Deviations in Silica Geothermometry at Wairakei

Abstract

Analysis of feed-zone temperatures and discharge chemistry data for Wairakei production wells indicates that reservoir silica concentrations have departed from the quartz equilibrium that existed prior to development of the field and are now oversaturated with respect to quartz. As a result quartz equilibrium temperatures are 10-20?C above measured temperatures in 220-230?C liquid feeding wells. Supersaturation occurs primarily as a result of boiling but the ingress of chalcedony-saturated waters may also be a contributing factor. Oversaturated conditions are sustained because the kinetics of silica precipitation are not sufficiently rapid to allow fluid to reequilibrate before it reaches the wellbore. The analysis suggests that while the geothermometer may be accurate in the early stages of development when reservoir conditions are stable, divergent temperatures may result when a system is in a more dynamic state. The main implications of silica disequilibrium at Wairakei are that quartz geothermometer temperatures can no longer be used to accurately monitor changes in feed-zone temperature and can distort interpretation of reservoir hydrology when combined with chloride data in chloride-enthalpy plots. As with other fluid-mineral geothermometers, application of the quartz geothermometer should be circumspect and supported by regular verification against measured temperatures.