The Enhanced Geothermal System of Soultz-sous-Forêts: A study of the relationships between fracture zones and calcite content

The Enhanced Geothermal System (EGS) of Soultz-sous-Forêts (France) is made of three boreholes (GPK2, GPK3 and GPK4). The hydraulic connection between the wells, which is crucial to get an efficient geothermal exchanger, may be unfortunately hindered by the more or less complete sealing of fractures by hydrothermal neoformed minerals. This paper takes over the recent work of the authors that quantified the amounts of calcite on a random sampling within the three boreholes in order to assess the role of this mineral on the exchanger zone and in particular its influence on the permeability of the flow pathways. In the present study we focused the sampling on the fracture zones that have been identified as flow pathways. Now, all the three wells show high (above 10 wt.%) and moderate to low (between 2.0 and 7.0 wt.%) calcite anomalies. In GPK2, fracture zones with a high conductivity are characterized by high calcite content, and those with a low conductivity by low calcite content. The fracture zones of GPK3 show the opposite relationship: the most efficient flow pathways have a low calcite content, whilst the less conductive show a high calcite content. Fluid flow in GPK4 is mainly accommodated by the fracture zone at 4775 m MD that is not calcite rich. The other fracture zones of GPK4, which are interpreted with a similar and low conductivity, show low, moderate and high calcite contents. These data show clearly that GPK2 is different from GPK3 and GPK4, these two later being more comparable. It suggests that the fracture zones themselves may be of different type, and that the connectivity to the fracture network may be different too. This difference of behaviour is consistent with microseismicity data showing that GPK2 is characterized by a rather compact and well structured network of medium-scale fracture whereas GPK3 and GPK4 are altogether characterized by more localized and discrete fracture zones. Some issues remain such as the occurrence of calcite anomalies with no evident relationship with fracture zones, as well as the occurrence of identified flow pathways with no abnormal calcite content.

Research Highlights
► Calcimetry is a simple and low-cost analytical method.
► It can be used directly on the drilling site.
► It should be used for the description of fracture networks in granitic reservoir.