Chemical modelling programs for predicting calcite scaling, applied to low temperature geothermal waters in Iceland

Abstract

Three chemical models, the WATCH program, the VDATA program and the Ryznar index are used for the study of calcite scaling potential in low temperature geothermal wells in Iceland. Fields with known calcite scaling in some wells were chosen for the study. They are: Laugarnes field, Sudureyri field, Hrisey field, Selfoss field and Seltjarnarnes field. The study involved training in the use of chemical models to predict calcite scaling and to determine the allowable supersaturation before calcite scaling formation becomes a problem. This is done by comparing the degree of calcite supersaturation and actual scaling status in the selected wells. The saturation index, log (Q/K) = 0.36 - 0.5 was found to be the boundary between calcite scaling and no scaling. If the saturation index, log (Q/K), is higher than 0.5, calcite scaling problems occur, if it is lower than 0.36, no calcite scaling will occur. Between 0.36-0.5, calcite scaling may and may not occur, depending on other factors. This implies a calcite supersaturation ratio, Q/K, of 2.3-3.2 and thus geothermal waters can contain 2.3 times more CaCO3 in solution than the theoretical amount. The information from changes of chloride concentration and temperature as well as mixing calculations show that mixing of different waters is the main reason for the high degree of calcite supersaturation in the low temperature wells selected in this study. The results from the WATCH program agree with the results from the Ryznar index. Calcite scaling occurs at high calcite saturation index values and low Ryznar index values. Based on these findings more accurate predictions of calcite scaling can be made.