| Title | Updated Geologic and Geochemical Assessment of the Olkaria III Field Following Recent Expansion to 110MW |
|---|---|
| Authors | Lara OWENS, Enrique PORRAS, Paul SPIELMAN, Patrick WALSH |
| Year | 2015 |
| Conference | Stanford Geothermal Workshop |
| Keywords | Kenya, Olkaria, OEC, ORC, stable isotopes, gravity, helium isotopes |
| Abstract | The Olkaria III geothermal field has been under development by the ORMAT geothermal group since 1998. The field is located on the western side of a large Kenyan-rift caldera complex hosting several distinct geothermal systems. Recent drilling and expansion to 110MW at Olkaria III has enhanced our geologic and geochemical understanding of the reservoir in the western side of the caldera. Production within the reservoir draws largely from fractured and silicified Mau tuffs as well as interbedded trachyte to rhyolitic pyroclastics layers, as opposed to the predominantly trachyte reservoir found in Olkaria east. Permeability is mainly controlled by rift parallel north-south trending faults which support high productivity wells. High resolution gravity surveys demonstrate that felsic intrusive plugs occur near major permeable fault intersections in the field which correlate with some of the largest wells in the field. High chloride primary upflow is found along the north-south Olkaria Hill fracture which feeds much of the production in the field, with CO2-rich fluids infiltrating from the west across the Kolingi fault. Oxygen and deuterium isotopes indicate that 18-35% of the mature recharge in the system is supported by Lake Naivasha drawing from the north, while western, more peripheral discharge is largely comprised of meteoric recharge from the Mau escarpment. Near magmatic CO2-carbon isotopes occur in the bicarbonate rich fluids to the western wells. Additional isotopic studies support that the center of the Olkaria III production field is strongly heated by a shallow magmatic heat source demonstrated by mantle-rich He-isotopes(R/Ra = 6.7) in these central wells. Recharge from the constant pressure boundary to the east and northeast contains degassed, less magmatic chemistries suggesting upflow from a mature deep circulating system similar to Olkaria East. Less than 4 bar of drawdown has been observed in Olkaria over 14 years of production. Enthalpy is predicted to change from 1500kJ/kg to 1300kJ/kg over 30yrs as a result of the expansion to 110MW, however this is managed by the flexibility and specific design of the Ormat OEC and 2-phase power station. |