Geochemical techniques for the exploration and exploitation of geothermal energy / Геохимические методы разведки и эксплуатации геотермальной энергии

Before discussing the geochemical techniques applied in the exploration and exploitation of geothermal resources, it is convenient to recall few general concepts.  Geothermal energy is the natural heat of the earth, which is transferred towards the surface through conductive and convective processes. Because of these processes, the temperature within the earth crust generally increases with depth of ∼30 °C/km. Assuming that the yearly average temperature at the earth surface is 15°C, a temperature close to 105°C should be found at ∼3 km depth. However, in the present economic and technologic framework, it is generally not convenient to extract these relatively low-temperature fluids from these comparatively great depths. From this simple consideration, it is clear that geothermal energy can be economically exploited in geologically favorable contexts, where a natural thermal engine, which is generally represented by the heat released from magma batches situated at depths of few kilometers, is present. This natural thermal engine triggers the convective circulation of the waters contained in the overlaying rocks. Thus, these fluids ascend towards the earth surface and are stored in relatively shallow reservoirs. These geothermal reservoirs are separated from cold waters, which are generally present at shallow depths, by either primarily impervious layers or by rocks whose poor permeability is due to self-sealing phenomena. 
Since the energy contained in geothermal fluids is much smaller than that contained in the same amount of oil, a further requirement for the economic exploitation of geothermal energy is the storage of geothermal fluids into reservoirs of comparatively high permeability.  <...>