In 2007 ProSep provided the Ekofisk 2/4 J platform with a CTour produced water treatment package. CTour is proprietary technology that uses existing gas condensate combined with ProSep’s high-efficiency mixers to enhance oil and water separation. The implementation of the CTour began in 2004 with a FEED study followed by pilot tests during 2004 and 2005. The results of the pilot test showed significant reductions of oil in water and polycyclic aromatic hydrocarbons (PAH).

These results led to the installation of a full-scale system in 2007 that is still in operation.

Evaluating operational data from the period after installation in 2007 until 2014, it can be seen that CTour technology reduced the oil-in-water concentration down to about 9 ppm in 2014, which is a 60% reduction from installation in fourth-quarter 2007. In CTour’s first year of operation a 29% reduction of oil in water and a 39% reduction in PAH was observed compared to conventional treatment alone.

Discovered in 1969 by Phillips Petroleum Co., Ekofisk remains one of the most important oil fields in the North Sea. It was the first discovery of oil in Norway after the drilling of more than 200 exploration wells in the North Sea, when Phillips started producing directly to tankers from four subsea wells.

Oil production is planned to continue until at least 2050.

The Ekofisk Complex comprises installations connected with bridges on the central Ekofisk Field. The complex is a center for the Ekofisk Field itself as well as other fields in the Greater Ekofisk area.

CTour process

The CTour process removes dispersed oil and dissolved aromatic components from large volumes of produced water. Condensate (typically from the compressor knock-out drums) is injected and mixed into the produced water to extract dispersed and dissolved hydrocarbons. The condensate is then separated from the produced water using conventional separation technology such as hydrocyclones, induced gas flotations or compact flotation units. Typically, the reject from these units is routed back upstream to the first-stage or second-stage separator, or it can be directed to the crude line from the separators.

The CTour process enhances conventional oil/water separation equipment by the addition of the condensate. Following the absorption of condensate, the oil density decreases, leading to a larger density difference between oil/condensate and produced water, and the oil droplets grow larger. With the addition of the CTour process the same conventional oil/water separation equipment will see increased efficiency because of these characteristics.

On Ekofisk 2/4 J CTour is used to clean produced water from the Ekofisk Field. The facility treats about 150 Mbbl/d of water before discharging it to sea. From fourth-quarter 2007 until 2014 all produced water in the Ekofisk Field was discharged after treatment via CTour. Since 2014 only produced water from Ekofisk 2/4 J, Ekofisk 2/4 X and Ekofisk C wells is processed for discharge on Ekofisk 2/4 J, accounting for about 65% of all produced water from the Ekofisk Complex. CTour also has been used on Eldfisk since 2015 as well as on Aker BP’s Valhall since 2013.

Process description and conditions

Ekofisk 2/4 J processes production in a high-pressure (HP) and a low-pressure (LP) separator. There is also a test separator for testing of Ekofisk 2/4 X and Ekofisk 2/4 C wells. The produced water from the HP and LP separator enters primary produced water treatment (de-oiling hydrocyclones) for preliminary treatment. The underflow of the hydrocyclone with oil-in-water of 70 mg/l at 950 cu. m/hr (34,000 cf/hr) is mixed with 0.3% condensate, where the high-efficiency mixer allows the oil droplets to become larger and less dense. This change in oil droplets is the main principle in the CTour process for increasing performance in the de-oiling hydrocyclone. This new mixed water stream is routed to the CTour hydrocyclones for treatment. The underflow then enters the degasser before being discharged to sea. The reject flow from the CTour hydrocylones is collected into a CTour condensate separator and treated by a smaller CTour system.

Operational data

Figure 1 displays the volume of produced water in cubic meters per year that the Ekofisk Field discharged to sea from 2005 to 2014. From 2008 all produced water was treated via CTour for discharge, with flow rates as high as about 210 Mbbl/d of water. From 2014 65% of the total produced water was discharged via CTour on Ekofisk 2/4 J, which was about 150 Mbbl/d of water.

In the Norwegian sector of the North Sea operators and the OSPAR Commission legislation must comply with specific Norwegian legislation calling for zero environmental impact from overboard discharges. The Environmental Impact Factor (EIF) is used as a tool to quantify the harmful effect of discharge to the environment.

The EIF was developed as a management tool on internationally agreed procedures for hazard and risk management. The tool models the dispersion of produced water and calculates the predicted environmental concentration vs. predicted no-effect concentration ratio, accounting for the volume and composition of oil, aromatic components and production chemicals of the produced water.

Figure 2 highlights the improved performance of the produced water treatment on Ekofisk 2/4 J since the installation of CTour in fourth-quarter 2007. CTour reduced the oil-in-water discharge by 29% in 2008, and a 55% reduction was observed in 2014 compared to 2007. During this time, the produced water volume also increased by as much as 3.9 MMcm (137.7 MMcf) from 2008 to 2012. However, it has been reported that the increase in 2010 was a result of operational problems.

Figure 3 presents the concentration of PAH in the produced water discharged to the sea from 2005 to 2014. During the first year the CTour system was in operation, a reduction of 39% was observed for PAH. By 2014 the PAH concentration had dropped by more than 60% compared to 2007, the last year of operation without the CTour process.

It can be concluded that CTour contributed to about a 60% reduction of EIF value in 2014 compared to 2007.

References available.

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