NEAR-SURFACE GAS GEOCHEMISTRY TECHNIQUES TO ASSESS AND MONITOR CO_2 GEOLOGICAL SEQUESTRATION SITES

摘要

As is well known the long-term effects and stability of a man-made CO_2 geological storage facility is very difficult to predict with laboratory or modeling experiments due to the size and long time scales involved. Instead attractive additional sources of information are natural sites where CO_2 produced at great depths is either trapped in porous reservoirs or leaks to the surface. These sites can be considered as "natural analogues" of what may occur over geological time spans within an engineered CO_2 geological storage site. The study of these sites can be subdivided into three broad fields: ⅰ) understanding why some reservoirs leak while others don't; ⅱ) understanding the possible effects of CO_2 should it leak into the near-surface environment; and ⅲ) using the leaking sites to develop, test and optimise various monitoring technologies. The present article summaries many of the near-surface gas geochemistry results obtained in central Italy during the EC-funded NASCENT project (Natural Analogues for the Storage of CO_2 in the Geological Environment). These include a comparison of leaking (Latera) and a non-leaking (Sesta) CO_2 reservoirs, detailed soil gas surveys to outline migration pathways, the development of a geochemical continuous-monitoring station to study temporal changes in CO_2 concentrations, and field experiments involving the injection of a gas mixture in the shallow subsurface to outline migration pathways and to understand the behaviour of various gas species based on their different chemical-physical-biological characteristics. Put together this data provides useful information for site selection, risk assessment and monitoring techniques, which is needed if CO_2 geological storage is to become an accepted and widely-applied technology.