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5| CO 2 injection modelling within saline aquifers
In order to better understand how the available storage volume within a saline aquifer might be affected by injection of large amounts of CO 2 the Tay Sandstone saline aquifer (Figure 13) was selected for further study. Key issues governing whether large volumes of CO 2 can be safely, reliably and securely injected into and stored within a saline aquifer were investigated by modelling the injection of CO 2. The main areas of investigation were the potential of the saline aquifer to store the specified volumes of CO 2, the injectivity (including the number of wells required), the effect of orientation and location of wells, and the migration path of CO 2 away from the injection points. Sensitivity to injection rate, the number of wells, length, and spacing and location of wells were investigated. Two CO 2 injection scenarios were investigated addressing sources from Scotland and imported CO 2:
- a baseline case of 15 Mt/year
- a high-use case at 60 Mt/year.
5.1 Storage capacity of the Tay saline aquifer
Numerical modelling of the amount of CO 2 that can be stored in the Tay saline aquifer gives a wide range of possibilities according to whether the saline aquifer is considered 'open' or 'closed'. It is not yet clear which is the case. For this study, the Tay saline aquifer was modelled for both scenarios.
- Tay saline aquifer open (water naturally migrates out of the saline aquifer) - at an injection rate of 15 Mt/year for 25 years, the saline aquifer can readily store 375 Mt CO 2.
- Tay saline aquifer closed (water does not migrate out of the saline aquifer) - the saline aquifer can store 375 Mt CO 2 provided water is produced at a rate of 40, 000-60, 000 m 3/day.
Without water production it can store only 155 Mt CO 2 (0.4% of total pore volume) because of the increase in pressure as the CO 2 is injected.
5.2 Investigation of sensitivities in modelling the saline aquifer
- Although the total volume of CO 2 injected at 15 Mt/year for 25 years injection is less than 1% of the total volume of water in the Tay formation, the average reservoir pressure increased 50% by the end of the injection period. The pressure then reduced gradually as the CO 2 dissolved in water.
- The injectivity of the Tay saline aquifer is very good, based on the reservoir properties collected from current oil fields.
- Injection pressure varies with the porosity, permeability and the amount of reservoir rock.
- To reduce pressure at the injector well, additional injectors are recommended for the 15 Mt/year plan. The injection rate for one well should be less than 6 Mt/year.
- Unless it is 'open', injection of 60 Mt/year CO 2 over 25 years into the Tay saline aquifer is likely to lead to excessive pressure.
- The movement of CO 2 within the saline aquifer can be controlled by appropriate location of injector wells.
- Injected CO 2 tends to move higher within the reservoir, but when it dissolves in water it tends to move downwards.
- If producing hydrocarbon fields are connected to the saline aquifer, and CO 2 is injected into their vicinity, the CO 2 tends to move towards them, as they may be at a lower pressure than the saline aquifer, and are usually higher on the structure.
- The presence of a relatively impermeable layer within the saline aquifer did not act as a significant barrier to the lateral movement of CO 2.
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