Extraction of geothermal fluid at the Wairakei geothermal power plant, New Zealand, has induced large-scale subsidence in the Wairakei-Tauhara area. Two-dimensional finite element analysis has been applied to the geothermal subsidence in order to investigate the causal mechanism, and to predict likely future subsidence and the associated risk. The software used has the facility to accommodate variable rock properties, including non-linear stress-strain behaviour, and the pre-consolidation history. It was found necessary to use strongly anisotropic permeabilities, which emphasise the importance of using a multidimensional model. The subsidence models showed a good match in time and space to historical subsidence observed to date. This has been achieved using a consistent set of reference material properties for each geological unit, except where evidence warrants use of different properties for a limited zone. The reference material properties were adjusted where appropriate to account for stress state, temperature and unusual geological features. The analysis provided an insight into sensitivity of subsidence to changes in reservoir pressure, and demonstrated the geological control of the compressible material on the rate and location of subsidence. Predicted future subsidence formed a basis for evaluation of the long term risks associated with subsidence. This case study demonstrated that 2-D subsidence models have distinct advantages over 1-D models. It is recommended that non-linear stress-strain behaviour of geo-materials be routinely incorporated into subsidence modelling.
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