FAWAG: A Breakthrough for EOR in the North Sea

Abstract

Abstract The application of FAWAG (Foam Assisted WAG) on the Western fault block at the Snorre field has qualified foam as a gas mobility agent for North Sea reservoirs. Results from the foam mobility control trial on WFB at Snorre, well pair P32-P39, were conclusive. The results show that gas breakthrough was delayed and the Gas-Oil-Ratio (GOR) was considerable lowered compared to pre-foam gas injection cycles. Large volumes of gas have been stored in the reservoir. Analysis has shown that gas material balance, decline analysis, and simulations all give consistent results. The expenses for FAWAG on Western Fault block (WFB) was 1M USD, and additional oil recovery value is estimated to ~ 25–40M USD at current oil prices. The FAWAG on WFB has been history matched using a commercial foam simulator. Assuming the same foam properties as in the history match, simulations have been performed to evaluate further potential for FAWAG on WFB of the Snorre field and on the Visund field. These predictions show a significant potential for increased oil production and storage of gas: Simulations of a 2nd FAWAG injection in the same well pair, P32-P39, estimate an increased gas storage at the same level as for the 1st FAWAG injection, and indicate a maximum increased oil recovery potential in the range of ~ 400 000 Sm3. The potential for FAWAG in a neighboring well pair in WFB indicate an increased gas storage of ~ 106 MSm3 and a maximum increased oil recovery potential up to ~ 650 000 Sm3. The potential for FAWAG injection in the N1 segment of the Visund field suggest an increased gas storage potential of 191 MSm3 and an increased oil recovery potential of 217 000 Sm3, when injecting 400 tons of 38% active surfactant. Introduction Foam is applied for gas shut-off and to improve of sweep efficiency during gas injection. The experience with foam in the North Sea involves foam for mobility control and production well treatments1–3. The production well treatments have shown varying degree of success, reducing production GOR for weeks up to more than 6 months. The best GOR reduction has been observed in a gas coning situation4. Both the Snorre oil field and the Visund oil field is located in the Norwegian sector of the North Sea, the Tampen area, about 150 km off the coast. The Snorre reservoir is a massive fluvial deposit within rotated fault blocks5. The reservoir is a high-pressure reservoir (>300 bar) and the formation temperature is 90°C. The FAWAG (Foam Assisted Water-Alternating-Gas) project has been a full-scale field demonstration of the use of foam to improve gas sweep efficiency during WAG injection, partly funded by the European Commission's Thermie program. The project was initially started in 1997 on the central fault block (CFB) of the Snorre field (Figure 1). During planning of the test experience was gained during the production well treatment in P-18 and the injection tests in P25A3–7. The pilot area had a downdip WAG injection from the injection well P25A towards the producer P18. This is the largest ever foam application in any oil field, injecting a total of ~ 2000 tons of commercial grade surfactant and consisted of two injectivity tests and two full-scale treatments. The FAWAG injection on CFB was stopped in early 1999 due to a gas leakage in P25A. It was decided during the autumn of 1999 to change the pilot area from the CFB to Western Fault Block (WFB) of the Snorre field. The selected well pair was the injector P-32 and the production well P-39. The objectives of the FAWAG in the Western Fault Block was to increase sweep efficiency during gas injection, thus to increase the storage of gas in the reservoir and to reduce the producing GOR in production well P-39.