Factors controlling the kinetics of crystallization: supersaturation evolution in a porous medium. Application to barite crystallization

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Abstract
The nucleation of barite has been studied in a system involving the doublediffusion of Ba2+and SO42-in an Na-rich aqueoussolution, through a porous medium. The evolution of the concentration profiles in the medium, which is a column of porous silica gel, has been determined as a function of time by direct chemical analysis of the diffusion-controlled mass transfer. By measuring the pH evolution, a Debye–Huckel treatment of ionic complexing has enabled the supersaturation evolution to be determined. The location of barite precipitation in the column is controlled both by the need to exceed a threshold supersaturation, as well as achieve an ‘equality range’ in which [Ba2+]/[SO42−] is close to unity. The value of the threshold supersaturation is a kinetic parameter and depends on the rate at which supersaturation increases. The experimental system described here has wide application to the study of crystallization phenomena in rocks. Experiments on the effect of additives designed to inhibit nucleation of barite in North Sea oil wells are used to quantify the resultant increase in supersaturation threshold.

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