Influences on the Barite Precipitation from Geothermal Brines

Abstract

Barite is one of the toughest scaling minerals in geothermal power plants all over the world using saline wells of deep sedimentary basins. It is highly insoluble, can incorporate radioactive isotopes, and its scaling persists on the walls causing constriction or even blockage of system components. Removing the scales is only possible by mechanical means. To help preventing barite scale formation a profound understanding of precipitation and scaling processes is necessary. This study focusses on the parameters that influence the velocity of barite precipitation. These include: • Supersaturation • Overall salt concentration • Shear rate (as a measure for the flow conditions) • Contact surface • Crystallization nuclei • Molar ratio n(Ba2+)/n(SO42-) To determine the barite precipitation rate at different shear rates, a special annular gap apparatus has been developed. This apparatus ensures constant shear rates in the solution. Overall salt concentration was varied by using three model geothermal brines. These are solutions with different salt concentrations. Each resembles the water composition of one of three European geothermal sites (total dissolved salt between 90 and 265 g/L). The molar ratio of barium ions and sulfate ions was varied from 1:1 up to a twelvefold sulfate excess. Furthermore, the influence of different contact surfaces like glass and polymethyl methylacrylate (PMMA) as well as crystallization nuclei have been examined. Experiments were carried out at room temperature. Supersaturation was attained by adding barium and sulfate ions separately in the form of barium chloride (BaCl2) and sodium sulfate (Na2SO4) solutions in the necessary excessive amounts. To measure the precipitation progress, the remaining barium ions were analyzed by probe sampling. Barite precipitation rate rises with higher shear rates. This influence is more distinctive when regarding brines with lower overall salt concentration. At these concentrations, barite precipitates more rapidly than at higher salt concentrations. A high sulfate excess as well as a large supersaturation accelerate precipitation. Precipitation is slower when using glass instead of PMMA. When adding crystal nuclei before starting, precipitation is unexpectedly fast.