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CO2 can be injected in closed structures or in open aquifers. In a closed structure, the amount of CO2 injected will be restricted by the maximum fracing pressure of the structure with a safety margin. Some of the CO2 will be trapped as free CO2 by the seal of the structure, and a certain amount will be dissolved in the water. In an open aquifer the amount of CO2 will not be restricted by pressure, but it can gradually be trapped as residual and dissolved CO2 in the water phase. In the Trøndelag Platform and Nordland Ridge, both alternatives have been studied. The map shows the outlines of five large closed structures which have been identified in the study. Structures A and B are located SE of the Helgeland Basin and comprise only the Åre-Tilje Trøndelag Platform aquifer. Structure C is bounded by the Ellingråsa Graben to the west, and it could trap CO2 in all the aquifers of the Trøndelag Platform. Structure D belongs to the Nordland Ridge Åre Formation aquifer, while structure E is located in the Ellingråsa Graben, outside the Trøndelag Platform aquifer. The volumes of structures D and E are listed in the table. The volumes of prospects A, B and C are included in the calculation of the Trøndelag Platform aquifers. In a closed aquifer, the limiting factor for the volume which can be injected is the total pore volume of the aquifer, not the pore volume of the structure. Seismic mapping was also carried out east of the Frøya High, south of the Draugen Field, to search for closed structures suitable for CO2 trapping in that area. It was concluded that such structures may exist, but there is uncertainty related to their definition on 2D seismic data and to how far petroleum has migrated into the area east of the Frøya High.
The rectangle in the map shows the model area for the study of open aquifer injection into the Ile and Garn Formations.