Processes controlling aquifer fluid compositions in the Olkaria geothermal system, Kenya

Abstract

The volcanic geothermal system of Olkaria in Kenya has been extensively drilled for development purposes. The well data show that the reservoir is liquid-dominated. Often downhole temperatures in thermally stabilized wells follow the boiling point curve with depth to at least 2 km. In one part of the field (Olkaria East), a vapor-dominated cap existed above the liquid reservoir when the first deep wells were sunk in the area. Many of the producing wells have excess discharge enthalpy, i.e. the enthalpy is higher than that of steam-saturated water at the temperature of producing aquifers. In some cases, inflow from feed zones in the vapor cap may contribute to the excess enthalpy. Geothermometer results indicate, however, that excess enthalpy is dominantly produced by phase segregation in the depressurization zone around producing wells, the liquid being partially retained in the aquifer by its capillary adsorption onto mineral grain surfaces while the steam flows into the wells. The flashed water discharged from wells is of the Na–Cl or Na–HCO 3 types containing 50–1200 ppm of Cl. The most abundant dissolved constituent (average on a molal basis) in the aquifer fluid is CO 2, followed by Na, Cl, Si, F, K and H 2S. Modelling of aquifer fluid compositions using the phase segregation model reveals that the concentrations of only one of the major reactive components (CO 2) may be externally controlled, i.e. by its flux to the hydrothermal fluid. All the other reactive components (Al, Ca, F, Fe, K, Mg, Na, Si, S) are controlled by close approach to local equilibrium with hydrothermal minerals. In the Olkaria West and Domes sectors, the CO 2 aquifer fluid concentration is controlled by its flux from the magma heat source, but in other parts of the Olkaria field, it closely approaches equilibrium with the epidote–prehnite–calcite–quartz hydrothermal mineral assemblage. The mineral assemblage pyrite–pyrrhotite–magnetite controls aquifer water H 2S and H 2 concentrations. Departure from equilibrium is larger for individual hydrothermal minerals than for mineral pairs and especially mineral assemblages. In the case of mineral pairs, the only aqueous species entering the reactions are either a pair of ions or gases, but for minerals assemblages it is a single gas only. If a gas is in equilibrium with a mineral assemblage, all its minerals should also be in equilibrium with the gas. There are several reasons for the observed larger departure from equilibrium between solution and individual minerals than between mineral pairs and assemblages:(1) the stoichiometry of the reactions, (2) in the case of iron, inadequate quality of thermodynamic data on its hydrolysis constants and(3) changes in fluid component concentrations between undisturbed aquifer and wellhead due to precipitation/dissolution reactions. This last mentioned process is most important for components present in low concentration in the fluid. Specifically for Ca-bearing minerals, the apparent under-saturation in the reservoir water is considered to be an artefact caused by removal of Ca from solution by calcite precipitation in the depressurization zone around wells where the water is strongly degassed by its extensive boiling. The initial vapor fraction in the reservoir fluid has been estimated from the modelled H 2S and H 2 concentrations in that fluid assuming their concentrations to be fixed by specific mineral-solution equilibria in the initial aquifer liquid water. The results indicate that very small amount of vapor is present in the initial aquifer fluid, or 0.03% by weight on average (a few % by volume). This number is thought to carry considerable uncertainty when all the errors and approximations made in the calculations are taken into account. Yet, some vapor must be present, generated by depressurization boiling of rising hot water.