Changes in containment properties of claystone caprocks induced by dissolved CO2 seepage

Abstract

Abstract The capture and the storage of CO2 in deep geological media are proposed to reduce the emission of CO2 to the atmosphere. While the argillaceous formations have been extensively studied since more than two decades as potential host-rocks for nuclear waste disposal, only few studies have dealt with the potential degradation of their containment properties as caprocks induced by a CO2 attack. This paper focuses on the impact of a CO2-enriched fluid on the diffusive transport parameters of indurated argillaceous samples originating from the massif of Tournemire (Aveyron, France). Two through-diffusion experiments were carried out, the first one with a CO2-enriched fluid and the second one with an equilibrated fluid, chemistry of which is very close to that measured in-situ. Non-radioactive (deuterium and bromide) and radioactive tracers (tritium and Cl-36) were successively used. Throughout the duration of the experiments, the evolution of water chemistry was monitored by measuring pH, Ca, Na, Mg, K, Cl, SO4, TOC and alkalinity. The effect of the CO2 stress has been clearly evidenced. On the one hand, the tracer fluxes in the downstream reservoir of the cell having undergone CO2 attack were up to 3 times higher than in the reference cell, indicating a significant increase of the porosity and effective diffusion coefficient values (up to 100% for Cl-36 porosity and up to 300% for Cl-36 effective diffusion coefficient). On the other hand, the chemical monitoring showed that magnesium, calcium and carbonate concentrations in the collection reservoirs increased up to one order of magnitude, suggesting a possible dissolution of some carbonate minerals such as calcite, reacting with the acid fluid.