Mfathematical Modelling of Subsidence at Wairakei Geothermal Field

Abstract

A multi-dimensional model is required at Wairakei in order to account for the significant observed horizontal deformations. From the subsidence models and subsidence/compaction relationships examined, a finite element model was chosen for its potential to reproduce such deformations and for its ability to utilise either linear or nonlinear, nonhomogeneous material models. Using the ADINA finite element program, a simple two-layer model was developed incorporating the Pumice and Wairakei pumice breccia layers as the overburden, and the Huka formation as the reservoir . The deformation in the Waiora formation was considered to be less significant and was neglected. Using suitable estimates for the elastic properties of each layer (Young's modulus and Poisson's ratio) and by depressurising in the reservoir a zone whose lateral extent corresponds approximately to the point of inflection on the observed vertical deformation profile, a good fit to both the horizontal and vertical deformation profiles was obtained. An alternative model examined the effect of a lateral variation in Young's modulus, as opposed to a lateral variation in pressure decline. Again, a good fit was obtained, particularly to the vertical profile. Features of both models are apparent at Wairakei.