Dipping open aquifers-The effect of top-surface topography and heterogeneity on CO2 storage efficiency

The Forties aquifer underlying the North Sea is used as an exemplar base case to provide a quantitative framework for assessing the CO2 storage efficiency of dipping open aquifer storage units. Storage under a set of putative regulatory constraints is considered: pressure, migration distance and migration velocity. The effects of permeability, heterogeneity, aquifer dip and top-surface topography are all assessed and the results presented in terms of quantitative storage regimes. Permeability and aquifer dip are key determinants of storage efficiency, since they control the flow speed of the CO2 and the amount of pressure build-up. Heterogeneity reduces storage efficiency due to localised pressure build-up, if this is a constraint. However, where pressure does not limit capacity, vertical heterogeneity improves storage efficiency through boosting the lateral sweep of CO2. Top-surface topography introduces structural closures, regions of higher and lower dip than the model average, and channels. When compared to smooth models, structural closures increase efficiency and channels generally decrease efficiency. The net effect of all the competing topographical effects depends on which storage regime describes the smooth model. Overall, it is demonstrated that the storage regime and topography play important roles in determining storage capacity.