An Interpretation of the Dilution at Wairakei

Abstract

The hydrology of the Wairakei borefield is characterized by an initial pressure drawdown followed by two distinct increases in dilution. The dilution is the result of the lateral inflow of essentially fresh water at two different levels in the system. The deeper and hotter inflow occurs at a depth of about and has a temperature of shallower inflow occurs at a depth of about 450m and has undergone considerable cooling from about 220?C to 190?C since the mid 1970s as a result of exploitation. A The source of the increase in sulphate content in the reservoir fluid since exploitation began is not associated with the dilution, but is instead the result of a corrosive reaction between the reservoir rock and geothermal gases in the steam zone.