42 Geophysics in Geothermal Exploration the surface, which is then circulated through a primary network. This hot geothermal fluid is directed to a heat exchanger, where its heat is transferred to a secondary network used to distribute warmth to buildings across the district. After the heat has been extracted, the now-cooled geothermal fluid is directed into the reinjection well, where it is returned to the underground reservoir. This reinjection process is critical for maintaining the pressure balance within the geothermal reservoir, ensuring that the resource remains stable and sustainable over the long term. A list of the good practice guidelines on deep geothermal drilling and exploitation from experience in the Paris Basin (Dogger and Albian aquifers) is regularly published (Hamm et al., 2022). Figure 1.11 Localization of the geothermal doublets taping the Albian or Neocomian sands (pink) or the Dogger Limestones (green and blue) in Paris area (Hamm et al., 2022). In the Alsace region, geothermal energy in fractured granite and crystalline rock represents a promising frontier for accessing geothermal resources. The dense granite or crystalline rock basement of the Upper Rhine Graben sometime lacks adequate permeability. To overcome this, water is injected into the rock to create or expand existing fractures (Enhanced geothermal systems or EGS). The high thermal conductivity of granite and crystalline rock makes them efficient at transferring heat, allowing them to reach elevated temperatures that are suitable for both direct heating and electricity generation. France’s Soultz-sous-Forêts project, for example, has demonstrated the feasibility of EGS in fractured granite, highlighting the potential of these formations to supply substantial geothermal energy.
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