New 2-D Subsidence Modelling Applied to Wairakei-Tauhara

Abstract

Two-dimensional finite-element analysis has been applied to the question of subsidence in the Wairakei-Tauhara area, caused by abstraction of geothemal fluid. The software used has the facility to accommodate variable rock properties, including non-linear stress-strain behaviour, and the preconsolidation history. It was found necessary to use strongly anisotropic permeabilities, which emphasises the importance of using a 2-D model. Results show a good match in time and space to historical subsidence. This has been achieved with a single set of rock properties for each geological unit, apart from locally enhanced vertical permeability under the Wairakei subsidence bowl. Compared to previous 1-D subsidence modelling, the current study shows a greater sensitivity to changes in reservoir pressure and strong control over the location of subsidence by the morphology of the lowest unit in the Huka Falls Formation. It is predicted that subsidence will continue for longer, and so eventually be larger, than previous modelling has predicted. However major subsidence is not anticipated to encroach on the Taupo urban area.