Abstract
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.
Highlights
The estimation of CO2 storage capacity is used to assess the economic and geological viability of CO2 storage at different scales from individual sites to national assessments (Gammer et al, 2011; Norwegian Petroleum Directorate, 2012; US Department of Energy, 2010)
The storage capacity of potential storage sites can depend upon many characteristics
Its scale was chosen as representative of a potential licensable open aquifer storage unit: smaller than the basin scale ∼100 km × 100 km, but larger in areal extent than most oil and gas fields
Summary
Volumetric estimation methodologies, e.g. CSLF, (2008), Vangkilde-Pedersen et al (2009), provide a simple and applicable storage capacity estimation method. These methods do not directly take into account any temporally or many spatially varying effects that may affect storage capacity, but assess the CO2 storage capacity as the mass of CO2 that could be stored, as a fraction of the aquifer pore volume. A number of national capacity estimation studies (e.g. Radoslaw et al (2009), Lewis et al (2009), Ogawa et al (2011)) have used these volumetric estimation methodologies
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