CO2 geologic storage
Hydro-Chemo-Thermo-Mechanically Coupled Phenomena
Injecting supercritical CO2 in geologic formations triggers a complex cascade of interconnected events such as: CO2 advection, buoyancy in response to density differences, convection of CO2-dissolved water, mutual diffusion and dissolution between CO2 and water phase, viscous fingering of CO2, capillary trapping of CO2 phase by the water-saturated porous formation, and reactive fluid transport due to the water acidification following CO2 dissolution. Moreover, injection of cold CO2 will cause thermall-induced stress changes as well as poroelastic stress changes following pore-fluid pressure buildup.
CO2 storage reservoir and caprock
The CO2 storage reservoir can be analyzed into four different zones around a CO2 injection well (see the Figure above). The far-field Zone I is not affected by CO2 injection and brine saturation is 1. Acidified brine dominates Zone II and mineral dissolution prevails over precipitation; loaded with dissolved CO2 and minerals, denser brine experiences convection and sustains further dissolution in this zone. In Zone III, the ionic strength in brine increases as water is removed by the injected CO2, and salt precipitation may occur; in fact, brine acidification by CO2 dissolution and brine dissolution into CO2 coexist in the transition Zone III and there is partial compensation between mineral dissolution and precipitation. The continuous influx of “dry” CO2 in Zone IV around the injection well, first displaces brine and then dries all the residual brine, resulting in salt precipitation while CO2 saturation approaches 1 (Kim, S. and Santamarina, J. C. 2014. "CO2 geological storage: Hydro-chemo-mechanical analyses and implications." Greenhouse Gases: Science and Technology, 4, 528-543).
CO2 geologic storage and an overview of the research areas in need defined by NETL, DOE. Advanced technologies that can improve containment and injection operations, increase reservoir storage efficiency, and mitigate unwanted migration of CO2 in all types of storage formations are needed for the succes of CO2 geologic storage. Image is from NETL website.