Modern Research Methodologies for Geothermal Systems

Author: Nino Kapanadze
Co-authors: -
Keywords: Geothermal systems, Geochemistry, Geothermometers, Environmental Isotopes, Isotope dating
Annotation:

The geochemical techniques are very important in investigation geothermal systems. In order to characterize the thermal state of a geothermal reservoir when in situ temperature measurements are not available, chemical geothermometers can be applied. Application of geochemical techniques is a common practice to investigate the equilibrium states of minerals and reservoir temperatures of geothermal systems. Such techniques involve various fluid-mineral equilibrium models based on ionic (Fournier and Truesdell 1973; Fournier and Potter 1979, 1982; Arniston 1985; Giggenbach 1988) or isotopic compositions (Bottinga1969; Richet et. Al. 1977; Ohmoto and Rye 1979; Giggenbach 1982) of discharging fluids. The calculation of subsurface temperatures from chemical analyses of water and steam collected at hot springs, and shallow water wells is a standard tool of geothermal exploration. Interpretation of the calculated temperatures requires knowledge of the most likely reactions to have occurred between the water and the surrounding rocks. Geothermometer-based temperature estimates in the geothermal resource assessment rely primarily on silica and cation geothermometers. In natural environments, it is often difficult to choose the correct geothermometer because it is not clear which mineral is controlling the dissolved silica concentration. Environmental Isotopic measurements, especially when carried out in parallel with conventional geochemical analyses, provide information on the characteristics of the geothermal fields. Studies proved that the δD, δ18O and δ3H values of waters, especially when combined with the concentrations of conservative solutes (for example Cl) are the best geochemical indicators of the origins, recharge locations, and flow paths of subsurface waters ( Truesdell and Hulston, 1980; Kharaka and Thordsen, 1992). The isotopic ratios of 18O/16O, 2H/H of waters from thermal and cold water can be used to interpret the origin of the groundwater system. Correlation or non-correlation between temperature and δ2H, δ18O and EC, or CL- and δ18O or other values can indicate some interaction between surface cold waters and thermal waters. Thermal samples plotted together with meteoric line may show some water mixings or water origin. All these geothermometers, including the isotopic ones, are affected to differing extents by processes occurring during the ascent to surface of the geothermal fluids. The parallel use of as many geothermometers as possible helps in evaluating correction terms and in attaining the right result.