Scaling and corrosion mitigation in Olkaria IAU by codensate-brine mixing method

Abstract

Monitoring and management of pH is an integral part in geothermal operations and utilization. At the Olkaria geothermal field, reinjection of geothermal wastewater from the power plants is a routine practice and an integral part of sustainable geothermal utilization. In order to safely and efficiently reinject the water without any problems of scaling or corrosion, the 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 of the wastewater is maintained within the required limits of 6.0 to 8.5. 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 company in 2001 in Olkaria II and later replicated in other 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 study, 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 than 8) before reinjecting 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. This assessment is done using PHREEQC which is a geochemical modelling program code and the WATCH program. The PHREEQC program simulates the mixing of various fluids while assessing their scaling potential for silica, calcite and anhydrite. The results from the study show that mixing the geothermal wastewater from the power plant and the separated brine from the reinjection considerably lowers the calcite and silica scaling potential in the reinjection aquifers and also raises the pH of the fluid. Upon producing from the mixed fluid after heat up, the resultant fluid has low scaling and corrosion effect. The scaling potential of anhydrite is increased considerably but this does not pose any danger to geothermal operations.