Jeremiah Kipngok1, Leakey Auko, Sylvia Malimo, George Igunza, Shilla Kangogo Lawrence Ranka, Evans Bett, Janet Suwai and Koji Matsuda2
Geothermal Development Company
jkipngok@gdc.co.ke
This paper presents an overview of the chemical characteristics of Menengai geothermal reservoir from wells discharge fluid. The fluid discharge from Menengai generally exhibits widely varying chemical properties from well to well. This observed variability points to heterogeneity of the reservoirs feeding Menengai wells. The fluid discharge belongs to the NaHCO3 facies although some of the wells located in the northern part of the central sector, in the vicinity of well MW-19, display a Na-HCO3-Cl type. Most wells in Menengai have excess enthalpy which is largely attributed to phase segregation. Three models were used to assess gas-mineral equilibria assuming a closed, isolated and an open system and unit activity. The models show that H2S in the reservoir(s) in Menengai is largely controlled by hydrothermal minerals but H2 is higher suggesting the presence of significant vapor fraction in the reservoir(s) feeding Menengai wells, albeit in varying proportions. CO2 may have contributions from the magmatic heat source as well as organic sources besides being fixed by hydrothermal minerals. The water recharging the Menengai reservoir is meteoric in origin while a mantle source is inferred for the gases. The main upflow is located within or in the locality of well MW-13 situated in the central part of the Menengai geothermal field. The main process affecting the chemistry of Menengai wells discharge fluid is reservoir boiling and phase segregation. Conductive heat transfer/cooling and to some extent addition of magmatic fluids are other factors that significantly contribute in some wells.
Keywords: Menengai, chemical characteristics, geothermal reservoir, isotope data, geothermal wells
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