Aquifer models of pressure drawdown in the Wairakei‐Tauhara Geothermal Region

Abstract

The analytical theory of porous horizontal aquifers is applied to the problem of pressure response of the Tauhara geothermal field to drawdown from Wairakei. In a ‘bounded’ model the Tauhara field is regarded as a sector from an axisymmetric solution in the horizontal plane, having a permeability contrast at the curved outer boundary. The straight sides of the sector are impermeable. In a second model the only barrier to horizontal flow is a straight boundary situated to the southeast of the Tauhara field, the presence of which is suggested by a high gradient in the field resistivity. Features shared by both models include a steady horizontal outflow from Wairakei, which was present before development and probably arises from natural convection, and a superimposed downward pressure gradient, which may be a consequence of the efficient horizontal propagation of the pressure drawdown at Wairakei. Statistical tests indicate that these features are important. The best fit to the field pressure data is obtained by using the second model with impermeable upper and lower boundaries.