Statoil has begun a pilot program of foam-assisted water alternating gas (Fawag) injection on the Statfjord field, seeking to extend the benefits this technology has brought on the Snorre field.
The injection program was completed in June using the B5 well on Statfjord B platform; however, it will take another year before the results can be assessed, according to Senior Reservoir Engineer Tone Botnen.
Fawag technology was pioneered on Snorre by Saga Petroleum, the original operator, in the mid 1990s. It was first implemented on the field in 1997. Norsk Hydro maintained the program when it took over Saga in 2000, and Statoil in turn when it assumed the Snorre operatorship at the beginning of this year.
Fawag can only be introduced in reservoirs on which water alternating gas (WAG) injection is already in use. In WAG, the water displaces the lower part of the oil-bearing sands, and gas fills the upper part and attics. Although WAG is proven as a means of enhancing oil recovery, it has been observed that the gas often rises to the top of the reservoir relatively quickly, and its presence can be detected in the oil produced from this zone.
Fawag is intended to create a foam barrier that impedes the upward passage of the gas, forcing it to spread laterally and in the process contact previously unswept parts. To achieve this, water and surfactant are injected simultaneously over several days, followed by gas. The foam is created in the area close to the wellbore, at first making it difficult to inject gas. Injectivity gradually improves as the gas finds paths unimpeded by the foam.
On Snorre, WAG injection is used over the whole field. The injection wells are mainly down-dip, so the gas has a tendency to go up-flank fairly quickly and make an early breakthrough into the production wells, according to leading reservoir engineer Lars Ras-mussen, who has worked on the Fawag development on Snorre since the beginning. The original Fawag pilot was carried out in 1997, using a well from which gas breakthrough during WAG had taken place in the relatively short period of four weeks. The results of the pilot were not totally conclusive, however, as the gas arriving in the production well was identified as coming from several sources.
A second injection attempt in 1999 using the P32 injector and P39 producer in the western fault block was successful. Over one year, the injection program was responsible for adding some 1.3 MMbbl of oil to production. The additional volume was worth NKr 200 million and was achieved at a cost of less than NKr 10 million.
Fawag uses ordinary surfactants such as those employed in the cosmetics industry. For the Snorre tests, Statoil screened surfactants beforehand, but the effect of the technique appears to be relatively insensitive to the composition of the surfactant. Statoil selected the surfactant C14C16 AOS, which the company has used for subsequent Fawag programs.
The surfactant was purchased from a supplier in France, delivered to the Snorre operations base at Florø on Norway's west coast, and shipped out to the platform by supply vessel. A total of 1,800 metric tons of surfactant was used for the two tests. The volume did not present a problem, as there was spare storage capacity available in the ballast tanks of the Snorre TLP. However, with such large volumes involved, it is clear that there are logistical issues to be considered, Rasmussen points out.
Injection trials on Statfjord B using the Fawag technique were completed in June.
Further Fawag injection will be carried out on Snorre, though the wells used in the previous program are not available because the injector cannot take gas for the time being. There are plans to extend injection to take in new well pairs, perhaps beginning in early 2004, Rasmussen says.
The Fawag experience has been limited to the Statfjord formation but is likely to be extended to the Lunde formation. Operationally, there is no challenge, but as the Lunde is more stratified than the Statfjord, it is more difficult to track the passage of fluids in the reservoir. In the case of Statfjord, the pilot is being carried out in the Lower Brent formation, which is similar to the upper Statfjord formation in the Snorre field.
On Snorre, batch Fawag treatments are being considered, with injection taking place through a number of wells in series, involving perhaps 10- to 15-well pairs.
Statfjord B platform, site of recent Fawag injection tests by Statoil.
On Statfjord B, the injection program was carried out in two steps. A total of 126 tons of surfactant were injected in January and a further 189 tons in May and June. In the first phase, the surfactant was stored in mud tanks, as no drilling was taking place on the platform at that time. In the second phase, completion tanks were used, but storage was limited and surfactant supplies had to be delivered to the platform during the injection program.
The B5 injector supports a number of production wells, and it is planned to inject tracers into the gas to track its course. Further Fawag programs are under consideration, possibly on one of the other Statfjord platforms.