The importance of baseline surveys of near-surface gas geochemistry for ccs monitoring, as shown from onshore case studies in Northern and Southern Europe [Importance des lignes de base pour le suivi géochimique des gaz près de la surface pour le stockage géologique du CO2, illustration sur des pilotes situés à terre en Europe du Nord et du Sud]

The monitoring of the integrity of onshore geological carbon capture and storage projects will require an approach that integrates various methods with different spatial and temporal resolutions. One method proven to be quite effective for site assessment, leakage monitoring, and leakage verification is near-surface gas geochemistry, which includes soil gas concentration and gas flux measurements. Anomalous concentrations or fluxes, relative to the natural background values, can indicate the potential occurrence of a leak. However the natural background can be quite variable, especially for CO2, due to biological production and accumulation in the soil that changes as a function of soil type, land use, geology, temperature, water content, and various other parameters. To better understand how these parameters influence natural, near-surface background values, and to examine the potential of different sampling strategies as a function of the survey goals, this paper reports results from two highly different case studies, one from northern Europe (Voulund, Denmark) and one from southern Europe (Sulcis, Sardinia, Italy). The small Voulund site, with its homogeneous soil, climate, and topography, was surveyed twice (in fall and in spring) within the EU-funded SiteChar project to examine the effects of different land use practices and seasons on baseline values. Forested land was found to have lower CO2 concentrations during both campaigns compared to cultivated and heath land, and higher CH4 values during the spring sampling campaign. Continuous monitoring probes showed much more detail, highlighting seasonal changes in soil gas CO2 concentrations linked primarily to temperature variations. The much larger Sulcis site, studied within an ENEAfunded project on potential CO2-ECBM (Enhanced Coal Bed Methane) deployment, was surveyed at the regional scale and on detailed grids and transects for site assessment purposes. Despite the completely different soil and climate conditions, the statistical distribution of the Sulcis data was similar to that of Voulund. Much higher soil gas CO2 anomalies were found at this site, however, due to the less permeable sediments (i.e., better water retention and greater gas accumulation) and the warmer temperatures. Detailed surveys at this site highlighted various significant anomalies, some of which can be explained by near surface biological processes, whereas others, especially helium anomalies, were more difficult to explain. These results show the utility of baseline surveys, and highlight the need for follow-up studies to clarify any unexplained anomalies before any CO2 storage. © S.E. Beaubien et al.