Design of carbon dioxide storage in aquifers

We propose a carbon storage strategy where CO2 and brine are injected into an aquifer together followed by brine injection alone. This renders 80–95% of the CO2 immobile in pore-scale (10 s m) droplets in the porous rock. Over thousands to billions of years the CO2 may dissolve or precipitate as carbonate, but it will not migrate upwards and so is effectively sequestered. The CO2 is trapped during the decades-long lifetime of the injection phase, reducing the need for extensive monitoring for centuries. The method does not rely on impermeable cap rock to contain the CO2; this is only a secondary containment for the small amount of remaining mobile gas. Furthermore, the favorable mobility ratio between injected and displaced fluids leads to a more uniform sweep of the aquifer leading to a higher storage efficiency than injecting CO2 alone. This design strategy is demonstrated through the incorporation of a recently developed trapping model into a field-scale streamline-based simulator. The new model includes gas trapping and relative permeability hysteresis and is based on pore-scale modeling results. One-dimensional results are verified through comparison with analytical solutions. Results are then shown for storage in a North Sea aquifer. We design injection to give optimal storage efficiency and to minimize the amount of water injected; for the cases we study, injecting CO2 with a fractional flow between 85% and 100% followed by a short period of chase brine injection gives the best performance. Sensitivity studies were conducted for different rock wettability and using the Land trapping model. The effectiveness of our proposed strategy is very sensitive to the estimated residual CO2-phase saturation.