Review and implications of relative permeability of CO2/brine systems and residual trapping of CO2

The adoption of carbon capture and storage (CCS) as a method of mitigating anthropogenic CO2 emissions will depend on the ability of initial geological storage projects to demonstrate secure containment of injected CO2. Potential leakage pathways, such as faults or degraded wells, increase the uncertainty of geological storage security. CCS as an industry is still in its infancy and until we have experience of industrial scale, long term CO2 storage projects, quantifying leakage event probabilities will be problematic. Laboratory measurements of residual saturation trapping, the immobilisation of isolated micro-bubbles of CO2 in reservoir pores, provides an evidence base to determine the fraction of injected CO2 that will remain trapped in the reservoir, even if a leakage event were to occur. Experimental results for sandstone, the most common target lithology for storage projects, demonstrate that 13–92% of injected CO2 can be residually trapped. Mineralisation, the only other geological trapping mechanism which guarantees permanent trapping of CO2, immobilises CO2 over hundreds to thousands of years. In comparison, residual trapping occurs over years to decades, a timescale which is more relevant to CCS projects during their operational phase and to any financial security mechanisms they require to secure storage permits.