Title: A reservoir study of Olkaria East geothermal system, Kenya

Abstract

The conceptual model of the Eastern Olkaria geothermal system comprising of
Olkaria Northeast and Olkaria East fields has been reviewed. The 3-D natural state
model developed by Bodvarsson and Pruess (1987), has been updated to include the
natural state thermodynamic conditions of all the wells drilled to date, with special
emphasis to Olkaria Central wells. Both lumped parameter and distributed parameter
models have been used to study the reservoir response to 20 years of production at
Olkaria East field and some performance prediction for the next 20 years has been
done.
Based on these studies, the following is concluded:
• That Olkaria East reservoir is an open system with a good pressure support and
can be approximated by a simple first order differential equation whereby the
recharge can be modelled as a direct proportion of pressure drawdown. In the
natural state, the hydrology is controlled by convection.
• Three upflow zones seem to exist in the Eastern Olkaria geothermal system with
two in the Northeast field and one in the East field.
• In the natural state, the Eastern geothermal system can be simulated by a
recharge of 320 kg/s of 1290 kJ/kg water and the Western system by 245 kg/s of
1200 kJ/kg water. Steam amounting to 128 kg/s is lost along the Ololbutot fault
and Olkaria Central zones resulting in cold temperatures deep down in the wells.
• Pressure drawdown in the Olkaria East field is localised within the producing
zones. The deep reservoir still appears to be intact and can be exploited further
to boost up the generating capacity of the field.
• A reasonable preliminary match to the history data is achieved from a coarse grid
by lumping together many wells within a specified grid block and producing the
sum out of one well. With this match, it is predicted that mean enthalpies will
fall to about 1700–1800 kJ/kg in the next 20 years if production is maintained at
the same rate and pressure drawdown will eventually stabilize as the fluid
recharge rates equalize the production rates. However, a better prediction would
be obtained from an extended grid producing from deeper aquifers.