CO2 Storage in Saline Aquifers: In the Southern North Sea and Northern Germany

CO2 storage in depleted gas fields is attractive but gas fields are unequally distributed geographically and can be utilized only within a restricted window of opportunity. Therefore, CO2 storage in saline aquifers can be expected to become an important element of CO2 capture and storage (CCS) systems. CO2 storage in saline aquifers is studied in the European DYNAMIS project, where special attention was paid to the relation between the geological structure, geological properties and achievable injection rates. This paper presents the results from simulations of CO2 storage in two aquifer structures. One aquifer structure is located in the English sector of the Southern North Sea and one aquifer structure located in Northern Germany. The geological model of the Southern North Sea structure was built using existing knowledge of facies geometry and distribution from literature and well logs and the overall geometry from interpreted horizons. The distributions of properties like lithology, net-to-gross ratios, porosity and permeability were stochastically modelled to represent the braided and meandering river and fan systems within the Bunter Sandstone Formation. High-porosity and high-permeability zones model fluvial channels running through the reservoir. A different approach was chosen for the North German structure. Because there is no information on the facies or petrophysical parameter distribution of the aquifer, three different model scenarios were generated. One model was created with homogeneous properties throughout the aquifer. Two other model scenarios were generated allowing the porosity and permeability to vary within certain ranges. This was done using a sequential Gaussian simulation with the known mean, minimum and maximum values of porosity and permeability, respectively. Due to the size of the aquifer structures and computing limits, up-scaling of the geological model of the Southern North Sea was required. The up-scaling was aimed to retain as much as possible of the modelled geological features such as braided river and fan systems. Several reservoir simulations were carried out in collaboration between TNO and IFP to assess CO2 injection. Given the injection rate constraints, the aim of the injection simulations was to: assess the achievable rate of injection; study the migration and behavior of the CO2 in the reservoir; assess the most appropriate location(s) for the CO2 injection wells using alternate injection strategies. The injection strategy is relevant especially for the Southern North Sea structure, where river and fan systems may results in high-permeability zones through the reservoir. Accessing such systems may significantly improve a reservoir's value for CO2 storage. The effect of such systems on the injection rates is demonstrated. The reservoir simulations also demonstrate the injection rates that can be achieved in tight reservoirs as in the North Germany case, and the effect of spatial variability of geological parameters. This will show whether it is reasonable to assume spatial geological homogeneity, when data on the reservoir formation are scarce. (C) 2008 Elsevier Ltd. All rights reserved