Modelización geoquímica directa métodos teóricos y numéricos aplicados al cálculo del equilibrio químico, transferencia de masa y caminos de reacción en procesos de interacción agua-roca

Abstract

This work tackles the rock-wa:er interaction modelling by Direct Modelling (DM), and more specifically the Modelling of the Reaction Paths (MRP). This modelling quantitatively analizes the evolution of the system towards the total equilibrium through consecutive states of partial equilibrium based on the progress reaction variable, and requires the definition of the irreversible processes of the reacting system, the reversible processes of the equilibrium system and the progress reaction variable. Calculations require the relation of the mass balance equations of each element, the mass action law equations for each species, as well as the equations of mass balance of hydrogen, oxygen, charge balance and the conservation of electrons or constant mass according to the code to be used. The thermodynamic properties of the system are tackled according to the ionic association (Debye-Huckel) or to the specific interactions (Pitzer coefficient), and the chemical equilibrium by Gibb's free energy method or by the approximation method of the equilibrium constant. Based on the numeric resolutions used by the codes, three groups of codes have been generated. WATEQ series, which uses the successive approximation numeric method, MINEQL series, that uses the Newton-Raphson method and PATH series which directly integrates the differential equations. One or other code family is applied according to the specific characteristics of the problem. The direct modelling is a very important tool in the interpretation of the water-rock interaction processes. However, the space-time variation of the water composition is not taken into account, and it has no capacity to model advection, dispersion and kinetic processes.