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Prospect A is defined as a closed structure located east of the Loppa High in the southernmost part of the Bjarmeland Platform, west of the Nysleppen Fault Complex. The structure is drilled by the7125/1-1 well. 1 m oil saturation was encountered in the top of the main reservoir, with a residual oil zone below. The main reservoir zone evaluated for CO2 storage is the Stø Formation with a thickness of 130 m in well 7125/1-1. The Stø Formation is part of the Realgrunnen Subgroup, which thickens westwards into the Hammerfest Basin. Depth to top of the interpreted structure is about 1400 m. The Stø Formation overlies a thick Triassic succession of the Sassendalen Group. No shallow gas indications have been observed along the boundary faults to the south. However, the residual oil observed in the exploration well 7125/1-1 indicates that leakage or seepage has taken place. As discussed in section 6.2.1, this seepage is believed to be a slow process, and the seal risk is characterized as relatively low. The geomodel of the Realgrunnen Subgroup is based on interpretation of 3D seismic data and data from the exploration well. The geomodel is developed into a reservoir simulation model in order to study the behaviour of CO2 injection in this reservoir with brine and residual oil.
The simulated CO2 injection well is located down dip with plume migration towards south-southeast, but alternative locations with different injection rates have been simulated. The injection period is 50 years, and simulation continues for 1000 years to follow the long term CO2 migration effects. CO2 will continue to migrate upwards as long as it is in a free, movable state. Migration stops when CO2 is permanently trapped, by going into a solution with the formation water or by being residually or structurally trapped (mineralogical trapping is not considered here).
Confinement of CO2 requires prevention of migration of the CO2 plume to potential leakage areas. For Prospect A, the fault/graben system to the west and south will seal the structure in that direction. The structurally highest point on the Bjarmeland structure is located along this fault.
To obtain confinement of CO2, the injection pressure must not exceed fracturing pressure. The fracturing pressure increases with depth. The depth of the maximum acceptable pressure increase was calculated for the shallowest point of CO2 plume migration during the period of injection (1400m). The structure is hydrostatically pressured. Fracture gradients established from the North Sea and Norwegian Sea indicate that a maximum acceptable pressure increase of 75 bar could be applied at that depth. However, as discussed in section 6.2.1, the fracture gradients in the eroded regions of the Barents Sea could be lower, and the effects of a maximum pressure of 30 bar were also investigated. The pressure build-up depends on the volume and connectivity of the surrounding aquifer. The aquifer used for modelling covers the area of the thick Stø Formation and has excellent reservoir properties. Further north in the Bjarmeland Platform, the Realgrunnen Subgroup is thinning, but good porosity and permeability is developed in a large area.
The volume of the active aquifer system is conservatively estimated to be 25 times the volume of the geological model, and this volume is added to the simulation model volume. In the simulation model, CO2 injection was stopped when the plume reached the eastern boundary of the model. This boundary was regarded as the spill point of the structure. East of this boundary there is only seismic coverage by 2D lines, and the spill point is regarded as conservative.
Prospect B is located in the transition zone between the Hammerfest and the Nordkapp basins, about 70 km northeast of the Goliat Field. It is defined at a NW-SE trending fault block with a structural closure. The main reservoir is in the Stø Formation (Realgrunnen Subgroup). The structure has been drilled by the well 7124/4-1 S, where the Stø Formation was encountered at a depth between 1259 and 1312m. The formation consists of a 52m thick homogeneous unit of mainly fine to medium grained sandstone with good reservoir properties. The well was water-bearing and there are no indications of hydrocarbons. Interpretation of the prospect is based on good 3D seismic data and data from the 7124/4-1S well. The 3D seismic data set does not cover the spill point SE of the structure, which means that the calculated volume is conservative.
The geosection illustrates the geometry of aquifers (yellow) and sealing formations (green). The primary seal is the Hekkingen Formation, and thick Cretaceous shaly sediments act as a secondary sealing layer. The reservoir quality and storage capacity is summarized and illustrated in the table below.
The reservoir properties used in the evaluation are based on the 7124/4-1S well. Prospect B is defined as a half open structure, where the boundary towards the west is structurally closed by a major fault and a graben structure. The structure is segmented by several smaller WSW-ENE trending faults.
Approximately 50 metres of Hekkingen shale overlie the sand-rich Stø Formation. The faults cutting through the Stø Formation seem to terminate in the Hekkingen shale, hence the seal risk is considered to be relatively low.
The structure consists of two main segments. If a CO2 injector is placed in the northern segment, the CO2 plume can migrate and spill into the structurally higher segment to the south. The calculated CO2 storage capacity for both segments is 19 Mt based on a constant thickness of the Stø Formation.