Foam-Assisted WAG: Experience from the Snorre Field

Abstract

Abstract The Foam Assisted WAG (FAWAG) has been a large-scale demonstration of foam for gas mobility control. Foam has been applied at the Snorre, North Sea Brent-type sandstone reservoir, for different purposes. First initiated as a gas shut-off production well treatment, thereafter as two large-scale gas mobility control processes. Combined water and gas injection (WAG) is the main oil recovery method at the Snorre field. Early gas breakthrough in some production wells has limited the oil production. Gas production control is one of the major reservoir management challenges on Snorre. Foam for mobility control has the potential to improve gas sweep in the Snorre WAG process. Results from the last trial on the Western fault block were very conclusive. Gas breakthrough was delayed, but equally important the Gas-Oil-Ratio (GOR) was considerable lower than gas injection cycles prior to the foam treatment. The paper summarizes all the foam field trails at the Snorre field, with emphasis on interpretation of the last foam application on the Western fault block. The Snorre FAWAG is the world's largest application of foam in the oil industry. The logistics involved transport of 2000 tons of chemicals from central Europe to the field location. The application of FAWAG has qualified foam as a gas mobility agent for North Sea reservoirs. Large volumes of gas have been stored in the reservoir. The expenses for FAWAG on Western Fault block (WFB) was 1M USD, and additional oil recovery value was ~ 25–40M USD at current oil prices. 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 treatments1,2. Table 1 gives a summary of the tests performed. The production well treatments have 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 situation3. The foam tests on Snorre are summarized in Figure 1. The Snorre oil field is located in the Norwegian sector of the North Sea. The reservoir is a massive fluvial deposit within rotated fault blocks4. The reservoir pressure is 300 bars 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. During planning of the test valuable experience had been gained during the production well treatment as mentioned and also injection tests in P25A. 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. Results Production well treatment In mid-1996 a foam treatment was performed on production well P18 located in the Central Fault Block of the Snorre Field. The P18 well had suffered high GOR due to premature gas breakthrough. The purpose of the field pilot was to reduce the production GOR in P18. The surfactant used in the foam treatment was a C14/16 commercial grade alphaolefin sulfonate. Production well treatment In mid-1996 a foam treatment was performed on production well P18 located in the Central Fault Block of the Snorre Field. The P18 well had suffered high GOR due to premature gas breakthrough. The purpose of the field pilot was to reduce the production GOR in P18. The surfactant used in the foam treatment was a C14/16 commercial grade alphaolefin sulfonate.