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Abstract

Electrolytes, dissolved in aqueous solutions, have the tendency to associate at near-critical temperature, which causes a removal of free charge carriers from the solution. This behavior is intensified in the “low” pressure range of the supercritical field and has become noticeable in a reduction of fluid conductivity by an order of magnitude. Thus, deep resistivity surveys are regarded to provide a convenient means for detecting supercritical roots of geothermal high-enthalpy reservoirs. In previous decades, the temperature dependence of electrical properties of single-component brines has been extensively studied up to supercritical conditions. Very few data are available for two-component brines, but none for multicomponent mixtures, representing natural geothermal fluids. Thus, we have developed a flow-through set-up to measure both the intrinsic temperature dependence of electrical properties of hydrothermal solutions and the effect of fluid-solid interactions on fluid conductivities in a temperature range of 23–422 °C at 31 MPa. Experimental conditions were adapted to those of two geothermally exploited areas on Iceland – the Krafla and the Reykjanes field – which are characterized by meteoric and by seawater controlled fluid systems, respectively. Our study shows that the intrinsic temperature dependence of mixed brines exhibit a similar left-skewed curve characteristic like those of single component brines with conductivities decreasing precipitously to a minimum at critical temperature. At further isobaric temperature rise conductivities remain on this level. The opposite was observed for reactive experiments, where fluid – solid interaction was allowed. In this case, conductivities re-increased by up to factor 7 within seconds, indicating a significant and immediate increase in solubility of rock and composite materials, which are in contact with supercritical aqueous solutions and suggest high reaction kinetics of this process. However, at supercritical conditions conductivities did not remain steady and the electrical properties of supercritical fluid-rock systems are characterized by a wide range of conductivities. The competing processes of mineral dissolution and new mineral formations are also evident from complementary micro-structural investigations as well as chemical analyses of the percolated fluids.