The Domes well field at Olkaria, Kenya : Reservoir characteristics with emphasis on fluid chemistry

Abstract

A total of 23 wells have now been drilled in the Domes Sector of the Olkaria geothermal system. Their current total steam yield is approximately 840 tons/hour which is sufficient to produce 120 MW of electric power. Data on the chemical composition of liquid water and vapour discharged from this well field have been used to reconstruct the composition and species distribution in the initial aquifer fluid using the Phase Segregation Model for wells with excess discharge enthalpy. 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 the close approach to equilibrium with specific hydrothermal mineral assemblages or source controlled i.e. by its flux from the magma heat source. The results for the other major reactive gases, 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. The collective use of geothermometers, measured well discharge enthalpy, subsurface pressure distribution, temperature build-up in wells after completion tests and circulation losses during drilling are weighed against spatial distribution of outputs in the well field. This has led to the development of a conceptual model that tends to sequestrate promising from non-promising areas. As the model demonstrates, well outputs are highest where reservoir pressures are highest and where the producing aquifers are deepest.

A low-temperature zone runs northsouth through the middle of the well field. In this zone, well discharges have liquid enthalpy. But discharge enthalpy increases both to the west and east, while good wells are observed to lie preferentially to the east of the cold zone. The proffered conceptual model suggests that new wells should be drilled in the southeast part of Domes.