Abstract
The Olkaria geothermal field is located in the Kenyan Rift valley, about 120 km from Nairobi. Development of geothermal resources in the Olkaria area, a high temperature field, started in the early 1950s. In the subsequent years numerous expansions have been carried out with additional power plants being installed in Olkaria. These include a binary plant at Olkaria South West (Olkaria III) in 2000, a condensing plant at Olkaria North East (Olkaria II) in 2003, another binary plant at Olkaria North West (Oserian) in 2004 and finally condensing plants in the year 2014 within East production field (EPF) and Olkaria Domes (OD) areas. The total generation from this field is about 730 Mw. The study considered samples from 4 producing wells from 3 fields of the Olkaria geothermal area (OW-44 from the Olkaria East, OW-724A from the Olkaria North East, and OW-914 and OW-915 from the Olkaria Domes field). The chemical data were first analyzed using SOLVEQ. This helped in the determination of the equilibrium state of the system, the reservoir temperatures and the total moles to be run through CHILLER. The run CHILLER considered the processes that have been proven to be occurring in the Olkaria field i.e., boiling and condensing processes, fluid-fluid mixing rocks and titration resulting from water-rock interaction. The effects on gas evolution were evaluated based on the resulting recalculated gas pressures. The results indicate that the gas species are not in equilibrium with the mineral assemblages. The CHILLER evaluation shows boiling as the major process leading to the evolution of gases. OW-44 had the least gas concentrations, arising from the considered reservoir processes due to degassing, and near surface boiling, besides the removal of NH3, H2 and H2S are through the reaction with steam condensate. The gas breakout is most likely in OW-914 and least in OW-44. The study proposes different reservoir management strategies for the different parts of the Olkaria geothermal field. That is by increasing hot reinjection in the eastern sector around well OW-44. The reservoir around OW-914 is to be managed by operating the wells at a minimum flow rate (or even to close them) or the use of chemical inhibitors to prevent calcite scaling.
Highlights
Evaluation of gas concentrations in Olkaria geothermal field revealed that on average gases are the highest in the Domes, intermediate in Northeast production field and lowest in East production field (West JEC [1])
The results indicate that the gas species are not in equilibrium with the mineral assemblages
West JEC [1] considered three factors for the variations: 1) CO2 gets added at great depth to Northeast production field and Domes waters as a result of volcanic degassing below these areas, with less input of deep CO2 to the East production field; 2) CO2 addition at depth is similar in all three areas, but waters of the East production field lose CO2 during the onset of boiling in the reservoir such that the CO2 escapes to fumaroles which lie in the west and south of East production field; 3) cap-rock conditions in the Northeast production field and Domes are somewhat more restrictive than in the East production field, leading to more entrapment at shallower levels of CO2 that has been released by boiling
Summary
Evaluation of gas concentrations in Olkaria geothermal field revealed that on average gases are the highest in the Domes, intermediate in Northeast production field and lowest in East production field (West JEC [1]). The gas concentration in systems that are unexploited is generally controlled by temperature dependent equilibria with various mineral buffers (Giroud and Arnórsson [4]). The gas ratios like in the case of H2S/H2 activity ratio correspond closely to equilibrium with the mineral pairs of pyrite/pyrrhotite and pyrite/magnetite that is generally under saturated with respect to the individual minerals. These findings agreed with Karingithi [5] in the study on the Olkaria field which found out that activity of aqueous H2 and H2S gases generally corresponded closely to equilibrium with the mineral buffer pyrite, pyrrhotite, and magnetite. The H2S/H2 activity ratio corresponded closely to equilibrium with the mineral pair pyrite, pyrrhotite and pyrite and magnetite
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