Hydrodynamics in subsurface CO2 storage: Tilted contacts and increased storage security

Abstract

Hydrodynamic aquifers with horizontal variations in overpressure and brine flow have been reported from sedimentary basins worldwide. In a hydrodynamic aquifer pore-waters flow in the direction of overpressure reduction, whereas trapped hydrocarbons remain static. The main effect of this pressure disequilibrium in the aquifer is a tilt of the free water levels (FWL) in the direction of lower overpressure.Although the impact of hydrodynamics on the petroleum system is established in the oil industry and provides important information for exploration and field development, the interaction of CO2 storage and hydrodynamic flow has hardly been investigated. Supercritical CO2 has a density comparable with oil densities in known hydrodynamic systems in the North Sea. Hence, a tilted CO2-water FWL is expected and has to be taken into account during the planning of CO2 storage. Hydrodynamic spill points will replace structural spill points as a parameter in CO2 storage capacity calculations.Storage security is dependent on CO2 dissolution rates, among other factors. The more effectively CO2 dissolves into brine the more CO2 will be permanently stored in the subsurface. Numerical simulations show that CO2 storage in hydrodynamic aquifers enhances the dissolution of CO2 compared with a static regime. When CO2 dissolves into brine, the brine density increases and the CO2-saturated brine sinks to the bottom of the reservoir and is replaced by under-saturated brine. In a hydrodynamic system the brine saturated with CO2 will be removed more effectively from the CO2-brine interface. Hence, CO2 comes in contact with more under-saturated brine and dissolves at a greater rate.The presence of hydrodynamic flow in a reservoir requires a high degree of understanding but research on oil reservoirs has proven that it is an uncertainty that can be adequately characterized. For long term CO2 storage operations hydrodynamic flow can even be an opportunity for more secure CO2 storage.