Untitled - Orkustofnun

Olkaria reservoir response30OfwonaDrilling started in 1973 with well OW-1 located to the southeast of the Olkaria hill that was to a depthof 1003 m with a temperature of 126°C but never discharged. An attempt was made to stimulate thewell to discharge by air-lift, but failed. Following this unsuccessful result, OW-2 was drilled 3.5 kmto the northeast of this well. The major considerations in locating well OW-2 were:1. To make one more attempt to locate a discharging well in the area.2. To move up along the ground water gradient (without going into Hell’s Gate), but also to keepthe wellhead elevation as low as possible in order to minimize depth to static water level.3. Favorable resistivity and gas chemistry. The chemical data obtained from fumaroles gaveindications of high underground temperatures and the resistivity surveys showed a pronouncedlow at 1000 – 1200 m (Sweco and Virkir, 1976).Drilling of well OW-2 gave positive results. It was drilled to 1350 m and encountered a 246°C steamzone at 650 m. Maximum temperature recorded was 280°C at the bottom. Discharge at atmosphericpressure gave 70–75 % steam and total flow rate was 9 kg/s (32.4 t/hr) at a pressure of 6 bar-abs. It isdue to the success in this well that further appraisal and production drilling were done in its vicinityand 1976, a feasibility study for utilization of geothermal steam for generation of electricity at Olkaria(Sweco and Virkir, 1976) was done. The study indicated that the development of the geothermalresource was attractive and it was decided to construct a 30 MWe power plant of two 15 MWe unitswith possible extension by addition of a third 15 MWe unit (Svanbjörnsson, et. al., 1983). The firstunit was brought on line in July 1981, the second in December 1982 and the third in April 1985. Thisplant is owned by Kenya Electricity Generating Company Ltd. (KenGen). Since then, Olkaria Eastfield (Olkaria I) has been producing steam for generation of 45 MWe and in most occasions stretchedto 48 MWe.Further exploration and development work has shown that the Olkaria field is very extensive (morethan 80 km 2 ) and is now divided into several sectors namely: Olkaria East field (Olkaria I), Northeastfield (Olkaria II), Central field, West field (Olkaria III), Northwest field, Southeast field and OlkariaDomes field (Olkaria IV). A 13 MWe binary plant was commissioned in Olkaria III in August 2000by ORPOWER4 Inc (a subsidiary Company of Ormat International). KenGen commissioned a 70MWe conventional steam power plant in Olkaria II in October 2003. Oserian Development CompanyLtd has been utilizing well OW-101 to supply heat and CO 2 for flower growing in greenhouses sinceMay 2003. They also commissioned a 2.1 MWe binary power plant in June 2004. The total number ofwells drilled to date is 98 and appraisal drilling will now be focused in Olkaria Domes field. Totalelectric power now generated at Olkaria is 130 MWe.2. RESERVOIR CHARACTERISTICS2.1 GeologyOlkaria geothermal field is a remnant of an old caldera complex which has subsequently beenintersected by N-S normal rifting faults that have provided loci for later eruptions of rhyolitic andpumice domes. Eruptions associated with Olkaria volcano and Ololbutot fault zone (Figure 1)produced rhyolitic and obsidian flows while eruptions from Longonot and Suswa volcanoes ejectedpyroclastic ash that has blanketed much of the area. NW, NNW, N-S, NNE and NE trending faults areobserved in the geothermal complex (Muchemi, 1999; Odongo, 1993). The most prominent structuresare the NE trending Olkaria fault, N-S trending Ololbutot fault, Olkaria Hill fault, Suswa fault andGorge Farm fault.Subsurface stratigraphy of Olkaria wells show that from the surface (which is at an average elevationof 2000 m.a.s.l) to about 1400 m.a.s.l, the rocks consist of Quaternary comendites with an extensivecover of pyroclastics. Below these, the dominant rocks are trachytes with basaltic lava flows and tuffs