Scaling and corrosion potential of selected geothermal waters in Serbia

Abstract

A study of scaling and corrosion potential of selected geothermal waters in Serbia is presented in this report. Reinjection will be an integral part of the design for any future geothermal development in Serbia. Two of the important problems in connection with reinjection are scaling and corrosion, since the chemicals dissolved in geothermal waters may have a tendency to precipitate or cause corrosion in a reinjection well. The selected geothermal waters are characterized by a wide range of temperatures (25-94 C), are mainly of the sodium-bicarbonate type, with important amounts of calcium ions as well as dissolved carbon dioxide. Therefore they are prone to calcite scale deposition. With the aqueous speciation computer programme WATCH, different thermal and chemical conditions can be simulated and the resulting changes in water chemistry predicted. The results show that geothermal waters in Serbia are slightly supersaturated with respect to calcium carbonate. After adiabatic boiling the extent of supersaturation increases significantly. Heating of spent geothermal waters from 25 C to the production well discharge temperature or the predicted chalcedony geothermometer temperature gives rise to a slight calcite supersaturation. Some of the geothermal waters have a significant corrosion potential. Their carbon dioxide and oxygen concentrations are high. These two parameters are of the greatest importance from the point of view of corrosion in geothermal systems. The ferrous ion is the stable form of iron in geothermal waters. Therefore corrosion would be expected to proceed when iron comes into contact with the above dissolved gases. These results agree with those of several reinjection tests in Serbia during the past few years.