Soil-gas concentrations and flux monitoring at the Lacq-Rousse CO 2 -geological storage pilot site (French Pyrenean foreland): From pre-injection to post-injection

Abstract

Soil-gas concentrations and flux were measured during 20 separate measurement campaigns at the TOTAL Lacq-Rousse carbon capture and storage (CCS) pilot site, southern France, where 51,000 tons of CO 2 were injected in a depleted natural gas field. Baseline data (September 2008 to December 2009) are compared to monitoring data from the injection (March 2010 to March 2013) and post-injection (February 2014 to December 2015) periods. CO 2 soil-gas concentrations varied from atmospheric concentrations to more than 16% vol. with 1.4% as median value. Summer data showed high CO 2 concentrations in the soil that remained quite high during winter. Median CO 2 flux at the soil/atmosphere interface was close to 4.4 cm 3 ·min -1 ·m -2 . Carbon-isotope ratios measured on CO 2 in soil gas had a mean value of -23.5 ± 3.1‰, some deviation being due to atmospheric CO 2 . Comparison between different gas species and the influence of temperature, pressure and soil-water content suggest that gases in near-surface environments are produced locally and naturally, and are unrelated to CO 2 ascending from the storage reservoir. Monitoring of CO 2 injection and the use of threshold levels is discussed as part of a practical approach considering specific regulations for the Lacq-Rousse CCS pilot experiment and constraints for the site operator.