A Supercritical Model of the Menengai Geothermal System

Abstract

The Menengai geothermal system is hosted in a ring-like caldera a short distance north of the city of Nakuru in Kenya. In 2010 it became the second field in Kenya to be developed for energy generation, following Olkaria, which has proven to be successful and is currently undergoing expansion. In 2013 a preliminary numerical model of the Menengai field was developed (Kipyego et al., 2013) which highlighted the structural control of the system and provided initial insights into its behaviour. The model also demonstrated that the standard TOUGH2 simulator was not sufficient for modelling the supercritical conditions known to exist at depths below 3200m. In this work a new model is developed in which supercritical conditions are imposed in the upflow region of the bottom boundary. The model is run using the University of Auckland supercritical version of the TOUGH2 simulator (Croucher and O’Sullivan, 2008) with the air-water equation of state. Comparisons of modelled downhole temperatures with field data show that the supercritical model achieves a significantly better match than the preliminary model and that artificially low permeabilities are not required at the bottom of the model to replicate the high temperatures encountered. The possible production and reinjection scenarios investigated previously are implemented using the supercritical model and the differences discussed.