Steam gathering system for the NE-Olkaria geothermal field, Kenya : preliminary design

Abstract

The Northeast Olkaria geothermal field occupies an area of 12 km2. It is one of three sectors which together form the Greater Olkaria geothermal field, located about 125 km northwest of Nairobi and situated within the East African Rift Valley. The other two sectors are the East and the West Olkaria geothermal fields. Production drilling is nearly complete with 27 wells already drilled in this sector. The developmental process is approaching the Detailed Design Stage. Due to environmental reasons it has been decided to reinject the effluent obtained from separation stations and from the cooling towers. The reinjection operation will involve pumping of the effluent to the reinjection well(s). The objective for this study was to design a steam gathering system which would provide an efficient arrangement for collecting the effluent at a pumping station before being pumped to the reinjection well(s). The gathering system has been looked at in terms of cost operational convenience. The two-phase lay-out has been carefully selected ensuring that the flow is always downhill. In sizing the two-phase pipelines, the superficial steam velocity has been restricted to between 20 and 40 m/s and slug flow regime has been avoided. Two-phase pressure drop calculations have been done using two correlations, one by Lockhart-Martinelli and the other by Friedel. The sizing of major equipment that constitutes the system has been done. The calculated operational wellhead pressures are possible as seen from the output curves of the wells. The study has shown that it is possible to design a central separation system for utilizing the steam from the field. The total cost of the equipment required for the gathering system has been estimated at approximately USD 13 million.