Static and dynamic estimates of CO2 storage capacity in two saline formations in the UK

Estimation of CO2 storage capacity is a key step in the appraisal of CO2 storage sites. Different calculation methods may lead to widely diverging values. The compressibility method is a commonly used static method for estimating storage capacity of saline aquifers: it is simple, easy to use and requires a minimum of input data. Alternatively, a numerical reservoir simulation provides a dynamic method which includes Darcy flow calculations. More input data are required for dynamic simulation, and it is more computationally intensive, but it takes into account migration pathways and dissolution effects, so is generally more accurate and more useful. For example, the CO2 migration plume may be used to identify appropriate monitoring techniques, and the analysis of trapping mechanism for a certain site will help to optimize well location and injection plan. Two hypothetical saline aquifer storage sites in the UK, one in Lincolnshire and the other in the Firth of Forth, were analysed. The Lincolnshire site has a comparatively simple geology, while the Forth site has a more complex geology. For each site both static and dynamic capacity calculations were performed. In the static method, CO2 was injected till the average pressure reached a critical value. In the migration monitoring case, CO2 was injected for 15 years was followed by a closure period lasting thousands of years. The fraction of dissolved CO2 and the fraction immobilised by pore scale trapping were calculated. The results of both geological systems show that the migration of CO2 is strongly influenced by the local heterogeneity. The calculated storage efficiency for the Lincolnshire site varied between 0.34% and 0.61% of the total pore volume, depending on whether the system boundaries were considered open or closed. Simulation of the deeper, more complex Forth geological system gave storage capacities as high as 1.05%. This work was part of the CASSEM (CO2 Aquifer Storage Site Evaluation and Monitoring) integrated study to derive methodologies for assessment of CO2 storage in saline formations. Although, static estimates are useful for initial assessment when less data is available, we demonstrate the value of performing dynamic storage calculations, and the opportunities to identify mechanisms for optimising the storage capacity.