Microseismic Activity at Wairakei and Ohaaki Geothermal Fields

Abstract

A 23 month study of microseismicity at Wairakei, and a 35 month study at Ohaaki show the natural activity within the two fields to be completely different. Wairakei displays continuous, low to moderate activity, while at Ohaaki, only two microearthquakes have been recorded within the field. A 13 month reinjection test in the borefield at Wairakei gave no detectable change in the occurrence of microearthquakes within the geothemal field. This contrasts with the seismicity induced by the injection test at in 1984. The difference appears to be related to the wellhead pressure used on each occasion. During 1984 this ranged from 20 to 30 bars, while during the 1988-89 test, the well accepted fluid under gravity and no pumping was required. At Ohaaki the continual fluid withdrawal and reinjection that followed the commissioning of the power station gave no increase in seismicity, even though the wellhead pressures used for reinjection were similar (if not greater) than the 1984 test. One explanation could be that the deep water table at Wairakei m below the ground surface) due to the effect of 30 years of drawdown. This meant that the formation overpressures were 24 bars above the wellhead pressure during the injection. If this is the critical factor detexminingthe seismicity, then this implies the local must have adjusted to the effects of the drawdown. An alternative explanation is that the low natural seismicity at Ohaaki is due to a relatively weak layer which cannot accumulate strain, and slips aseismically. Such behaviour could be caused by the presence of partial melt in the lower crust, or rocks with a high clay content.