Diffusion Properties of Carbonated Caprocks from the Paris Basin
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 Géocarbone-INTÉGRITÉ 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 Nacrée 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 1H3HO 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, H1CO-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 CO2storage 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.
Domains
Physics [physics]
Origin : Publication funded by an institution
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