The Role of Helium and Other Noble Gases in the Modelling of Geothermal Systems

Abstract

Amodel of the geothermal system in which deep circulating groundwater containing noble gases at air saturated water concentrations mixes with hot fluids of mantle origin at depth is described. It is proposed that the ratio should be similar to that observed in mid-ocean ridge systems, in which case the to atmospheric argon ratio in geothermal discharges would be an indicator of the likely heat content of a system. As a first test of this hypothesis the noble gas results of Mazor et ai.(1990) have been presented as the ratios for Wairakei and early Mokai wells and fumaroles. Their simplified boiling model has been used to correct for the effects of gas-water separation which occurs in underground boiling. At Wairakei, the resultant range of values is 140-8500 atoms similar (except for the lowest values) to the range measured in mid ocean ridges. Further work is needed, but the available data show that the technique has promise for the modelling of deep geothermal systems and providing input to simulation models currently being used in reservoir engineering. The technique has potential to distinguish between stored heat systems and renewing systems.