Modeling and field studies of radon-222 in geothermal reservoirs

Radon-222 was shown to be useful in-situ tracer for studying thermodynamic, geologic, and transport properties of geothermal reservoirs. The relationships to reservoir properties are based on steady-state and non-steady-state models which incorporate the thermodynamic state and relative mobility of the fluids, and partitioning between the fluid phases. The steady-state thermodynamic models assume equilibrium between radon emanation from the reservoir rock and the geofluid concentration. The non-steady-state model solves the partial differential equations describing radon transport under transient two-phase geothermal flow conditions. Radon concentrations of fluids from nine geothermal reservoirs showed strong correlations to the specific volume of the reservoir fluid. Lowest values were associated with all-liquid wells at Cerro Prieto, Wairakei, and Los Azufres reservoirs, and the highest values for steady-producing wells at Serrazzano, Geysers, and Matsukawa reservoirs. Greater variability in radon concentration compared to fluid specific volume and individual reservoir correlations were attributed to geologic differences affecting radon volumetric emanation of the formations. Laboratory emanation studies on reservoir cores indicate porosity variations cause these changes in volumetric emanation. Sensitivity analysis indicates the usefulness of the model in designing future reservoir experiments