Design of carbon dioxide storage

Carbon capture and storage (CCS) – the collection of carbon dioxide (CO~2~) from industrial sources and its injection underground – is an important technology to reduce CO~2~ emissions to the atmosphere, mitigating climate change. The North Sea, with mature hydrocarbon fields and saline aquifers offers an attractive storage location for CO~2~ produced by the UK’s gas and coal-fired power plants. The principal concern with CCS is to ensure that the CO~2~ does not leak into the oceans or atmosphere over hundreds or thousands of years. We propose a storage strategy where CO~2~ and brine are injected together followed by brine injection alone. We predict that using this technique around 95% of the CO~2~ can be rendered immobile in pore-scale (10s micron) droplets in the porous rock. Over thousands to billions of years the CO~2~ may dissolve or precipitate as carbonate, but it will not migrate upwards and so is effectively sequestered. This design is demonstrated through numerical simulation of field-scale flow through a North Sea aquifer coupled to an experimentally-based model of small-scale trapping. The CO~2~ is trapped during the decades-long lifetime of the injection phase, avoiding the need for extensive monitoring for centuries. The method does not rely on impermeable cap rock to contain the CO~2~; this is only a secondary containment for the small amount of remaining mobile gas. Furthermore, the reduced mobility ratio between injected and displaced fluids leads to a more uniform sweep of the aquifer leading a larger storage capacity than injecting CO~2~ alone