Assessment of Olkaria-Northeast geothermal reservoir, Kenya base on well discharge chemistry

Abstract

Chemical data from twenty-one deep wells in Olkaria-Northeast field have been used to evaluate the reservoir processes and to try to build a field reservoir model based on geochemical evidence. The field under consideration produces basically a sodium-chloride type of water with deep reservoir Cl concentrations ranging from 300 to 550 ppm and deep water pH in the range of 6.7-7.4. The hot geothermal fluid upflow seems to be located in the central part of the drilled field with cold recharge coming from the east and northwest, including nearby Lake Naivasha water, while the hot fluid flows southward at deeper levels under a continuous tight caprock, to be tapped from the current producing field, Olkaria-East located a few hundred metres to the south. Both solute and gas geothermometers indicate temperatures of about 270-290°C in this reservoir. Fluid-mineral equilibria indicate an amorphous silica saturation temperature of about 170°C, while calcite solubility is limited to temperatures of about 200°C. It has been shown that no scaling problems are expected, at least for these minerals, and silica deposition may only be a problem as regards discharge of effluent with no problems in production if proper separation is carried out.