Status of Olkaria 1 Geothermal Reservoir, Kenya: Results from Precision Gravity, Enthalpy and Mass Output Measurements

Abstract

The Olkaria 1 Geothermal Field consists of some 30 wells providing steam for power production of 45 Mwe. Geophysical measurements indicate a resistivity low covering an area of approximately 100 km2, with the largest amplitude resistivity low covering an area of 12 km2. Most of the wells have been drilled in this area and a working reservoir model has been developed. Data from the wells indicate that below the caprock a 50-150 m thick vapour zone exists at 700-800 m below ground surface. This vapour is underlain by a thick water dominated reservoir.

Precision gravity, enthalpy, temperature, pressure and mass production measurements of wells have been carried out at regular intervals since the early 1980s to monitor reservoir changes as a result of geothermal fluid withdrawal. In this paper we review well output measurements. We then describe the observed relation between fluid mass withdrawal, enthalpy and gravity changes. Our purpose here is to examine the changes in detail over the different parts of the reservoir and to deduce an integrated model for the hydrologic changes that have taken place since 1981 when the 1st unit was commissioned.