We investigate the characteristic properties of porous media that influence the entrapment of carbon dioxide (CO2) by capillary forces. It is known that different geological formations can trap different quantities of CO2 but the relationship between formation properties and trapping is poorly understood at present. Advances in micro computed tomography (µCT) techniques now allow the porous media and trapped CO2 clusters therein to be visualised and characterised on the micro meter scale. The context of this work is the geological storage of CO2 where the entrapment of injected CO2 by capillary forces on the pore scale is proposed as a fast and safe method to store injected CO2.
We analyse a series of saturated and unsaturated porous media using µCT; four glass bead packs, a sand pack and a sandstone. In the saturated images the pore space contains brine and residual CO2 (Sr) at subsurface storage conditions. We quantify Sr and cluster size distributions and determine characteristic properties of the porous media through image analysis and the extraction of representative networks. We show that media with narrower pore throats, such as sandstones, trap more CO2 than media with wider pore throats. Numerical simulations performed on the extracted networks do not accurately predict the measured residual CO2 saturations. We discuss the important implications of these results for CO2 storage site selection, containment security assessments, and storage capacity appraisal.