A quantitative description of barite thermodynamics, nucleation and growth for reactive transport modelling

Abstract

The regression of available thermodynamic data in the BaSO4-NaCl-H2O system yielded Pitzer ion interaction parameters that accurately describe the activities of aqueous species and mineral solubilities in this system. This thermodynamics description is compared with published Pitzer parameter sets, and combined with a model for the kinetics of barite nucleation and growth, based on classical nucleation theory. Both the thermodynamic and nucleation/growth models have been incorporated into the PHREEQC computer code to facilitate calculation of the extent and consequences of barite formation in natural and engineered systems. Results of geochemical modelling calculations agree adequately with the amount of barite scale thicknesses derived from calliper measurements from an oil well if the effective surface free energy of barite nuclei is assumed to be similar to 50 mJ m(-2). Better results, however, are achieved using a temperature dependent effective surface free energy. In contrast, calculations performed by ignoring the effects of barite nucleation lead to a substantial overestimation of the amount of scale formed in our modelled systems. The success of our mineral nucleation and growth model to describe scaling in our modelled system suggests this description of precipitation rates can be applied to many other mineral-aqueous fluid systems, in particular where supersaturation is slight and the solids forming have substantial surface free energy.