Abstract
Supercritical fluids exist in the roots of many active high-temperature geothermal systems. Utilization of such supercritical resources may multiply energy production from geothermal systems; yet, their occurrence, formation mechanism, and chemical properties are poorly constrained. Flow-through experiments at 260°C and 400-420°C were performed to study the chemical and mineralogical changes associated with supercritical fluid formation near shallow magmatic intrusions by conductive heating and boiling of conventional subcritical geothermal fluids. Supercritical fluids formed by isobaric heating of liquid geothermal water had similar volatile element concentrations (B, C, and S) as the subcritical water. In contrast, mineral-forming element concentrations (Si, Na, K, Ca, Mg, and Cl) in the supercritical fluid were much lower. The results are consistent with the observed mineral deposition of quartz, aluminum silicates, and minor amount of salts during boiling. Similar concentration patterns have been predicted from geochemical modeling and were observed at Krafla, Iceland, for the IDDP-1 supercritical fluid discharge. The experimental results confirm previous findings that supercritical fluids may originate from conductive heating of subcritical geothermal reservoir fluids characterized by similar or lower elemental concentrations with minor input of volcanic gas.
Highlights
Volcanic geothermal systems are associated with magmatic intrusions in the upper part of the Earth’s crust characterized by increased temperature, specific fluid enthalpy, and convection of groundwater [1]
Studies of alteration mineralogy and fluid composition in geothermal systems show that equilibrium is closely approached between the geothermal fluids and secondary minerals formed in the systems, except for mobile elements such as Cl [35,36,37,38,39]
The chemical and mineralogical changes associated with supercritical fluid formation by conductive heating and boiling of subcritical geothermal fluids were studied experimentally
Summary
Volcanic geothermal systems are associated with magmatic intrusions in the upper part of the Earth’s crust characterized by increased temperature, specific fluid enthalpy, and convection of groundwater [1]. Conductive heat transfer from a magmatic intrusion to the surrounding groundwater occurs in the roots of the geothermal system below the depth of typical conventional geothermal wells. Recent modeling suggests that supercritical fluids with temperatures and enthalpies exceeding ~400°C and ~3000 kJ kg-1, respectively, exist at the boundary between geothermal systems and the magmatic heat source, with such fluids possibly capable of generating up to ~30-50 MW of electricity from a single well or ten times more than conventional geothermal wells [4]. The critical 22 01 MPa temperature Tc = 373 976°C of pure water (H2O) [5] Such a definition can lead to an artificial boundary in the phase diagram of water, across which there is a continuous change in fluid properties.
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