Nature of Geothermal Resources


Dated: Ocak 24th, 2008

Geothermal energy is the heat of the earth generated from the core and emission to crust which can be extracted by naturally circulating water from the anomalously heated shallow part of earth crust, or water that is injected into the ground to sweep the heat from rock (hot dry rock).

Geothermal springs are the naturally emerged hot springs and steam with gases generated in relation with a heat anomaly formed by magmatic intrusions in geologic structure in shallow part of the crust, and have a temperature continuously more than mean atmospheric temperature of the region, and can have more dissolved solids and gases than that of surrounding groundwater and cold springs.
Hot dry rocks, which do not contain fracture and fluid, can also be considered as geothermal energy sources.
In resent years heat pumps are considered as geothermal energy sources too.

 Geothermal systems; are the systems consist of

•         Heat source in the crust (to form a heat energy by emplacement of magma and young volcanism or others such as radioactive decay)

•         Working fluid which is mainly meteoric water with a contribution of minor amount of magmatic, metamorphic, fossil waters circulating in the crust to transfer the heat energy by sweeping the heat of rock and bring to the reservoir zone, 

•         Fractures and reservoir zones to transfer the heated fluid and to accumulate it at geologically porous and permeable zones, at economically shallow depth in earth crust,

•         Impermeable cap rocks mostly to keep the heat and fluid in reservoir zones,

•         and recharge area to feed the system.

 The source of the heat can be described as

1-     mantle plumbs in the crust,

2-     magmatic chamber either itself or which can produce volcanic activities,

3-     decay of radioactive elements,

4-     and average geothermal gradient.

 These thermally anomalous areas are certain places that are formed by geologic processes at or around plate boundaries, intra plate rifts and young volcanic activities, deep basins for geo-pressured system or a foot of mountainous range in where radioactive heat source can be expected. In these areas subsurface temperature increases anomalously with depth, while below the upper several meters affected by solar radiation and infiltration of surface water. The rate of increase in temperature with depth (the geothermal gradient) varies widely according to the regional geology. High geothermal gradients typically are found along the boundaries of global “tectonic plates” often-characterized young or active volcanism, igneous intrusive activity and earthquake focals(Fig. 1). High gradient can also be found associated with certain intraplate rift zones and isolated centers of upwelling molten magma (mantle plumes or “hot spots” especially in oceans). Areas within the stable continental interiors far from plate boundaries and rifts, exhibit a lesser geothermal gradient than that of above mentioned areas. The geothermal springs or formation of geothermal systems in these areas are of the conclusion of deeply circulating meteoric water thorough fractures and/or pores. 
 Radioactive elements (notable radioactive elements, which can produce heat, are U238, U235, Th232, K40. These elements are mainly found in acidic rocks. Heavy radioactive elements such as U, Th can intrude to structure of minerals such as zircon, rutil, titan that are formed by having big atomic diameter elements, during the dissociation of magma. Major amount of U, Th can accumulate at suitable temperature (T) and pressure (P) conditions in acidic magma during solidification phase) present within rocks of the earth crust and mantle located in intraplate far from plate boundaries, generate some heat to form geothermal system with low enthalpy.

 The classification of energy types in geological environment, which can produce geothermal energy are the,

1-     Hydrothermal energy all result from the concentration of

2-     Geo-pressured energy earth’s heat in discrete regions of the

3-     Magma         subsurface by geologic processes.

 

4-     Earth energy (is thermal energy at the normal temperature of the shallow ground, without enhancement due to geologic processes. And can be tapped by geothermal heat pumps (1oC/33 m.)),

5-     Hot dry rock (occurs at depths of 5 to 10 miles beneath the surface everywhere. It is also occurs at shallower depths in areas of thermal enhancement due to geologic processes.

 
 

 

 

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