Successful Use of Hydrothermal Alteration Mineralogy in Appraising a Geothermal Reservoir: Case Study: Olkaria Domes Geothermal Field, Kenya

Abstract

Three geothermal exploration wells OW-901, OW-902 and OW-903 were drilled in Olkaria Domes field to evaluate its geothermal potential. The three wells were drilled to a depth of 2200 m and all encountered a high temperature system and discharged on test. Completion tests and hydrothermal alteration mineralogy indicated a cold zone between 900-1150 m in all the three wells. Permeability of the wells was low compared to wells drilled in the neighbouring Olkaria East and Olkaria North East fields hence low productivity. The analyses of well completion tests and alteration mineralogy indicted that the resource was deeper compared to the other fields in Olkaria. Six appraisal wells OW-903A, OW-904A, OW-905A, OW-906A, OW-907A and OW-908 were recommended to delineate the geothermal reservoir and also site the best localities to drill production wells. Deep casing set at 1200 m recommended from the exploration wells studies for the appraisal wells was successfully, with the cold zone being isolated completely. Directional drilling was able to intersect the faults and fractures thus enhancing permeability and average production in the field increased from 2.5 MWe to 8 MWe per well, with the highest producer being over 14 MWe. Deep drilling to 3000 m was able to tap the deeper aquifers. Alteration mineralogy studies from the six appraisal wells were able to delineate the field and currently production drilling is underway to provide enough steam for a 140 MWe power plant. This paper therefore discusses the successful use of hydrothermal alteration mineralogy in to optimize the geothermal resource in Olkaria Domes field.