Enhanced-Geothermal-Systems, Hot-Dry-Rock Systems, Deep-Heat-Mining

Abstract

With Enhanced-Geothermal-Systems (EGS) the deep underground is used as a source of heat for the production of electrical and thermal energy irrespective of the hydraulic properties of the deep heat reservoir. In other words the rocks are hot at depth irrespective of whether or not they qualify as aquifers or aquitards. Weakly fractured granites at depth are best described as “hot dry rocks” (HDR systems). However, keep in mind that also the few fractures present are interconnected and filled with hot pore water. The term “hot dry rock” originates from the early-days of geothermal energy utilization where the concept was to drill a deep wellbore into hot but assumingly dry rocks for extraction of thermal energy. Later it was found that the fracture porosity of continental rocks is always saturated with hot water so that the term hot-dry-rock became rather misleading. The upper continental crust is always fractured; its fracture density differs however. A saline, occasionally gas-rich fluid is typically present in the fractures. The geothermal utilization of the hot underground with low hydraulic conductivity is sometimes also referred to as “deep heat mining” (DHM). Because the continental crust is predominantly granitic or gneissic, HDR systems strongly focus on granitoid heat reservoirs. Typical target temperatures for HDR systems are above 200 °C. This means that wellbores of 6–10 km have to be drilled in continental crust of average geothermal gradient.