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 distribution which has a great impact on the output of the simulated results in this study.