Estimation of reactive surface area of the minerals during fluid-rock interaction in Galicia (Spain) : Analog for artificial geological sequestration of CO2
Résumé
The understanding of complex reactions between CO2 rich fluids and rocks is basic in prospecting safety for long term CO2 storage in natural geological reservoirs. Hydrothermal natural fields are considered one of the best analogue for carbon dioxide mineralization because integrate the signal of long term interaction. The Spanish hydrothermal field of Galicia is characterized by co-genetic fluids issued from a quite-homogeneous granitic reservoir. Here, fluids are characterized by an increase on major elements (Ca, Mg, K and Na) and alkalinity associated to the 4-fold decrease on fluid pH (pH from 10 to 6). This natural situation has been used to evaluate the effect of CO2 on the long term fluid -rock interaction. In this work is reported the result of an original methodology applied to the estimation of the reactive surface area of the minerals participating to CO2 neutralization. The adopted methodology is based on an inverse model approach using the chemistry of the sampled fluids. The irreversible mass transfer process is defined by the overall degree of reaction advancement assuming that ion activity of dissolved silica and aluminium ions are limited by the equilibrium with quartz and kaolinite. A system of dissolution equation for the main rock mineral is solved in terms of a transposed reaction rate vector, introducing experimental kinetic rate constants and solving for the surface area of dissolving minerals. Preliminary results indicate that dissolution rate of oligoclase, albite, K-feldspath, and biotite remains quite constant. On the contrary the reactive surface area of oligoclase and biotite changes by 1 - 2 order(s) of magnitude and the reactive surface area of albite and K-feldspar changes 2 - 3 orders of magnitude in the estimated 50 000 years of interaction. Those results suggest that mineral surfaces reactive area can varied by several order of magnitude over the water-rock interaction process, while classical geochemical model integrate a constant reactive surface mineral in there simulations. Keywords: Natural analog, CO2-water-rock interaction, kinetic rate dissolution, inverse geochemical modeling