Review (1968-2016) of Subsidence at Ohaaki Geothermal Field, New Zealand

Abstract

Parts of the Ohaaki geothermal field have been adversely affected by subsidence resulting from 30 years of geothermal fluid extraction. At the centre of the area of largest subsidence, the total has reached 6.7 ± 0.1 m, with maximum subsidence rates decreasing from over 510 mm/year during the 1993-1996 period to 150 ± 10 mm/year during 2012-2016. The general location of the subsidence anomaly has not changed significantly, although the location of the area of maximum subsidence rate has shifted over time. Following a review of the spatial and temporal variation of subsidence rates, along with supporting evidence such as reservoir pressure, subsurface geology, and material compressibility, we postulate an explanation for the changes. To appropriately manage infrastructure and environmental effects, reliable forecasts of subsidence are important. Consequently, we also test previous predictive models of ground deformation against actual levelling data to assess predictive uncertainty.