Pore-scale imaging of trapped supercritical carbon dioxide in sandstones and carbonates

Geological carbon dioxide storage must be designed such that the CO2 cannot escape from the rock formation into which it is injected, and often simple stratigraphic trapping is insufficient. CO2 can be trapped in the pore space as droplets surrounded by water through capillary trapping. X-ray microtomography was used to image, at a resolution of 6.6 μm, the pore-scale arrangement of these droplets in three carbonates and two sandstones. The pressures and temperatures in the pore space were representative of typical storage formations, while chemical equilibrium was maintained between the CO2, brine and rock phases to replicate conditions far away from the injection site. In each sample substantial amounts of CO2 were trapped, with the efficiency of trapping being insensitive to pore-morphology and chemistry. Apart from in one extremely well connected sample, the size distribution of residual ganglia larger than 105 voxel3 obey power law distributions with exponents broadly consistent with percolation theory over two orders of magnitude. This work shows that residual trapping can be used to locally immobilise CO2 in a wide range of rock types.