Evaluation of fluid-mineral interaction in the Menengai geothermal system, Central Rift, Kenya

Abstract

The geothermal reservoir fluid composition, water-rock interaction and possible causes of excess enthalpy were studied in the Menengai geothermal system, Kenya. The reservoir fluid composition and aqueous speciation distribution were assessed based on two models: first assuming that the excess enthalpy was caused by twophase reservoir fluids (water and vapour) and, secondly, assuming single liquid phase reservoir fluids. The calculated chemical composition of major non-volatile components is relatively similar in both models except when the well discharges approach dry steam. The concentration of the volatiles in reservoir water is significantly lower when assuming a liquid and vapour reservoir and higher when assuming a liquid only reservoir. For these reasons, H2S geothermometers give significantly lower values when assuming a two-phase reservoir. Moreover, the geothermometer temperatures show a remarkable discrepancy as a result of mixing of fluids from different feed zones that may affect the fluid equilibrium at a given temperature. The exception to this is for wells where the discharges approach dry steam.

While there is some uncertainty due to the model calculations, virtually all the common Ca-bearing minerals observed in Menengai are under-saturated. However, andradite and epidote, which also contain Fe (III), show mixed saturation states, whereas Fe (II) bearing minerals are oversaturated; this might be affected by the calculated Fe activity that is sensitive to precipitation and dissolution. Menengai aquifer waters are saturated with respect to albites and K-feldspars. The calculated activity of volatiles in the aquifer water in relation to mineral buffers is model-type dependent. Therefore, activities of the volatiles are close to equilibrium with volatile mineral buffers when assuming two-phase reservoir fluids, while a departure is observed when assuming a liquid reservoir. 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 predominantly caused by phase separation and conductive heat transfer from hot rock or magma to the circulating fluids, also compounding to form superheated steam.