Diffusion Properties of Carbonated Caprocks from the Paris Basin

The efficiency of geological storage of CO2 lies in the confinement properties of the caprock. Among these properties, diffusive characteristics play an important, though sometimes underestimated, role. Although diffusion is a slow process, it can play a significant role in the long run. Besides, the desirable properties for a caprock – mainly high entry pressure and low permeability – are by no means contradictory with relatively high diffusion coefficients; the reason is that all these quantities do not depend on the same geometrical properties of the porous matrix (pore size in one case, tortuosity in the other). Within the framework of the Geocarbone-INTEGRITE project, financed by the National Research Agency ANR from 2006 to 2008, the diffusion coefficients for water and bicarbonate ions have been measured in caprock samples from three levels (Comblanchien and Dalle Nacree formations). These two species cover the main situations encountered in storage site conditions (neutral/charged species, interacting or not with the solid matrix). The model used for interpretation of the experiments is presented; this enables review of various quantities of interest: effective diffusion coefficient, pore diffusion coefficient, apparent diffusion coefficient, retardation factor. The pore self-diffusion coefficient for water was measured by two different techniques (Nuclear Magnetic Resonance and tritiated water 1 H3 HO tracing) on two samples. The results are comparable: porosity about 6%, pore diffusion coefficient 2 × 10−10 m2 /s, tortuosity about 10. The radioactive tracer method is then applied with tritiated water and carbon-14-tagged bicarbonate ions, H1 CO- 3 , to samples from three depth levels. Reduction of accessible porosity is evidenced for bicarbonate ions, which is attributed to anionic exclusion. Interaction between bicarbonate ions and the solid matrix is also shown. This suggests that additional CO2 storage capacity might be offered by carbonated caprocks (regardless of the problem of alteration by acidified water, not addressed here). Lastly, a few simple rules for extending the results presented here to other experimental conditions are proposed.