Borehole geology with case examples from Kenya

Abstract

Hydrothermal alteration minerals have been used successfully to determine the relative permeability, temperature, fluid composition, pressure changes and thermal history of a geothermal reservoir. The alterations noted in the reservoir rocks have been due to changes in the prevailing conditions leading to change from primary to secondary minerals deposited in veins, vugs and also within the rock matrix. In Olkaria Domes geothermal area, OW-921A, OW-916 and OW-922 have been discussed to show how alteration minerals have been used to determine the thermal history of the reservoir surrounding the wells. In the Olkaria East Field, OW-35, OW-37A and OW-39A give a good contrast on the characteristics of the wells in the upflow and downflow zones. In Menengai geothermal area, wells MW-04 and MW06 are located in the upflow zone as depicted by the abundance of high temperature alteration minerals and good permeability from the fractured formation. On the other hand, MW-02 and MW-07 are sited in the downflow zones of the field as shown by the low permeability and low enthalpy hence used are reinjection wells.