Aquifer Fluid Chemistry Characteristics for the Domes Geothermal Wellfield at Olkaria, Kenya

Abstract

The initial aquifer fluids feeding Olkaria Domes wellfield in Kenya have been modelled using the analytical data of the wellhead samples. The chemical composition of liquid water and vapour discharged were used to reconstruct the composition and species distribution in the initial aquifer fluid using the Phase Segregation Model at a temperature of 180°C in the case of wells with excess discharge enthalpy. The fluid enthalpy reveals that some of the wells have liquid enthalpy while the majority having “excess enthalpy” with the geothermometers indicating that excess enthalpy of well discharges is largely produced by phase segregation in producing aquifers or occurring by gravity under natural conditions. The measured pH was corrected for silica polymerization and subsequently used to calculate aquifer water pH and mineral saturation. The modelled fluids is characterized by Na-Cl-HCO3 water type and it exposes the existence of a wide-ranging difference in composition with discharge wellhead waters showing a general increase in contents of most elements at surface conditions. As described in this paper, the results were further used to assess how closely they have approached equilibrium conditions with respect to various mineral-gas and mineral-solution reactions that may occur in the aquifer. The aquifer water is close to saturation with respect to calcite but under-saturated with fluorite except for the lowest temperature waters (~200°C). Aqueous carbon dioxide is either controlled by close approach to equilibrium with specific hydrothermal mineral assemblages or source controlled. Hydrogen and hydrogen sulphide show large scatter for their modelled concentrations in the initial aquifer fluid making it difficult to deduce the processes that govern them.