Geological CO2 Sequestration

Work on carbon capture and storage (CCS) has addressed the critical question of how much CO2 can be injected in deep subsurface formations by developing storage capacity estimates that, unlike previous estimates, are based on the fluid mechanics of CO2 injection, migration, and trapping. The two key trapping mechanisms are residual trapping (the result of capillary forces snapping the CO2 into immobile ganglia) and solubility trapping (the result of a density-driven convective instability that takes place when CO2 dissolves in brine). Our effort required feeding the detailed physics from the centimeter scale (at which residual and solubility trapping operate) into mathematical models of how CO2 migrates and is trapped in deep saline aquifers at the 100-km scale (characteristic of entire geologic basins). A fundamental result from our work is that storage capacity can be limited by both the ability of the formation to dissipate injection overpressures and by the migration of the injected CO2 underneath a sealing caprock. A key insight from our analysis is that storage capacity must be understood as a dynamic quantity, which depends critically on the duration of injection. This insight allowed us to frame the problem in a new way: as a supply vs. demand problem. The lifetime of CCS as a climate-change mitigation technology can then be understood as the crossover between storage capacity (“supply curve”) and cumulative injection (“demand curve”).