Revision of the Conceptual Model for the Olkaria Geothermal System, Kenya

Abstract

Olkaria is an unusually large and powerful geothermal system located inside the East African Rift Zone, about 100 km WNW of Nairobi, the capital of Kenya. It is situated inside a major volcanic complex that is associated with a buried caldera and cut by N-S trending normal rifting faults and other major faults and fractures. The Olkaria system is presently under intense development with the electrical generating capacity having increased from about 200 MWe in 2010 to more than 670 MWe at the end of 2016. This development has been dominated by Kenya Electric Generating Company (KenGen) in its 204 km2 license area. Utilization in Olkaria started when the Olkaria I power station (45 MWe) was commissioned in the 1980’s, while the Olkaria II power station (105 MWe) was not commissioned until the early 21st century. In 2007 drilling of deeper wells (to about 3000 m), in an extended area, identified a much larger and more widespread resource than was previously believed to exist. Extensive drilling since then (about 165 deep wells) has been the basis of the intense development for electricity generation in recent years. The conceptual model of the Olkaria geothermal system has been under constant revision in recent years based on available geological- and geophysical information, temperature- and pressure data, various reservoir testing and monitoring data as well as information on the chemical content of reservoir fluids. Most important are the comprehensive data from the new wells drilled in the area since 2007. At least three deep magmatic heat sources are assumed in the system with hot water up-flows into the four main well fields. Comprehensive management is essential for the successful future utilization of the Olkaria geothermal system; including monitoring, reinjection, research and modelling as well as environmental and social awareness. Further exploration of the parts of KenGen’s license area where exploration has been limited is essential before they will be considered for further development.