Precise Gravimetry and Geothermal Reservoir Management

Modern portable gravimeters can routinely achieve a 5 ugal uncertainty with careful measurement procedures involving multiple station occupations in the same day, and stacking of readings over at least 15 minutes during each occupation. Although further improvements in gravimeter accuracy are feasible, other practical factors relating to repeat surveys of geothermal fields make such improvements of limited value. The two most important factors are benchmark elevation variations (3 ugal/cm) and groundwater level fluctuations (5-10 ugal/m). Dual frequency GPS receivers can give elevations to about 2 cm after 30 minutes of recording, and for reducing groundwater uncertainties, repeating the surveys during the same season coupled with checks of groundwater monitor wells is advisable. Simple models are presented for the gravity effects of evolving steam zones at Wairakei field, Dixie Valley field, and The Geysers field. Changes during the main pressure-drawdown phase of development probably caused gravity decreases of 200 – 1000 ugal, but subsequent changes would have been much smaller in amplitude. Gravity monitoring is able to discriminate between steam zone dry-out and steam zone resaturation. However the amplitude of gravity increases associated with individual injection wells is often small and may not be resolvable with annual surveys. Once fields have passed their initial pressure-drawdown phase and the rate of gravity change decreases, the frequency of gravity surveys should be decreased.