Modelling of Subsidence at the Wairakei Geothermal Field, New Zealand

Abstract

Significant localised ground subsidence related to geothermal production has been experienced within Wairakei geothermal field since the 1950s. Much of current knowledge of the cause of this subsidence anomaly has come from data gathered recently during an intensive subsidence investigation program. This anomaly is understood to be caused by both a reduction in pore pressure and hydrothermally altered weak formations. In order to quantitatively study the subsidence and predict its future behaviour, a subsidence modelling project is being carried out. A new radially symmetric subsidence model has been created combining geomechanics modelling using the finite element ABAQUS code and mass and heat flow modelling using TOUGH2. The coupled modelling approach calculates the local fluid pressure and temperature and then the related stress state using a nonlinear elasto-plastic constitutive model. The model parameters were determined using field data, laboratory measurements and calibration. The results show that the model is capable of accurately representing the subsidence that has occurred at Wairakei and that it will be a valuable tool for predicting rates of subsidence as a result of future production.