Effect of Boiling and Cooling of Geothermal Fluids on Precipitation of Secondary Minerals: A Case Study of Olkaria Fields, Kenya

Abstract

The main drawback in the utilization of geothermal resources arises from the precipitation of secondary minerals within wells, pipelines, steam separators, turbines and other surface equipment in form of scales. Scale formation is an outcome of the alteration of various rocks dissolved in geothermal fluids that find their way into a reservoir. Once geothermal fluids ascend to the surface, hydrostatic pressure decreases toward a phase separation level that permits the dissolved gases such as CO2, H2S and H2, and steam to separate from the liquid phase by “boiling”. Stripping of these volatiles may increase fluid pH, leading to precipitation and deposition of secondary minerals. The study sought to establish the relationship between water-rock interaction and secondary mineral precipitates at the surface and deep fluid at different temperatures during depressurisation boiling and cooling. Samples were collected from selected Olkaria wells; OW-38A, OW-910 and OW-910A. The analysis of the results outlined deep fluid Alkali-Chloride waters and surface steam-heated Alkali-Bicarbonate and acidic Sulphate-Chloride waters. Various models suggested adiabatic boiling, conductive cooling and possible mixing and dilution in the wells. Hydrothermal alteration minerals were found to be in equilibrium with the geothermal fluids at varying temperatures, and the secondary minerals controlled the chemistry of the reservoir. Silica-saturated solutions precipitated silica in OW-910 and OW-910A, which may have resulted from rapid cooling following mixing with cold surface water.