In planning to inject CO2 underground it is important to be able to predict where and how quickly CO2 will move, and where it will be trapped. These predictions are made difficult by a combination of the complexities of fluid dynamics and the unknown and heterogeneous nature of the subsurface rocks in which CO2 will be stored. To improve our ability to design CO2 storage projects and estimate the capacity of a formation for CO2 storage, we are studying key problems at a range of size scales of importance to CO2 storage. This includes questions such as: How does rock heterogeneity impact the flow pathways of injected CO2? How quickly does CO2 dissolve into water in the subsurface? What can we measure about rocks in the laboratory that would enable us to answer these questions at the larger scales associated with underground CO2 plume movement?
CO2 saturation and flow processes at multiple scales impose fundamental constraints on CO2 migration and trapping efficiency. This controls both the speed and extent of plume migration, and the amount of CO2 that is residually trapped within a reservoir formation. Due to the low viscosity of supercritical CO2, relative permeability and residual and dissolution trapping rates are controlled by micron to metre scale capillary heterogeneity. At larger scales CO2 percolation and the fluid front are controlled by the largest 10% of the permeability distribution, in contrast to hydrocarbon migration which is controlled by the modal permeability. The impact of capillary heterogeneity on fluid flow and trapping in the laboratory will be quantified using 3D micro-scanning of core representing the range of storage reservoirs in the UK. Scaled up models will then incorporate these impacts into prediction of fluid migration and storage efficiency. Ground-truth will include datasets from the Otway, Sleipner, CMC Field Research Station, Ketzin, and Decatur CO2 injections. At these sites, downhole CO2 saturation distributions can be obtained through geochemical methods of major isotope and minor noble gas partitioning, linked to geophysical methods with time-lapse seismic imaging and novel characterisation of CO2 migration fronts, using spectral attenuation analysis.