DETERMINATION OF RESERVOIR PROPERTIES AT WAIRAKEI GEOTHERMAL-FIELD USING GRAVITY CHANGE MEASUREMENTS

Exploitation of a liquid-dominated geothermal system generally results in the transfer of mass within, or out of, the system and causes measurable changes in gravity. When the rate of mass transfer is controlled by the permeability of the reservoir rocks, then the analysis of measured gravity changes can yield values for reservoir properties. For two such cases we derive permeability or permeability-thickness (transmissivity) and storativity. One case is during the early stages of exploitation when a two-phase zone is rapidly expanding. Calculations using a numerical reservoir simulation model show that for Wairakei the gravity changes associated with permeabilities of 50 and 100 md would be clearly distinguishable (> 50 muGal) in less than 2 years. A measured gravity change of -415 muGal between 1950 and 1961 suggests a permeability of 100 md, which is consistent with values obtained from well tests. The second case is during reinjection into a deep-liquid zone overlain by a two-phase zone. Gravity changes of up to 120 muGal associated with reinjection at Wairakei are analysed using analytical models (Theis solution). The measured gravity changes are matched by those of a model with isotropic permeability having a permeability-thickness (kh) of 9.9 d-m and a storativity of 9.2 x 10(-6) m Pa-1. If a model having anisotropic (horizontal) permeability is used the derived values for kh are 18.2 and 5.4 d-m, and the storativity is 8.7 x 10(-6) m Pa-1.