Achievable storage capacity assessment for multiple trap storage site – a case study of carbon dioxide storage in the German North Sea sector

<p>In order to dampen the effects of anthropogenic climate change, a significant reduction of atmospheric carbon dioxide (CO₂) has to be achieved. Geological sequestration of CO₂ in porous formations represents a potential long-term and large-scale carbon sink. The suitability of a storage site depends upon of wide range of geological parameters, like adequate depth, sufficient storage capacity, favourable petrophysical parameters and safe containment. Existing storage facilities, however, demonstrate that in some cases the initially assessed storage capacity cannot be reached during the implementation stage, which is commonly linked to the hydrodynamic nature of the multiphase flow regime induced. Numerical simulation of the injection dynamics should thus allow to reduce the uncertainty in capacity estimations. This study, therefore, aims to develop a workflow and methods to allow for improved quantification of the achievable dynamic storage capacity.</p> <p>An existing geological structure and its large-scale subsurface setting within the German North Sea sector is investigated as a potential storage site for this study, which is currently characterised within the GEOSTOR research project. The storage structure was mainly formed by salt pillow growth in the Zechstein (Late Permian) and formed the anticline trap in the Middle Buntsandstein (Lower Triassic) sandstone formations with an average pay thickness of about 40 m. Three structural closures at the site could be identified. Using depth-dependent parameters, static storage capacities of 184 Mt, 106 Mt and 96 Mt are estimated. Two closures can be combined into one connected trap at a depth of about 2 km (structural spill-point), which allows to double the static capacity.</p> <p>To quantify the site-specific hydrodynamic effects, a storage aim of 300 Mt CO₂ injection over 30 years at a constant rate of 10 Mt/a, followed by a 100-year post-injection period, is assumed as a basis for the injection strategy development. Numerical simulations are performed using a 3D reservoir model with a model domain of 25 km x 30 km, accounting for the regional boundary conditions and the regional hydrostatic pressure gradient. Simulation results show strong density stratification in the storage reservoir as well as effects of gravity override and pressure superposition. The consideration of the far-field geological boundary conditions leads to a large-scale and long-term pressure accumulation in the model, which limits injection rates. Based on these, suitable well positions and completions ensuring long-term confinement are identified.</p>