Chemical changes in natural features and well discharges in response to production at Wairakei, New Zealand

Development of the Wairakei geothermal resource has resulted in changes to the chemistry of the discharged fluids. The output of hot chloride springs declined rapidly and eventually ceased 10 years after the commencement of drilling. Deep pressures were drawn down by well discharge resulting in an increase in size and pressure of the shallow steam zone. Natural heat flow in the Karapiti Thermal Area increased from 40 MWt in 1950 to 420 MWt in 1964. Reduced reservoir pressures have also caused cool and less-mineralised water to enter the Western Borefield production reservoir. This reduced the average chloride concentration in the well discharges from 1580 g/t in 1960 to 1230 g/t in 2001. By this time 34% of the well discharge was from cool inflowing water and this has decreased to 27% in 2004. Since the mid-1980s production at Wairakei has been increasingly supported by fluid extracted from the Te Mihi sector. “Dry” steam wells, with gas concentrations up to 3.2 wt%, were brought into service in 1989, and in the last decade production has been added from deep liquid wells. By October 2007 the total gas flow to the Wairakei and Poihipi Power Stations was 9500 kg/h. Temperatures (∼250 °C) and compositions of the deep Te Mihi wells are close to the inferred initial reservoir conditions. The commencement of injection in 1995 and an increase in shallow groundwater drainage have increased concentrations of calcium and CO2 in total discharge. This has resulted for the first time in problems of calcite scaling in some wells although chemical evidence of injection returns is inconclusive.