Results of Injection Testing at Wairakei Geothermal Field, New Zealand

Abstract

Well WK301 was drilled to 1440 m depth near Wairakei Power Station as part of reinjection feasibility investigations. The well bottomed in unexpectedly thick, lower Waiora pumice breccias and ignimbrites, suggesting a basin of down faulted structure in this part of the field. Although the maximum temperatures was only 138°C at well bottom, a temperature gradient of 15°C/100 m over the lower 300 m suggests a temperature of close to 230°C at 2 km depth. Static pressure in the well is the same as in the eastern production bore field. Three separate pump tests, which injected cold water at wellhead pressures up to 35 b.g. and flows up to 668 t/h, caused a permanent improvement in the injection capacity of the well. The injectivity increased in sudden jumps, presumed to be hydrofracturing events, and stabilized at 22 t/h/b. Both inflation of the ground surface (maximum of 4 mm) and induced seismicity (>70 events) accompanied the pump tests.