Calcium Sulfate Scale Formation: A Kinetic Approach
Authors: Nancollas George H. , Eralp Atal E. , Gill Jasbir S. 
Published: December 2007

Abstract
The growth and phase transformation of calcium sulfate dihydrate and hemihydrate crystals were studied at temperatures from 70 to 130 deg. C. At 70 deg. C the second-order rate constant for dihydrate crystal growth did not change by more than 20 percent over a pH range of 3.2 to 9.2. It was also independent of ionic strength up to 2.0M. Growth in stable supersaturated calcium sulfate solution was completely inhibited by 7 x 10-7 M phytic acid for about 24 hours at 70 deg. C. The seeded crystallization of calcium sulfate hemihydrate at temperatures from 90 to 140 deg. C and The phase changes from a- to beta-hemihydrate were investigated by X-ray diffraction, specific surface area analysis, and scanning electron microscopy. Organic phosphonates were found to be effective inhibitors of crystal growth of all the phases at high temperatures.

Introduction
The phases that form during the crystallization of many sparingly soluble salts evidently are determined much more by kinetic factors than by thermodynamic considerations. Thus, in the case of calcium phosphate crystal growth, an amorphous precursor is formed rapidly at the beginning of the precursor is formed rapidly at the beginning of the reaction and undergoes slow transformation to the thermodynamically stable phase, hydroxyapatite. Significant changes with time are observed in such factors as chemical composition, crystallinity, and specific surface areas of the solid phases. The simple equilibrium studies do not reveal the factors that may be important in determining whether these phases will precipitate in the field. phases will precipitate in the field. The case of calcium sulfate, which is important in desalination, geochemistry, and petroleum engineering, is complicated further by the fact that it can crystalize from aqueous solutions in three forms- dehydrate (CaSO4 - 2H2O), hemihydrate (a-CaSO4 1/2 H2O or beta-CaSO4 - 1/2 H2O), and anhy-drite (CaSO4). These phases may be stable or unstable depending on temperature or ionic strength, and they have decreasing solubilities with increasing temperatures above about 40 deg. C. To understand the formation of these scale minerals, high-temperature laboratory methods must be used for the kinetic studies, allowing both solutions and solid phases to be sampled without spurious temperature effects. The kinetics of transformation of one hydrate to another is particularly important in determining the nature of the scale formed under field conditions as a function of both temperature and background electrolyte concentration.
This investigation studied the formation and dissolution of calcium sulfate phases under some typical field conditions. Kinetic investigations were emphasized since these frequently can be used to predict the nature of the phases formed under specific conditions of concentration or temperature. Moreover, unlike the results of spontaneous precipitation experiments, such studies are highly reproducible. The effects of factors such as ionic strength, temperature, supersaturation, and effectiveness of scale inhibitors may be studied quantitatively. In addition, the influence of the nature of the seed crystal phase and morphology on the subsequent growth process can be investigated. The morphology of the crystals comprising scale deposits may be particularly important in determining whether they pack together as hard, destructive scale or remain as a sludge to be swept away by the liquid phase. Seeded-crystal growth processes are better models than are spontaneous processes are better models than are spontaneous precipitation studies for the scale formation reactions precipitation studies for the scale formation reactions in which the solid phase is formed heterogeneously either on a foreign substrate or on crystals of scale already present.
The growth rate of calcium sulfate dihydrate seed crystals is independent of the fluid dynamics in the system, suggesting that the rate is not diffusion-controlled but depends on a surface reaction rate. This has particular significance for the formation of scale in the oil well because the scaling rate is expected to be independent of the dynamics of fluid flow at the metal surface.
SPEJ
P. 133