Dissolution Trapping of Carbon Dioxide in Reservoir Formation Brine – A Carbon Storage Mechanism

Carbon Capture and Storage (CCS) is a method to reduce anthropogenic greenhouse gas emissions thereby mitigating global warming. In CCS, carbon dioxide (CO2) is captured from fossil fuel-fired power plants or other large point-source emitters, purified, compressed and injected deep underground into subsurface formations at depths of or greater than 800m. At such depths CO2 is in a supercritical (sc) state increasing storage capacity (IPCC 2005). In CCS, there are four main mechanisms which keep the buoyant CO2 underground: 1. Structural/stratigraphic trapping – here an impermeable caprock prevents the CO2 from flowing upwards, 2. Capillary trapping, where micrometer-sized disconnected CO2 bubbles are formed and held in place by local capillary forces in the rock pore-network, 3. Dissolution trapping, where CO2 dissolves in the formation brine and sinks in the reservoir as the CO2-enriched brine has an increased density, 4. Mineral trapping, where the dissolved CO2 reacts with the formation brine, forms carbonic acid which dissociates generating protons, 3 HCO − and 3 CO − ions; these species subsequently react with the formation brine and/or host rock to form solid minerals which trap the CO2 very safely. The focus of this text is on dissolution trapping; how much CO2 dissolves under which geothermal conditions and what happens to the CO2-enriched brine, which is slightly denser than the original formation brine, in the formation. Important open questions in this context are: How fast are these mass transfer processes in real geological porous media under realistic CCS conditions? Are there means of accelerating CO2 dissolution? How do separate gas and/or oil phases (oil and/or gas reservoirs) in the reservoir affect CO2 dissolution processes and reservoir fluid dynamics? How does the pressure drop due to CO2 dissolution affect injectivity and storage capacity of CO2?