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Title: Geological control on the reservoir characteristics of Olkaria West Geothermal Field, Kenya

Abstract

The reservoir of the West Olkaria Geothermal Field is hosted within tuffs and the reservoir fluid is characterized by higher concentrations of reservoir CO{sub 2} (10,000-100,000 mg/kg) but lower chloride concentrations of about 200 mg/kg than the East and North East Fields. The West Field is in the outflow and main recharge area of the Olkaria geothermal system. Permeability is generally low in the West Field and its distribution is strongly controlled by the structures. Fault zones show higher permeability with wells drilled within the structures havin larger total mass outputs. However, N-S and NW-SE faults are mainly channels for cold water downflow into the reservoir. Well feeder zones occur mostly at lava-tuff contacts; within fractured lava flows and at the contacts of intrusives and host rocks.

Authors:
Publication Date:
Research Org.:
The Kenya Power Company, Ltd., Naivasha, KE
Sponsoring Org.:
USDOE
OSTI Identifier:
889133
Report Number(s):
SGP-TR-147-18
Resource Type:
Conference
Resource Relation:
Conference: Proceedings, nineteenth workshop on geothermal reservoir engineering, Stanford University, Stanford, CA, January 18-20, 1994
Country of Publication:
United States
Language:
English
Subject:
Geothermal Legacy

Citation Formats

Omenda, Peter A. Geological control on the reservoir characteristics of Olkaria West Geothermal Field, Kenya. United States: N. p., 1994. Web.
Omenda, Peter A. Geological control on the reservoir characteristics of Olkaria West Geothermal Field, Kenya. United States.
Omenda, Peter A. Thu . "Geological control on the reservoir characteristics of Olkaria West Geothermal Field, Kenya". United States. doi:. https://www.osti.gov/servlets/purl/889133.
@article{osti_889133,
title = {Geological control on the reservoir characteristics of Olkaria West Geothermal Field, Kenya},
author = {Omenda, Peter A.},
abstractNote = {The reservoir of the West Olkaria Geothermal Field is hosted within tuffs and the reservoir fluid is characterized by higher concentrations of reservoir CO{sub 2} (10,000-100,000 mg/kg) but lower chloride concentrations of about 200 mg/kg than the East and North East Fields. The West Field is in the outflow and main recharge area of the Olkaria geothermal system. Permeability is generally low in the West Field and its distribution is strongly controlled by the structures. Fault zones show higher permeability with wells drilled within the structures havin larger total mass outputs. However, N-S and NW-SE faults are mainly channels for cold water downflow into the reservoir. Well feeder zones occur mostly at lava-tuff contacts; within fractured lava flows and at the contacts of intrusives and host rocks.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Jan 20 00:00:00 EST 1994},
month = {Thu Jan 20 00:00:00 EST 1994}
}

Conference:
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  • The bipole-dipole, Schlumberger and in line dipole-dipole electrical resistivity configurations were used to delineate the Olkaria geothermal reservoir with the view to site boreholes for the production of electric power using the geopressurized hot water. The dipole-dipole resistivity data provided the least ambiguous and most usable data for assessing the resource. Deep drilling into two of the anomalies outlined by this survey has proved the existence of high-temperature reservoirs and a 15MW power station is under construction.
  • Olkaria Geothermal area is located in the central sector of the Kenya, Rift Valley. A 45MW Geothermal power station has been operational at Olkaria since 1985 supplied by 22 of the 26 wells drilled in the Eastern production field (EPF). Between 1988 and 1993, eight more wells referred to as {open_quote}replacement wells{close_quote} were drilled in the same field to boost steam supply to the station. Petrographic analyses of the drill cuttings is usually done to determine detail stratigraphy of the field, extends of hydrothermal activity, subsurface structures and other parameters which may influence production potential of a well. Analyses ofmore » the drill cuttings from the EPF wells show that: Variations in the whole rock alteration intensities correlate with differences in rocktypes. Permeable horizons, especially the productive feeder zones are well marked by enhanced hydrothermal minerals depositions, mainly quartz, calcite, pyrite and epidote. Other aspects of state of reservoir like boiling are signified by presence of bladed calcite.« less
  • A detailed three-dimensional well-by-well model of the East Olkaria geothermal field in Kenya has been developed. The model matches reasonably well the flow rate and enthalpy data from all wells, as well as the overall pressure decline in the reservoir. The model is used to predict the generating capacity of the field, well decline, enthalpy behavior, the number of make-up wells needed and the effects of injection on well performance and overall reservoir depletion. 26 refs., 10 figs.
  • The Olkaria geothermal field has been under continuous development since 1970. A feasibility study, completed in 1976, after six wells had been drilled and tested, indicated that development of the Olkaria field was feasible. The feasibility study was followed by production drilling and the construction of three 15 MW generating units. The first unit was brought on stream in July 1981, the second in December, 1982, and the third is scheduled to be completed in early 1985. The current output of 19 productive wells is equivalent to 46 MWe. Distribution of fumaroles and resistivity surveys indicate an areal extent ofmore » some 80 km{sup 2} for the Olkaria geothermal field. Gas chemistry of fumaroles indicates comparable underground temperatures over the whole field, 200-250{degrees}C. The capacity of the resource has been estimated to be 500-1000 MW electric for a production period of 25 years. Most of the drilling has been confined to a small part of the geothermal field. Here maximum recorded downhole temperature is 339{degrees}C and temperatures follow the boiling point curve with depth. A thin steam zone at 240{degrees}C is observed in the top of the reservoir at approximately 600-700 m depth. The reservoir fluid is dilute, of the sodium chloride type, contains chloride in the range of 200-700 ppm. The reservoir rocks consist of a sequence of near horizontal lavas and tuffs of trachytic composition, but basaltic andesites have also beenidentified. The drilled rocks at Olkaria are of relatively low permeability, the average yield of wells being equivalent to about 2.5 MWe. Exploratory drilling is presently in porgress in the Olkaria field, the aim being to locate new production areas withing the field. Three holes have been completed and the forth and last hole under the present plan is being drilled.« less
  • The use of numerical models for the evaluation of the generating potential of high temperature geothermal fields has increased rapidly in recent years. In the present paper a unified numerical approach to the modeling of geothermal systems is discussed and the results of recent modeling of the Krafla geothermal field in Iceland and the Olkaria, Kenya, are described. Emphasis is placed on describing the methodology using examples from the two geothermal fields.