Enhanced CO2 Dissolution in Heterogeneous Porous Media

Long-term and secure geological storage of CO₂ through technologies such as Carbon Capture and Storage (CCS) within reservoirs is seen as a technological means to reduce anthropogenic CO₂ emissions. The long-term viability of this technology is reliant on the structural and secondary trapping of supercritical CO₂ within heterogeneous reservoirs. Secondary trapping, primarily through the dissolution of CO₂ into ambient reservoir brine to produce a denser fluid, is capable of retaining CO₂ in the subsurface and thus reducing the risks of storage. To model secondary trapping we need to understand how the flow of CO₂ through heterogeneous reservoir rocks enhances dissolution of supercritical CO₂ in reservoir brines.

Here we experimentally investigate the dissolution of CO₂ in reservoir brines in layered, heterogeneous geological formations. Using analogue experiments, designed to approximate an enhanced oil recovery (EOR) setting, the processes of mixing, dispersion and dissolution are examined. These are compared against test results from non-layered, homogeneous porous media experiments. We find that heterogeneities significantly enhance mixing, particularly between adjacent porous layers. During fluid propagation, pore-scale viscous fingers grow and retreat, thereby providing an increased surface area between the flow and the ambient reservoir fluid. This enhanced mixing is predicted to substantially increase the dissolution of CO₂ in reservoir brines. Both permeability and viscosity differences are found to have a significant effect on the interface between the two fluids, and therefore the likely amount of dissolution of CO₂.