Estimation of reservoir permeability using gravity change measurements

Exploitation of a liquid-dominated geothermal system generally results in a transfer of mass that causes measurable changes in gravity. When the rate of mass transfer is controlled by the permeability of the reservoir rocks then analysis of measured gravity changes, using numerical reservoir simulation models, can yield values for reservoir properties. One such case is during the early stages of exploitation, during the formation and expansion of a 2-phase zone. Calculations using MULKOM models show that for Wairakei field the gravity changes associated with permeabilities of 50 and 100 md would be clearly distinguishable (> 50 microgal) in less than 2 years. A measured gravity change of -415 microgal between 1950 and 1961 suggests a permeability of 100 md for rocks in the upper part of the 2-phase zone. This value is consistent with those obtained from well tests