Modelling the Gravity Changes at Wairakei Geothermal Field

Abstract

The principal cause of gravity change at Wairakei has Seen the drawdown of liquid and the formation of steam zone in the upper part the reservoir. have also been caused by subsequent saturation changes in the steam zone, groundwater level decline and changing pore water density due to temperature changes. compaction* pore water decompression and mineral precipitation ate unlikely to have caused significant gravity changes An example of modelling the gravity changes I n the eastern production bore f indicates a residual sat u ration in the steam zone, immediately after liquid drawdown, of 0.6. Dry o u t of the steam zone during the caused the s at u ratio n to decrease to less than 0.5. During the 1970's there appears to have been s at u ratio n change. Continuing pressure and temperature declines in the steam zone are apparently now caused mainly by groundwater invasion.