Large Scale Numerical Models of TVZ-Type Geothermal Fields

Abstract

Several models for the large scale structure of a typical Taupo Volcanic Zone, New Zealand, geothermal field are presented. In these models, ëmagmaticí water is injected over a region at 8 km depth at a rate of 20 k@s, and heat is supplied to the same region at a rate of 400 MW. Meteoric water is entrained to form a buoyant plume which extends to the surface. The mass output, mean enthalpy, ratio of magmatic/meteoric waters and the outflow area of this plume provide constraints on both the size of the heat source and the large scale permeability distribution. The most satisfactory models have low vertical permeability (5-10 millidarcy) and small source regions (< 2 km radius). In most cases, these systems reach a steady state 10 - 40 thousand years after the heat source is turned on. In some models however, rather chaotic behaviour is seen, with the thermal output at the surface varying by a factor up to 3 on a timescale of a few thousand years.