Title: Interpretation of feed zones to map sub-surface permeability structures and natural state simulation: A case study of Olkaria Domes Geothermal System in Kenya

Abstract

The Olkaria geothermal area is a high temperature geothermal field located to the west of Longonot
volcano in the southern sector of the Kenya Rift system. Exploration of the geothermal resources in
Olkaria started in the 1950´s. The structural domains of the Greater Olkaria volcanic complex
depict diverse structural trending patterns. Currently, Olkaria has a total installed capacity of
654 MWe.
In this thesis, sub-surface permeability structures were mapped according to major feedzones at the
depths of 1000-2000 m and 2000-3000 m and their distribution across the Olkaria Domes geothermal
field. It is clear that the structures in this field are mostly trending in NW-SE, N-S and ENE-WSW.
They also confirmed the location of the existing structures as earlier mapped such as the ring
structure and Gorge Farm fault. Analysis of temperature distribution across the Olkaria Domes field
coupled with the knowledge of how permeability is controlled by sub-surface structures can be used
to site both make up and re-injection wells. Re-injection wells are highly recommended for this
field for pressure support and to enhance energy extraction efficiency. Analysis of isotope data
from boreholes should be conducted in order to draw conclusions regarding the flow patterns within
the Olkaria Domes geothermal system.

A numerical model for this field was developed to simulate the natural state of the system in its
pre-exploitation state. The primary purpose of a natural state model is to verify the validity of
conceptual models and quantify the natural flow within the system. The simulated results of some
wells did not match the observed data. Most of these wells are located at the inferred colder
regions of this geothermal field. This could partly be attributed to the permeability distribution
of the reservoir rock domain and the intensity of the heat sources assigned in the numerical model.
Mapping of sub-surface structures is recommended in this field to accurately assign permeability
distrib on the output of the simulated
results in this study.