Title: Numerical Reservoir Modelling and Application of Tracers in Calibration and Infield-Reinjection Design: Case Study Olkaria East and South Geothermal Field, Kenya.

Abstract

Numerical simulation of geothermal reservoirs is a very useful instrument for developing strategies
of field exploitation, production well location and reinjection scheme design. Reinjection of spent
geothermal fluids serves to maintain reservoir pressure and enhance energy extraction efficiency
over the life of the resource. Tracer testing is used as tool for tracing flow within a geothermal
system for the purpose of characterizing the system and to gain good understanding on inherent
heterogeneity. Tracer transport is orders of magnitude faster than cold-front advancement around
reinjection boreholes and can be used as a cooling prediction tool.
This study involved interpretation of tracer test data collected during cold injection into well
OW-12 in the Olkaria geothermal field in Kenya. A single fracture model showed well OW-15 to be the
most affected by the cold reinjection and OW-19 to be the least affected. A pessimistic version of
the model predicts well OW-15 to cool by more than 20°C and wells OW-18 and OW-19 to cool by about
6°C for a forecast period of 15 years. An optimistic model version predicts well OW-15 to cool by
16°C, OW-18 to cool by 2°C and OW-19 to cool by 4°C for the same period. A numerical reservoir
model was developed for the Olkaria East and Southeast fields, covering an area of 27 km2. The
natural state model matched well available temperature and pressure data as well as being validated
by production history data. Thermal front advance compares well for both models, it takes about a
year to be felt in production well. Onset of cooling is immediate for single fracture model but in
numerical model there is temperature rise followed by decline. This temperature increase before
decline in the complex model is attributed to steam cap collapse.
Well OW-12 in Olkaria can be used as a cold reinjection well, but it has to be used intermittently
according to the results of the study, injection for one year followed by a period of recovery. The
current injection depth in well OW-12 is shallow, but model calculations show that if reinjection
depth is considerably greater (-2600 m a.s.l.), longer injection periods are possible without
collapse of the steam cap
involved.