| Abstract |
In order to exploit dry hot rocks or enhanced geothermal systems, it is critical to accurately locate an area (or zone) with a sustainable geothermal temperature (greater than 150ºC), which is buried as shallow as possible. Current study has revealed the search criteria for the viable enhanced geothermal systems, after examining the exploitation of the existing enhanced geothermal systems worldwide. 1.The target area should be focused on the active deep fault zones as much as possible, preferably the lithospheric fault zone and the crustal fault zone. This will ensure the accessibility and control of high temperature geothermal energy from deep crust regionally. 2.There are secondary deep fracture zones intersecting the active deep fault zones. These fracture zones serve as heat conduction channels. 3.There are hydrothermal activities or modern volcanic activities in the intersections. This indicates that the intersections are well connected to the deep crust geothermal sources. 4.The target area is covered with a certain thickness of sedimentary caprock, which is relatively enclosed to insulate and maintain the high temperature from the deep crust geothermal systems. Two types of geological structures (zones) are identified to meet all the above criteria. The first ones are located near the boundaries of grabens, which are formed as a result of lithospheric or crustal faults. The other ones are located in the modern volcanic zones, which are accompanied with hydrothermal activities. Our study has shown that shallow high temperature geothermal reservoirs (3~5 km) are mostly of hydrothermal convection system. In these areas, fractures are highly developed and the geothermal gradients are steep. Generally, it is advantageous to exploit the enhanced geothermal systems in the regions that have a geothermal fluid temperature of greater than 150ºC and a burial depth of lesser than 3000 m. On the other hand, the dry geothermal environment formed by the intact rock geothermal reservoirs, deep away from the fractured zone, has low geothermal gradients. Exploitation of such dry hot rock geothermal by using hydraulic fracturing method faces not only considerably high cost but less than satisfactory outcomes. |