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The Snøhvit Field is located in the central part of the Hammerfest Basin in the Barents Sea. The water depth is 330 m, and the reservoirs are found in the Stø and Nordmela Formations (Early and Middle Jurassic age), at depths of approximately 2300 m. The hydrocarbon phase in the Snøhvit main field is largely gas with minor condensate with a 10-15 m thick oil leg.
The Stø Fm is mainly shallow marine, while the Nordmela Fm was deposited in a coastal environment. Maximum burial of the reservoirs was approximately 1000 m deeper than the present depth, resulting in massive quartz cementation of the sandstones and a poorer reservoir quality below 2900-3000 m. The reservoir quality in the fields is fairly good. Porosity as high as 20% and permeability at 700 mD have been interpreted on logs in the best zones of the Stø Formation. The Snøhvit field developments include the Askeladd and Albatross structures. These structures have reservoirs in the same formations. In addition the 7121/4-2 Snøhvit North discovery contains gas and condensate which is still not in production.
The natural gas produced from the fields contains about 5-8% CO2. CO2 is separated from the gas at Melkøya in an amine process. Compressed CO2 in liquid phase is returned to the field in a 153 km long pipeline, to be stored 2500 m below sea level.
CO2 storage at the Snøhvit Field started in 2008, and CO2 was until April 2011 injected in well 7121/4F-2H in the Tubåen Fm, which is dominated by fluvial sandstone. After a while the pressure built up faster than expected, and an intervention was performed to avoid fracturing of the seal. In 2011, the injection in the Tubåen formation was stopped, and the shallower Stø formation was perforated as the new storage formation for CO2.
After the intervention in 2011, all CO2 from the Snøhvit Field has been injected in the water zone of the Stø Formation. Until 2013 a total of 1.1 Mton CO2 has been injected in the Tubåen Fm and 0.8 Mton in the Stø Formation.
In contrast to the Tubåen Formation, the Stø Formation is in pressure communication with the gas producers on Snøhvit, and no significant pressure build-up is expected in the injection site. However, a new injection well for CO2 is considered in segment G (SW-SE profile) to prevent future migration of injected CO2 into the natural gas of the main Snøhvit Field. This segment is located between the Snøhvit main structure and Snøhvit North.
The new well will inject into the Stø Formation. In order to investigate the storage potential for the new well, the minimum and maximum pore volumes with good communication to the planned well area have been estimated. The maximum connected pore volume, “Snøhvit 2800”, represents the pore volume of the water zone in the Stø, Nordmela and Tubåen Formations in the Snøhvit and Snøhvit north area down to 2800 m.
2800 m was selected because permeability deteriorates below this depth. The minimum pore volume, “Snøhvit central Stø”, was calculated as the pore volume of the Stø Formation in the areas surrounding the G segment. This is interpreted to represent a water volume where good communication to the new injection site is very likely. Communication through major faults is not expected where the throw is larger than the thickness of the Stø Formation, but in this minimum case, structural ramps create corridors of communication within the Stø Formation.
The calculation of maximum and minimum pore volumes resulted in 6400 Mm3 for the “Snøhvit 2800” case and 680 Mm3 for Snøhvit central Stø. These pore volumes indicate that there are sufficient aquifer volumes available to support the planned CO2 injection in the Stø Formation at Snøhvit.
The expected flow direction for the injected CO2 will be towards the west. As seen in the well section profile, thick packages of shale seal the Stø Formation and are expected to prevent vertical leakage of CO2. Seepage of gas along the faults is regarded as a risk, in particular in the areas with shallow gas clouds. Monitoring of the injection (section 9) will be important to control the injection and the movement of the CO2 through time. Data quality in the area is good, except in the areas with gas clouds. There is sufficient experience with injection in the Stø Formation to conclude that the area has been matured as a storage site.
In addition to the CO2 storage potential related to the ongoing injection in the Stø Fm, interpretation and calculations were performed to evaluate the storage potential in the Snøhvit Jurassic aquifer consisting of the Stø, Nordmela and Tubåen Formations above the spill point for the main Snøhvit Field. This pore volume case is called the Greater Snøhvit area. It may represent the pore volume which has been filled with hydrocarbons in geological history and is analogous to the Greater Albatross and the Greater Askeladd areas. The results show a pore volume of 4100 Mm3.
All parameters used in the calculations and presented in the table, are based on well information. Key wells are 7121/4-1, 7121/4-2, 7120/6-1 and 7121/4F-2H. Porosity and permeability trends and input to depth conversion were derived from several wells in the area. The reservoir quality varies in the different formations in the “Snøhvit 2800” case. The best quality is seen in the lowermost part of the Stø Fm, but more shaly zones in the middle part of the formation most likely act as an internal barrier or baffle for injected CO2.
Data quality is good, as indicated in the table. Due to possible conflicts with the petroleum activity, maturation is shown in blue colour. This represents a theoretical volume of the CO2 storage potential calculated for the Jurassic aquifer. Uncertainty in the calculation is mostly related to interpretation, depth conversion and a simplified approach to the distribution of the aquifer.
The Snøhvit development includes several gas discoveries within the greater Snøhvit, Askeladd and Albatross structures. The potential of CO2 storage after abandonment of the smaller of these discoveries was calculated from the pore volume of their gas zones. It was assumed that after production there will remain residual gas and minor amounts of free gas and that injected CO2 can occupy 40 % of the initial pore volume. Based on this assumption, which is regarded as conservative, the storage capacity of the abandoned field is 200 Mtons.