Application of Precision Gravity Measurement to Reservoir Monitoring of Olkaria Geothermal Field, Kenya

Abstract

Precision gravity surveys at Olkaria Geothermal Field began in 1983 to monitor gravity changes as a result of geothermal fluid withdrawal. A review of the observed gravity data over each benchmark indicates changes over the years during monitoring. Most benchmarks show an increase in gravity between 1983 and 1988. Gravity increases up to 120 µgals were observed over benchmarks situated to the S-E of the power station. However, from 1988 up to 1996, decreases in gravity have been observed for most parts of Olkaria. Decreases down to 180 µgals were observed on benchmarks to the west and north-west of the power station. Maximum gravity changes show a constant trend in time, but different characteristic distributions from zone to zone. Unfortunately, these figures were not corrected for subsidence as no recent precise levelling survey has been done. Nevertheless, from the data, subsidence alone cannot explain the changes in gravity during this period. The negative gravity values could be attributed to a mass deficit in the reservoir rocks in the 1988- 96 period. Figures of average mass output and enthalpy of some of the wells indicate presence of excess mass in the reservoir triggered by the initial exploitation followed by a deficit in mass.