Study of the environmental variability of gaseous emanations over a CO₂ injection pilot-Application to the French Pyrenean foreland

Prior to the CO₂ injection in the Rousse pilot operated by TOTAL Exploration Production France, baseline monitoring has been performed in near surface environments in order to characterize soil gas concentrations and soil gas fluxes behavior along with time. The baseline measurements were conducted in 2008 and 2009 on a quarterly basis. All investigations were performed under weather conditions representative of temperate climate yearly cycles. CO₂ fluxes and CO₂ soil concentrations followed an annual cycle with enhanced emissions during high season (summer) and lower emissions during low season (winter) corresponding with the annual cycle of biological activity in soil. The CO₂ is predominantly of organic origin with atmospheric dilution during low season times with an absence of an upward gas flux as suggested by nearly constant helium concentrations in soils and depleted carbon-13 isotope ratios. Even considering quite long baseline monitoring, it is not possible to find a significant correlation between the flux measured at the soil/atmosphere interface and the concentration measurements at 1m depth. From the knowledge gained at the site, a monitoring strategy is proposed in order to match both scientific and operator requirements.