An Initial Resource Assessment of the Menengai Caldera Geothermal System Using an Air-Water TOUGH2 Model

Abstract

The Menengai geothermal field in Kenya is hosted in a ringlike caldera of approximately 12 km diameter. A threedimensional numerical model of the system has been developed based on a conceptual model derived from the available surface exploration and field data. The model was developed using AUTOUGH2 with the air-water equationof- state EOS3 allowing the shallow unsaturated zone to be included. The model domain covers an area of 432 km2 with higher resolution in the upflow regions and extends to a depth of 3200 m where supercritical conditions are known to begin. Natural state modelling was carried out and good agreement with downhole temperatures has been achieved. The position of the unsaturated zone is represented well, including the position of surface outflows. The explicit inclusion of caldera structures was found to be necessary in order to match the strongly differing downhole temperature profiles of neighbouring wells. The model also predicts the formation of steam zones at the correct elevations. Production history matching simulations were performed using a limited amount of field data and the model results agree well with measured discharge enthalpies. The model was then used to estimate the performance of the reservoir under a number of possible production regimes. These included targeting feedzones at different depths and testing various reinjection strategies. The results of these scenarios are presented.