| Abstract |
The Olkaria III geothermal field has been under development by ORMAT since 1998. A 12 MWe binary plant was commissioned in July 2000. Long term stability of the reservoir and the well output suggested that the field is able to sustain a larger load; therefore ORMAT continued field development by drilling new wells and increased power output by an additional 36 MWe by early 2009. A series of well tests have been carried out in Olkaria III since the commissioning of the new power plant, including down hole pressure and temperature measurement, continuous reservoir pressure monitoring, geochemical sampling and tracer tests to define the hydrologic flow paths and rate of return/inflow in the Olkaria III geothermal reservoir. Around 3 x 100 kg of liquid-phase tracers were injected in July 2009: 2,6-NDSA, 1,5-NDSA and 2,7 NDSA into wells OW-401, OW-307 and ORP-B1 respectively. Of the nine production wells monitored for tracer recovery during eight months, only two wells show positive tracer responses from two injection wells. The tracer mass recovered to date is however only about 3-5 % of the total tracer mass injected. The TRINV computer software program was used to calibrate properties of two flow channels connecting wells ORP-B1 and OW-301. The analysis suggests that around 5 kg/s of the total 80 kg/s injected into ORPB1 flow towards OW-301. A small cross section area for the two flow channels (5 m2) is modeled and relatively high longitudinal dispersivity (280-300 m). Average flow velocity from ORP-B1 to OW-301 is about 15 m/day for the bulk of the injected mass, to be compared with total distance of 1000 m. Such a slow flow velocity, high dispersivity and long distance imply that immediate cooling effects are not of concern for this well dipole. To verify this conclusion, a simple parallel plate fracture model, of the same cross section area as the larger tracer flow channel, implies that maximum cooling of production well OW-301 is about 15 ºC over 10 years of continuous injection. The study therefore concludes that injection to ORP-B1 should eventually terminate and be transferred to a more southerly location. When coupling the minimal tracer recovered with the observed lateral pressure gradient from North to South, it appears evident that most of the injected tracer is flowing straight south and consequently out of the ORMAT production sector. This is highly unusual for most high temperature geothermal reservoirs, in particular for Olkaria West where 100 % reinjection has been practiced from the onset of power production. The outer reservoir margins therefore must respond to the current mass production by a healthy and strong inflow of hot fluids from the North. |