An Integrated Leapfrog/Autough2-SuperCritical Natural State Model of the Menengai Geothermal Field

The Menengai caldera is an ellipsoidal depression with minor and major axes measuring approximately 8 and 12 kilometres respectively. The geothermal field is near Nakuru town along Kenya’s Rift Valley. The caldera encompasses the Ol’rongai volcanic area in the northwest and parts of the Solai graben in the North East. A deep drilling program has been in place since early 2011, from exploration at the beginning to production drilling currently. This drilling programme has resulted in more than 55 deep geothermal wells within the caldera aiming to characterise the deep reservoir properties, delimit the field extension and provide steam for power development. The geoscientific, reservoir and drilling data are continually reviewed and integrated to produce an updated geothermal model of the field. However, supercritical temperatures of above 370C were encountered in some of the wells drilled in the Menengai geothermal field which pose a challenge in developing a numerical model of the field. The supercritical temperatures present difficulties for conventional reservoir simulators as a result of changes in the properties of water under these conditions. The problem can be attributed to the complex nature of the system which occurs in a particular zone with a deep vapour dominated high-temperature reservoir just on top of the igneous intrusion at temperatures higher than 300°C. This zone is disconnected from the shallower liquid-dominated reservoir (at temperatures ranging from about 130C up to approximately 210°C) by a relatively thin impermeable layer. An integrated Leapfrog/AUTOUGH2-SuperCritical model of the Menengai geothermal field was developed with a complete geological conceptual and natural state reservoir models. The findings demonstrate two primary heat sources for the Menengai caldera and the Ol’rongai field, and it also indicates how major faults govern heat and fluid flow directions of the Menengai geothermal field. The reservoir model shows that the main feed zones are from ~1500 masl to about 500 masl and ~300 masl to -500 masl and the extent of the reservoir is around the summit of the caldera and towards the West of the caldera. The AUTOUGH2-SuperCritical was able to match some wells with preliminary calibration signifying that it is the ideal tool for reservoir modelling of the Menengai geothermal field.

Key words: Conceptual modelling, Natural state modelling, AUTOUGH2-SuperCritical, Supercritical temperatures, Menengai