Abstract
The kinetic behaviour of an ideal solid solution is modelled by two competing reactions: the stoichiometric dissolution of the existing solid and the precipitation of the least soluble compound, i.e. that with respect to which the oversaturation of the fluid is maximum; both reaction rates are expressed as a function of the corresponding departure from equilibrium, with a pH-dependent kinetic constant. Within this ideal model, the saturation curves and the fluid evolution of a simple reacting system may be displayed conveniently in chemical potential diagrams. The simulation of a geochemical system, constituted of a glauconitic sandstone aquifer infiltrated through by a sea water enriched in carbonates, allows to analyze the impact of taking into account two ferro-magnesian solid solutions, a calcite and a chlorite, on Fe-Mg exchanges during carbonates dissolution. The model accounts well for the tendency of solid solutions to dissolve congruently before precipitation starts and drives the fluid composition towards equilibrium.
Highlights
As long as a particular ideal solid solution exists in the geochemical system, whatever stoichiometric saturation state the fluid has with respect to it, and as the aqueous solution is at total oversaturation with respect to the solid solution, two particular solid solutions have to be taken into account at every moment: the existing one and the least soluble one
In the context of CO2 sequestration into saline aquifers of the subsurface, the computer code has been used to study the evolution of a glauconitic sandstone aquifer (11), in which a sea water enriched in carbonates is injected (12) continuously, at 60 ◦C and 100 bar
Since the current aqueous solution and the injected sea water have very close compositions, aqueous concentrations do not any longer evolve significantly. This application illustrated the ideal solid solution model proposed in this paper, as the overall behaviour of two solid solutions, a carbonate and a chlorite one, was explained
Summary
The kinetic behaviour of multiphasic solid-fluid assemblages is usually described by competing mineral dissolutions and precipitations, and is governed by departure from equilibrium. It takes into account the behaviour of ideal solid solutions as well as fixed compounds. J subscript denoting an aqueous secondary species k subscript denoting an aqueous basis species kdm kinetic dissolution constant of the mth mineral [number of moles per unit of surface and unit of time]. Xιs mole fraction of the ιth end-member component in the sth solid solution [dimensionless]. Xιs mole fraction of the ιth end-member component in the sth solid solution of fixed composition [dimensionless]. Αli number of moles of the lth element in one mole of the ith aqueous species βlm number of moles of the lth element in one mole of the mth mineral ιs subscript denoting an end-member component of the sth solid solution. ΩSs(Xιs) stoichiometric saturation degree solution of fixed composition [dimensionless]
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