Managing the risk in the Vadose zone associated with the leakage of CO2 from a deep geological storage

CO2 capture and geological storage (CCS) is seen as a promising technology to achieve large reduction in atmospheric greenhouse gas emissions. But a prerequisite to its industrial scale implementation is demonstrating safety of deep reservoir storage. In this context, the need to know “what can be done” in case of abnormal behaviour of the reservoir has been outlined by the recent European directive on geological storage operations (directive 2009/31/EC, issued in April 2009). In the present article, we consider the remediation environmental problem of restoring the vadose zone from the potential impact associated with the CO2 leakage. In this view, we use an analogy-based approach. Considering the hydrodynamic impact, linked with the formation of a CO2 leakage plume and with the potential emission of the CO2 in the atmosphere, CO2 presents several similarities with Volatil Organic Compounds (VOC) for which remediation strategies have been developed for the last 20 years. These strategies can be divided into two main categories: (1) based on natural attenuation (i.e. concentration and global mass reduction) combined with active monitoring techniques; (2) based on active extraction techniques such as “Soil Vapour Extraction”. Generic modelling using the multiphase and multi-component TOUGH2/EOS7CA (Oldenburg et al., Transport in Porous Media, 2009) show good effectiveness of such strategies in case the CO2 leakage source is either of limited spatial extent or of moderate flux rate. Nevertheless, more complex strategies should be undertaken in case of large scale pollution (so-called « diffuse pollution »). Guidelines are proposed based on the lessons learned from diffuse pollution case studies with a special focus on Radon diffusion and pesticide contamination. Considering the geochemical impact, linked with the acidification owing to the CO2 dissolution and with the potential release of heavy metals, we define an intervention strategy based on the study of complex environmental remediation real cases in the fields of acidic mine drainage and industry contamination.