Produced water was one of the reasons for Benin’s only offshore field, Sèmè, being shut in after 14 years of production. Vince Buchanan explains how technology is helping the field return to production.
|The free water knock out vessel in fabrication at ProSep's fabrication facility in Houston. Photos from PWA ProSep.|
By the end of this year, for the first time in over a decade, the Sèmè field in the Republic of Benin, West Africa, is due to be operational.
The field was shut-in after 14 years of production, during which time more than 21MM bbl were produced. At the time of field dismantlement, Sèmè was delivering several times more water than it was producing oil and it was no longer economical.
Chemical injection technology and oil recovery in water treatment systems will enable the field to produce a peak 6000 boe/d and it is estimated that an additional 16 years of production can be achieved.
Sèmè is the only producing field offshore the Republic of Benin. It sits in the Gulf of Guinea and consists of three platforms, one well head and two production facilities. The field produced 7627 boe/d at its peak in 1984, but, at the lowest in 1997, just before abandonment, it was delivering only 1207boe/d.
Benin, which shares a border with Nigeria, started producing oil in the 1970s, but output remained low and stopped by the end of the 1990s, when the price of oil became uneconomical and funds for operations dried up. South Atlantic Petroleum (SAPETRO) took over the block in 2004, and earlier this year awarded ProSep, through Procegas, a US$5.1 million contract to provide both the oil and water process packages.
A major issue to overcome was limited storage at the site. The tankage available was only sufficient to store the produced oil with no real facility at site for water storage and every three to four months tankers would have to collect the oil. Using the tankers to transport the water to a disposal site as well as the additional cost for utilizing that site would have made the project not economically feasible. Local disposal wells were not a viable solution due to environmental concerns. The only solution available was to dispose of the produced water into the ocean.
A 1.5km subsea pipeline will be used to move the produced water from the onshore separation facilities offshore. The onshore water treatment plant is designed to meet the standard water quality for the Gulf of Guinea of 20ppm oil in water. This solution allowed this project to become economically feasible.
|The vessel schematics. Image from PWA ProSep.|
To meet the final oil specification of 0.5% basic sediment and water (BS&W), an oil train was developed consisting of a free water knockout (FWKO) feeding two trains of scavenger heat exchangers and thermal electrostatic treaters. This system was developed to provide an efficient and environmentally sound method for heating the process. This was by reducing the total heat load and recapturing energy already expended by cross exchanging the dehydrated oil with oil coming from the FWKO.
The skid-mounted FWKO is fully automated and was provided to remove the bulk water from the process. As the name implies, only free water is removed from the production inlet stream, leaving the emulsified water in the outlet oil, to be treated downstream. The design allows the user to adjust the interface level so retention time can be adjusted to the current flow conditions of the water and oil.
ProSep’s thermal treaters combine both heating and coalescing capabilities in one process unit. The design allows the heating section to meet the treating viscosity requirements of the coalescing section with electrostatic coalescing elements. The treating temperature for a thermal treater is considerably higher than that for an FWKO, in order to meet the stricter water content specification of 0.5% BS&W, by removing the remaining emulsified water from the process stream.
ProSep was tasked with treating 11,000bbl/d of water with a 2000 ppm inlet 100 ppm inlet solids. Charge pumps will be provided to pump the water from the storage tank to the first water treatment vessel, which is a corrugated plate interceptor (CPI).
CPIs are designed to provide the same removal efficiency as a traditional gravity separator in a much smaller vessel, making them ideal for fixed-platform as well as land-based installations. In this application, 95% of solid particles greater than 5micron will be removed from the inlet stream.
As the water exits the CPI, there is a secondary stage of treatment; an induced gas flotation (IGF) vessel. To recover oil and condition water for overboard discharge, re-injection or further polishing through filtration, ProSep’s ProFloat Flotation Systems is used for efficient removal of oil and solids (capacities from 3000-100,000bbl/d of water, with a separation efficiency of up to 98%), while completely containing the process. This is where the stringent discharge quality of 20ppm of oil in the water will be met, which is now common law environmental regulation in West Africa following years of pollution.
|The oil separation train in fabrication.|
The government’s requirement is that the field needs to have at least 20 years life expectancy. But, after five years, SAPETRO need to start producing from a second block, as the amount of water produced from the current block will continue to increase while the oil decreases. After five years, there will be less than 1000bo/d so the longevity will need to come from another block.
This is the first time ProSep technology will be used in West Africa. The contract includes training local staff. About 20 workers are expected to be involved in the operations and maintenance, on and offshore. Almost 1000 Benin workers will be involved in the construction phase and around 50 people will be working on the operations. Sèmè is due to be operational by the
Vince Buchanan is ProSep’s Oil Technology Manager, based in the company’s Houston office. He has worked for ProSep since its inception in 2005 and has over 40 years’ experience in the oil and gas industry.
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