Significance of volatile constituents from the geothermal fields of Olkaria

Abstract

Volatile constituents in geothermal fields provide valuable information on the physical and chemical processes occurring in volcanic geothermal systems. This includes classification of geothermal waters, which is done by analysis of water and steam from the exploration and appraisal wells. Physical processes such as ore deposition may take place because these volatiles are carriers of metals from the depth. Volatile constituents, especially the gases such as CO2 and CH4 can be utilized commercially from their relative abundance. Volatile constituents in this project report are analyzed from the geothermal well fluids of Olkaria Fields. The Olkaria region of the Kenya rift valley is a potentially important geothermal area due to the occurrence of young volcanic activity and surface geothermal manifestations which include hot springs, fumaroles and thermally altered ground. Olkaria is a high-temperature geothermal system, located within the central part of the Kenya Rift valley. It has been explored extensively for geothermal development purposes. The geology is dominated by PleistoceneHolocene with the Holocene rhyolites flows on the surface while basalts, trachytes and tuffs dominate the subsurface. Geology of Olkaria falls on the broader sense of the Rift Valley which is a crustal extension feature, the rifting being as a result of magma upwelling. This project focuses on the significance of the volatile constituents which are present in the Olkaria fields. These volatiles include the hydrothermal fluids at depth, steam being extracted for geothermal energy, and those manifested on the surface as gas emissions. These volatiles interact with wall rock as they move from depth towards the surface, become saturated with different minerals, and due to differing temperatures and pressure, some of these dissolved or adsorbed minerals are deposited within the rock. Some of these deposits might be precious metals such as gold and silver. The amount of some volatiles such as the gases may be high enough to allow extraction for commercial use. These include carbon dioxide and methane, which make a significant percentage of the geothermal steam of Olkaria. New technological innovations such as Hycap (Wyoming) method are predicted to be able to extract and separate methane from other geothermal gases and use it commercially. Carbon dioxide is the most abundant of the gases from the Olkaria wells. From the estimation it is evident that there are high levels of CO2, most of which escapes into the atmosphere. This gas can be utilized in enhanced geothermal systems (EGS), where in supercritical condition, it is used as an efficient fluid for extraction of geothermal heat instead of water. This is significant in that more geothermal energy will be produced and at the same time, it will serve as a disposal unit for this greenhouse gas.