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The first approach was to calculate the total pore volume and use a storage efficiency representing a closed system. The second approach was to calculate the pore volumes of the largest closed structures A, B and C presented below, and assume that they are in communication with the larger aquifer (half-open system). The third approach was to simulate injection in the Garn-Ile aquifer presented above, where the injected CO2 volume is restricted because it is not allowed to reach the coastal subcrop.
In the table below, showing the results for the Garn – Ile aquifer, a half-open case and a closed case for the whole aquifer are presented to illustrate how important this is for the estimates of storage volumes. Large volumes can theoretically be stored if the aquifer is in pressure communication with additional large water volumes. In the Garn-Ile case, such pressure communication could take place with the sea along the subcrop line. Another alternative to creating a half-open system might be to inject CO2 and produce water. The most optimistic case would be to assume that closed structures with a large storage capacity exist and could be filled with CO2, without any migration to the half-open eastern boundary. Although interesting structures exist, we have not been able to identify such large storage volumes in closed structures in our mapping of the Garn-Ile aquifer. Based on the structures we can map and the simulations we have performed, we have chosen the lower estimate (closed aquifer) as the most likely scenario.