Explorations
Geothermal is a system involving a circulation of water from the earth's surface down to the earth's interior where the water picks up heats from the depth. The hot fluids then stay at depth and create geothermal reservoir. A fraction of the fluids often emerge at the surface in the form of hotsprings, geysers, and fumaroles. The hot fluids from the reservoir are useful for electric generations and other direct usage. To explore this system, The knowledge on geology, geochemistry, and geophysics is essential.
The flowline begins with geology and geochemsitry field works. And from their studies, when a direct evidence of deep reservoir containing high enthalpy fluids is found. the geophysical work could be started with the objective is to image the geometry, and the location of the prospect. A 3-D join inversions of multi-phycisal properties is a common practice today.
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Types of geothermal systems
There are various classifications of the systems, which based on various parameters, such as enthalpy, tectonic settings, fluid content, and entropy. Enthalpy, or spesific heat content of the reservoir fluids is commonly used to describe a geothermal system, namely high enthalpy, medium, and low enthalpy systems. In the SI systems enthalpy has the unit of kilojoules/kilogram (kJ/kg), thus the higher the enthalpy the more heat is brought by the carrying fluids to tthe surface compared to those low enthalpy systems. Therefore, high entalpy systems have more commercial values. The fluid content inside the reservoir is also commonly used to describe the systems, where three types are well known, i.e. vapor dominated, liquid dominated, and two-phase systems.
Permeability structures together with the heatsource, they drives the development of a geothermal system. Consider a reservoir with a reasonably high porosity and permeability. the reservoir is surrounded by a recharge belt and equipped with an overlying cap having a certain range of permeability. The reservoir is filled with a cold water, having a temperature that matches with the normal geothermal temperature gradient. When a heat pulse is conducted to the bottom of the reservoir at a right quantity and at the right duration, various geothermal systems are produced.
- When the permeability of both the cap and the recharge belt is high, the heat pulse yields only a liquid dominated medium enthalpy system. This is caused by the heat and the hot fluids that are continuously exposed to the surface. The hot fluids escapse to the surface are quickly replaced by the cold fluids from the recharge belt, and thus the heat pulse is unable to increase the reservoir temperature. A massive warm springs occur at the surface, but when the heat pulse dies off, these warm springs will cool off too.
- When the permeability of the cap is rather limited but the recharge belt is high, the heat pulse produces a liquid dominated high enthalpy system. This is caused by the importat role of the cap to keep the heat and the hot fluids within the reservoir. A fraction of the hot fluids still emerge at the surface in the form of fumaroles or boiling springs. When the heat pulse dies off, the hot reservoir is still able support the life of the fumaroles and hotsprings for a quiet long time.
- When the permeability of both the cap and the recharge belt is rather limited, the heat pulse produces a liquid dominated high enthalpy system that later it transforms into a vapor dominated system. Since a small fraction of the hot fluids escape to the surface and there is no recharge support to replace the fluids, the hot liquid in the reservoir undergoes a continuous evaporation. The life of this a vapor system is short, especially those volcano hosted systems. The vapor system perishes right away after the heat pulse dies off.
