Provenance of Phase Segregation and Conductive Heat Transfer Processes in Menengai Geothermal System

Abstract

Menengai geothermal field wells displays heterogeneous physico-chemical properties. For instance, the enthalpy of individual wells ranges from water enthalpy of less than 1000 kJ/kg to dry stream enthalpy of about 2700 kJ/kg. Whereas, individual wells’ chemistry of volatiles and non-volatiles components vary spatially. This may be ascribed to various physico-chemical processes in the geothermal system. This paper therefore, presents the qualitative evaluation of the provenance of phase segregation and conductive heat addition in Menengai geothermal system with the view of providing insights into the possible cause of excess enthalpy (i.e the enthalpy of the discharged fluids is higher than that of the enthalpy of steam saturated liquid at the aquifer temperature; hd,t more than hf,l). The trends displayed by the non-volatile Cl concentration in the total well discharge and in the liquid phase as a function of discharge enthalpy suggests that Menengai has a heterogeneous reservoir, with excess enthalpy dominantly caused by phase separation and conductive heat transfer from hot rock to the circulating fluids. Moreover, SiO2 vs. enthalpy was used to evaluate the mixing scenario between the shallow end-member feed zone and a deep end-member steam dominated feed zone and ultimately identify the aquifers of provenance of the geothermal liquids feeding the selected wells.