Crystal Growth and Dissolution of Barium Sulfate
Authors: Nancollas G.H. , Liu S.T. 
Published: December 2007

Abstract
The kinetics of crystallization and dissolution of barium sulfate seed crystals were investigated conductimetrically. Growth is characterized by an initial surge caused by secondary nucleation, followed by a rate that is proportional to The square of the supersaturation. Studies were made using seed material of differing morphology; in all cases, the crystallization was surface controlled. A surface reaction also appears to be rate-determining for the corresponding dissolution process, but the over-all rate constant is considerably greater than that for growth. Crystallization and dissolution were studied in the presence of potential phosphonate and polyphosphate scaling inhibitors phosphonate and polyphosphate scaling inhibitors in some cases, both processes were markedly inhibited. The incorporation of the antiscalant into the developing crystals may pose problems in down-hole application.

Introduction
The state of knowledge of adsorption and desorption phenomena and of reactions at the solid-liquid interfaces under wellbore conditions is extremely limited. Consequently, the procedures used for eliminating scale by the chemical treatment of surface waters that are frequently injected into an oil-bearing formation are often based on empirical considerations. In the absence of knowledge of the mechanism of scale formation and its inhibition, the choice of additive is usually made on the basis of the results of spontaneous precipitation experiments made in the laboratory. precipitation experiments made in the laboratory. Although attempts are made to reproduce such experimental data, extreme sensitivity to factors such as the methods used to mix reagents, rates of stirring, and concentrations of reactants make it impossible to do so. Also, it is difficult to avoid heterogeneous nucleation in such systems, and this process also may be influenced by the presence of the additive. Interpretations of the course of the precipitation reactions solely in terms of the precipitation reactions solely in terms of the thermodynamic solubility products of the precipitating minerals also is questionable. Such precipitating minerals also is questionable. Such treatments assume that, at all stages of the scaling process, the systems are effectively at equilibrium process, the systems are effectively at equilibrium and are amenable to treatment using experimental solubility products. It has been shown that kinetic factors often are considerably more important in determining the course of a precipitation process. Thus, in the case of calcium phosphate crystal growth, an amorphous precursor is formed rapidly at the beginning of the reaction and undergoes a slow transformation to the thermodynamically stable phase, hydroxyapatite. Significant changes with phase, hydroxyapatite. Significant changes with time are observed in such factors as chemical composition, crystallinity, and the specific surface areas of the solid phases. The nature of the initially precipitated phases and the course of the subsequent precipitated phases and the course of the subsequent crystal growth reaction is markedly dependent not only on the degree of supersaturation of the solution, but also on the ionic strength of the solution and the type of neutral or inert electrolyte present. Simple equilibrium solubility studies reveal nothing of these factors that may be important in determining whether scale will form in the field.
Not only is the growth of crystals important for studies of scale formation, but a knowledge of the mechanism of the reverse process, dissolution, also is essential if the results of laboratory experiments are to be used to predict the behavior in actual scaling situations. At first, the growth and dissolution of crystals may be considered to be exactly reciprocal processes. The dissolution process usually has been considered to be a simple process usually has been considered to be a simple diffusion-controlled process, with the transport of lattice ions away from the crystal surface as the slow step in the reaction. in terms of diffusion following Fick's law, the rate of reaction would be expected to be proportional to the subsaturation, mo - m, where m is the molar concentration of electrolyte in the solution and mo is the equilibrium (solubility) value. Although a number of salts follow this kinetic path, there is now appreciable evidence that the dissolution of many slightly soluble salts is controlled by a process other than film diffusion of the crystal lattice ions. Whereas scale inhibitors would be expected to have little influence on a dissolution process that depends on the diffusion of crystal lattice ions away from the surface, a surface-controlled process may be markedly retarded in their presence.
SPEJ
P. 509