Abstract
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.
Highlights
Molecular diffusion is a slow transport process at the time scale of injection in a storage site
2 EXPERIMENTAL TECHNIQUES We describe here the Nuclear Magnetic Resonance (NMR) and radioactive tracer techniques used on several samples from a caprock formation (Charmottes, France)
The effective diffusion coefficients for water and for bicarbonate ions were measured in three types of carbonated caprocks from the Paris Basin, under ambient room conditions
Summary
Molecular diffusion is a slow transport process at the time scale of injection in a storage site. If we take a simple system of packed grains of uniform grain size, permeability typically depends on the square of the grain size whereas porosity and diffusion will be constant in this particular case. Despite the very low permeability of caprocks, diffusion coefficients can be significant and only reduced by one or two orders of magnitude compared to free water values. Caprocks may have a significant porosity (e.g. 20%) but a permeability reduced by six orders of magnitude or more compared to the reservoir zone. This is due to the fact that caprocks are made of fine grains such as clay particles, and/or they have been subjected to specific diagenetic processes such as dissolution/recrystallization
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