator. If this equipment could be placed directly in the well subsea, it would be much easier to install and operate than a multiphase pump system.”
IFP has also been working with Total to improve flow assurance modelling, Mabile says. “We are engaged in a four-year study with Total to examine an alternative, open-software platform, based on the CAPE-OPEN standard developed by the process industry.
“The aim is to promote easier interaction, allowing the operator to simulate and optimize a full field development system by simultaneously tying together modules such as near-wellbore modeling, multiphase flow, hydrate formation, fluid dynamics, gas lift and so on, right up to the surface installation.
“Still on the subject of hydrates, IFP is working a lot now on transportation issues. We have launched proposals for two joint industry projects to study the risk of pipeline blockages for companies planning to transport oil and gas as hydrate slurries over long distances. The pressure drop of the proposed mixture also needs investigating. We will be doing a combination of experimental work on the Lyre loop at Solaize. We need four or five companies to sponsor this research.”
