Prediction of Anhydrite Precipitation in Field Water-Heating Systems
Authors: Templeton Charles C. , Rodgers Jane C. 
Published: December 2007

A key step in feed water treatment for generating wet steam for thermal oil recovery is the removal of calcium and magnesium hardness by cation-exchange series softening. Knowing the solubility of any scale-forming salts in brines at elevated temperatures is necessary for fixing the level to which the feed water must be softened. Such calcium sulfate solubility data, previously not available above 392F, were determined by the authors in a flow equilibrium apparatus and will be reported elsewhere. These data were used to develop a method for predicting the solubility of anhydrite in hot water or steam droplets for saturated steam pressures as high as 2,000 psig (637F). (The calcium sulfate solubility product is represented by a combination of two factors, one reflecting the effects of ionic strength and the other accounting for the effects of complex ion formation in either calcium-magnesium-rich or sulfate-rich brines.)

The method is applied to a calcium-magnesium-rich brine of moderately high salinity from a pilot hot-water flood, and to several sulfate-rich, low-salinity feed waters and blowdown (cooled steam droplets) samples from steam soak operations. The predicted calcium hardness levels corresponding to the calcium sulfate solubilities agreed reasonably well with the results of laboratory solubility determinations run on the field samples. Further testing of the method is needed for brines of other composition classes. Existing field cation exchange softeners in the cases tested are performing adequately since all the samples were found to be undersaturated with respect to calcium sulfate at their operating temperatures.

Introduction

Prevention of scaling caused by precipitation of calcium sulfate (anhydrite) is of considerable concern in connection with thermal recovery processes using wet steam or hot water. To avoid anhydrite precipitation in a heated system, an engineer must keep the product of the calcium and sulfate concentrations in the water or steam droplets below the value of the solubility product of anhydrite for the temperature and brine composition in question. Usually it is most practical to keep the concentration product lower than the solubility product by keeping calcium low in the presence of high sulfate, or by keeping sulfate low in the presence of high calcium. This can be done by a choice of combinations of natural waters and water treatment processes (such as series cation exchange softening to remove calcium).

Until recently, few anhydrite solubility data, particularly for solutions containing other salts, were available for temperatures above 392F (211 psig steam); Marshall, Slusher and Jones studied the CaSO4-NaCl-H2O system up to 392F and surveyed the work of previous investigators. To model natural brines, one needs to study the solubility of anhydrite in aqueous solutions of sodium chloride. sodium sulfate, calcium chloride, magnesium chloride and their mixtures. Since steam pressures as high as 2,000 psig (637F) may be involved in thermal oil recovery projects. a solubility study was conducted between 482 and 617F. Discussed in this paper is the application of these data to the prediction of anhydrite precipitation in some practical steam soak and hot-water injection projects.

Any simple method for predicting the solubility of an inorganic compound over a wide range of temperatures and solution compositions must be based on some assumptions, and therefore must yield approximate results. On the one hand, natural brines contain too many ionic species for all to be included in a simple scheme; on the other hand, there is no adequate theoretical basis for the exact prediction of solubility in even simple solutions of mixed electrolytes. However, it is possible at a given temperature to base a reasonable prediction scheme on two phenomena: the increase in solubility with increasing total concentrations of all ions (as measured by the ionic strength; see the Appendix). and an increase due to formation of complexes between calcium ions and sulfate ions and between sulfate and magnesium ions. Stiff and Davis developed such a scheme for predicting the solubility of gypsum (CASO4 * 2H2O) in brines at temperatures up to 212F. However, for the higher temperatures of present interest, the stable solid phase is anhydrite (CaSO4).

This study involves a two-part method for predicting anhydrite solubility products. First, one predicts a value K, for a given ionic strength I and a given temperature T corresponding to mca/mso4= 1 from data of the CASO4- NaCl-H2O system. Second, one determines a group of factors F = F(Ca) * F(Mg) * F(SO4). where the individual factors account for increases in the solubility product due to complex formation by high concentrations, respectively, of calcium, magnesium and sulfate. Combining the two parts, one obtains the solubility product in molalities as