Upscaling of Capillary Trapping in a Buoyant Plume: Application to CO2 Sequestration in Aquifers

CO2 sequestration refers to the capture and long-term storage of anthropogenic CO2 in order to limit its emission to the atmosphere. Injection into geological formations is one option to store CO2. Deep saline aquifers are prime candidates for CO2 storage because they have a huge storage capacity and they are widely distributed. One of the major concerns in any sequestration project is the potential leakage of the CO2 into the atmosphere. CO2 tends to migrate to the top of the geologic structure. This upward migration is sometimes delayed or suppressed by low permeability layers that impede the vertical flow of gas - hydrodynamic or structural trapping. However, the success of a sequestration project relies to a great extent on the magnitude of capillary trapping (Juanes et al., Water Resour. Res. 2006), in which the CO2 phase is disconnected into an immobile (trapped) fraction. Here, we investigate, by means of laboratory experiments and numerical simulation, the scale dependence of capillary trapping. The basic flow unit is a transparent glass-bead pack - a Hele-Shaw cell filled with glass beads. This design enables simple visualization techniques to be used to monitor the flow. The present paper confirms that current field-scale simulation models of CO2 storage overestimate the amount that is actually trapped, because they do not capture the small-scale variability due to viscous and gravity instabilities, and permeability channeling.