Solubility of Calcium Sulfate in Brine

Abstract

Abstract Brine solubility of calcium sulfate is a function of several variables, including brine composition, temperature, and the hydration state of the precipitating phase. Solubility data covering these variables is included, and the need for inclusion of an additional variable—turbulence—is shown for temperatures above the normal boiling point. These data are then used to derive an expression for that solubility to about 250 degrees F. Introduction Calcium sulfate is one of several sparingly soluble salts commonly deposited from oil field waters. That deposition is the result of a supersaturated condition approaching equilibrium by precipitating some of its dissolved salt burden. Precipitation continues until stability has been achieved. A number of parameters influence the solubility of calcium sulfate. Certainly one of these is the hydration state of the calcium sulfate molecule. Figure l shows solubility of the three most common forms in distilled water as a function of temperature. Solubility of calcium sulfate is also affected by temperature; salinity; excess common ion; concentration of magnesium ion; and, to some extent, pressure. This latter effect has been well covered by Vetter. In this study of calcium sulfate solubility, we have searched the literature for data on the effects of the above-mentioned parameters. We have supplemented the available parameters. We have supplemented the available information with new, experimental data. The purpose of this paper is to present these purpose of this paper is to present these data, to note the trends revealed, and to use the information to improve prediction of calcium sulfate deposition over a wide range of temperatures, salinities, and magnesium ion concentrations. With this procedure, we can now calculate calcium sulfate scaling tendencies from waters with temperatures as high as 225 degrees F. At the elevated temperatures, an additional solubility parameter—turbulence— must also be considered. CALCIUM SULFATE SOLUBILITY Anhydrite Solubilities of anhydrous calcium sulfate in water and in sodium chloride solutions are shown in Figures 2 and 3. The bulk of these data were taken from the literature shown in the attached bibliography.