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Approximately 1 Mt CO2 from the Sleipner Field has been successfully injected annually in the Utsira Formation since 1996, proving that the formation is an excellent reservoir for CO2 storage. Due to its size, the formation has been regarded as attractive for storage of large volumes. However, the formation is part of a much larger sandy deltaic complex located at both sides of the UK-Norway boundary. The upper parts of this system are buried to less than 200 m below the sea floor, and the communication between the different sandy formations has not yet been studied in detail. In this atlas we present the results of an NPD study based on 3D seismic interpretation and biostratigraphy. The Miocene and Pliocene aquifer is subdivided into four major units which are in communication towards the west. The largest pore volumes in the system are in the Utsira and Skade Formations, which appear to be separated by a Middle Miocene shale in the eastern/distale parts. There is a regional dip upward towards the west, and consequently there is a risk that injected CO2 will migrate updip to levels which are too shallow to be accepted for storage. Three areas are assumed suitable for CO2 injection.
Pore volumes for this aquifer are presented together with storage capacities calculated for the three suggested sub-areas.
To estimate the capacity of CO2 storage in a southern part of Utsira/Skade aquifer, a reservoir model was built to simulate the long-term behavior of CO2 injection. The model covers 1600km2 in the southern part of the Norwegian sector. The study illustrates potential migration and forecast possible migration of CO2 from the Skade Formation into the Utsira Formation above.
CO2 injected in the Skade sand may penetrate through a intermediate clay layer into Utsira sand if the clay has permeability from 0.1 mD or higher. Approximately 170 Mt CO2 can be injected in Utsira-Skade aquifer within the segment model, with four horizontal wells injecting over 50 years, with BHP change of 10 bars, and with no water production. After 8000 years of storage, the dissolved part is nearly 70%, residual trapping is less than 1%, and mobile CO2 has decreased to 29% of the total amount of injected CO2.
These results are based on a residual saturation of CO2 of 0.02. If a residual saturation of CO2 is 0.3, CO2 trapped by residual mechanisms is 13% of total CO2 injected after 8000 years. Mineral trapping by geo-chemical reactions was not considered in the simula-tion, but will add additional storage capacity.
The NPD has calculated that 0.5-1.5 Gt of CO2 can be stored in the southern area of the Utsira-Skade aquifer, based on in house simulation calculations.