Advances in Subsidence Modelling of Geothermal Fields

Abstract

One dimensional subsidence modelling that was standard 20 years ago has recently given way to more advanced two and three dimensional modelling using soil mechanics software packages, which now run on standard office computers. Accurate modelling requires a good knowledge of the subsurface geology, reservoir conditions, and rock properties, including porosity, permeability and compressibility. With good input data it is possible to obtain a close match to historical subsidence, and thus to produce reliable future predictions. The latter is however dependant on the reliability of future reservoir pressure predictions, which requires more detailed reservoir modelling than would normally be the case for geothermal resource estimation or management.
Subsidence at Wairakei-Tauhara due to almost 50 years of geothermal fluid extraction was the first to be modelled by two dimensional finite-element analysis. The software accommodates variable rock properties, including non-linear stress-strain behaviour, and pre-consolidation history. A good match to historical subsidence in time and space was achieved with a single set of rock properties for each geological unit, apart from two local zones with different permeability. Compared to previous 1-D subsidence modelling, this study showed greater sensitivity to changes in reservoir pressure and strong control over the location of rapid subsidence by the formation morphology. Analysis of Ohaaki and Kawerau geothermal fields indicates that the method can be applied to other systems where there is sufficient geological, geomechanical and pressure data.