3D Natural state model of the Menengai geothermal system, Kenya

The Menengai caldera is an elliptical depression with minor and major axes measuring about 8 km and 12 km respectively, located near Nakuru, Kenya's Rift Valley. Geophysical investigations (micro-seismic monitoring, gravity and MT surveys), geological surveys, geochemical surveys (fumaroles sampling, soil gas surveys) and a soil temperature survey allowed GDC to locate the first exploratory well MW-01 which intercepted a deep reservoir with temperatures exceeding 300°C, found beneath a shallower liquid-dominated reservoir at temperature in the range 190-200°C. The exploratory and development drilling campaign, which is undergoing, provided the drilling of some 20 deep wells by December 2013 aimed at characterizing the deep reservoir properties, delineate the field extension and provide additional steam for power development. The geochemical data acquired with production well testing allowed not only to characterize the chemistry of the shallow aquifer and of the deep steam-dominated reservoir, but indicates also an 'intermediate temperature zone' that could be interpreted as a 'rim' of the deep reservoir. Surface exploration surveys and the findings derived from drilling, logging and testing of geothermal wells were used to build a 3D numerical model of the geothermal system natural state by means of the TOUGH2 reservoir simulator, supported by Petrasim 5.3 graphical interface. The numerical model was intended to check the consistency of the conceptual model, give a better picture of the reservoir thermodynamic conditions, and constrain the mass and heat flows through the system. Making in numbers the idea arisen in the conceptual model, the 3D natural state has been reconstructed by simulating the heating up due to a hot intrusion in a complex system. The complexity of the system arises in a particular structure consisting in a deep vapor-dominated high temperature reservoir just on top of the hot intrusion at temperatures higher than 300°C, separated from the shallower liquid-dominated reservoir (with temperatures ranging from 130 up to 210°C) by a relatively thin impermeable layer. The model was run to steady state conditions, and calibrated against data obtained from geochemical investigations, well pressure and temperature profiles recorded under warm-up and flowing conditions, as well as available production test results. The details about the 3D model construction within TOUGH2 and Petrasim and the results of the simulated natural state are presented and discussed.