Numerical Modelling of the Alum Lakes Geothermal Surface Features, Wairakei

Abstract

In this study we use a two-dimensional (2-D) numerical model to show that pressure decline in the Wairakei geothermal reservoir is related to a decreasing geothermal outflow from the overlying Alum Lakes. The conduit between Alum Lakes and the reservoir originally was originally a zone of geothermal upflow, but now hosts a down flow of groundwater into the reservoir. The TOUGH2 (1) numerical model is a vertical 2-D slice through the main production zone of Wairakei (the Waiora Valley). It is derived from a large, complex 3-D computer model of Wairakei system, which is used extensively as a reservoir management tool by Contact Energy, the operators of the Wairakei geothermal wells and associated geothermal power stations. However, surface features and the shallow subsurface are not modeled in detail. We have refined the vertical grid close to the surface from 50 m layers to 2 m thick layers, and also refined the grid columns where necessary, and calibrated the 2-D model with reservoir and geothermal surface data.