Scaling and Corrosion Mitigation in Olkaria Using Brine and Condensate Mixing Method

Abstract

pH monitoring and management is an integral part in geothermal operations and utilization. In Olkaria geothermal field, reinjection of the spent geothermal waters from the power plants is a routine practice and is an integral part of sustainable geothermal utilization. In order to safely and efficiently re-inject the waters without any problems of scaling or corrosion, pH and temperature of the condensate need to be maintained within the design requirements so that it does not corrode the carbon steel components of the power plant or the casing of the cold reinjection wells. To achieve this, the pH is maintained within the required limits of 6.0 to 8.5 for the power plants reinjection fluid. Current practice in Olkaria is treatment of the acidic condensate with sodium carbonate (Na2CO3). This method was introduced in the Olkaria field by the Sinclair Knight Merz in 2001 in Olkaria II and later replicated in other new plants that have come online since then. The suspected major concern with this method is side reactions from the impurities in the chemical such as silicates and sulphides. In this paper, an alternative method of pH modification in geothermal power plants is explored which is mixing the spent geothermal waters from the power plants with the separated waters (brine which has a higher pH of more than 9) before re-injecting into the cold reinjection wells. This approach assesses the scaling potential before and after mixing the fluids and how the pH changes upon mixing the condensate and brine in different mixing ratios from 90% condensate and 10% brine to 50% condensate and 50% brine. Since the aim is to raise the pH of the condensate while maintaining the optimal operating temperatures for silica saturation, more condensate to brine is taken. This is done using PHREEQC which is a geochemical modelling program code, and WATCH. The PHREEQC program allows for simulation of mixing of various fluids while assessing their scaling potential for silica, calcite and anhydrite. The results from the study show that mixing the spent geothermal fluid from power plant and the separated water from the reinjection considerably lowers the calcite and silica scaling potential in the reinjection aquifers and also raises the pH of the fluid thus preventing corrosion. Upon producing from this fluid after heat up, the mixing does not affect the scaling tendencies much. The scaling potential of anhydrite is increased considerably but this does not pose any danger to geothermal operations